Download The relationship between field-aligned currents and the auroral

THE RELATIONSHIP
CURRENTS
AND
BETWEEN
FIELD-ALIGNED
THE AURORAL
ELECTROJETS:
A REVIEW
Y. K A M I D E *
NOAA/ Space Environment Laboratory, Boulder, Colo. 80303, U.S.A.
(Received 9 October, 1981)
Abstract. The recent development of several new observational techniques as well as of advanced
computer simulation codes has contributed significantly to our understanding of dynamics of the
three-dimensional current system during magnetospheric substorms. This paper attempts to review the
main results of the last decade of research in such diverse fields as electric fields and currents in the
high-latitude ionosphere and field-aligned currents and their relationship to the large-scale distribution
of auroras and auroral precipitation. It also contains discussions on some efforts in synthesizing the vast
amount of the observations to construct an empirical model which connects the ionospheric currents
with field-aligned currents. While our understanding has been greatly improved during the last decade,
there is much that is as yet unsettled. For example, we have reached only a first approximation model
of the three-dimensional current system which is not inconsistent with integrated, ground-based and
space observations of electric and magnetic fields. We have just begun to unfold the cause of the
field-aligned currents both in the magnetosphere and ionosphere. Dynamical behaviour of the magnetosphere-ionosphere coupling relating to substorm variability can be an important topic during the coming
years.
Contents
l. Introduction
2. 'Equivalent' Three-Dimensional Current Systems
2. I. Importance of Field-Aligned Currents
2.2. Three-Dimensional Current System Inferred from Ground Magnetic Observations
2.3. Calculation of Magnetic Perturbations by Model Three-Dimensional Current Systems
2.3.1. Substorm Current Systems
2.3.2. Relatively Quiet Conditions
3. Observations of Field-Aligned Currents
3.1. Early Observations of Transverse Magnetic Perturbations
3.2. Gross Field-Aligned Current Pattern
3,3. Recent Polar-Orbiting Satellite Observations
3.3.1. TRIAD Satellite Observations
3.3.2. ISIS-2 Observations
3.4. Projection of the Field-Aligned Current Region into the Magnetosphere
3.5. Indirect Observations of Field-Aligned Currents
3.5.1. Field-Aligned Currents as Divergence of Ionospheric Currents
3.5.2. Polar Cap Electric Field
3.5.3. Meridian Line Magnetometer Data
4. Spatial Relationship of Field-Aligned Currents to the Distribution of Auroras and Electrojets
4.1. Diffuse and Discrete Auroras
4.2. Field-Aligned Currents and Auroras
4.2.1. Large-Scale Field-Aligned Currents
* On leave of absence from Kyoto Sangyo University, Kyoto 603, Japan.
Space Science Reviews 31 (1982) 127-243. 0038-6308/82/0312-0127 $17.55.
Copyright (~ 1982 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.
128
Y. K A M I D E
4.2.2. Small-ScaleField-Aligned Currents
4.2.3. Field-Aligned Currents and Radar Auroras
4.3. Global Auroral Features and Currents
4.4. Field-Aligned Currents and the Auroral Electrojets
5. IonosphericElectric Fields and Currents
5.1. Electric Field Pattern
5.2. Electric Field Associated with Auroras
5.3. Electric Field Near the Harang Discontinuity
5.4. IonosphericConductivityand Currents
5.4.1. IonosphericCurrents and Ground Magnetic Perturbations
5.4.2. Altitude Dependence of IonosphericCurrents
6. Modelingof the Three-Dimensional Current
6.1. Electric Fields
6.2. Field-AlignedCurrents
6.3. Ionosphere-MagnetosphereCoupling
6.4. Estimation of the Three-Dimensional Current System From Ground Magnetic Perturbations
6.4.1. Latitudinal Profile of the Auroral Electrojets
6.4.2. Advanced Methods
6.4.3. IMS Alaska Meridian Chain
7. ConcludingRemarks
7.1. Empirical Current Model
7.2. Model Current System and Auroral Distribution
7.3. Future Problems
1. Introduction
The determination of the three-dimensional electric current configuration associated with magnetospheric substorms has been one of the central problems in
magnetospheric and ionospheric physics. Until very recently, attempts at inferring
the current distribution have relied primarily upon magnetic observations made on
the Earth's surface. By using the worldwide distribution of the ground magnetic
perturbation vectors, several equivalent current systems for the polar magnetic
substorm have been put forward, which are quite successful to identify the patterns
or modes of the global magnetic disturbance field. By their definition, such
equivalent currents are supposed to flow on a spherical shell which is concentric
to the Earth. If they were real currents, they would flow in the ionosphere. General
reviews on the proposed current systems have been published, including the work
of Fukushima and Kamide (1973), Matsushita (1975), and Mishin (1977). However,
it must be emphasized that magnetic perturbations observed on the Earth's surface
arise from ionospheric, field-aligned and magnetospheric currents, as well as from
an induced current flowing within the Earth. In principle, the separation of the
magnetic fields of these different current sources cannot be properly made from
ground magnetic measurements alone (Chapman, 1935). Therefore, it is difficult
to deduce the 'real' three-dimensional current system only from ground magnetic
measurements.
Subsequently, a combined study of ground magnetic perturbations, simultaneous
auroral displays, auroral precipitation and electric fields in the ionosphere made it
FIELD-ALIGNED CURRENTS AND AURORAL
E L E C T R O JETS
129
possible to construct a first-approximation model of the three-dimensional current
flow. However, such correlated studies of spatial relationships among the auroral
luminosity, the field-aligned currents and the auroral electrojets have been limited
by the lack of simultaneous data for a variety of substorm conditions and for wide
local time intervals. In addition, the sparse network of ground-based magnetic
observatories makes it difficult to study the auroral electrojet configuration with
sufficient detail. Therefore, although several possible three dimensional current
systems for the substorm have been proposed, they must be regarded as 'equivalent'
three-dimensional current models at best.
In the last decade, with the advent of new techniques, attempts have been made
to remove these difficulties to a significant degree. Of particular importance are
the polar-orbiting ISIS, DMSP, T R I A D , and $3-2 satellites which can provide us
with plentiful information on characteristics of the field-aligned currents and their
relationship to both auroral intensity and auroral electrojet flow. Incoherent scatter
radars at auroral latitudes can also determine simultaneously most of the electromagnetic properties in the ionosphere including the electric fields, conductivities,
currents, neutral winds and Joule heat dissipation (see Banks and Doupnik, 1975).
Recent efforts to improve the ground-based magnetometer network by operating
several meridian chains have made it possible to determine the auroral electrojet
locations with an accuracy of 1~ in latitude (see Akasofu et al., 1980). By combining
these new observations, we are just beginning to construct the 'real' threedimensional current model that can account for the complexity in behaviour of the
ionospheric electric fields and currents and the field-aligned currents during different
substorm periods. The plausibility of the model current system can then be tested
by theoretical and computer simulation studies, in which efforts are made to clarify
problems as to what parameters are essential to reproduce the observed characteristics of the electric fields and currents.
In this paper, an attempt is made to sum up the main results during the last
decade of constructing three-dimensional models which link the field-aligned and
ionospheric currents.
2. 'Equivalent' Three-Dimensional Current Systems
2.1. IMPORTANCE OF FIELD-ALIGNED CURRENTS
At the beginning of this century, Birkeland (1908, 1913) explained the world
distribution of magnetic field variations during a negative polar elementary storm
(which is now called a negative bay in latitudes in the midnight sector during a
magnetospheric substorm) by a simple flow of electric currents; the current flowing
down along the field lines in the morning sector is connected to a westward
ionospheric current at a certain altitude above the Earth and then recedes again
as another flow of field-aligned current in the evening sector. As will be seen in
this review paper, Birkeland's (1908) ideas are basically valid even today, except
130
Y. KAMIDE
that his field-aligned currents were provided by a beam of electrons coming directly
from the sun. H e considered two different possibilities for the cause of the magnetic
perturbations in high latitudes: (1) The entire current belongs to Earth, or rather
at some height above it, which implies the conductive ionosphere. (2) The current
consists principally of vertical portions, which we now call field-aligned currents.
The following are direct quotations from Birkeland (1908) which seem worth
repeating:
"It is true that systems of plane curves can always be arranged for a given field, which, from a pure
mathematical point of view, would be able to explain the field; but when we consider the physical
conditions for the formation of such a system, we meet with great difficulties for it is not easy to maintain
a current with this peculiar form which moreover remains constant for several hours."
"If we assume, as from a physical point of view, we might legitimately do, that the current is of a
cosmic nature, and consists of negatively and positively charged corpuscles, the trajectories of the
separate corpuscles must more or less approximately follow the magnetic lines of force, moving in
spirals around them."
Birkeland assumed that the bulk of the precipitating particles were able to
return upwards thus giving rise to a descending current, explaining the magnetic
perturbation on the Earth's surface by a current system with two vertical currents
in opposite directions connected by a horizontal section. As Birkeland pointed out,
this current system can explain many of the properties of the polar storm. The
disturbance field in the auroral zone shows large and sudden spatial and temporal
variations but at lower latitudes the changes take place very gradually. At the
auroral stations, we are near the ionospheric current which may move and change
intensity locally, but at the lower latitude stations we observe the integrated effect.
Alfv4n (1939, 1940) studied the motion of particles in combined magnetic and
electric fields, and proposed a new idea to explain the SD field by the auroral
electrojets connected to field-aligned currents. The SD field can be regarded as an
equivalent ionospheric current system obtained for averaged substorm conditions.
In his original theory, the earthward approach of the drift of charged particles
under the dusk-to-dawn electric field. The charged particles emitted from the Sun
undergo the gradient drift in an Earth's magnetic field, leaving a forbidden region
around the Earth that is not symmetric with respect to the Sun-Earth line. The
difference in size of the forbidden regions for electrons and protons causes discharge
along the field lines toward the ionosphere, where the current tends to flow along
the auroral zone which is a conductive strip. The magnetic effect of Alfv6n's model
current on the Earth's surface was checked experimentally by using a wire model
with a small movable search coil. It was shown that the measured magnetic field
at various locations was to be in reasonable agreement with the SD field.
In the sixties after the historical discovery of the solar wind and the magnetosphere, there have been at least three major works which pointed out the importance
of the three-dimensional closure of the auroral electrojets.
Fejer (1961) postulated that the ring current particles are injected into the
trapping region in the sunlit sector and that the asymmetric ring current thus
produced completes its circuit by generating the Pedersen and Hall currents in the
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
131
polar ionosphere. Since there is much evidence now that the ring current particles
consist of mainly protons of a few keV and that they often form the ring current
belt well inside the trapping region, they cannot be directly injected from the
solar wind. If the partial ring current grows in the day sector, a current tends to
flow from the sunset meridian to the sunrise meridian. This incomplete circuit
generates space charges at both ends of the ring current, and consequently a
potential difference across the magnetosphere is produced. The resulting electric
field is directed from the dawn to the dusk sectors. Such a system may complete
the circuit by driving currents along the field lines and in the ionosphere. The
pattern of the currents in the ionosphere depends on the distribution of the
conductivity. Fej er (1961) suggested that the resulting Hall current system resembles
the SD current system.
The current pattern would be greatly distorted if the conductivity is not uniform.
For example, if there is a highly conductive strip, like the auroral oval, between
the 06 and 18 LT meridian, the Pedersen current 0-1E flowing westward will be
channeled along the oval in the dark hemisphere. Furthermore, the Hall current
-o-2E x B/B, which flows across the high conductive strip, generates space charges
near the poleward and equatorial boundaries of the oval. In the northern hemisphere,
a positive space charge appears on the poleward boundary of the oval and a negative
space charge appears on the equatorial boundary; the electric field associated with
the space charges is directed equatorward and drives an additional westward Hall
current in the dark sector. This process is often expressed as an increase of the
2
conductivity. The total current J is then given by J = o-E, where 0-3 = 0-1-{-o"2/o1
and is called the Cowling conductivity.
Bostr6m (1964, 1968) pointed out that there are possibly two types of threedimensional current systems, type I and type II. In type I, a current flows into the
ionosphere at the end of the east-west auroral electrojet and out at the other end.
This system is essentially the same as the field-aligned current system proposed
originally by Birkeland (1908). In the type II system, however, the driving electric
field is meridional so that a sheet current flows into the ionosphere on the northern
(or southern) side of the auroral electrojet and out of the southern (or northern)
side of the electrojet. Bostr6m proposed, in the early sixties, to decide whether
type I or II is nearest the real current, that a possible measurement be needed of
the magnetic field using a magnetometer onboard a polar orbiting satellite.
Atkinson (1967) considered a similar current system, but some modification to
simulate poleward expanding bulge and the westward travelling surge was included.
H e demonstrated, by an analog computer calculation, that the combination of a
field-aligned current and an ionospheric Hall current reproduces well the extremely
complicated distribution of magnetic vectors in and near the region of active auroras.
In the auroral arcs, the ionospheric conductivity is strongly enhanced. However,
since the Hall current must be continuous across the boundary of the electrojet, a
southward polarization electric field will be produced which lowers the northward
current component in the electrojet. It will also drive a westward Hall current in
132
Y. KAMIDE
the region of enhanced conductivity, such that the net effect is an intense and
confined electrojet.
2.2.
THREE-DIMENSIONAL
C U R R E N T SYSTEM I N F E R R E D F R O M G R O U N D
MAGNETIC PERTURBATIONS
In earlier studies (see Sugiura and Chapman, 1960, and references therein), it was
tacitly assumed that in mid- and low-latitudes the Dst field of the H component
perturbations is produced by an axially symmetric ring current in the magnetosphere
and that the DS field results from an ionospheric current. From the detailed analysis
of a number of large storms, however, Akasofu and Chapman (1964) gave several
reasons why the DS part cannot be entirely ascribed to the ionospheric return
current of the auroral electrojets, and suggested that the storm-time ring current
belt may sometimes be quite asymmetric. Figure 1 shows the iso-intensity contours
of the H component perturbations at each UT hour for the September 13, 1957,
storm which was one of the most intense storms during the IGY period. A
considerable longitudinal (local time) asymmetry can be noticed, with the maximum
depression around 18:00 MLT. Similar contour plots were employed later by Meng
and Akasofu (1971), Kawasaki and Akasofu (1973), and Kamide (1976) to examine
the progressive change of the global pattern of the asymmetry. The local time
shift of the phase of DS in conjunction with the growth and decay of the storms
was examined by Sugiura (1968).
Akasofu and Chapman (1964) concluded that the observed longitudinal asymmetry is in many cases too large to attribute it totally to the ionospheric return
current, but the ring current itself must have different intensities at different local
times, being largest in the evening sector. This suggestion was first confirmed by
Cahill (1966) and Frank (1970) by direct observations in the magnetosphere of the
asymmetric inflation of the storm-time magnetic field and the largest particle
population in the evening sector.
From a simple law of current continuity, such an ~asymmetric ring current'
requires field-aligned currents at both edges of the 'partial' ring current. Cummings
(1966) constructed a wire model of the partial ring current system, consisting of
the field-aligned currents and the partial ring current with their longitudinal extent
to be variable. Assuming exponential decays for both symmetric and partial ring
currents, the total magnetic effect was measured on a model Earth's surface. It was
shown that even such a simple wire loop model can account for most of the observed
features of the recovery phase of magnetic storms.
In the early work of Akasofu and Chapman (1964), they did not connect the
field-aligned currents with the westward electrojet, because it was found that the
mid-latitude H asymmetry is observed even when the electrojet activity is quite
low. This relation between the auroral electrojet and the asymmetric ring current
was also emphasized by Kawasaki and Akasofu (1971), who showed that Asy index
(similar to DS in mid-latitudes) is maximum when the intensity of the westward
electrojet is growing and rather weak. Fukushima and Kamide (1973) suggested
-O0:LO [~Aaolu! oql ~o] suo~leqanl~d tu~uodtuoo H 0!l~u~tuoo~ jo sJno~,uoo ~!su~,u!-~.nb~
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Y.
KAMIDE
that the lack of a definite quantitative relation between these two might come from
the fact that the parameter representing the auroral electrojet depends on the
latitudinal current density of the electro jet, whereas the ring current field is produced
by the total current of the equatorial ring current in the magnetosphere. Hence,
regardless of the lack of the definitive relation, we may be allowed to consider a
possible connection between the auroral electrojets and the ring current.
I
I
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Fig. 2. Three-dimensional current flow proposed by Akasofu and Meng (1969) to represent the
substorm current system. (Akasofu, S.-I. and Meng, C.-I.: 1969, J. Geophys. Res. 74, 293.)
Figure 2 illustrates a three-dimensional current model for the world magnetic
disturbances during substorms, which was derived from an extensive study of
magnetic records from more than 70 observatories (Akasofu and Meng, 1969). It
was noted that such a current system can reproduce surprisingly well the observed
distribution of magnetic perturbation vectors over the Earth's whole surface at
about the maximum epoch of polar magnetic substorms. This implies that the
ionospheric return current of the auroral electrojet, if any, seems to have only a
minor contribution in mid- and low-latitudes, since there exists a striking similarity
between the magnetic records from the synchronous satellites and those from
ground observatories located in nearly the same longitude as the satellites (Cummings and Coleman, 1968). Meng and Akasofu (1969) supported the existence of
such a current system on the basis of the distribution of the D component variation
in mid-latitudes.
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELEC~ROJETS
135
Another possible and perhaps more plausible current system is a modified version
of the tail current by bringing a part of it to the auroral oval (Akasofu, 1972). This
current configuration can be accomplished by supposing that a part of the tail current
is suddenly disrupted at the onset of a substorm. Akasofu (1972) considered that
the so-called positive H bay in the night sector can be explained by the disappearance of the dawn-to-dusk tail current. Because of the lack of the asymmetric ring
current, however, this model fails to reproduce ground magnetic variations in midand low-latitudes in the sunlit hemisphere.
More recently, McPherron et al. (1973) summarized their concepts of substorm
current flow and proposed a phenomenological model current system, which is
essentially the same as the one proposed by Akasofu (1972). They suggested that
the disruption of the tail current causes a local collapse of the tail magnetic field
to a more dipolar configuration. Clauer and McPherron (1974a, b) further showed
that positive bays in mid-latitudes during substorms can be explained mainly by
the field-aligned currents which are connected to the westward electrojet in the
nightside ionosphere.
It should be noted that the actual relation between the auroral electrojet and
the ring current is not at all so simple (Davis and Partharathy, 1967; Grafe, 1972)
as all these models imply. There is no simple correlation between high-latitude and
mid-latitude magnetic variations. The complicated relation may partially be due
to inadequacy in measuring the auroral electrojet, which is usually recorded as the
H component perturbation at a high-latitude observatory that represents only a
local concentration of the electrojet along the auroral oval. It is also due to the
fact that two auroral electrojets, eastward and westward, can be enhanced
significantly during a substorm, but each has its own characteristic time for growth
and decay.
The latitudinal width of the auroral electrojet is an important factor in estimating
the total electrojet current intensity. Pudovkin et al. (1968) suggested that the
correlation between the DP and DR fields becomes better if the statistical width
of the westward electrojet (Starkov and Feldstein, 1967) is taken into account.
Feldstein and Sheven (1966) found that the longitudinal asymmetry of the lowlatitude H decrease is large or small according to whether magnetic activity in the
auroral zone is strong or very weak. They explained the observed low-latitude
asymmetry of the H decrease as the effect of (1) a possible eccentricity of the
symmetric ring current, (2) an increase in the magnetospheric surface current, and
(3) the return current of the auroral electrojets. Later, Troshichev and Feldstein
(1972) confirmed a good parallel relation between auroral electrojet intensity and
maximum equatorial H decrease, where the latter is thought to include the effect
of a partial ring current. These papers seem to emphasize a close relationship
between the longitudinal asymmetry of the H decrease in low latitudes and the
auroral electrojet. Shevnin (1970) attributed further the low-latitude A H asymmetry to a partial ring current flowing in the equatorial plane, and concluded that
such a partial ring current is centered on the night side of the magnetosphere and
136
Y. K A M I D E
shifts westward with a speed of 30~
with respect to the rotating earth.
Troshichev and Feldstein (1972) studied the progressive changes during magnetic
storms in the meridian of maximum H decrease and concluded that the maximum
H decrease is seen first at the meridian of 16h-18 h LT and that it shifts westward
with the expansion of the nightside westward electrojet.
By separating ground magnetic effects into symmetric and partial ring currents,
Kamide and Fukushima (1971) noticed the following characteristics: The partial
ring current seems in general to develop and decay earlier than the symmetric ring
current, which is responsible for the worldwide uniform decrease in H. The intensity
of the partial ring current is very often comparable to that of the symmetric
equatorial ring current during the main phase of magnetic storms; sometimes the
partial ring current is even stronger than the symmetric component. The time
variation in the partial ring current intensity is quite similar to that of electrojet
intensity throughout the storm.
The asymmetric development of the H component perturbations in mid-latitudes
in connection with the auroral electrojets has been studied also by Crooker and
Siscoe (1971) and further by Crooker (1972) with the idea of separating the
mid-latitude field into two components; the geomagnetic bay current system and
a partial ring current system. They referred to the equivalent current model by
Silsbee and Vestine (1942) for the bay current system. It was found that a systematic
movement exists of the local time of the maximum depression toward earlier local
time with an increase in A E and statistical time lag of the mid-latitude depression
behind the A E index. To explain the actual longitudinal distribution of the midlatitude H variation, Crooker and McPherron (1972) reached the conclusion that
the positive H near midnight is caused by 'short circuiting' of the tail current along
the field lines and the negative H near the dusk meridian is due mainly to a partial
ring current.
This double current system is quite similar to that proposed independently by
Kamide and Fukushima (1972). The dual current system shown in Fig. 3 is based
on a peculiar relation of the eastward and westward electrojets in high latitudes
of the evening sector. The westward electrojet flows along the auroral oval while
the eastward electrojet flows along the so-called auroral zone, which is several
degrees equatorward of the auroral oval in the evening region. This feature was
pointed out first by Harang (1946) and later by Akasofu et al. (1965), Akasofu
and Meng (1967, 1968), and many others. The development and decay of
positive and negative bays in high latitudes are usually different from each other.
Kamide and Fukushima (1972) also found the following characteristics of positive
bays in high latitudes, which are favourable to the hypothesis of a connection
between the eastward electro jet and a partial ring current: (1) the eastward electro jet
center shifts equatorward during the expansive stage of a substorm in spite of the
poleward expansion of the westward electrojet near midnight; and (2) the positive
bay region shifts westward as it grows.
FIELD-ALIGNED CURRENTS AND AURORAL ELECTROJETS
137
Model Current System for
Polar Magnetic Substorm
Fig. 3. Three-dimensional current configuration porposed by Kamide and Fukushima (1972) to
represent substorm-associated current flow. (Kamide, Y. and Fukushima, N.: 1972, Rep. Ionos. Space
Res. Japan 26, 79,)
More recently, Rostoker (1974) presented a new interpretation of magnetic field
variations in which he described changes of the real three-dimensional current flow
associated with substorm activity. The key to his model is the suggestion that during
the substorm expansive phase the region of current outflow moves westward in an
impulsive fashion (Wiens and Rostoker, 1975) without a dramatic increase in the
upflowing current. The onset of a substorm is characterized only by the westward
stepping of the upward current region. However, we know that during the substorm
expansive phase, the magnetic field in the entire magnetosphere undergoes a
significant change that cannot totally be attributed to the localized current loop
near midnight. Perhaps, Rostoker's model is applicable to small-scale
intensifications in the course of a global substorm.
2.3. M A G N E T I C PERTURBATIONS PRODUCED BY MODEL THREE-DIMENSIONAL
CURRENT SYSTEMS
2.3.1. Substorm Current System
Magnetic fields of any model three-dimensional current system can be calculated
to examine whether or not the proposed current system can reproduce reasonably
well the actual observed distribution of the magnetic disturbance vectors over the
world. Technically, such a numerical calculation is quite complicated. In fact,
138
Y. KAMIDE
Kirkpatrick (1952) and others worked analytically on how to derive simple
forms of equations. In these days, however, the use of high-speed computers
has made it possible to calculate the magnetic field at any point in the magnetosphere and on the Earth's surface caused by model current systems with relative
ease.
Akasofu and Meng (1969) adopted a slightly modified version of Kirkpatrick's
(1952) model, which attempted to reproduce the SD field. The only major difference
is the clockwise rotation of Kirkpatrick's system by 90 ~ such that the westward
electrojet has its maximum at the midnight meridian. This three-dimensional current
system consists of (1) the equatorial ring current flowing along a circle at synchronous
distance, (2) the field-aligned current, and (3) the corresponding auroral electrojet.
Without taking earth's induction effect into account, Akasofu and Meng (1969)
calculated the latitudinal distribution of the resulting H and D components along
the noon-midnight meridian and dawn-dusk meridian, respectively, and compared
them with the observed values at the maximum epoch of an intense substorm; see
Figure 4. The model calculation reproduced fairly well the observed distribution
for the two components especially in mid- and low-latitudes. It is quite obvious
that the adopted model is too simple to reproduce the distribution of the magnetic
vectors in high latitudes, where many local structures are expected. Akasofu and
Meng (1969) noted only that the discrepancy between the observation and the
model calculation might be caused by inadequacy of the model in the vicinity of
the auroral oval. Note also that while the model current is symmetric with respect
to the noon-midnight meridian, the actual electrojet should have a strong asymmetry
in terms of the local time dependence of the current intensity.
Kamide and Fukushima (1971) made a detailed calculation of the magnetic fields
of a three-dimensional current system in order to see which element (of the partial
ring current, the field-aligned current, or the ionospheric current) contributes most
to the magnetic perturbations observed on the Earth's surface. Further, Fukushima
and Kamide (1973) and Siscoe and Crooker (1974) showed that the field-aligned
current portion makes a larger contribution to the logitudinal asymmetry of the
magnetic field than do the other elements of the circuit.
Magnetic disturbances near the auroral electro jet have been examined extensively
by Bonnevier et al. (1970) and Kisabeth and Rostoker (1971) who modeled a
three-dimensional current loop as a superposition of simple current elements which
are essentially the same as Bostr6m's (1964) type I and type II current systems.
Bonnevier et al. (1970) showed that magnetic disturbances Qbserved by a meridian
chain of magnetometers in northern Europe during substorms can be fitted by type I
current system. Kisabeth and Rostoker (1971) analysed extensively magnetic data
obtained by the Canadian meridian chain of observatories during substorms and
found that the simplest ionospheric currents that produce the observation field
must be closed by type I field-aligned current. A mathematical method of complicated three-dimensional current systems has recently been summarized by Kisabeth
and Rostoker (1977).
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E
UT
100
150 L
L
I
I
I
90
80
70
60
I
I
50
40
DIPOLE LATITUDE
i
I
I
50
zo
Io
J
0
Fig. 4. (a) Latitudinal distribution of the H component for the model current system, together with
the observation. (Akasofu, S.-I. and Meng, C.-I.: 1969, 3". Geophys. Res. "/4, 293.) (b) Latitudinal
distribution of the D component for the model current system, together with the observation. (Akasofu,
S.-1. and Meng, C.-I.: 1969, J. Geophys. Res. 74, 293.)
140
Y. KAMIDE
One of the common defects of most of these works is that the field-aligned
currents are assumed to flow along the dipole field lines. Such an assumption is
certainly untenable, in particular, for currents flowing along high-latitude field lines.
Indeed, Haerendel et al. (1971) and Fairfield (1973) presented some evidence
which suggests the existence of field-aligned currents in the high-latitude lobe of
the magnetotail where magnetic field lines differ considerably from the pure dipole
field lines. Based on a realistic model of the magnetosphere Kamide et aL (1974)
showed that this deviation from the dipole configuration becomes serious when we
consider field-aligned currents in the magnetotail. They demonstrated that the
major parts of the well known 'positive' bays in low latitudes on the Earth's surface,
the positive H variations at synchronous orbit and the positive B z variations along
the negative X axis during magnetospheric substorms can all be caused by a
three-dimensional current system consisting of a field-aligned flow along tail-like
field lines and the auroral electrojet along the auroral oval.
Clauer and McPherron (1974a) showed that the pattern of the three-dimensional
current circuit varies significantly from one substorm to another such that the
current system does not stay in the same local time around midnight. Kawasaki et
al. (1974) assumed that the three-dimensional current pattern is quite variable
even during the lifetime of a single substorm, and showed that a combination of
its intensification and longitudinal movement gives rise to quite complex time
variations of the magnetic field in mid-latitudes even for relatively isolated substorms. They used a three-dimensional current model with time dependent spatial
variations to simulate one type of the complex mid-latitude substorm signature.
It was demonstrated that there is a systematic shift in the apparent onset of the H
component positive bay in mid-latitudes with increasing west longitude. The
calculated magnetic perturbations for the pre-midnight sector, such as at 21:00 LT,
caused by both temporal and spatial expansion of the simple three-dimensional
model system shows that the H component first goes negative and then positive.
The first negative change, in this case, can simply be interpreted as the substorm
effect, not as a small change preceding the substorm onset. This only means that
the region of negative H was located outside of the field-aligned current system
in the early stage of the substorm.
2.3.2. Relatively Quiet Conditions
There also exists a particular polar magnetic phenomenon, denoted by S~, in the
polar cap, which differs from what one would expect from the dynamo theory of
low latitude daily magnetic variation (Nagata and Kokubun, 1962). This daily
variation was examined in great detail by Kawasaki and Akasofu (1967) and
Feldstein and Zaitsev (1967). They showed that the polar cap daily variation is
essentially a daytime phenomenon and that it occurs even during extremely quiet
days (Y~Kp = 0). On the other hand, it has been proposed by Nishida et al. (1966)
that there exists a distinctive type of worldwide magnetic variation which is not
directly associated with enhancements of the auroral electrojet. This variation,
FIELD-ALIGNED CURRENTS AND AURORAL
ELECTROJETS
141
subsequently called the DP2 variation (Nishida, 1968a; Obayashi and Nishida,
1968), has been found to be correlated with changes of the north-south component
of the interplanetary magnetic field (Nishida, 1968b; see also Brathwaite and
Rostoker, 1981). The DP2 equivalent current system, which resembles the SD
current system, consists of two current vortices without the concentration of the
current along the auroral region. Nishida and Kokubun (1971) stated that S~ and
DP2 are essentially the same, which represent magnetic effects of the Hall current
of the dawn-dusk convection electric field.
It has been pointed out that field-aligned currents play an important role also
in these vortex modes of current systems. Kawasaki and Akasofu (1973) proposed
that the charge distribution is continuously maintained by an external source through
an inward field-aligned current from the dawnside magnetopause to the forenoon
sector of the auroral oval (positively charged) and an outward field-aligned current
from the afternoon sector of the oval (negatively charged) to the duskside magnetopause. They showed that the S~ field obtained by Kawasaki and Akasofu
(1967) and Feldstein and Zaitsev (1967) can be reasonably well explained by a
model current system consisting of such field-aligned currents (of order 105 amp)
in addition to convection currents in the ionosphere. Leontyev et al. (1974) computed the total magnetic effect of field-aligned and ionospheric currents with
allowance for the different conductivity of the day and nighttime ionosphere. They
showed that the resultant equivalent ionospheric current system coincides with a
current system of the DP2 type.
It should be noted that several papers describe the possible existence of fieldaligned currents flowing from one hemisphere to the other to produce some
characteristics of the Sq field in low latitudes (Van Sabben, 1966; Maeda and
Murata, 1966; Yanagihara, 1972).
3. Observations of Field-Aligned Currents
It is only in the last decade that the presence of the field-aligned currents has been
confirmed with particle and magnetic field observations acquired from rocket and
satellite instruments. The observations of the field-aligned currents can be classified
into three groups in terms of the regions where the measurements are made; rocket
observations at ionospheric altitudes, low altitude (<3000 km) observations by
polar-orbiting satellites, and observations in the magnetosphere. A large number
of review papers (Arnoldy, 1974; Armstrong, 1974; Anderson and Vondrak, 1975;
Cloutier and Anderson, 1975; Sugiura, 1976; Russell, 1977; Potemra, 1977)have
already been published on observations of the field-aligned currents and their main
characteristics in terms of local time variations and their relation to auroral display
and energetic particle precipitation. Therefore, only the main results are discussed
in this section with special emphasis placed on the connection between the fieldaligned currents and the auroral electrojets.
142
3.1.
Y, KAMIDE
EARLY
O B S E R V A T I O N S OF T R A N S V E R S E M A G N E T I C P E R T U R B A T I O N S
The first satellite measurements of the field-aligned currents, more accurately, of
transverse magnetic perturbations, were made by Zmuda et al. (1966, 1967) with
a fluxgate magnetometer aboard the satellite 1963-38C at an altitude of 1100 kin.
The magnetometer yielded field variations orthogonal to the local geomagnetic
field direction to within several degrees, but the disturbance direction within this
transverse plane was unknown. In more than 90% of all satellite passes through
the auroral region, transverse fluctuations of several hundred nanotesla (nT) magnitude were observed. They also reported the region of the transverse magnetic
disturbances was confined to the auroral oval. Although initially these fluctuations
were interpreted as hydromagnetic waves, it was soon realized that their latitudinal
extent was too small for waves of the appropriate wave lengths (Cummings and
Dessler, 1967), and they were thereafter explained in terms of magnetic fields of
field-aligned currents.
Zmuda et al. (1970) suggested that their observations could be explained by
sheet currents of strength 0.024 to 0.7 A m -L, or 0.2 to 6.0 x 10 .6 A m -2. To obtain
these values they assumed that the current flows in an infinite sheet lying roughly
in the east-west direction with a north-south latitudinal average width of 1~
However, as will be seen in Section 3.3, the latitudinal extent of the field-aligned
currents are not confined only to the area of visible auroral arcs but are extended
in the diffuse auroral region, indicating that the above intensities of the field-aligned
currents are overestimated. It was also noted by Zmuda et al. (1970) that the
greatest intensity was observed near 23:00 MLT with a secondary peak around
12:00 MLT, suggesting the existence of two different sources of the field-aligned
currents, one located in the night sector and the other in the day sector.
3.2. GROSS F I E L D - A L I G N E D
CURRENT PATTERN
A model of the field-aligned current system that fits the satellite magnetic observations at 1100 km altitude for a single event was presented by Armstrong and Zmuda
(1970). Their model consists of a double-sheet configuration; one current sheet on
the poleward side is directed into the ionosphere and in the late morning sector
the other sheet is out of the ionosphere on the equatorward side of the region.
Coleman and McPherron (1970) reported further evidence for the field-aligned
currents in data obtained at the synchronous orbit. Magnetic perturbations on
Earth's surface and the electric field at 12.5RE observed by means of a barium
cloud experiment (HEOS satellite) were analysed by Haerendel et al. (1971). They
suggested that in the morning sector, a downward field-aligned current feeds the
westward electrojet, which is the dominant feature of the polar substorm.
Fairfield (1973) and Iijima (1973) studied magnetic fields of the field-aligned
currents in the magnetosphere and their gross patterns as functions of local time
and substorm activity using magnetometer data of the IMP-4 and 5, and ATS-1
satellites, respectively. As Birkeland (1908) suggested, the westward electrojet can
FIELD-ALIGNED
R= t3,7
GM LAT = - 4 ~
GM LONG9 460~
CURRENTS AND AURORAL
14.8
t~
160"
ELECTROJETS
t5.8
5 9
164 ~
4O
AB(),)
20
143
16,7
10"
162"
I
-
-30 o 0
1
I -60 ~
-~20(~K)o,
,
1
0o
D
-60"
8 ( • ) 8 -izo,
1
0
1o 3
c
o
~o2
(/)
.(,,~ w 3
o
~01
U_i
400 ~
22 UT
23
17/18
J.
JAN
24
1971
0t
Fig. 5. Magnetic field and energetic electron flux observations in the magnetotail during substorms.
The H component record from the midnight meridian station is also shown. (Fairfield, D. H.: 1973,
J. Geophys. Res. 78, 1553.)
144
Y. K A M n ) E
be thought to be connected to a pair of the field-aligned currents originating in the
magnetosphere; they are directed toward the Earth in the morning sector and away
from the Earth in the evening sector. When these currents are intensified in harmony
with the growth of magnetospheric substorms, one would expect an eastward
deflection of the magnetic field in the morning sector of the high-latitude tail lobe
and a westward deflection in the evening sector. Fairfield (1973) showed that such
a feature was indeed observed by satellites in the magnetotail during substorms.
Figure 5 shows an example of such magnetic changes in the evening sector; the
upper part of this figure is taken from Fairfield (1973). It is seen that the large
negative D perturbation at 00:15 UT occurred in association with a substorm
recorded at Leirvogur. It should be noted, however, that this sudden change in the
D component did not occur at the onset time of the substorm, implying that the
D change indicates the encounter of the satellite with the expanding plasma sheet
boundary. In fact, the simultaneous energetic electron data (the bottom of the
figure) show a considerable increase at the time of the magnetic deflection (see
Akasofu, 1977). Therefore, the observed changes were associated with spatial
variations rather than time variations. These observations may suggest that such a
magnetic deflection can be observed only in a limited region near the plasma sheet
boundary.
Figure 6 shows the locations of the D events observed by the satellite projected
to the Earth's surface along with the statistical auroral oval (Fairfield, 1973). It is
noticeable that the directions of the inferred field-aligned currents are systematically
separated by different local time sectors with respect to premidnight.hour. There
is a clear preference for current flow toward the Earth in the morning sector and
current flow away from the Earth in the evening sector. It is important to note that
these field-aligned currents are seen on the majority of orbits near the high latitude
boundary of the auroral oval. As will be seen in the next section describing more
recent imformation based on data from polar-orbiting satellites, the pattern shown
in Figure 6 represents only the high latitude portion of the overall field-aligned
current system (i.e., region 1 current, see Section 3.3.1).
A survey of auroral energy electrons was carried out with instruments on the
OGO-4 satellite at 412-908 km altitude. The observations of the field alignment
of 0.7-and 2.3-keV electrons were first reported by Hoffman and Evans (1968)
and Hoffman (1969). Berko et al. (1975) examined three regions, namely the
regions where high fluxes of the field-aligned 2.3 keV precipitations were observed
(from Berko, 1973), regions where the OGO-4 magnetometer recorded fluctuations
(Burton et al. 1969) and regions where Zmuda et al. (1970) observed large transverse
magnetic disturbances. They indicated that all these regions share the spatial feature
of an oval shaped auroral belt in that the lower boundaries of the three regions
are located at higher latitudes during the day-time hours than during the nearmidnight hours. It was further noted also that upward field-aligned currents in late
evening hours were detected in the region of high field-aligned particle precipitation,
and that much of the current in this region was carried by particles with energies
FIELD-ALIGNED CURRENTS AND AURORAL ELECTROJETS
145
12
I
18 - -
--06
1
24
o
Current
out
9
Current
into
of
Ionosphere
Ionosphere
Fig. 6. Locations of field-aligned current events, which are most frequently seen near midnight
and near the northern boundary of the auroral oval (dashed line). (Fairfield, D. H.: 1973, Z Geophys.
Res. 78, 1553.)
greater than 0.7 keV. Berko and Hoffman (1974) examined the dependence of the
occurrence of field-aligned currents on season and altitude by using electron
precipitation data from more than 7500 orbits of OGO-4.
Theile and Praetorius (1973) described the results of an analysis of two component
magnetometer data on board the A Z U R satellite, which is in polar orbit to 400
to 3000 km altitude. They showed that the regions of transverse magnetic perturbations coincide with the regions of measured emission of 3914 A radiation, presumably excited by precipitating auroral electrons.
Evidence of field-aligned currents in the dayside cusp has been reported as well.
For example, Fairfield and Ness (1972) observed transverse fluctuations with
magnitudes up to 45 nT at about 7RE in a region where the total field strength
equals about 200 nT, which could be reasonably attributed to paired sheet currents
with the downward sheet on the poleward side.
146
Y. KAMIDE
3.3. R E C E N T POLAR-ORBITING SATELLITE OBSERVATIONS
3.3.1. T R I A D Satellite Observations
The TRIAD satellite, launched into a nearly circular polar orbit at 800 km altitude
in November 1972, is the first satellite that carries a tri-axial, high resolution
magnetometer, allowing us to determine the current flow directions, spatial distribution, and intensities of field-aligned currents at all magnetic local times. The
characteristics of the TRIAD magnetometer experiment have been described in
detail by Armstrong and Zrnuda (1973), along with sample magnetometer records.
Armstrong (1974) noted that in most cases, the total magnetic perturbation vector
at auroral latitudes is transverse to the main field to within experimental sensitivity,
confirming the earlier suggestion that the magnetic perturbations result from fieldaligned currents.
iiiiiiiiiiiiiiiiiiiiiiiii~CURRENT OUT OF
i::i::iii:::ii::i::i~i::i!i~IONOSPHERE
N. i! CU
E,T,.TO
IONOSPHERE
2
18~ ~
~i
:i~!i
06
04
50
22
02
O0
Fig. 7. Diurnal flow pattern of field-aligned currents along the auroral oval observed by the TRIAD
satellite. (A) Both types of current-patterns found in this region. (B) Irregular region. (Zmuda, A. J.
and Armstrong, J. C.: 1974b, 3. Geophys. Res. 79, 4611.)
FIELD-ALIGNED
CURRENTS
AND
AURORAL
147
ELECTROJETS
According to Zmuda and Armstrong (1974a), the magnetic field perturbation in
the evening sector is eastward, indicating a current flow away from the Earth at
the poleward part and a flow toward the Earth at the equatorward part of the
perturbation region. The current direction is reversed in the morning sector. Either
set can appear in the transition period around noon and midnight. The diurnal flow
pattern is schematically shown in Fig. 7, which is reproduced from Zmuda and
Armstrong (1974b). The figure indicates that the current densities of the two
oppositely-directed, field-aligned currents are essentially equal (Zmuda and
Armstrong, 1974b). They claimed that when the satellite traversed the auroral
I
I
I
i
I
--',~ :
J
I
. - - 0 8 0 6 UT F E B I 5 1975
2J40 MLT
Kp=2
~
(b)
0557 UT MAY 5 F375
1630 MLT
Kp = 2
E
g
]~00 T
(c)
v
0207 UT APR 28 1975
1650 Mt-T
Kp=2+
Z
O
O
(D
(d)
- - 0217 UT MAY I ]975
1620 MLT
Kp= 3_
{e)
0527 UT MAY 8 1973
1550 MLT
Kp = 3
. . . . .
~
--
(f)
~,/_..._,.._~_~
0150 UT MAY 22 1973
1520 MLT
Kp = 4
Zmudo and Armstrong (19741
500y
0858 UT FEB [0 1975
2305 MLT
Kp = 5L
L
80
z5
I
i
~
1
[
70
65
60
5s
50
INVARIANT
LATITUDE
(degrees)
Fig. 8. Typical examples of the magnetic field perturbations observed by TRIAD; the invariant latitude
is shown at the bottom. The dashed lines are extrapolation from both the poleward and equatorward
portions of the traces. The arrows o~ and /3 on the top trace indicate the poleward and equatorward
edges of the perturbation region. The last trace is one of the examples studied by Zmuda and Armstrong
(1974b) in which the dashed lines shows their base line. (Yasuhara, F., Kamide, Y., and Akasofu, S.-I.:
1975, Planetary Space Sci. 23, 1355.)
148
Y. KAMIDE
region from a high latitude side to a low latitude side, the east-west component of
the magnetic field returned to the previous level at high latitudes even after passing
through the current flow region. However, Yasuhara et al. (1975) argued that this
equality of the oppositely directed current intensities exist less often than the
inequality cases. They showed also several 'typical' examples of the magnetic field
perturbations observed by TRIAD (see Fig. 8), in which the eastward deviation
does not recover fully at the end of the data; namely, the trace does not merge
with the extrapolated line from the poleward side. This tendency can be explained
by supposing that the intensities of the inflow and outflow currents are not equal.
The upward and downward field-aligned currents are not simply balanced in a
meridian plane.
Figure 9 shows the relationship between the upward and downward field-aligned
currents in the evening sector, together with three lines giving the ratio between
these currents. Although the ratio varies considerably, most points lie between the
two lines 1.0 and 2.0, a clear evidence that the upward current is in general greater
than the downward current in this local time sector.
There have been well-documented definitive studies carried out by Sugiura and
Potemra (1976) and Iijima and Potemra (1976a, b) who reached a similar conclusion
using bulk data from the TRIAD satellite. Figure 10 shows a summary of the
average distribution in 'MLT and invariant latitude' coordinates of the large-scale,
field-aligned currents determined from TRIAD magnetometer data obtained on
several hundred passes during weakly disturbed conditions (Iijima and Potemra,
EVENING-MIDNIGHT
SECTOR
~0,4
"
'
o
0,2
O0
9 0,0
I
0,2
I , , OUTWARD
I
0.4
I
FIELD-ALIGNED
I
0,6
I
0,8
CURRENT(omp/m)
Fig. 9. Relation between the intensities of the poleward and equatorward field-aligned currents. All
these data points are from passes in the evening-midnight sector. (Yasuhara, F., Kamide, Y., and
Akasofu, S.-I.: 1975, Planetary Space Sci. 23, 1355.)
FIELD-ALIGNED CURRENTS AND AURORAL
Number of Passes
4
l
3.0
11
16
~
l
149
ELECTROJETS
Number of Passes
26
~1
9
r
0
13
I
i
16
i
18
-7 . . . . . .
16
11
T
T~ -
0400 - 0900 M L T
1300 - 1800 M L T
9 Current Into Ionosphere
O Current Away from Ionosphere
9 Current Into Ionosphere
O Current Away from Ionosphere
G" 2.0
E
"E
g
yi
Q
1.0
ot
...-
--~'r-i
0
J
f
•
i
2
_.1
3
Kp
._L.
.l
4
~5
0
[
1
1
2
I
3
1
4
~5
Kp
Fig. 10. Spatialdistribution and flow directions of large-scale field-alignedcurrents determined from
data obtained on 493 TRIAD passes during weakly disturbed conditions. (Iijima, T. and Potemra,
T. A.: 1976b, J-. Geophys. Res. 81, 5971.)
1976b). As summarized by Potemra (1977), the principle features include the
following: (1) Field-aligned currents are concentrated in two major areas, regions
1 and 2, which are located on the poleward and equatorward side of the auroral
belt, respectively. The region I field-aligned currents flow into the ionosphere in
the morning sector and away from the ionosphere in the evening sector, whereas
the region 2 currents flow in the opposite direction at any given local time. (2) The
areas of maximum current density in region 1 are approximately coincident with
the location of the S~ associated electrojet current. (3) The currents in region 1
are statistically larger than the currents in region 2 at all local times, indicating an
unbalanced or 'net' current flow. (4) The region 1 currents appear to persist even
during very low geomagnetic activity with a value of current density ~>0.6x
1 0 - 6 A m -a for Kp = 0; see Figure 11. (5) A region of field-aligned currents has
been discovered in the dayside between 10:00 and 14:00 M L T and poleward of
region 1 between - 7 8 ~ and 81 ~ invariant latitude. These 'cusp' field-aligned currents
(called as region 3 currents) flow away from the ionosphere in the pre-noon sector
and into the ionosphere in the post-noon sector.
By means of O G O - 5 magnetometer data, Sugiura (1975) has also found the
existence of the paired field-aligned currents in the magnetosphere. In the nightside
magnetosphere, the polar cap boundary was identified by a sudden transition from
a dipolar field to a more tail-like configuration; a field-aligned current layer exists
150
Y. KAMIDE
IALI < 1007
12
18
•
Currents into Ionosphere
Currents Away from Ionosphere
Fig. 11. Relationship between field-aligned current densities and the general level of geomagnetic
activity in the forenoon and afternoon sectors. (Iijima, T. and Potemra, T. A.: 1976a, J. Geophys. Res.
81, 2165.)
in such a transition region. Most recently, Frank et aI. (1981) have reported the
encounter by the ISEE satellite of field-aligned current sheets at the northern
boundary of the plasma sheet.
The MAGSAT satellite launched into a low-latitude (190-560 km), near polar
orbit on October 30, 1979, should also provide information on the global distribution
of the field-aligned currents (Langel et al., 1980).
3.3.2. I S I $ - 2 Observations
Simultaneous particle and magnetic field measurements made on the ISIS-2 satellite
have been reported by Burrows et al. (1976), Klumpar et al. (1976), and McDiarmid
et al. (1977). The ISIS-2 satellite is in a nearly circular polar orbit at an altitude
of approximately 1400 kin. The energetic particle experiment onboard includes an
FIELD-ALIGNED CURRENTS AND AURORAL
ELECTROJETS
151
electrostatic analyzer measuring particle fluxes in the energy range from 0.15 keV
to 10 keV in 8 channels and a Geiger counter with an electron energy threshold
of 22 keV (Venkatarangan et al., 1975). The satellite instrumentation also includes
an orthogonally-mounted system of flux-gate magnetometers, two of which have
their axes aligned in the direction of the spin axis.
Klumpar et al. (1976) presented sample data of magnetic field signatures of
field-aligned currents and simultaneous charged particle measurement, and found
that in the post-midnight sector, the equatorward region of upward field-aligned
current flow coincides with the region of a nearly isotropic precipitation of kilovolt
electrons. They also noted that the poleward region of downward-directed current
flow is often associated with fluxes of low energy electrons, some having pitch
angles near 180 ~ with sufficient upward flux to account for the downward current
density inferred from the simultaneously observed magnetic field perturbation.
In an attempt to identify the charge carriers of the field-aligned currents, Maier
et al. (1980) used both ISIS 2 magnetometer records and electron flux data obtained
by the retarding potential analyzer in the suprathermal energy range (>1 eV). It
was concluded by them that net upward field-aligned current was derived by
combining the downward fall of energetic keV electrons with the upward flux of
the suprathermal electrons, meaning the partial cancellation is taking place in
auroral arcs. On the other hand, when the magnetometer data show the field-aligned
currents to be downward, the upward suprathermal electrons escaping the ionosphere contribute substantially to the current density.
About 300 satellite passes in local time intervals of 6 h centred on the dawn and
dusk meridians were examined extensively by McDiarmid et al. (1977). Figure 12
shows a dawn-dusk pass in which magnetic field perturbations corresponding to
the typical field-aligned current pattern are observed. The field perturbation shown
in the upper panel of the figure was obtained as the observed field minus a model
field (IGRF 1965.0 model). As indicated by arrows, the field perturbation can be
modeled by two oppositely-directed, field-aligned current sheets in both the morning and the evening sectors. In this example the currents are again not balanced;
net flows into the ionosphere on the morning side and out of the ionosphere on
the evening side are present. By comparing the field and particle measurements it
is evident that the high-latitude upward current in the evening sector coincide with
the high-latitude part of the plasma sheet, called the boundary plasma sheet (BPS)
by Winningham et al. (1975) using SPS (soft particle spectrograms) onboard the
ISIS-2 satellite. The BPS portion is characterized by highly structured particle
fluxes and is the region where inverted V's and discrete aurora are observed. More
recently, McDiarmid et al. (1978) showed similar examples, but in these cases, with
more reliable determination method of the base line for the magnetic field.
McDiarmid et al. (1977) also found that in a few percent of the morning sector
passes, the current pattern is reversed from the normal configuration; that is, the
high-latitude current is upward, and the low-latitude part downward. It was noted
that these perturbations are observed only at times when the interplanetary magnetic
152
Y. KAMIDE
,oolI~"*!t
......
1 .... ' .... I ........
l .... i .... i .... , ........
J
kJ
,X
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...............................
!
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52
,
55
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54
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58
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59
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I
. . . . . . . . . .
2
3
4
5
6
7
25'5
20'8
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8 4 21
82"9:5
78"51
18"5
18'0
7:5'28
67'82
8
17'7
6228
72/69/1:5
Fig. 12. Example of a dawn-dusk pass in which field-aligned current directions are shown by arrows.
Electron fluxes at five different energies are also shown. The average electron energy in keV is shown
at the bottom on a linear scale. (McDiarmid, I. B., Budzinski, E. E., Wilson, M. D., and Burrows,
J. R.: 1977, ar. Geophys. Res. 82, 1513.).
9
FIELD-ALIGNED
CURRENTS AND AURORAL
153
ELECTROJETS
field has a strong northward component, and they are found along the contracted
auroral belt.
3.4.
PROJECTION OF THE FIELD-ALIGNED CURRENT REGION INTO THE
MAGNETOSPHERE
It may be possible to discuss magnetospheric processes associated with large-scale
currents by projecting the region of the observed large-scale, field-aligned current
onto the equatorial plane of the magnetosphere. Assuming that the pressure
gradient is solely responsible for the field-aligned currents, Bostr6m (1975) showed
the direction of pressure gradients in the equatorial plane of the magnetosphere.
Using the spatial distribution and flow direction pattern of the field-aligned
currents determined by Iijima and Potemra (1976a), Potemra (1977) has attempted
to map these current regions to the equatorial plane along field lines of magnetic
field model of Mead and Fairfield (1975) and Fairfield and Mead (1975) for
conditions corresponding to quiet and disturbed periods. In Figure 13, the regions
of the large-scale, field-aligned currents are indicated along with the boundary of
the inner edge of the plasma sheet. It is noticed that the boundary between the
flow direction of the field-aligned currents on the dusk side statistically coincides
with the earthward edge of the plasma sheet. Potemra (1977) emphasized that in
the dusk sector, the region of field-aligned currents flowing away from the ionosphere maps onto the region within the plasma sheet, implying that the flow of
electrons from the plasma sheet to the auroral region can adequately account for
the flow pattern of these currents. On the other hand, the region of field-aligned
currents flowing into the auroral ionosphere in the dusk maps onto the earthward
side of the plasma sheet boundary, where no electrons are available in transporting
Quiet
conditions/IAL1 < 100 3'
....
Dawn ~
/
' 1 5
-
,
Inner edge of
~.'u~:~ , ^ I
plasma sheet;
~
- - - Vasyliunas 1968
x
"
1972
Sun
- -
~
~ " j
.
T :::"! Current into
fonosphere
~mm~J Current away
from ionosphere
Fig. 13. The projection of the regions of large-scale field-aligned currents onto the equatorial plane.
(Potemra, T. A.: 1977, in B. Grandal and J. A. Holtet (eds.), Dynamical and Chemical Coupling between
the Neutral and Ionized Atmosphere, D. Reidel Publ. Co., Dordrecht, Holland, p. 337.)
154
Y. KAMIDE
the charges to the auroral region. Thus, it could be inferred that upgoing low-energy
electrons from the ionosphere carry the downward field-aligned currents in the
equatorward half of the evening auroral belt. Note, however, that this inference
contradicts significantly some evidence that the diffuse aurora in the evening sector,
with which downward field-aligned current collocated, originates from the nearearth plasma sheet (Kamide and Winningham, 1977; Lui et al., 1981).
It is also important to note that Sugiura (1975) suggested based on OGO-5
magnetometer data that the region 1 currents are associated with the distant
boundaries of the plasma sheet, while the region 2 currents should close via the
equatorial (ring) currents in the magnetosphere.
3.5.
I N D I R E C T O B S E R V A T I O N OF F I E L D - A L I G N E D C U R R E N T S
3.5.1. Field-aligned Currentss as Divergence of Ionospheric Currents
The Chatanika incoherent scatter radar can measure simultaneously both the
electric field E and the electric conductivity Y~ so that it can be used to deduce
horizontal currents in the ionosphere (Brekke et al., 1974). de la Beaujardi~re et
al, (1977) have estimated the spatial variation of horizontal ionospheric current in
the vicinity of an east-west aligned auroral arc which moved in the north-south
direction above the radar, and by taking the divergence, inferred the field-aligned
current.
In the work of Kamide and Horwitz (1978) and de la Beaujardi~re et al. (1981),
an attempt was made to develop a technique for deducing field-aligned current
densities from measurements of the horizontal ionospheric currents at two or more
latitudes using the Chatanika incoherent scatter radar. By computing the 'onedimensional' divergence of the current in a suitable coordinate system, an estimate
of the field-aligned current density was obtained. In addition, direct comparison
was made of measurements by the Chatanika radar and those from simultaneous
TRIAD satellite over Chatanika. Figure 14a shows the ionospheric current at three
invariant latitudes, in which the following points of interest are noted. First, the
horizontal current is generally northeastward in the evening sector, and southwestward in the morning sector. The eastward and westward auroral electrojets in
evening and morning hours thus have, respectively, northward and southward
components as well. Second, the transitions from the evening to morning features
in the north-south and east-west components do not necessarily occur at the same
time. In the region where the east-west component changes its sign (i.e., at the
Harang discontinuity), an intense northward current prevails at all three latitudes.
This predominance of the northward current is presumably caused by the westward
electric field and the Hall conductivity (see Horwitz et al., 1978a). Third, the main
features of the currents at the three latitudes are generally similar, but sizable
latitudinal variations can be seen on several occasions. For example, at about
06:35 UT, the eastward current component was about 2 A m 1 at the lowest
latitude, but was only 1 A -1 over Chatanika, and 0.5 A m -1 at the highest latitude.
FIELD-ALIGNED
CURRENTS
AND
AURORAL
ELECTROJETS
155
MAY 'i7, ~974
E
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7
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--
I
o
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-I
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n
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/8
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24
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46
{
I
~8
{
04
{
UT
{
06
MLT
Fig. 14(a). Northward and eastward components (in geomagnetic coordinates of height-integrated
ionospheric currents at three latitudes. (Kamide, Y. and Horwitz, J, L.: 1978, J. Geophys. Res. 83, 1063.)
MAY
}
g"
{
Triad
E
~ N. 5-"
{
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{
I
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1974
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PASS
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downward
--
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Fig. 14(b). Field-aligned current densities estimated from the horizontal ionospheric currents. Solid
lines represent the current densities which are the divergence of the horizontal current in the direction
perpendicular to the statistical auroral oval; whereas dotted lines are obtained by the divergence of the
Pedersen current alone. (Kamide, Y. and Horwitz, J. L.: 1978, J. Geophys. Res. 83, 1063.)
156
Y. K A M I D E
The currents in Figure 14a were used to estimate the field-aligned current
densities by assuming that current variations along the auroral oval of Feldstein
and Starkov (1967) are much smaller than variations across the oval. The average
field-aligned current densities in latitudes lower than Chatanika and in those higher
than Chatanika are shown in Figure 14b. Also shown are the current densities
computed under an alternative assumption, that the current density results entirely
from a divergence of the Pedersen current ( = div(Y.p E)). The general agreement
between the two estimates points to the fact that the electric field usually points
perpendicular to the auroral oval and most of the current obtained in this way is
due to the divergence of the Pedersen current.
The salient features found by Kamide and Horwitz (1978) are: (1) The magnitudes
of the field-aligned current densities, 10-6-10-5 A m -2, are comparable to those
observed with more direct methods, such as rocket and satellite detectors. (2) The
current tends to be directed upward in the morning sector in a latitudinal range
near Chatanika. Equatorward of Chatanika, the currents are directed generally
downward in the evening sector. This sense corresponds to the equatorward half
of the field-aligned current system constructed on the basis of T R I A D data (Zmuda
and Armstrong, 1974b), which is expected to be present at these latitudes under
moderately disturbed conditions (see Iijima and Potemra, 1976a). The currents
were highly irregular in the midnight sector, perhaps due to real fluctuations as
well as to a more complicated current structure, invalidating the procedure used
to complete the field-aligned currents. (3) Intensifications of the field-aligned
currents appear to be associated with substorm activity as observed in highlatitude ground magnetic disturbances in the midnight sector (not shown here).
(4) During periods of very large disturbances, the poleward half of the fieldaligned sheet current system expands equatorward to the vicinity of the radar
site.
In a recent paper by Yashura et al. (1981), a similar method has been employed
to obtain the global distribution of the field-aligned current in high latitudes from
the divergence of ionospheric currents. However, the ionospheric currents are
estimated from Millstone Hill incoherent radar observations of ion drifts as well
as from a realistic model of the ionospheric conductivity. During the IMS, the
Millstone Hill (invariant latitude A = 56 ~ radar was upgraded such that measurements of the drift velocity can be made with a spatial resolution of 1-2 ~ in latitude
over the region of A = 60 ~ and 75 ~ (Evans et aI., 1979, 1980). Since the results
seem to be quite sensitive to the choice of the ionospheric conductivity model, the
method has been applied only to quiet days for which no large local enhancements
in the conductivity are expected. The results obtained reproduce not only large-scale
current patterns being consistent with T R I A D observations, but also feature smallscale patterns. Although the method is more or less indirect in obtaining the
field - aligned current distribution, Yasuhara e t a I. ( 1981) emphasized the importance
of such a trial, since the use of the ground-based radar data covering all local times
in 24 hr can overcome the shortcomings of the polar orbit satellites which need
6 months to cover all the local times.
FIELD-ALIGNED
CURRENTS
AND
AURORAL
ELECTROJETS
157
3.5.2. Polar Cap Electric Fields
Wescott et al. (1970) and Haerendel and Last (1970) pointed out that the polar
cap horizontal magnetic disturbance vector cannot be explained in terms of ionospheric currents driven by polar cap electric fields as measured by barium cloud
experiments. Heppner et al. (1971) reported the results of twelve barium cloud
release experiments in the polar cap at invariant latitudes 76 ~ to 78 ~ for the study
of polar cap electric fields and their relationship to the simultaneous magnetic
disturbances on the Earth's surface. They showed that the electric field is typically
between 20 and 40 mV m -1, directed roughly from the dawn to dusk direction.
There is, however, a noticeable disagreement between the observed ground magnetic perturbation vectors and the expected vectors on the basis of the measured
electric field and the assumed ionospheric conductivity, indicating that a significant
part of the polar cap magnetic disturbances are caused by sources other than
overhead ionospheric currents. Heppner et al. (1971) thus proposed that in order
to resolve this disagreement, the net field-aligned currents are required, which are
equivalent to two sheet currents flowing 'out of' and 'into' the auroral belt ionosphere, respectively in the MLT sectors 20:00-24:00 and 08:00-12:00.
3.5.3. Meridian Line Magnetometer Data
A quantitative modelling of the ionospheric and field-aligned currents to estimate
the distribution of the auroral electrojets using magnetometer data from a meridian
r--
0QY345
16HR 0 MIN. 0 8 E C
co
cE
)-}.cE
C_b
/
I
>I--.
co
o ~. .............. .v y,
y
bJ
,,
Z
I
z
F-- ' .....
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'7,
tO
b_
I
..~.....
............
l
~176176176176
"2LI
/
g
g
g
t
LQTITUOE
Fig. 15. Hourly average latitude profile of ground magnetic perturbation taken near local dawn.
Latitude is given in corrected geomagnetic latitude. (Hughes, T. J. and Rostoker, G.: 1977, Jr. Geophys.
Res. 82, 2271.)
158
y. KAMIDE
chain has been discussed by Oldenburg (1976, 1978). See also Nopper and
Hermance (1974) and Homing et al. (1974) for a similar treatment of modeling
of three-dimensional current systems.
Most recently, Hughes and Rostoker (1977, 1979) demonstrated that although
the separation of the effects of both ionospheric and field-aligned currents cannot
be made uniquely in principle from the ground magnetic perturbation data, it may
be possible to find the ground-based signature of the 'net' field-aligned current
flow using the latudinal profile of the three-component ground magnetic perturbations. Figure 15 shows an example of such profiles taken near local magnetic dawn
in which a clear level shift appears in the D component profile. The level shift, or
step, was considered to be one of the ground-based signatures of the net field-aligned
current flowing into the ionosphere. Hughes and Rostoker (1977) showed, using
this technique, the diurnal variation of the deduced net field-aligned currents and
demonstrated that there is a remarkable similarity between the behavior of the
field-aligned current flow as inferred from this technique and the diurnal variation
of the average electric field observed at auroral zone latitudes by Mozer and Lucht
(1974).
4. Spatial Relationship of Field-Aligned Currents to the Distribution of Auroras
and Electrojets
One of the important problems of auroral physics is to find how the field-aligned
currents are related to different auroral features. This problem has a crucial
importance in understanding some of the dominant physical processes occurring
in the upper atmosphere in association with precipitating and upgoing particles.
This problem is also related closely to the questions of, "What are the main charge
carriers responsible for the field-aligned currents?" and "where do these particles
originate?"
The distribution of visible auroras can be observed by ground-based all-sky
cameras and TV systems, airborne all-sky cameras, and satellite scanners from
above the polar region. Early studies of the distribution and forms of auroras relied
principally upon all-sky cameras. An all-sky camera has only a limited field of view
and often fails to detect the diffuse aurora. Further, even the combined data from
the extensive IGY all-sky camera network were not extensive enough to provide
the opportunity to observe the distribution of auroras over the entire polar region.
Auroral physics has been advanced to a significant extent by studies of photographs
from a scanning camera aboard the ISIS-2 and DMSP satellites (Anger et al.,
1973a, b, 1978; Shepherd et al., 1973; Anger and Lui, 1973; Lui et al., 1973;
Lui and Anger, 1973; Pike and Whalen, 1974; Snyder et at., 1974; Rogers et al.,
1974; Snyder and Akasofu, 1974; Mizera et al., 1975; Murphree et al., 1980). This
type of camera reconstructs a map of auroral luminosity from horizon-to-horizon
scans of the Earth by a photometer sweeping perpendicular to the orbital path.
Efforts have been made to improve computer programs for transforming the
FIELD-ALIGNED CURRENTS AND AURORAL ELECTROJETS
159
obtained data into various coordinate systems and for reproducing latitude profiles
of auroral intensities at different local times (Murphree and Anger, 1977; Hays
and Anger, 1978; Harrison and Anger, 1977a, b). Reviews on the development
of recent morphological studies of the aurora have been published by Akasofu
(1974a, 1976) and Vallance Jones (1974). The scanner data have revealed several
new auroral characteristics which had not been clearly recognized in all-sky camera
studies.
4.1.
DIFFUSE
AND
DISCRETE
AURORAS
Akasofu (1974a) examined how various auroral patterns (such as rayed arcs,
patches, torches, g2 bands) in the combined all-sky camera records are represented
by large-scale structures in the satellite-viewed pictures. Based on several montage
photographs of the DMSP auroral forms, Akasofu (1976) also examined overall
features of auroral characteristics in different local time sectors. From these studies
has emenged a schematic distribution pattern showing the main characteristics of
auroras during an auroral substorm.
There are essentially two auroral belts, diffuse and discrete auroras (Akasofu,
1974a). The diffuse aurora can be defined as a broad band of auroral luminosity
with a width of, at least, several tens of kilometers which is separated from the
discrete aurora. The diffuse auroral belt has been studied in detail by Lui and
Anger (1973), Shepherd et al. (1973), Lui et al. (1973), Pike and Whalen (1974),
and Snyder et al. (1974). The diffuse aurora often covers a significant part of the
field of view of an all-sky camera, making it difficult to recognize even its presence
from a single ground station. Thus, the Feldstein and Starkov's (1967) auroral oval
does not necessarily represent accurately the distribution of the diffuse aurora
particularly in the evening sector. The fact that the region of the diffuse aurora
covers a large latitudinal range well beyond the view of a single all-sky camera
also makes it difficult to obtain the global distribution of the diffuse aurora. It was
found by Stenbaek-Nielsen et al. (1973) that the diffuse aurora contains complicated
fine structures and that such fine structures in one hemisphere can be identified in
the other hemisphere. This character of the diffuse aurora can be contrasted to
that of the discrete aurora, for which conjuqacy breaks down at times during intense
substorms.
The poleward boundary of the diffuse aurora develops in various wavy forms,
such as omega (.O) bands and torch structures. In contrast with the complex
configurations of its poleward portion, a striking feature is the remarkable continuity
of the equatorial boundary of the diffuse aurora. This boundary tends to align most
closely along L-shells when the auroral oval is largest in diameter; it is more
oval-like when the auroral oval is smallest (Davis, 1974). Lui et al. (1981) reported
that there sometimes occur large amplitude undulations near the equatorward
boundary of the evening diffuse aurora during the main phase of large magnetic
storms. There is a general consensus that the equatorward boundary of the diffuse
aurora represents the earthward boundary of the plasma sheet, or, according to
160
Y. K A M I D E
Winningham et al. (1975) of the central plasma sheet (CPS); see Meng et al. (1979),
Burke et al. (1980). The equatorward boundary has been used to express the size
of the auroral oval (e.g., Kamide and Winningham, 1977; Sheehan and Carovillano,
1978; Slater et al., 1980; Gussenhoven et al., 1981).
In studying the ISIS-2 scanning photometer data, Anger and Murphree (1976)
found that discrete auroral arcs lie within the diffuse aurora, not outside of it,
contrasting to previous conclusions by Snyder et al. (1974) and Snyder and Akasofu
(1974) that there is often a clear separation between the discrete and diffuse auroras.
It was thought by Anger and Murphree (1976) that the apparent discrepancy is
probably a result of the finite intensity threshold (-2kR) of the DMSP satellites
(see Berkey and Kamide, 1976). The terms such as the mantle aurora (Sandford,
1968) the continuous aurora (Pike and Whalen, 1974), and the diffuse aurora are
essentially interchangeable, in the sense that they all express the continuous and
relatively uniform belt of auroral emissions, which is persistently present even
without discrete auroras (see Shepherd et al., 1976).
One of the prominent features of the diffuse aurora when viewed from a global
perspective is its well-defined equatorward boundary (Lui and Anger, 1973;
Creutzberg, 1976). Its luminosity is enhanced significantly during substorms (Murphree and Anger, 1978). Anger and Murphree (1976) cautioned that the auroral
intensity does not actually cut off as sharply as one might expect from looking at
the satellite pictures, because each has a specific intensity threshold. Berkey (1980)
reported that the diffuse aurora periodically (less than 1 min) changes its luminosity.
Pulsating auroras can also be found in the diffuse auroral region both in the evening
and morning hours (Siren, 1975; Royovik and Davis, 1977). The patchy aurora
can be formed in the diffuse aurora, and the pulsating auroras represent a subset
of the class of the patchy aurora. We note that although the identification of the
diffuse aurora in the evening sector is relatively easy, it is difficult to define it clearly
in the morning sector where complicated post-breakup auroral signatures are
generally observed.
4.2. FIELD-ALIGNEDCURRENTS AND AURORAS
4.2.1. Large-Scale Field-Aligned Currents
Armstrong et al. (1975) examined the spatial relationship between field-aligned
currents and auroras for a few TRIAD satellite passes, and found that the poleward
discrete arc marks the poleward boundary of the field-aligned current region. It
was also found that all the visible arcs lie within the latitudinal region occupied
by the field-aligned current flows. Kamide and Akasofu (1976a) examined a number
of such simultaneous satellite and ground-based observations with special reference
to the distinction of discrete and diffuse auroras.
In Figure 16 we combine the TRIAD magnetometer (A sensor) data together
with the locations of the arc crossing. On the right-hand side the superimposed H
component magnetic records from nine auroral zone stations are shown to indicate
161
FIELD-ALIGNED CURRENTS AND AURORAL ELECTROJETS
AURORA
II
(a)
ARCS
. . . . . . . . . . . . . . . . .
, l j
o
3M~T
-
(b) _.__._.
(c)
2030 MLT
Kp = 3
(d)
0711
(e)
i
i
Mor 9, F973
09%. .3. . . . . . .2535
..
MLT
(f)
0922 Feb 6, 1973
Kp=3+
2325 MLT
(g)
0851
Feb 17, 1973
2310 MLT
Kp = 4
(h)
r052
d o n 24, 1 9 7 3
Kp=4
0130 MLT
{h}
0702
(i)
(i)
i
~
Mor 6, 1973
(i)
Kp; 5
2140 MLT
=
1950 MLT
[
(k)
(k}
I
8o
I
I
I
I
I
I
75
I
I
I
I
70
I
I
I
I
rNVARIANT LATrTUDE
I
65
I
I
I
I
(degrees)
I
60
I
I
I
I
~uF
i
r
i
i
J
i
J~
i
i
i
i
i
r
Fig. 16. T R I A D magnetometer data in the A sensor (approximately in the geomagnetic east-west
direction) as functions of the invariant latitude, along with the locations of the discrete arc crossing,
and the superposed H component magnetic records from nine auroral zone observatories. (Kamide, Y.
and Akasofu, S.-I.: 1976a, Z Geophys. Res. 81, 3999.)
magnetic activity during a few hours before and after the time of passage, which
is marked by a vertical line. Most of the data were taken during evening and
premidnight hours in MLT, but the corresponding magnetic conditions (expressed
by the Kp index) are quite different. There is a significant latitudinal expansion of
the field-aligned current region as magnetic activity increases. It can also be seen
that (1) discrete arcs are, in general confined within the region of the upward
field-aligned current, (2) no discrete arcs are seen in the region of the downward
field-aligned current, and (3) the region of downward field-aligned current corresponds to the region of the diffuse aurora.
However, it was difficult in their study using all-sky camera data to associate
individual arcs with the irregular features of the TRIAD magnetic perturbations,
162
Y. K A M I D E
which presumably indicate concentrated upward and downward currents within the
large-scale, upward field-aligned current region. More recently, Kamide and
Rostoker (1977) made a detailed study of the spatial relationship between the
field-aligned currents and the distribution of nightside auroras on the basis of
nearly simultaneous sets of the T R I A D magnetometer data and auroral imagery
and information on precipitating electrons in the energy range between 200 eV
and 20 keV obtained from the DMSP satellites. In Figures 17 and 18, we show
such comparisons for morning and evening sectors, respectively. The distribution
of vectors of ground magnetic perturbations at the DMSP passage time is also
given. It is noticed in Figure 17 that optical auroras appear to be confined to the
region of the field aligned currents as defined by the T R I A D data. The downward
current flow in the morning sector occurs in the region of auroral luminosity
generated by precipitating electrons, but the strength of the downward current and
the auroral intensity are anticorrelated. On the other hand, the region of upward
field-aligned currents coincides well with the region of the visible aurora in the
equatorward half of the morning auroral belt.
Fig. 17. DMSP auroral imagery data for a morning pass together with the magnetic perturbation
vectors, measured at ground observatories.The trajectory of the TRIAD satellite is also indicated along
with the field-alignedcurrent direction (upward or downward) inferred from the TRIAD magnetometer
record. (Kamide, Y. and Rostoker, G.: 1977, J. Geophys. Res. 82, 5589.)
F I E L D - A L I G N E D CURRENTS A N D A U R O R A L E L E C T R O J E T S
163
Fig. 18. DMSP auroral data for an evening pass together with the magnetic perturbation vectors
measured at ground observatories. The trajectories of the DMSP (southbound) and TRIAD (northbound)
satellites are also indicated along with the field-aligned current directions inferred from the TRIAD
magnetometer records. (Kamide, Y. and Rostoker, G.: 1977, Y. Geophys. Res. 82, 5589.)
A n e x a m p l e of t h e D M S P i m a g e r y in t h e e v e n i n g s e c t o r a n d t h e c o r r e s p o n d i n g
d i s t r i b u t i o n of t h e f i e l d - a l i g n e d c u r r e n t s is s e e n in F i g u r e 18, in which a b r i g h t
w e s t w a r d t r a v e l i n g a u r o r a n e a r 2 0 : 0 0 M L T with s e v e r a l arcs e x t e n d i n g into e a r l y
e v e n i n g h o u r s a n d diffuse a u r o r a a r e seen. A l t h o u g h the p o l e w a r d b o u n d a r y of
t h e diffuse a u r o r a m e r g e s into t h e d i s c r e t e a u r o r a in a c o m p l i c a t e d fashion, its
e q u a t o r w a r d b o u n d a r y d e l i n e a t e s an o v a l - s h a p e d belt. A striking f e a t u r e is t h a t
t h e r e a r e s e v e r a l h i g h - l a t i t u d e , s m a l l e r - s c a l e , f i e l d - a l i g n e d c u r r e n t r e g i o n s (of
100 k m l a t i t u d i n a l e x t e n t o r less) which m a y c o r r e s p o n d to the d i s c r e t e arcs visible
in t h e D M S P data. T h e s e s m a l l e r scale c u r r e n t f e a t u r e s d o n o t always s e e m to
c o i n c i d e with a d i s c r e t e arc. This was i n t e r p r e t e d as b e i n g a t t r i b u t a b l e to t h e
164
Y. KAMIDE
north-south motion of the discrete arc (or the relatively short lifetime of any discrete
arc) and the time separation between the DMSP and TRIAD passes. The weak
downward field-aligned current with density of less than 1 x 10 .6 A m -2 is collocated
with the diffuse aurora and the eastward electrojet.
Figure 19 shows the differential electron fluxes for three selected energies along
the DMSP trajectory. The northernmost arc corresponds to the sharp rise of the
electron flux from the background level in the polar cap at all energy channels, for
example, an increase by a factor of about 10 in the 0.2 and 8.0 keV channels. Such
a good correlation cannot be found for other auroral arcs. In particular, there are
no visible auroral arcs apparent for the enhancement in 0.2 keV electrons which
peaked at about 72.4 ~ and 71.0 ~ corrected geomagnetic latitude (almost identical
to the invariant latitude in this sector). It was found, however, that each auroral
arc coincides well with an increase in the energy flux more than 10 o erg cm -2 s -1 sr -1,
indicated by the dashed line in Figure 19.
It is seen that the equatorward boundary of the diffuse aurora can be determined
by that of 8 keV electron precipitation, in agreement with the results by Meng
(1976). The energy flux of the precipitating electrons in the diffuse aurora is about
one order of magnitude lower than that for the discrete aurora.
The latitudinal profile of the electron number flux is shown at the bottom. On
the right-hand side of the figure the scale of the estimated current density is given.
By comparing the estimated current density with the field-aligned current density
inferred from the T R I A D magnetic perturbation it may be said that the upward
field-aligned currents in the region of the discrete auroral arcs can be explained
by the precipitating electrons.
Two other points seem worth mentioning: (1) Before the satellite encountered
the sudden increase in the electron precipitation corresponding to the northernmost
arc, a sharp decrease in the electron flux, especially at low energies (0.2 and
1.3 keV), was observed. An intense downward current was also detected by the
TRIAD satellite in this region. It can be inferred that the current may be carried
by upward flowing thermal electrons escaping from the ionosphere. (2) Although
considerable upward field-aligned currents are carried by the precipitating electrons
even in the diffuse aurora region, a new downward field-aligned current is actually
flowing in the latitudinal regime, indicating that upgoing ionospheric electrons may
well be carrying the downward current. Most recently, Shuman et al. (1981) gave
simultaneous measurements of the field-aligned current densities by the transverse
magnetometer and the auroral precipitating electron detector on board the $3-2
satellite (Smiddy et al., 1980). They found that in the nightside auroral oval, current
densities calculated from electron fluxes in the energy range 0.08 to 17 keV can
account for 50-70% of the upward current indicated by the magnetometer data.
4.2.2. Small-Scale Field-Aligned Currents
Measurements of field-aligned currents by sounding rockets have several advantages
compared with measurements by polar-orbiting satellites, in spite of a shorter range
FIELD-ALIGNED
Corrected Geomc,gnet/c Lofi'fua'e
8f
CURRENTS
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OCTOBER
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Fig. 19. Differential electron flux for three selected energy channels, the precipitating electron energy
ftux integrated from 0.2 to 20 keV, and the total electron n u m b e r for the D M S P pass shown in Figure
18. The aororal distribution along the D M S P subtrack is also indicated in negative, (Kamide, Y. and
Rostoker, G.: 1977, Z Geophys. Res. 82, 5589.)
166
Y. KAMIDE
that rockets can traverse as compared with satellites. Rocket observations can give
fine spatial details concerning the particle precipitation and the magnetic field
configuration for selected auroral forms. A great deal of work has been carried
out over the last decade using rocket-borne particle and magnetic field detectors
to estimate small-scale (<100 km) field-aligned currents in the near auroral forms
(e.g., Cahill et al., 1974; Kintner et al., 1974). Some of the studies have already
been reviewed by Arnoldy (1974) and Anderson and Vondrak (1975), and so only
the results which relate closely to the gross field-aligned currents and auroras are
noted in this paper.
Arnoldy and Choy (1973) found that the net flux of low-energy electrons was
upward on the poleward side of the visible aurora, which coincides with the narrow
sheet of downstreaming electrons. The suggestion that the upgoing electrons are
the main carriers of net downward field-aligned current that is the return of the
upward current carried by more energetic electrons to the south. Field-aligned
fluxes have also been measured by Bryant et al. (1973), Maehlum and Moestque
(1974), and Bosqued et al. (1974), who reported a region of energetic and intense
electron flux over auroral arcs.
Several sounding rockets over relatively stable auroral arcs were launched by
the group at Rice University, as summarized by Anderson and Vondrak (1975)
and Anderson and Cloutier (1975). They found that there are oppositely directed
field-aligned current sheets associated with a bright auroral arc, in which the total
measure number flux jumped abruptly and the energy of maximum flux rose to
approximately 10 keV as the pitch angle distribution became more field-aligned
at all energies (Pazich and Anderson, 1975; Spiger and Anderson, 1975;
Casserly and Cloutier, 1975). Interpretation of the simultaneous magnetic
perturbations requires both field-aligned and horizontal ionospheric currents.
An upward current was collocated with the main arc and the region of intense
electron precipitation, the measured electrons (0.5-20 keV) carrying a significant
portion (15% to 50%) of the total upward current. The equal downward
'return' current was found to the south from the magnetic field data, although
a net downward electron flux was measured in this region. Thus, the energy
of the current carrier must be less than the lowest energy detectable by their
instruments (0.5 keV).
More recently, Arnoldy (1977) examined the question of the type of charged
particles that carry the upward and downward field-aligned currents within and
near an auroral arc. Data from rocket-borne particle detectors with higher sensitivity
(5 eV < Ee < 15 keV) indicated that the upward currents were in fact carried by the
precipitating electrons, but the upward currents, i.e., the downward electrons, were
located at the edges of the discrete auroral forms. His result shows that these
current carriers are not collocated with the auroral arc. Similar conclusions were
reached by Theile and Wilhelm (1980) using simultaneous observations of precipitating electrons in the energy range from 15 eV to 35 keV and magnetic field
onboard a sounding rocket launched into an auroral arc. It was found that the
FIELD-ALIGNED
CURRENTS AND AURORAL
ELECTROJETS
167
upward field-aligned currents deduced from both the electron precipitation and
the east-west magnetic perturbations occur just inside the borders of the auroral
arc, indication that there is only a little upward current flow in the arc. Theile and
Wilhelm (1980) also observed an intense downward field-aligned current poleward
of the northern borders of the discrete arc, where the current carriers may well be
upward-escaping cold electrons. Thus, there still remains the question concerning
detailed structures of the field-aligned currents and their carriers in the vicinity of
discrete auroral arcs.
Sesiano and Cloutier (1976) and Casserly (1977) further found definitive evidence
for multiple pairs of antiparallel current sheets associated with complicated discrete
auroral systems. Sheet thickness ranged from 20 to 60 km.
Based on rocket measurements, models of the field-aligned currents near an
auroral arc system have been put forward (Vondrak, 1975; Burch et al., 1976;
Carlson and Kelley, 1977). Two models constructed by Vondrak (1975) and Carlson
and Kelley (1977) suggest that the field-aligned currents are driven by divergence
of the horizontal currents arising from electric field gradients. These models differ
from a previous model by Atkinson (1970) who considered the field-aligned currents
to be driven by variations in the horizontal currents arising from conductivity
gradients. The model of Vondrak (1975) can explain also rocket observations in
which vertical currents were found adjacent to an auroral arc in a region where
no conductivity gradients were expected to be present.
4.2.3. Field-Aligned Currents and Radar Auroras
Tsunoda et al. (1976a) examined the spatial relationship of the evening radar aurora
to the field-aligned currents by utilizing data collected with a 398-MHz radar
located at Homer, Alaska, and with the TRIAD satellite. Figure 20 represents a
typical example in which the distribution of the radar aurora and the visual aurora
are shown together with the two transverse components of the magnetic peturbation
at 800 km altitude. It is clear that the poleward boundary of the radar diffuse band
is coincident with the location of maximum magnetic perturbation of the TRIAD
record, namely, the boundary between the upward and downward field-aligned
currents. They noted that while the latitudinal extent of the radar echo and current
regions are related, the downward current density is not necessarily proportional
to the auroral echo strength; the TRIAD magnetometer data contain oscillatory
structure within the downward current region that does not appear to correspond
to the auroral echo intensity. It was thus suggested by the authors that the downward
field-aligned currents must be carried by precipitating protons and/or upwardmoving, low-energy electrons. The relationship of the diffuse radio aurora and the
field-aligned currents together with proton and electron precipitation as observed
simultaneously by the ISIS-2 satellite has been examined by Unwin (1980). It was
found that there is a high correlation between the radio aurora in the evening
sector and the downward current strength, whereas there is no clear relation with
either ion or electron precipitation.
t68
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Fig. 20. Spatial relationships among the radar aurora, visual aurora, and field-aligned currents.
(Tsunoda, R. T., Presnell, R. I., and Potemra, T. A.: 1976a, J. Geophys. Res. 81, 3791.)
4.3. G L O B A L A U R O R A L FEATURES A N D CURRENTS
It is now possible to identify the position of the auroral electr0jets and field-aligned
currents with respect to the pattern of simultaneous large-scale auroral displays
observed from satellites (e.g., Weber et al., 1977). It should be noted, however,
that because the satellite auroral imagery is available only once per each orbital
period ( - 1 0 0 rain for the DMSP and ISIS-2 satellites) which is of the order of the
lifetime of one substorm, the progressive change of auroral features and of the
corresponding electrojet response must await the availability of a series of photographs from a satellite with a high polar apogee, capable of observing the entire
polar region continuously for several hours.
Kamide and Akasofu (1975) studied DMSP satellite auroral photographs and
the simultaneous ground magnetic records from a number of high-latitude observatories. By examining the distribution of the equivalent ionospheric current vectors
with respect to the auroral display, it was confirmed that there are two electrojets,
0
gap]
80 ~
70 ~
60 ~
0
12
-c
500~
- -
~Z
~Z
<0
t
O6
Fig. 21. Schematic diagram of auroral features during the typical auroral s u b s t o r m and the distribution of equivalent current vectors with reference
to the auroral features. T h e m a g n i t u d e of each current vector is normalized for the m a x i m u m m a g n i t u d e of the magnetic perturbation of 500 nT.
(Kamide, Y. and A k a s o f u , S.-I.: 1975, J. Geophys. Res. "/9, 3 7 5 5 3
(~a'~a
12
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170
v. KAMIDE
eastward and westward, flowing along the visible auroral display. The reversal of
the current direction occurs near the midnight meridian; eastward in the evening
sector and westward in the midnight and morning sectors. In Figure 21(a), we show
a schematic diagram of auroral characteristic features which is a composite of the
major features appearing on a large number of DMSP photographs (Akasofu and
Snyder, 1974). Note that this diagram shows the notable asymmetry with respect
to the midnight meridian. Structured discrete auroras are active near the poleward
boundary of the auroral oval, especially in the premidnight sector, whereas the
diffuse aurora delineates a relatively stable equatorward boundary of the auroral
belt. Using this auroral oval as the 'normalized' reference, the equivalent current
vectors are plotted in Figure 21(b) for a total of 20 satellite passes which took
place near the maximum phase of substorms. Figure 21(b) confirms that the
westward electrojet is most intense in midnight and early morning hours and that
it does not end in the midnight meridian, but extends into the evening sector along
the auroral oval. This is somewhat different from what the SD current system
indicates. The latitudinal width of the area where the westward electrojet effect is
observed in the morning sector is much larger than that in the evening sector. In
the diffuse auroral region, the eastward electro jet is a common feature in the
evening sector.
Wallis et aI. (1976) have recently made a similar study of the spatial relationship
between the ISIS-2 large-scale auroral display and the auroral electrojets, determined from the Canada meridian chain of magnetometer data. They defined the
latitudinal limits of auroral emissions in the evening sector on the basis of the
auroral intensity at 3914 N and 5577 ~. It was shown that the eastward electrojet,
determined from the latitudinal distribution of the H and Z components (see
Kisabeth and Rostoker, 1977) is contained within, and may be narrower than the
latitude range of auroral emission. It was also found that although discrete auroral
arcs within the electrojet may produce enhanced conductivity in their vicinity, this
does not necessarily lead to enhanced ionospheric current densities, implying the
importance of the electric field in determining the current intensity in the ionosphere.
Kamide et al. (1978) utilized magnetic field data from the TRIAD satellite at
800 km to define the boundaries of field-aligned currents when auroral luminosity
along the same meridian was scanned by the ISIS-2 satellite. For the maximum
phase of an intense substorm, a detailed comparison of the inferred field-aligned
currents and the auroral luminosity along the TRIAD subtrack is given in Fig 22,
which shows the TRIAD magnetometer record and the logarithmic profile of
albedo-corrected auroral intensities along the TRIAD trajectory. It is seen that
some structure can occur in the diffuse auroral region and may be due to discrete
arcs (in the sense described by Wallis et al. (1976)) imbedded in the diffuse aurora.
It is also evident that the luminosity does not go to zero between the diffuse and
discrete auroras. Kamide et al. (1978) pointed out that in the evening sector, the
latitudinal boundaries of the major portion of the field-aligned currents line up
well with the auroral luminosity boundaries of l - 2 k R at both the poleward and
FIELD-ALIGNED
CURRENTS
AND AURORAL
FEBRUARY
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1974
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Fig. 22. TRIAD magnetometer data, the estimated field-aligned current density, and logarithmic
latitudinal profiles of auroral intensity along the TRIAD subtrack. (Kamide, Y., Murphree, J. S., Anger,
C. D., Berkey, F. T., and Potemra, T. A.: 1979, J. Geophys. Res. 84, 4425.)
equatorward sides of the auroral distribution, and that the boundary between the
upward and downward field-aligned currents generally occurs at the minimum in
the auroral luminosity profile.
4.4. FIELD-ALIGNED CURRENTS AND THE AURORAL ELECTROJETS
Several 'equivalent' three-dimensional current systems had been inferred in the
past, based primarily on magnetic observations made on the Earth's surface, in
which there is an inflow of current into the morning half of the auroral oval and
an outflow from the evening half of the oval and these are connected through the
westward electrojet. However, as described in the previous sections, recent satellite
m e a s u r e m e n t s indicated the existence of both upward and downward flows at all
local times. Thus, the earlier current patterns must be revised considerably by
taking into account the recent new findings of the configuration of the field-aligned
172
,r KAMIDE
currents which exhibit distortions during substorms particularly in the region of
the Harang discontinuity.
Rostoker et al. (1975) examined the simultaneous magnetometer data from the
T R I A D satellite and ground-based meridian chains of observatories. They showed
by the use of modeling techniques employed by Kisabeth and Rostoker (1973) that
a region of intense upward field-aligned current encompasses the boundary between
the eastward and westward electro jets. They noted that there is a downward current
flow both to the north and the south of the boundary between the two electrojets,
i.e., the Harang discontinuity. Figure 23 shows the schematic picture of the fieldaligned current configuration in the substorm disturbed evening sector, inferred by
NORTH ~
1
WESTWAR D
E L E C T R O JET
~L . SOUTH
II
EASTWA R D
E LECT ROJET
I
Fig. 23. Schematicpicture of the field-alignedcurrent configurationin the substorm-disturbedevening
sector inferred from the TRIAD data. The E field configuration is inferred from the direction of
the auroral electrojet flow. (Rostoker, G., Armstrong, J. C., and Zmuda, A. J.: 1975, J. Geophys.
Res. 80, 3571.)
Rostoker et al. (1975). This configuration is interpreted in terms of the transition
in north-south electric field polarity at the boundary between the auroral belt and
the polar cap, as reported by Cauffman and Gurnett (1971) and Burch et al. (1978).
Iijima and Potemra (1978) showed that during periods when the westward
electrojet intrudes deeply into the evening sector, the T R I A D magnetometer data
exhibit complicated and fine-structured variations, indicating the presence of corn-
FIELD-ALIGNED CURRENTS AND AURORAL
ELECTROJETS
173
plex field-aligned currents in the Harang discontinuity region. Nevertheless, the
large-scale current configuration consists essentially of an upward field-aligned
current surrounded to the north and south by downward currents, which is a simple
superposition of morning-type and evening-type field-aligned current systems.
However, as pointed out by Kamide (1978), the loci defined by various methods
(such as reversals of the electric field, currents, and ground magnetic perturbations)
as the Harang discontinuity may not all be coincident, in particular, when we note
recent findings that the electric field does not show an abrupt change in the
north-south component across the Harang discontinuity but rather a gradual
rotation over a finite region. Kamide and Akasofu (1976b) showed several
examples of the TRIAD magnetometer data and the simultaneous ground magnetic
data from a meridian in which there is no significant field-aligned current in the
Harang discontinuity, i.e., the boundary between the eastward and westward
electrojets. Figure 24 shows the TRIAD data and the latitudinal profile of the
ground H and Z perturbations, together with the locations of auroral arcs. We
note that it is possible to determine the centre of each electrojet and the twoelectro jet boundary with a reasonable accuracy (<1 ~ in latitude) by knowing the
latitudinal profile of both the H and Z components (Oldenburg, 1976, 1978). It
is noticeable that the boundary between the upward and downward field-aligned
currents coincides well with the boundary between the westward and eastward
electrojets, which is inferred from the location of zlH = 0 and AZ = minimum.
In a recent detailed examination using joint meridian chain observations of
ground magnetic and ionospheric electric fields, Baumjohann et al. (1980) reported
that during one period, the eastward electrojet diverged up field lines in a local
area at the Harang discontinuity, while on another occasion it diverged northward
within the ionosphere and and joined the westward electrojet. These two observed
mechanisms of current divergence near the Harang discontinuity region may indicate that whether the ionospheric electrojet current becomes field-aligned current
or simply changes direction within the ionosphere depends strongly on local time
and substorm time. In any event, the ionospheric conductivity in the vicinity of the
Harang discontinuity plays a crucial role in determining the current configuration
(see Section 5.4). It is interesting to note in this connection that Vickrey et al.
(1981) have recently examined the auroral ionospheric conductivity at the Harang
discontinuity. In spite of the quite sharp encounter by the Chatanika radar with
the Harang discontinuity on the day examined, they found a decrease in the XH/Xp
ratio at the boundary indicative of a decreasing precipitating electron hardness,
which implies the decrease of upward field-aligned current.
Kamide et al. (1976c) and Sulzbacher et al. (1978) made a similar examination
of the relative location of the ionospheric and field-aligned currents in the morning
sector and found that during substorm periods, both the downward and the upward
field-aligned currents generally occur inside the westward electrojet region. It was
suggested, however, that the observed inequality of the intensities of the upward
and downward currents implies that their closure in the ionosphere cannot be
174
Y. KAMIDE
6 MARCH
0658UT
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Fig. 24. Relationship between the field-aligned currents and the auroral electrojets. Note that the
boundary of the upward and downward currents coincides approximately with the boundary of the
regions of positive and negative geomagnetic H perturbations.(Kamide, Y. and Akasofu, S.-I.: 1976a,
Planetary Space Sci. 24, 203.)
completed in the same meridian in the morning sector and must have a large
westward component, that is, the westward electrojet. In other words, the 'westward
electrojet' in the morning sector should flow in the southwestward direction.
Three-dimensional current configurations in the dayside cusp have been of
considerable discussion over the decade. Friis-Christensen and Wilhjelm (1975)
and Akasofu et al. (1980) undertook a statistical study of the so-called D P Y current
system associated with the By component of the interplanetary magnetic field.
Wilhjelm et al. (1978) correlated the D P Y system with the 'cusp' field-aligned
current. Rostoker (1980) has given a caution that the D P Y current system should
not be considered as a distinct system in the dayside cusp whose strength and sense
are regulated by By. Rather, By does redistribute the ionospheric current portion
FIELD-ALIGNED CURRENTS AND AURORAL ELECTROJETS
175
of the cusp current system by 'blending' the eastward current in the morning sector
into the current in the afternoon sector, a picture qualitatively in agreement with
the field-aligned current configuration proposed by McDiarmid et al. (1979). Energetic particles and convection associated with the dayside cusp field-aligned currents
have also been examined by Foster et al. (1980), Smiddy et al. (1980) and Bythrow
et al. (1981).
5. Ionospheric Electric Fields and Currents
5.1.
ELECTRIC
FIELD
PATTERN
The large-scale electric field plays a key role in the electrical coupling between the
ionosphere and magnetosphere, thus being crucial in discussing the connection of
the field-aligned currents to the auroral electrojets. General reviews concerning
the electric field distribution observed in the ionosphere include the work of Mozer
(1973a), Pudovkin (1974), Kane (1976), Gurevich et al. (1976), Stern (1977),
Pedersen et al. (1978), and Volland (1979). Experimental methods by which the
electric field in the vicinity of the Earth could be determined are described by Stern
(1977).
Early satellites in polar orbits equipped with an electric double probe, such as
OGO-6 and INJUN-5, have established the concept of the polar cap in terms of
the electric field pattern across which the dawn-to-dusk electric field exists permanently. The measurements are in agreement with models in which the plasma
convection in the polar cap is adequately expressed by a two-cell pattern, corresponding to the original idea of the magnetospheric convection envisaged by Axford
and Hines (1961). The electric field has usually a total potential drop of the order
of 20-100 kV across the entire polar region. The most prominent feature of the
electric field data from polar orbiting satellites is the persistent occurrence of steep
reversals in the field, and thus in the convection velocity at about 70 ~to 80 ~invariant
latitude (Gurnett, 1972b). It was found by Frank and Gurnett (1971) that the
so-called trapping boundary for electrons with energies E > 4 5 keV is located
essentially coincident with the electric field reversal. Heppner (1972a, b, c) noted
that the electric field pattern under very quiet conditions is basically the same as
during disturbed conditions, indicating that significant changes of the global scale
electric field distribution cannot be invoked as a direct cause of substorms. Foster
et al. (1981) have shown that even when midnight sector substorm activity exists,
the convection field pattern maintains a two-cell character.
One departure from such an 'average' pattern over the polar region is a systematic
non-uniform variation of the magnitude of the dawn-dusk component across the
polar cap, correlated with the By component of the interplanetary magnetic field
(Heppner, 1972d). The electric field tends to be stronger on the side of the polar
cap where By and the projection of the geomagnetic field into the equatorial plane
point in the same direction (Heppner, 1973), and vice versa. Based on OGO-6
176
Y. KAMIDE
data, Heppner (1977) presented an empirical model of high latitude electric potential patterns by including a modification at the Harang discontinuity.
Similar double probes were carried also by various sounding rockets into active
auroral forms (Maynard and Johnstone, 1974; Maynard et al., 1977; Whalen et
aI., 1974, 1975; Kelley et al., 1975; Carlson and Kelley, 1977; Evans et al., 1977),
and by various balloons in the upper atmosphere (Mozer and Serlin, 1969; Mozer,
1973a, b; Ogawa, 1973, 1976; Mozer and Lucht, 1974; Mozer etal., 1974; Ungstrup
et al., 1975; Madsen et al., 1976; Holzworth et al., 1977; Tanaka et al., 1977a, b;
JOrgensen et al., 1980). Figure 25a shows hourly averages of electric field components as a function of local time which were measured from 32 balloon flights
in the auroral zone (Mozer and Lucht, 1974). These average values mapped in the
equatorial plane of the magnetosphere (shown in Figure 25b) fit well into the
general picture of the sunward streaming plasma convection.
Haerendel et al. (1967) and Davis and Wallis (1972) discussed the method of
measuring the electric field by releasing barium vapor into the atmosphere. In
addition to the confirmation of the large-scale field pattern deduced from the double
probe methods (e.g., Wescott et al., 1969, 1970), it has become possible to inject
charges along magnetic field lines to great distances (Wescott et al., 1974), and to
IIK
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Fig. 25(a)
Fig. 25. (a) Hourly averages of electric field components measured in a non-rotating frames of reference
on 32 balloons flown in the auroral zone. (b) Hourly averaged electric field vectors plotted on the
equatorial plane of a non-rotating frame of reference, as viewed from above the north pole. (Mozer, F.
S. and Lucht, P.: 1974, J. Geophys. Res. 79, 1001.)
177
FIELD-ALIGNED CURRENTS AND A U R O R A L ELECTROJETS
I0
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Fig. 25(b).
examine substorm features of the electric field, in particular, the parallel field
(Wescott et aL, 1975).
It may also be possible to estimate the electric field by observing the bulk velocity
of ambient plasma from a spacecraft (Hanson et al., 1973; Hanson and Heelis,
1975; Heelis et al., 1976), and from a sounding rocket (Morgan and Arnoldy,
1978). Based on the AE-C satellite measurements, Burch et al. (1976) studied the
characteristics of pairs of oppositely directed spikes in ionosphere convection
velocities and found that these phenomena tend to occur near the reversal from
sunward to antisunward convection on the night side of the Earth, where inverted
- V type electron precipitation is observed. Heelis et al. (1980) made a study using
the A E - C instrument, of simultaneous measurements of auroral particles and ion
velocity in which it was shown that the convection reversal, which is the boundary
between the polar cap and the auroral oval, occurs within the boundary plasma
sheet (BPS). This result seems to coincide with that obtained by Winningham et
178
Y. KAMIDE
al. (1979) who compared the locations of the auroral electrojet boundaries using
electron spectra and ground-based magnetic records. Winningham et al. (1980),
however, appear to stress the possibility that the convection electric field is generated
on closed field lines.
Spiro et al. (1979) found evidence in the AE-C data, of large poleward-directed
electric fields in the region equatorward of the auroral oval. This intense poleward
electric field in sub-auroral latitudes was found to occur during substorm activity
(Smiddy et al., 1977; Maynard, 1978). Heelis and Hanson (1980) showed that such
large electric field events measured by AE-C, mostly occur in the 18:00-24:00 MLT
region. Banks and Yasuhara (1978) interpreted the large poleward field as a result
of the very low ionospheric conductivities.
Doppler shifts of radar waves scattered incoherently from the ionosphere can
give the ion bulk velocity at altitudes up to 500 km (Leadabrand et al., 1972;
Doupnik et al., 1972; Banks et al., 1973; Brekke et al., 1973, 1974; Horwitz et
aI., 1978a; Sojika et al., 1980). At altitudes above 160 km, ion velocities are almost
entirely due to E x B drifts. Therefore, the electric field can be obtained directly
from the velocity measurements by the radars at these altitudes. This field is mapped
down to the E region (Ecklund et al., 1977), where ion motions are influenced by
ion-neutral collisions as well. This method provides a powerful tool for deriving
the electric field, since it c~n monitor the field variations continuously. The facility
of this kind presently operating in high latitudes is an incoherent scatter radar at
Chatanika, Alaska (Banks and Doupnik, 1975). In addition to the electric field,
the incoherent scatter radar can offer most of the electrodynamic parameters in
the ionosphere. These include ionospheric conductivities and currents as well as
Joule heating (e.g., Banks, 1977; Brekke and Rino, 1978). Chatanika radar observations of latitude and local time variations in global heating rates, have shown the
presence of strong heating throughout the auroral regions (Banks et al., 1981).
Substorms can raise the auroral oval heating rates to large values as great as
50 mW m -2, which are shown to be an important heat source to the thermosphere
(e.g., Richmond, 1978, 1979; Mayr and Harris, 1978). During the IMS period, the
mid-latitude radar at Millstone Hill (A = 56 ~ has been upgraded to allow measurements of the electric fields in auroral latitudes over the interval 60 ~~<A ~<75 ~ (Evans
et al., 1979, 1980). From the obtained electric field patterns in high latitudes, it
was suggested that during disturbed periods, the electric field is not only enhanced
significantly, but also the convection cell in the dawn sector tends to be displaced
into the nighttime sector.
The STARE system can measure the drift velocity of electron density
irregularities at two sites in Finland and Norway, and hence it can provide us with
information on the electric field distribution in the two-dimensional region covered
by the two Doppler radar units (Greenwald et al., 1978). As will be described
in Section 6, the combination of the STARE operation with the successful
development of the Scandinavia magnetometer array has become increasingly
comprehensive in constructing the three-dimensional current system. Simultaneous
FIELD-ALIGNED CURRENTS AND AURORAL ELECTROJETS
179
measurements of the vector electric and magnetic fields have recently been made
by polar orbiting satellites. Bythrow et al. (1980) found it useful to compare plasma
drift velocities observed by the A E - C satellite and field-aligned currents observed
by a magnetometer onboard which was originally included in the AE-C payload
only for engineering functions. By examining several representative examples,
Bythrow et al. found good correlations between the field-aligned current density
and the gradient of the north-south electric field. Of particular interest is the
existence of an intense field-aligned current at the polar cap boundary where the
direction of the plasma velocity undergoes a reversal from sunward to antisunward
convection.
Shuman et al. (1981) compared the transverse magnetic field, the electric field,
and auroral electron flux, all observed simultaneously by the $3-2 satellite. It was
confirmed that the observed direction of the electric field component is consistent
with what the closure of the major field-aligned current by Pedersen current flow
indicates.
5.2.
ELECTRIC
FIELD
ASSOCIATED
WITH
AURORAS
It is reasonable to expect that the large-scale electric field and current are modified
locally in and near auroral arcs, since an arc is a region of enhanced conductivity.
However, the problem of the electric field and current inside an auroral arc
associated with energetic electron precipitation is not well settled.
As discussed by Rostoker (1978) and de la Beaujardi~re et aI. (1981), there
appear to be several conflicting sets of measurements of the electric field associated
with auroral arcs where an intense electron precipitation exists. Aggson (1969),
Potter (1970), Wescott et al. (1969), and Yau et al. (1981) indicated a decrease of
the electric field within the auroral form, whereas Mozer and Fahleson (1970),
Gurnett and Frank (1973), and Swift and Gurnett (1973) observed an increase in
the southward electric field inside auroral arcs. Most recently, Maynard et al. (1977)
showed by the use of a rocket measurement that electron precipitation is anticorrelated with the electric field intensity (both in the north-south and east-west components) inside the arc, in agreement with the earlier report of Maynard et al.
(1973). Evans et al. (1977) suggested that a polarization electric field is built up
within the arc such that current continuity holds at the arc boundary.
On the other hand, Carlson and Kelley (1977), using ion flow data of a rocket
double-probe, have found that within a substorm-activated auroral arc, the electric
field and energetic electron flux are correlated. Edwards et al. (1976) reported that
there was no simple relationship between the intensities of the electric field and
precipitating electron flux.
Stiles etal. (1980) and Cahill etal. (1980), based on radar and rocket observations,
reported that the northward electric field is large only on the equatorward side of
auroral arcs and is small or almost zero within the arcs. de la Beaujardi6re et al.
(1981) showed electric field data around an auroral arc measured simultaneously
by the AE-C satellite and the Chatanika radar at widely spaced longitudes, spanning
180
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FIELD-ALIGNED CURRENTS AND AURORAL ELECTROJETS
181
more than 3 hr in local time. Their data at these longitudes indicate remarkably
similar features of an intense northward field only at the equatorward side of the arc.
In most of these conflicting observations, it was not possible to derive a definite
conclusion concerning the field and current around auroral arcs, because only a
few ionospheric parameters were measured. On the other hand, de la Beaujardi~re
et al. (1977) recently presented a set of comprehensive observations made with the
Chatanika radar. These measurements were made with a relatively simple technique
in which the radar antenna was directed to the magnetic west and discrete auroral
arcs moved in the north-south direction, such that one could observe spatial
variations of the physical parameters of interest assuming no temporal variations
occurred during the observation period. They have noted that the electric field
data can be separated into two parts: an ambient field that is the large-scale electric
field and an arc-associated electric field within the arc form. It is essential to stress
this point in discussing the electric field intensity in relation to auroral arcs, since
as seen in the diurnal change of the electric field shown in Figure 25, the large-scale
electric field in the auroral latitudes is directed primarily northward in the evening
sector while southward in the morning sector. According to de la Beaujardi~re et
aI. (1977), evening sector arcs yield a reduced northward electric field in the region
where the enhanced electron density is present, indicating that the reduced northward field is due to an added southward field associated with the auroral arc. For
morning sector arcs, the southward field is stronger inside the arcs compared with
outside. These features are clearly demonstrated in Figures 26a and b.
Horwitz et al. (1978a) recently compared the electric fields probed at several
latitudes by the Chatanika radar with optical auroral data from DMSP and all-sky
photographs, and found that in the morning sector, sharp reductions of the southward electric field strength were seen in regions of bright, active auroras, with large
electric fields often appearing immediately poleward of the high-latitude borders
of these auroral regions. Mahon et al. (1977) recently observed a southward electric
field with 35-40 mV m -1 intensity in the region of 2kR diffuse aurora.
5.3. ELECTRIC FIELD NEAR THE HARANG DISCONTINUITY
Various magnetospheric and auroral phenomena significantly change their characteristics across the Harang discontinuity, as emphasized by Heppner (1972d). One
of the phenomena that is used to identify the discontinuity is the direction change
of the electric field. Figure 27 shows a schematic illustration of typical convective
flow and the corresponding electric field directions in the polar region (Maynard,
1974a). It can be seen that the Harang discontinuity occurs in higher latitudes at
earlier local times than at later local times. Maynard (1974a) showed that the
discontinuity in the electric field data obtained from OGO-6 double probe measurements, is present even during extremely quiet times.
It is also important to note that the reversal of the electric field cannot occur
across a mathematical line boundary, but the northward-directed field would
gradually rotate counterclockwise across the Harang discontinuity (Maynard,
7ig. 27.
0
MLT
12
-LECTRIC FIELD
SCHEMATIC
Schematic diagram of the convective flow and electric field distributions in the polar region. (Maynard, N. C.: 1974a, J. Geophys. Res. 79, 4620.)
0
ALT
.~ONVECTION
SCHEMATIC
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
183
1974a). Kamide (1978) stressed the importance of the predominance of the westward electric field in the Harang discontinuity 'region' in understanding the ionospheric current distribution in the premidnight sector; note that the electric field in
the evening and morning sectors is dominated by its north-south component.
However, predominance of the westward field has been reported earlier by Wescott
et al. (1969) and Banks et al. (1973). Maynard (1974b) further reported that the
latitudinal width varies in conjunction with magnetic activity; the width becomes
smaller under disturbed conditions and broadens under quiet conditions. Wedde
et al. (1977) showed that when the Harang discontinuity is traversed by groundbased instruments, such as the incoherent scatter radar, the discontinuity region
occurs over a fairly wide local time range, say, 1-2 hr. Horwitz et al. (1978b) have
recently examined the latitudinal distributions of the electric field configuration
near the Harang discontinuity region which were compared with those of isointensity
AH contours in latitude and time, and with the north-south component of the
interplanetary magnetic field. It has been argued by them that although previous
observations of the electric field in the premidnight sector stressed too much
enhancement of the westward component following the southward turning of the
interplanetary magnetic field, the latitudinal expansions of the oval may be far
more conspicuous than the enhancement of the westward field. Moreover, such a
westward field enhancement in the midnight sector could result from the simple
latitudinal shift of the electric field pattern near the Harang discontinuity without
a major change in the gross structure of the electric field.
The westward field in the discontinuity region has been shown to be modulated
in the presence of discrete auroras (Banks et al., 1974; Rino et al., 1974). Using
STARE data, Nielsen and Greenwald (1978) have shown marked distortions of
the electric field around discrete auroral forms near the Harang discontinuity. The
systematic longitudinal shift of the Harang discontinuity with respect to the
azimuthal component of the interplanetary magnetic field has been found by
Scourfield and Nielsen (1981). However, the ionospheric conductivity changes
across the Harang discontinuity have not been observationally established yet,
although Wedde et al. (1977) have reported that for at least one event, the
discontinuity encounter by the Chatanika radar is accompanied by an abrupt
increase in electron precipitation (thus in the Hall conductivity), the most intense
part being located slightly east of the center of the discontinuity. We note that it is
somewhat difficult for the auroral particle enhancement, detected by the radar
which is rotating with the earth, to be distinguished as being due to processes
closely connected with the Harang discontinuity region itself (i.e., spatial change)
or to a substorm-related energization (i.e., temporal effect).
An unsettled problem on the Harang discontinuity electric field lies also in the
nature of the discontinuity in the magnetosphere, in spite of the fact that several
attempts have been made to map the discontinuity onto the equatorial plane of
the magnetosphere (Maynard, 1974a; Fairfield and Mead, 1975; Brekke, 1977).
There is little doubt that the discontinuity represents the convection boundary
184
Y. KAMIDE
dividing the eastward and westward plasma drifts in the magnetosphere. However,
the question of how various phenomena (such as ionospheric currents, auroral
features and other substorm dynamics) change their characters across the discontinuity is unanswered. From a statistical study of plasma behavior at the synchronous
orbit, Lezniak and Winckler (1970) defined a 'fault line' near local midnight, west
of which inflation of the magnetic field occurs and east of which collapse is observed
during substorms. Maynard (1974a) suggested that when the Harang discontinuity
is mapped onto the magnetotail, it can be identified as the fault line. However, an
important question is how the fault line can agree with the well-known local time
dependence of the Harang discontinuity, which is located in different latitudes at
different local times. To resolve this question, detailed studies are needed as to the
large-scale convection characteristics related to ionospheric conductivities and
field-aligned currents.
Brekke (1977) has indicated that the Harang discontinuity defined by the electric
field reversal, corresponds to the substorm injection boundary in the magnetosphere. This inference was made by comparing the north-south component field
reversal observed by the Chatanika radar with the encounter of the injection
boundary by the ATS-5. It is noted that the injection boundary observed by the
ATS-5 (Mcllwain, 1974; Mauk and Mcllwain, 1974) and Explorer 45 (Konradi et
al., 1975, 1976) can be mapped to the equatorward boundary of the auroral belt
(Kivelson, 1976) rather than to the Harang discontinuity.
Doppler backscatter radars may become a new tool for inferring the ionospheric
electric field (Solvang et al., 1977). They measure the drift velocity of electron
density irregularities. Data from the STARE system, together with data from the
Scandinavian Magnetometer Array, should provide a comprehensive picture of the
ionospheric electric field and current near the Harang discontinuity. Greenwald et
al. (1978) have shown an interesting example of the STARE radar data in which
the eastward drifting region (southward electric field) penetrates to the north of
the westward drifting region (northward field) in the evening sector. The fairly
dense network of the Scandinavian magnetic stations during the IMS period has a
great ability to yield quantitative information on fine spatial variations of the auroral
electrojets in the region covered by the STARE radar system (Kiippers et al., 1978;
Baumjohann et aI., 1978).
5.4.
IONOSPHERIC CONDUCTIVITY AND CURRENTS
On the basis of the Chatanika radar observations of the electron density profiles
and atmospheric models tabulated by Banks and Kockarts (1973), Brekke et al.
(1974) obtained the typical altitude dependence of the radar-measured electron
density and derived conductivities. It was shown that the height-integrated Hall
conductivity (Xn) emphasizes the electron density in the region below 110 km,
whereas the Pedersen conductivity (Xp) attains its maximum contribution at an
altitude between 125 and 110 km. For quiet days the ratio ,~H/2;e is fairly constant
and close to 2, while during disturbed days, several peaks occur corresponding to
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
185
substorm activity. The ratio between the two conductivities gives a crude first
approximation to the energy of the precipitating particles (Rees, 1963), since
energetic auroral electrons penetrating the atmosphere reach different altitude
levels depending on their energy. Wallis and Budzinski (1981) conducted a computation of the two-dimensional distribution of the height-integrated Hall and Pedersen
conductivities for relatively quiet and disturbed conditions. Average precipitating
fluxes of electrons observed by the ISIS-2 satellite were used in conjunction with
the method of Rees (1963, 1969). This kind of model is extremely useful in
numerical modeling of high latitude electrodynamics (see Section 6).
The latitudinal distribution of the height-integrated conductivities has been
deduced from the data obtained by the Chatanika radar for the range of invariant
latitudes 62 ~ to 680 (Wedde et aI., 1977; Horwitz et al., 1978b). It was found that
the local time transition between the diffuse precipitation-conductivity zone in the
evening sector (Xp = 8-12 mho, Xz-/= 16-24 mho) and the harder, active precipitation-conductivity zone in the midnight and morning sector (Xe = 10-16 mho, XH =
20-60 mho) coincides with the Harang discontinuity defined by the electric field
reversal. Recently, Vickrey et al. (1981) utilized more comprehensive data of
space-time ionospheric conductivity variations obtained from the meridian scan of
the Chatanika radar. It was shown empirically that during quiet times, the solar
contribution to the conductivity is well represented by cos ~/2 (,g), where X is the
solar zenith angle. The enhancement of the Hall conductivity associated with
substorm onset was found to occur as abruptly as a few minutes.
Evans et al. (1977) utilized the auroral electron data obtained during a sounding
rocket flight over a stable auroral arc as an input to a computation of the Hall and
Pedersen conductivities. They used a different atmospheric model (see Bostrbm,
1973; Jones and Rees, 1973) from that used by Brekke et aL (1974). It was shown
that a change occurred in the ratio of the two conductivities from a value of 1.4
above the auroral form to 0.8 further northward, manifesting the softening of the
auroral electron spectrum outside the arc.
Knowledge of the electric field and conductivity leads to a description of the
ionospheric currents and thus to a clear demonstration of local time changes of the
auroral electrojets and their relationship with the intensities of the electric field
and the field-aligned currents. Thus, for the first time in the history of ionospheric
physics and geomagnetism, data from the Chatanika incoherent scatter radar made
it possible to deduce continuously the ionospheric current density (Brekke et aI.,
1974; Banks and Doupnik, 1975).
5.4.1. Ionospheric Currents and Ground Magnetic Perturbations
Several attempts have been made to correlate the ionospheric currents deduced
from the radar observations with ground magnetic perturbations, confirming that
ionospheric currents, called the auroral electrojets, can account for most of the H
component perturbations at auroral latitudes (Brekke et al., 1974; Kamide and
Brekke, 1975; Kamide et al., 1976a; Doupnik et al., 1977).
-0,5
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L--~
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FIELD-ALIGNED
CURRENTS AND AURORAL
ELECTROJETS
187
Figure 28 gives an example of the ionospheric currents in the northward and
eastward components at Chatanika together with the corresponding magnetic traces
of the H and D components observed at Poker Flat, which is only 3 km north of
the radar side. Moderate magnetic disturbances were observed all day. Time
variations of the east-west current density are quite similar to the corresponding
variations in the H component of the geomagnetic field near the radar site. There
are some disagreements in magnitude between the two quantities during the period
04:00-07:30 UT, but they can be reasonably explained by the fact that the eastward
electrojet had a small latitudinal width during that period.
The relationships between the north-south current density and the ground D
component appear, however, to be more complicated. From about 05:00 UT, the
Chatanika radar observed a gradual increase of the northward current followed by
a sudden intensification of it at about 07:30 UT. Until about 11:30 UT, the ionospheric current has essentially a northward component, regardless of the sign of
the D component perturbations. The disagreement between the north-south ionospheric current and ground D is most serious prior to local midnight, except for the
short time intervals 07:50-08:50 UT and 09:50-10:20 UT. That is, there are many
periods in which the Chatanika radar data give results completely contrary to the
observed D perturbations on the Earth's surface.
On the other hand, the disagreement becomes less serious in the morning sector.
There are at least two eastward magnetic deviations at Poker Flat which occurred
at about 11:45 and 14:20 UT associated with intense negative H perturbations. It
is noted that these eastward perturbations were observed at all the available Alaska
observatories except Sitka along the meridian, which are caused by the southward
ionospheric current actually observed at Chatanika.
This suggests that the main part of the ground D perturbation in the evening
sector is not caused by ionospheric currents but probably by field-aligned currents.
Then, why does the ground D perturbation indicate the incorrect direction of the
current in the evening sector? This peculiarity is at least partially explained by
supposing that the intensities 6f the upward and the downward field-aligned currents
are, in general, not equal (Yasuhara et al., 1975). After qualitative considerations,
Kamide et al. (1976a) inferred a three-dimensional current model including both
ionospheric and field-aligned currents. This situation is shown in Figure 29. The
dashed line representes the Harang discontinuity which divides the region of the
eastward current from the region of the westward current in the evening sector.
On the equatorward side of the discontinuity, the ionospheric current flows northeastward. On the other hand, the direction of the ionospheric current on the
poleward side is northwestward.. These should not be confused with the equivalent
current vectors derived from the ground magnetic perturbations.
If the upward field-aligned current in the evening sector is really intense enough
to cancel the westward D perturbation on the Earth's surface caused by the
northward ionospheric current deduced from the Chatanika radar, then the same
upward current should produce the westward perturbation in latitudes higher than
188
Y. K A M I D E
D
C
B
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-
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(b)
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Fig. 29. Schematicdrawing of ionosphericcurrent flow,together with the Harang discontinuity(dashed
line). (Kamide, Y., Akasofu, S.-I., and Brekke, A.: 1976a, Planetary Space Sci. 24, 193.)
the center of the current. This feature is indeed observed along the Alaska meridian
chain.
Finally, we note the applicability of the conventional overhead current approximation. In the course of a long history of study on geomagnetic disturbances, this
approximation has been commonly used to infer the ionospheric current density
(see Nagata and Fukushima, 1971). The determination of an equivalent ionospheric
current vector has usually been made by converting the magnitude (in nT) of the
horizontal magnetic perturbation at the point of observation on the Earth's surface
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
189
to the overhead current density (in m A m -1) by assuming that the ground magnetic
perturbation is produced by an infinite, uniform sheet current. Such an assumption
is undoubtedly incorrect, since it is well known that polar substorms are characterized by localized currents, called the auroral electrojets. Kamide and Brekke (1975)
examined how well the use of the equivalent ionospheric current approximation
can reproduce the 'observed' ionospheric current density. It was shown that the
overhead current approximation almost always underestimates the current density
by a factor 2 or more.
5.4.2. Altitude Dependence of Ionospheric Currents
A sounding rocket measurement of the ion flow velocity and the electric field by
the double probe was performed by Bering et al. (1973). The rocket was launched
right into a westward electrojet. Bering and Mozer (1975) have shown that above
140-km altitude, the electric fields deduced from the two data sets agree to an
accuracy within the uncertainties of the two measurements. The difference between
the two data at altitudes below 140 km provides an in situ measurement of the
ionospheric current density. It was shown that a maximum current density of
5 • 10-6A m -2 was observed at an altitude of 110 km in the westward electrojet.
More recently, high resolution ( - 1 0 km) measurements of the inospheric electric
fields have been made possible at the Chatanika incoherent scatter radar (Rino et
al., 1977). Height-versus-time vector plots in geomagnetic coordinates of the
deduced current density were presented, in which considerable height variations
of the current are noticed in both the magnitude and direction. During the midnight
and morning period, one can see the concentration of the electrojet at lower
altitudes with a tendency of the currents to rotate to the southwest at higher altitudes.
Kamide and Brekke (1977) have made a similar study in which iso-intensity
contours of the ionospheric current density are obtained as functions of altitude
(from 85 to 185 km) and universal time, as shown in Figure 30, where the following
two points of interest are noted: (1) The dashed line indicates the altitude of the
maximum current density as a function of time. It is seen that the altitude dependence of the eastward electrojet was rather stable in the evening sector even when
it grew in conjunction with substorms. In the midnight and morning sectors,
however, it became intermittently lower in conjunction with the growth of the
current that is a signature of the substorm intensification. (2) The current center
of the eastward electrojet is located at higher altitudes than that of the westward
electrojet. The average altitude of the maximum current density at the peak time
of the eastward electrojet is 120 km, while that for the westward electrojet is
100 kin. These results may not be unexpected if we combine previously observed
differences in various parameters between the eastward and westward electrojets.
The eastward electrojet is probably associated with the diffuse aurora caused by
relatively low energy particles. In that region, the Hall and Pedersen conductivities
seem to change in unison. In the westward electrojet region, however, there are
large changes in the Hall conductivity with only small increases in the Pedersen
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m i t is 10 5 A m 2. T h e h e i g h t - i n t e g r a t e d i o n o s p h e r i c c u r r e n t in t h e geomagnetic e a s t c o m p o n e n t a n d t h e c o r r e s p o n d i n g H c o m p o n e n t m a g n e t i c p e r t u r b a t i o n
at P o k e r F l a t ( t h e n e a r e s t s t a t i o n t o t h e r a d a r site) a r e a l s o s h o w n . ( K a m i d e , Y . a n d B r e k k e , A . : 1 9 7 7 , J. Geophys. Res. 8 2 , 2 8 5 1 . )
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FIELD-ALIGNED CURRENTSAND AURORAL ELECTROJETS
191
conductivity, implying the sporadic precipitation of particles with energies of several
keV and above.
6. Modeling of Three-Dimensional Currents
The ionospheric electric fields and currents can be reproduced quantitatively by
computer simulation models. The basic assumptions employed in reproducing the
observed characteristics can also be tested synthetically. Using a computer simulation study in this sense, we are attempting to obtain a set of numerical solutions
of the equations governing the system under consideration, in a way that includes
far less idealization or simplification of essential observational characteristics than
would be required for a simple theoretical study. Obviously, no simulation study
will predict in precise quantitative detail all the features of the observations, but
it may be possible to demonstrate how the basic assumptions lead to the main
observed features and to isolate certain aspects of the system so as to reproduce
some particular phenomena under study. It is also possible by the use of modeling
techniques and some observations to estimate or predict the distribution and various
features of some other parameters.
6.1. ELECTRIC FIELDS
A number of simulation studies have been conducted in an attempt to reproduce
the essential features of the current and field patterns in the ionosphere and of
ground magnetic variations by applying the electric field and obtaining the resulting
current flow. The relation is complicated mathematically, particularly when the
distribution of the ionospheric conductivity is not uniform over the polar region
where the field-aligned currents are supposed to flow. The problem has been
approached in various ways and with different simplifying assumptions by different
workers. However, the following are the most important assumptions to be noted
and are commonly employed in most of the past simulation studies:
(1) The ionosphere is regarded as a two-dimensional spherical current sheet with
height-integrated layer conductivities, since we are interested only in the large-scale
current and field patterns involving distances much greater than the thickness of
the ionosphere.
(2) The field lines are considered to be equi-potential lines.
(3) Only a steady state is considered.
The continuity of currents under such conditions requires that
div i =]ll sin X,
(1)
where i is the ionospheric height-integrated current density, ]11is the density of the
field-aligned current (positive for a downward and negative for an upward current),
and X is the inclination angle of a geomagnetic field line with respect to the
horizontal ionosphere. Ohm's Law for the ionospheric current is written as
i = tr 9 E = -er 9grad qS,
(2)
192
Y. K A M I D E
where E and q~ are the electric field and its potential, respectively, in the frame
rotating with the Earth, and ~ is the dyadic of the height-integrated ionospheric
conductivity. In view of the assumption (2), the distribution of the electric field
perpendicular to the magnetic field in the ionosphere must be an image of the
magnetospheric field distribution and vice versa. Thus, as envisaged by Swift (1967),
Bostr6m (1974), and Wolf (1974), it is possible to discuss the ionosphere-magnetosphere interaction in terms of the convection flow in the magnetosphere and the
ionospheric current flow coupled through the field-aligned currents. In this section,
however, we deal only with the current and field patterns in the ionosphere by
paying no attention to the sources of the field-aligned currents, because (1) the
origin of the field-aligned currents is not, at present, obvious, and (2) we concentrate
our discussion only on the comparison between the simulation results and observations, in which only 'total' effects are given.
Iwasaki and Nishida (1967) and Vasyliunas (1970a) examined the ionospheric
effects of the convection electric field by assuming a sinusoidal distribution of the
potential (or space charges) along a latitude circle representing the polar cap
boundary. For simplicity, symmetry between the two hemispheres was assumed
and day-night variations in the ionospheric conductivity were neglected. Figure
3 l a shows the resulting potential configuration in the ionosphere in the case of the
uniform conductivity over the world, whereas Figure 31b shows the potential
distribution for the case in which the conductivity is enhanced by a factor 10 at
latitudes just below the polar cap, corresponding to the auroral belt (Vasyliunas,
1970a). The former seems rather similar to the configuration inferred from the
DP 2 magnetic variation (Nishida et al., 1966; Obayashi and Nishida, 1968). It is
06 h
2h cash
06 h
oo 2
ooh
Fig. 31. (a) Calculated equipotential contours in the ionosphere. The outer circle is A = 50~ the inner
A = 72~ (b) Same as (a) but including effects of enhanced conductivitybetween A = 65~ and 72~ The
three circles are A = 50~ 67~ and 72~ respectively. (Vasyliunas, V. M.: 1970, in B. M. McCormac
(ed.), Particles and Fields in the Magnetosphere, D. Reidel Publ. Co., Dordrecht, Holland, p. 60.)
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
193
seen in Figure 31b that the potential pattern at latitudes below the enhanced
conductive belt is rotated counterclockwise.
The effects of inhomogeneity of the ionospheric conductivity on the convective
plasma flow in the magnetosphere was simulated by Wolf (1970). In his model, no
field-aligned current was allowed to flow outside the polar cap boundary. It was
shown that the computed ionospheric current patterns resemble the observed
patterns only if the auroral zone conductivity is assumed to be enhanced by an
order of magnitude or more over the mid-latitude nightside conductivity. This work
was extended to a significant degree by Jaggi and Wolf (1973) to discuss selfconsistently time-dependent electric fields including the effects of field-aligned
current from the inner edge of the plasma sheet (i.e., the equatorward boundary
of the auroral belt). In agreement with the conclusions of Karlson (1971), Block
(1966), Vasyliunas (1971), Swift (1971), and Maltsev (1974), the field-aligned
currents from the Alfv~n layer were found to reduce the electric field earthward
of the layer to a small value, indicating the 'shielding' of the high-latitude origin
of the electric field at mid-latitudes. Note that different parts of the electric field
earthward of the Alfv~n layer are shown to be eliminated at different rates; the
nightside relaxes to a low asymptotic field intensity in shorter times than the dayside.
Maltsev et al. (1972) and Lyatsky et al. (1974) computed the equivalent ionospheric currents for several models of the ionospheric conductivity to simulate
the ground magnetic disturbances observed in different phases of a typical substorm. In spite of comparatively simple assumptions including annular zones for
the auroral enhancement, some main features are reproduced quite nicely in the
world current systems. In particular, the Harang discontinuity in the premidnight
sector separating the eastward and westward electrojets was shown to be reproduced by assuming two auroral belts, perhaps corresponding to the discrete and
diffuse auroras.
In numerical calculations for the polar ionospheric fields and currents made by
Maeda and Maekawa (1973), two different driving forces are taken into account;
one of them is the dynamo ionospheric wind and the other is the solar wind. This
treatment indicates that in solving (1) and (2), the electric field E is decomposed
into the two parts, although ]'11had to be assumed to be zero except at two points
(source and sink) within the auroral belt. It was found that the solar wind-magnetosphere interaction is, in general, more effective than the ionospheric wind in producing the usual polar current systems such as DP-1 and DP-2. It was also pointed
out that the secondary polarization field caused by the non-uniform distribution
of the ionospheric conductivity is much larger than the primary field applied by
the solar wind.
Based on in situ measurements of the electric fields at ionospheric heights,
Volland (1978) has tried to construct simple semi-empirical models of electric
potentials, from which current configurations can be derived uniquely by assuming
the ionospheric conductivity. This model includes the Harang discontinuity region
where the convection electric field changes its direction. A general consistency
194
Y. KAMIDE
between the field-aligned current distributions predicted by the model and that
observed by satellites, was stressed by Volland (1979).
6.2.
FIELD-ALIGNED
CURRENTS
Instead of postulating the driving electric field, one can obtain the ionospheric
current pattern by assuming the field-aligned currents. This was the approach
effectively taken by Yasuhara et al. (1975), who noted that even the presently
available information on the field-aligned current distribution is extremely useful
in understanding the electric field and current configuration in the ionosphere,
while the electric field observation is at present incomplete in obtaining an accurate
potential distribution over the entire polar region. The field-aligned currents in the
poleward half of the auroral oval are on the average two times larger than those
in the equatorward half. Yasuhara et al. (1975) showed that this fact is quite
important in reproducing successively the observations of the ionospheric currents
in the auroral latitudes during substorms. As seen in Figure 32, the so-called
ionospheric return currents from the auroral electrojets, in middle latitudes and in
the polar cap, are surprisingly small for any of their ionospheric conductivity models,
indicating that the ground magnetic perturbations that have been ascribed to the
two-dimensional currents in mid-latitudes and in the polar cap are mainly due
to the magnetic effects resulting from the field-aligned currents and connected
currents in the magnetosphere.
The calculated ionospheric electric field was compared with the field observed
by the Chatanika incoherent scatter radar (Yasuhara and Akasofu, 1977). Since
the auroral belt was approximated by two conductive annular rings simulating the
discrete and diffuse auroral belts, their results may not necessarily be accurate in
the dayside. However, local time variations of the Chatanika electric field are
reproduced reasonably well by the numerical experiment particularly in the nightside auroral belt. The total potential difference across the polar cap does not seem
to increase very much even if the field-aligned currents are assumed to be considerably increased, in agreement with the observation of Heppner (1972a) who pointed
out that the electric field distribution does not differ significantly between quiet
and disturbed times.
Nisbet et al. (1978) conducted a numerical calculation of the electric fields and
currents in the global ionosphere produced by the field-aligned currents in auroral
latitudes. A two-dimensional network was constructed by using a number of
rectangular grids in which the distribution of the field-aligned currents and the
ionospheric conductivity was assumed to obtain the most suitable potential value
as a whole. The model has utilized 'as input', the field-aligned currents reported
by Iijima and Potemra (1976), for 2 - < K p < 4 +
as well as the conductivities
developed by Kirchhoff and Carpenter (1975). It was found that the electric fields,
due to the high-latitude field-aligned currents, p~netrate down to the equator but
are reduced from their values in the auroral latitudes by factors of 140 at 20 ~
latitude and 200 at the equator.
FIELD*ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
195
L2
A J = I 0 4 K amp
06
18
Fig. 32. Calculated ionospheric current pattern for a given distribution of the field-aligned currents
and for a model of the ionosphere. The arrow indicates the direction of the current flow. The amount
of currents flowing between two adjacent stream lines is 104 kA. (Yasuhara, F., Kamide, Y., and
Akasofu, S.-I.: 1975, Planetary Space $ci. 23, 1355.)
N o p p e r and Carovillano (1978) have m a d e a similar calculation of the ionospheric
electric fields and currents in relation to the field-aligned currents. The role of
the ionospheric conductivity was examined by the same authors (Nopper and
Carovillano, 1979). It was demonstrated that changes in the field-aligned current
pattern during disturbed periods can account for equatorial fluctuations in the
electric fields, indicating that magnetospheric dynamics which produce the fieldaligned currents given as the driving forces in their calculations, have a direct effect
on the equatorial ionosphere through the ionospheric conductivity. We contend
that it is of great interest to differentiate properly the roles of these two important
factors (i.e., the field-aligned currents in the auroral belt and the enhanced ionospheric conductivity) when discussing the ~polar-equatorial' coupling during active
periods. During the last several years, our knowledge about the distribution of the
large-scale field-aligned currents and the global auroral features has been advanced
196
Y. K A M I D E
to a significant degree. The availability of information on the ionospheric electric
field and conductivity by means of the radars located at auroral latitudes under a
variety of geomagnetic conditions yields also the possibility of isolating some
parameters in the Equations (1) and (2) that are crucial in determining the essential
aspects of the observations. In other words, although in the earlier simulation
studies, several simplifying assumptions are simultaneously employed, it may now
be possible to isolate in a quantitative detail what assumptions are required to
reproduce the essential features of the ionospheric and magnetospheric phenomena.
In a series of numerical simulations, Kamide and Matsushita (1979a, b) have
used, as faithfully as possible, the recent observations to examine the effects of
day-night asymmetry and auroral enhancements of the ionospheric conductivity
and changes in the intensity and location of the field-aligned currents. An algorithm
for solving numerically the steady-state Equations (1) and (2) was developed. The
simulation scheme has then been applied to more that fifty different models to
reproduce the main features of high-latitude phenomena by varying critically
important parameters to a realistic degree. In their model, the entire auroral belt
is divided into four regions, in each of which the height-integrated conductivities
and field-aligned currents were generally assumed to have Gaussian distribution
function. We show two examples which represent quiet and substorm times, respectively.
First, in order to simulate very quiet conditions, a fairly realistic distribution of
the conductivities developed by Tarpley (1970) and Richmond et al. (1976) was
used. The downward field-aligned current is flowing into the morning half of the
auroral ionosphere, whereas the upward current is flowing out of the ionosphere
in the evening half, representing the extremely quiet time configuration as observed
by Iijima and Potemra (1976a) who have shown that such a system called the
region 1 current persists during a very low geomagnetic activity such as K p = O.
The total field-aligned current amounts to approximately 2 x 105 A. Figure 33 shows
the calculated potential distribution in which we notice several significant changes
compared with the earlier studies that result from the conductivity gradients;
(1) The location of both the highest and lowest potential move toward darker local
times from the centers of the downward and upward field-aligned currents, respectively. This is simply because the nightside conductivity is smaller than the dayside
conductivity so that the electric field should be larger in the nightside than in the
dayside to maintain current continuity. (2) There is a considerable asymmetry in
the polar cap electric field strength, larger in the morning sector than in the evening
sector. This is produced by the conductivity decreasing toward nightside in the
polar cap. The same tendency is seen during periods of IMF (interplanetary magnetic
field) away sector (e.g., Heppner, 1973). That is to say, the polar cap field asymmetry
can be expected without changing the IMF conditions. This problem has recently
been discussed in detail by Atkinson and Hutchson (1978). (3) Accordingly, in
order for the ionospheric currents to be commensurate with the assumed symmetric
field-aligned current distribution between the morning and evening sectors, the
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTRO JETS
197
ELECTRIC POTENTIAL
•
~
aOo
90 ~
-90 ~
A=O ~
5~=3 kV
Fig. 33. Electricequi-potential distribution (3 kV contour interval) for the very quiet period without
auroral enhancement. (Kamide, Y. and Matsushita, S.: 1979a, Z Geophys. Res. 84, 4083.)
magnitude of the northward field in the evening auroral belt is larger than that of
the southward field in the morning sector. The ionospheric current pattern is shown
in Figure 34. The direction of the current flow in high latitudes is essentially
eastward, except in a limited longitudinal sector in early morning hours where small
( - 0 . 0 2 A m -1) westward currents are seen.
Secondly, a model calculation to represent the relationship of the field-aligned
currents to the ionospheric currents and fields during a period of, what we call the
'typical' substorm is shown. Since actual substorms, even isolated substorms, feature
complicated and highly variable processes in the ionosphere and magnetosphere, the
term 'typical' substorm here is employed to m e a n merely an idealized p h e n o m e n o n
198
Y. KAMIDE
IONOSPHERIC CURRENT VECTORS
+_180 ~
_90 ~
90 ~
so
A/kin
k=0 ~
Fig. 34. Vector distribution (50 A k m -1 vector scale) of the ionospheric current in latitudes higher
than 50 ~ during the very quiet period without auroral enhancement. (Kamide, Y. and Matsushita, S.:
1979a, Z Geophys. Res. 84, 4083,)
in which many recent observations that have been claimed as being made during
substorms, are reasonably fitted. Basic information on the field-aligned currents is
taken primarily from the recent observations by TRIAD (Armstrong and Zmuda,
1973; Zmuda and Armstrong, 1974a, b; Yasuhara et al., 1975; Iijima and Potemra,
1976a), OGO-5 (Sugiura, 1975) and ISIS-2 (Klumpar et aI., 1976; McDiarmid et
al., 1977) satellites. The following main results have been obtained as characteristics
of the current flow during periods of substorms: (1) The field-aligned currents are
confined statistically to the region of the auroral oval. (2) There is a downward
current in the poleward half of the auroral oval and an upward current in the
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
199
equatorward half in the morning sector; the current direction is reversed in the
evening sector. These currents are intensified during the substorms. (3) Intensities
of the upward and downward currents are in general not equal, so that there is a
net field-aligned current flowing into or away from the ionosphere depending
on local time. The total field-aligned currents are approximately 2 x 106A and
1 x 1 0 6 A for the poleward-half and equatorward-half field-aligned currents,
respectively.
The adopted conductivity model differs from the quiet case in that it has an
additional conductivity enhancement along the nightside auroral belt. The assumed
conductivity distribution in the dark sector is based primarily on the Chatanika
radar observations of the ionospheric conductivity as functions of local time and
substorm activity (Brekke et al., 1974; Banks and Doupnik, 1975; Horwitz et al.,
1978) and on auroral observations with respect to the location of the field-aligned
currents (Armstrong et al., 1975; Kamide and Akasofu, 1976; Kamide and
Rostoker, 1977; Kamide et al., 1978). In particular Kamide and Rostoker (1977)
have indicated that there are essentially four different regions with different auroral
luminosities corresponding to the different directions and intensities of the fieldaligned currents. This may be contrasted with the common previous calculations
which assumed only one conductive belt (e.g., Vasyliunas, 1970), or at most, two
conductive annular rings to represent the discrete and diffuse auroral regions
(Yasuhara et al., 1975).
Figure 35 shows the calculated potential distribution, in which several interesting
points are noticed. It indicates the coexistence of high and low potentials in the
morning and evening sectors, respectively, that characterizes the gross pattern. The
electric field is therefore directed from the dawn side to the dusk side in the polar
cap. This is more or less similar to those patterns obtained by early workers, except
that the equipotential lines are not symmetric, while the earlier models (that neglect
the conductivity gradients) showed symmetric vortices with respect to a certain
longitude which depends on the Hall to Pedersen conductivity ratio.
There is also a considerable distortion of the equipotential contours within the
nightside conductive belt, compared with the 'quiet' case. It is caused directly by
the assumed conductivity inhomogeneity which produces an accumulation of space
charges. Since all of these space charges cannot be removed by the imposed
field-aligned currents, they distort the electric field within and near the conductive
belt.
The following three main changes over the quiet-time pattern are worthwhile to
note: (1) The potential centers (locations of the highest and the lowest potentials)
undergo a significant shift; the high potential contours move toward midnight while
the low contours move away from the midnight. (2) The southward electric field
in the early morning sector is more intense than the northward field in the evening
sector. (3) Complicated structures of the electric field are seen in the vicinity of
the Harang discontinuity. Maynard (1974) has shown by using OGO-6 data of the
electric field, that the Harang discontinuity does not occur along a mathematical
200
Y. KAMIDE
ELECTRIC
POTENTIAL
_+180 ~
/
90 ~
_90 ~
),=0 o
A~=8
kV
Fig. 35. Electric equi-potential distribution (8 kV contour interval) over the northern hemisphere
(0 = 90 ~ is the equator and A = 0~ is the midnight meridian) for a typical substorm. (Kamide, Y. and
Matsushita, S.: 1979b, J. Geophys. Res. 84, 4099.)
line, b u t has a finite t h i c k n e s s w h e r e t h e w e s t w a r d electric field d o m i n a t e s o v e r
t h e n o r t h - s o u t h field. T h e H a r a n g d i s c o n t i n u i t y r e g i o n consists of two l a t i t u d i n a l l y s e p a r a t e d r e g i o n s with t h e a u r o r a l belt. In e a c h of t h e regions, t h e d o m i n a n t
w e s t w a r d field occurs in e a r l i e r l o c a l t i m e s at h i g h e r l a t i t u d e s a n d in l a t e r local
t i m e s at l o w e r latitudes. This is an effect of t h e a s s u m e d two c o n d u c t i v e belts at
e a c h local t i m e sector.
F i g u r e 36 shows t h e d i s t r i b u t i o n of t h e i o n o s p h e r i c c u r r e n t vectors, which a p p e a r s
to r e p r e s e n t s e v e r a l i m p o r t a n t f e a t u r e s a s s o c i a t e d with t h e typical s u b s t o r m . F i r s t
of all, t h e r e a r e two a u r o r a l e l e c t r o j e t s , e a s t w a r d a n d w e s t w a r d . T h e e a s t w a r d
FIELD-ALIGNED
CURRENTS
AND
AURORAL
201
ELECTROJETS
IONOSPHERIC CURRENT VECTORS
~
-
9
,
+180 ~
~----~
9
.
.
"
.
9
/
~0o
B
"',.
,,
~
a
.
_90 ~
....
~
,
. ~'~ . "
.
.
,.
. ..,
.
.
9
,
~
,.
"
o'~176
B
9
"""
~i
"" ..~ ." : i
'. '..'-. ".. "
~,,..
.....
/
i//
''.
2 A/m
I=0 o
Fig. 36. Vector distribution (2 A m 1 vector scale) of the ionospheric current in latitudes higher than
50~during a typical substorm. (Kamide, Y. and Matsushita, S.: 1979b, Y. Geophys. Res. 84, 4099.)
electrojet flows in the equatorward half of the evening auroral belt, while the
westward electrojet flows in wider regions in the evening and morning sectors than
does the eastward one. Secondly, the continuation currents for these electro jets
are supplied primarily by the assumed field-aligned currents, rather than by the
ionospheric currents in the polar cap and midlatitudes, which are very small. Thirdly,
the westward electrojet, which is the dominant feature of the polar substorm,
appears to have two peaks at premidnight and early morning hours. The m a x i m u m
current density of the westward electrojet in the premidnight sector is primarily
produced by the assumed high conductivity associated with the bright aurora (such
as the westward traveling surge) typically observed there during substorms. Note
)
202
Y. KAMIDE
that this westward electrojet has a small northward component as well. On the
other hand, the westward electrojet in the morning sector occurs primarily as a
result of the large electric field there, although the contribution from the enhanced
conductivity is not negligible. The morning electrojet has a considerable southward
component as well, which connects the downward field-aligned current (to the
north) and upward current (to the south) via the Pedersen current. Fourthly, the
eastward electrojet is about ~ less intense than the westward electrojet, a result
being in good agreement with the statistical results given by Kamide and Fukushima
(1972). It is also noted that the eastward electrojet has a northward component
which becomes more intense with the progress of local time; even a pure northward
current is seen in the premidnight sector. This means that a significant fraction of
the eastward electrojet turns northward, and joins eventually the westward electrojet. Finally, the eastward current can be produced even in the morning sector near
the poleward and equatorward edges of the auroral belt. This may be called the
morning eastward electrojet, since it is produced by the enhanced conductivity.
The eastward current near the poleward edge is caused by the northward electric
field and the Hall conductivity, while that near the equatorward edge in the late
morning sector is produced by the product of the eastward field and the Pedersen
conductivity (see Rostoker and Hron, 1976; Baumjohann and Kamide, 1981). The
current distribution near the Harang discontinuity is also complicated (Cahill et al.,
1980).
In Figures 37a and b, we show the distribution of the equivalent ionospheric
current vectors and the corresponding stream lines, respectively. The difference in
the patterns between the ionospheric and the equivalent currents stems from the
distribution of the field-aligned currents. In other words, the following characteristic
changes between Figure 37a and Figure 36 suggest the importance of the effects
of the field-aligned currents in discussing magnetic observations which reflect both
the ionospheric and field-aligned currents. Although the vector pattern in the
auroral belt is essentially unchanged in that there are two electrojets flowing along
the nightside conductive belt, there are two main differences. Firstly, the magnitude
of the electrojets decreases in the equivalent currents compared with that in the
real ionospheric current, indicating that the field-aligned currents tend to cancel
the ionospheric current effects. Secondly, although the electrojets have northward
and southward components in the evening and morning sectors, respectively, the
equivalent electrojets flow almost entirely in the east-west direction. This indicates
that the presence of only small D component ground magnetic perturbations at
auroral latitudes during substorms, does not necessarily mean that electrojets are
flowing almost in the east-west direction.
It is also interesting to note that in the polar cap, particularly near the auroral
latitude in the dark sector, the equivalent current vectors are directed from dusk
to dawn, consisting of 'return' currents of the auroral electrojets. Since there is
little ionospheric current flow in the region, these 'return' currents can be regarded
as the effects of the field-aligned currents. Therefore, it is noted that the sunward
FIELD-ALIgNED
CURRENTS
~kND AURORAL
ELECTROSETS.
203
W
>r"
Z
ILl
O
C)
E
0
w~
r~
0
Z
0
l-Z
..1
t'.-
5
0
Ul
u~
<3
o
ILl
hZ
W
n,
n,
r
n* %
I"
o<
+~
13.
ch
0
2'
0
.-X
Z
hl
-J
5
>
[..,
5
<3
bJ
O
t<
204
v.
KAMIDE
magnetic perturbations in the polar cap observed typically during substorms are
indeed produced by the field-aligned currents. These points are clearly demonstrated in the equivalent current system shown in Figure 37b.
Note that Kamide and Matsushita (1979b) have conducted other numerical
experiments in an attempt to determine the effects of variations in the field-aligned
current intensity and location and in the conductivity pattern (the Hall-to-Pedersen
conductivity ratio, the seasonal changes, and the expansion of the auroral belt) on
the electric field and current pattern. These effects are to be observed as variable
features in the course of substorms. Gizler et al. (1979) have attacked a similar
quantitative modeling of electric currents and fields in which the model distribution
of the ionospheric conductivity and the statistically obtained field-aligned current
configuration are given as the input. Most recently, Li et al. (1981) conducted a
simulation study of electric fields and currents in the ionosphere by taking into
account the field-aligned currents in the dayside cusp region which was modeled
on the basis of the TRIAD statistical results. To isolate and order the effects of
particular parameters (such as seasonal change of the ionospheric conductivity or
its local time dependence) on the overall electric field pattern, the distribution of
the field-aligned currents, which characterizes different models, is specified by three
indices: The total current, the ratio of the currents in region 1 to region 2 (see
Section 3.3.1), and the ratio of the currents in the morning to evening sectors. The
ionospheric current distribution, that is one of their outputs, is then compared with
reliable observations in terms of geomagnetic activity indices (Nisbet, 1981).
6.3.
I O N O S P H E R E - M A G N E T O S P H E R E COUPLING
The main dynamo processes which drive the field-aligned currents and the auroral
electrojets occur in the magnetosphere. The electric field and current in the
ionosphere thus far discussed, and those in the magnetosphere are mutually coupled
via very complex processes linked by the field-aligned currents. The treatment of
this interaction can be avoided by specifying either the electric potential at the
polar cap boundary (Section 6.1) or a priori the field-aligned currents (Section 6.2).
This essentially decouples the problem from the magnetospheric processes, such
as convection and the ring current formation, by concentrating on only a portion
of the entire magnetosphere-ionosphere system.
It is difficult to model the complexity of the entire system, in which many
non-linear processes are occurring, especially because of its strong time dependence.
Recently, a group at Rice University attempted to remove this difficulty to a
significant extent. Harel et al. (1981a) summarize the practical logic: Starting at a
given time t with a given hot plasma distribution in the magnetosphere, the gradient
and curvature currents are calculated. By calculating the divergence of the magnetospheric currents, they solve the field-aligned currents which are then to be
equated to the divergence of the ionospheric currents, where the conductivity and
electric potential are included. For a given conductivity tensor which was estimated
semi-empirically by using electron observations (0.08-17 ke'V) of the S3-2 satellite,
FIELD-ALIGNED CURRENTS AND A U R O R A L ELECTROJETS
205
the electric potential can be solved. This process is exactly the same as solving (1)
and (2) except for the boundary condition. Harel e t a l. (1981 a) specified the potential
distribution at their polar boundary by using the $3-2 observations. By neglecting
the field-aligned component of the electric field, the magnetospheric electric field
can be obtained (Holzworth et al., 1981). Given the electric field and the magnetic
field model, it is possible then to proceed to the next time t +At by calculating
drift velocities for plasma particles.
Harel et al. (1981a) and Spiro et al. (1981) have applied this scheme to a substorm
event which occurred on September 19, 1976. The outputs which are the electric
fields and currents as functions of time, are compared with various observations in
terms of, for example, growth of the ring current, high-latitude and low-latitude
electric fields and their substorm effects, field-aligned currents and Joule heating.
In most cases, they found general agreement with the computed results and
the observations, with some discrepancy in detail. The simulation efforts
represent satisfactorily a first-run model, which is, however, subject to an improvement in the magnetic field configuration including self-consistently all
the currents as a function of time, and in estimating the conductivity enhancement without using observed data of electrons. In this way, one may be able to
predict the distribution of the electric field as well as plasma processes in the
magnetosphere.
6.4.
ESTIMATION
FROM
OF THE THREE-DIMENSIONAL
GROUND
MAGNETIC
CURRENT
SYSTEM
PERTURBATIONS
As discussed already, we must realize that it is not possible to uniquely determine
the distribution of ionospheric and magnetospheric currents only from magnetic
observations made on the Earth's surface. In spite of this difficulty, however, the
systematic observations along meridian chain observatories during the last ten years
have contributed significantly to our understanding of the spatial relationship
between the auroral electrojets and field-aligned currents, since they give a unique
opportunity to scan continuously the entire polar region for unveiling the extent
to which the magnetosphere impresses the field-aligned currents to the ionosphere.
The continual increase in both the quantity and quality of ground-based magnetic
data has provided the incentive to more sophisticated modeling techniques. In
other words, the proposed models are capable of reproducing the original magnetic
observations in almost all features and seem to be consistent with other information
on the electric field and ionospheric conductivities as well as the field-aligned
currents.
In a series of theoretical papers by Fukushima (Fukushima, 1974a, b, c, d, e,
1975a, b), the relationship between three-dimensional current patterns and their
equivalent ionospheric conductivities are extensively examined for various types
of the ionospheric conductivity at various locations of the auroral electrojets. The
results are extremely useful in determining the extent to which the real threedimensional current system is inferred from ground magnetic records.
206
Y. KAMIDE
Hughes and Rostoker (1977) have demonstrated that it is possible to find specific
ground-based signatures of the net field-aligned current flow using the latitudinal
profile of the three-component magnetic perturbations. Figure 15 shows such an
example taken near local magnetic dawn in which a clear level shift appears in the
Y component profile, representing the magnetic eastward perturbations. The steplike level shift exhibits the signature of a downward net field-aligned current flowing
into the ionosphere in the meridian chains. Based on this technique, they have
given a histogram showing the distribution of the net field-aligned current as a
function of local time. Hughes and Rostoker (1977) noted that there is a remarkable
similarity between the behavior of the field-aligned current flow as inferred from
that diagram and the diurnal variation of the average electric field at auroral
latitudes obtained by Mozer and Lucht (1974).
Winningham et aI. (1979) and Rostoker et al. (1979) have organized the ISIS-2
electron precipitation data in the framework of the latitudinal distribution of the
auroral electrojets and the field-aligned currents, of which locations are estimated
by the use of ground magnetic perturbations along the meridian network. It was
found that precipitating energetic electrons are embedded within the poleward
portion of the eastward electrojet, and that in the evening sector, the downward
field-aligned currents flow into the ionosphere through the central plasma sheet,
while the upward-flowing currents are confined to the boundary plasma sheet.
6.4.1. Latitudinal Profile of the Auroral Electrojets
Quantitative modeling of the ionospheric and the field-aligned currents to estimate
the latitudinal distribution of the auroral electrojets using meridian line magnetometer data, has been discussed by Oldenburg (1976, 1978), and Kisabeth and
Rostoker (1978).
Oldenburg (1976) has used the linear inversion theory of Backus and Gilbert
(1970) to show how unique estimates of the auroral electrojets can be obtained if
the magnetic observations arise from the Birkeland-type or Bostr6m's type I current
system. Oldenburg (1978) demonstrated that although there exist many current
distributions J(O) which can generate the observations on the Earth's surface,
specific models may be obtained by imposing the condition that the current distribution minimizes some functional form, such as
I0 2 [dJ(0)/d0] 2 dO ~ minimum,
1
where 01 and 02 are the colatitude limits of the auroral electro jets.
The spatial structure of the estimated auroral electrojets during the course of
substorm sequences has practically been determined by Wallis (1976), Hughes et
al. (1979), and Bannister and Gough (1977, 1978). In particular, the array of
Gough and Bannister (1977) recorded three-component magnetic disturbances in
a two-dimensional network which has an ability to distinguish between latitudinal
and longitudinal variations of electrojet parameters and to determine spatial
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
207
motions of the three-dimensional current system. It was possible for them to find
how the field-aligned current system moved over the array as well as how the
intensity of the current circuit varied.
More recently, as one of the contributions to the IMS, a two-dimensional array
of magnetometers has been installed in Scandinavia (Maurer and Theile, 1978;
Untiedt et al., 1978; Kiippers et al., 1979). A considerable portion of the twodimensional region is monitored simultaneously by the STARE system as well.
Thus, the combination of the equivalent ionospheric current vectors from the
magnetometer chain and the electric field by STARE over a portion of the auroral
ionosphere has yielded important information relating to the ionospheric and
field-aligned current (e.g., Theile et al., 1980). Studies by Baumjohann et al. (1978),
showed that there exists the excellent spatial correspondence between the maximum
backscatter of STARE and the peak positive H magnetic perturbation, indicating
that the eastward electrojet in the evening sector is controlled mainly by the
northward electric field. Baumjohann et al. (1980) have shown cases of such
comparisons in the longitudinal gradient in the eastward electrojet intensity. It was
concluded that the central part of the substorm-intensified eastward electrojet is a
nearly pure Hall current driven by northward electric fields. Near the eastern end
of the eastward electrojet, however, the current configuration is complicated as
discussed in Section 4.4.
On the basis of these measurements over Scandinavia, the real three-dimensional
current system has been modeled for the region near the westward traveling surge
(Inhester et al., 1981). Figure 38 shows the parameters of the model system and
the resultant equivalent currents on the Earth's surface (i.e., the expected ground
magnetic perturbation vectors) which agree very well with the observations. The
intense upward field-aligned current can be intensified at the head of the traveling
surge. It is also important to note STARE observed a southeastward-directed
electric field at the head of the surge, while a southwestward electric field existed
behind the surge.
6.4.2. A d v a n c e d M e t h o d s
Mishin et al. (1980) outlined an algorithm to estimate the distribution of the electric
fields and currents in the ionosphere as well as of the field-aligned currents on the
basis of ground magnetic perturbations and a model of the ionospheric conductivity.
Spherical harmonic series were employed to solve an equation which relate these
parameters. It must be noted that the worldwide representation of a complex
perturbation field such as that during substorms requires many harmonic terms. It
was found by Mishin et al. (1980), however, by applying their method to observed
magnetic data that in generating any possible errors, the conductivity model to be
assumed plays a less role than does the choice of number of the harmonic terms.
A general agreement has been obtained between the field-aligned current distribution estimated by their method and that observed by polar orbit satellites, in
particular, for quiet periods.
4-
a O I
9
A
FIELD
. . . .
.
.
o o o = =
.
-
. . . .
o
. o o .
.
.
.
.
L3
.
.
.
.
.
.
.
5 ~,!CAIMI~2
+
.
. . . . . . . . . . .
ID~o
. . . . .
.
.
.
~ . . . . . . . . . . .
~ ~ . . . . . . . . . .
. . . . . . . . . . . . .
. . . . . . . . . . . . .
C U R R E N T S
;I!iiii1!!!
E L E C T R
~ I E L D - A L I G N
,,,)fK I
I O N O S P H
~ .
. .
B
EQ
X,!
CURR
I O N O S P H
GROUND
C O N D U C T I V I T Y
"
200
+~
NT
+
F
G
Fig. 38. Parameters of the model current system and the resultant equivalent current vectors on the Earth's surface. (A) Locations of negatively charged
field lines. (B) Ionospheric electric field vectors attributable to these charges. (C) Ionospheric Hall (sho~tt by square] and Pedersen (cross) eonductivities. (D)
Ionospheric current. (E) Upward (shown by square) and downward field-aligned currents. (F) The corresponding equivalent currents on the ground. Squares
and crosses denote negative and positive Z components, respectively. (Inhester, B., Baumjohann, W., Greenwald, Rs A., a , d Nielsen, E.: 1981, J. Oeophys.
49,155.)
IONOSPH
CURRENTS
--->. Y t ~
CHARGES
~saz~m_
k
,\
COLUMN
>
FIELD-ALIGNED CURRENTS AND AURORAL ELECTROJETS
209
Most recently, two advanced computer codes for numerical modeling of inferring
the three-dimensional current system have been developed independently by
Kisabeth (1979) and Kamide et al. (1981). For both methods, a set of assumptions
have to be given in addition to the distribution of magnetic field vectors observed
on the Earth's surface as the input to provide the distribution of the ionospheric
currents, field-aligned currents and other related quantities as outputs. In this
section, we first describe the two codes briefly which were developed on the basis
of entirely different principles, and then compare the two results by using a common
data set from the IMS Alaska meridian chain of magnetic observatories.
In Figure 39, we summarize the procedures taken by the two methods. The input
(ground magnetic records) data are exactly identical between the two codes and
the output (ionospheric and field-aligned currents and electric fields) are similar.
Both methods assume (i) geomagnetic field lines are equipotentials, (ii) the dynamo
effects of ionospheric winds can be neglected, and (iii) the magnetic field of the
magnetospheric ring current, the magnetopause current, and the tail current can
be neglected, although the Forward method contains the asymmetric portion of
the ring current.
COMPARISON
OF
FORWARD
AND
KRM
METHOD
INPUT
FORWARD
METHOD
I
OUTPUT
0....ROa
net,clI
.... ds
1,
..~.z
~
Th, e O,
I
(overhead)
Current
Ionospheric
and
E-W Current
.... ,ona,,~ ,
C..... t
Field-Aligned
Current)
(IonosphericCurrent+[
Due to Unit
Current density
[
[
~ " [
]
Electric Field
I
I
Conductivity
Ionospheric 1
[ Conductivity J
I~ ....Records
dMagne,o
H
K RM [
METHOD [
H, D, (Z)
Electric
Potential
External Current
Function
Equivalent
Ionospheric
Current
I
Field-Aligned ]
Current
Fig. 39. Flow chart showing the comparison of the Forward and KRM methods in estimating the
field-aligned and ionospheric currents from ground magnetic records. (Akasofu, S.-I., Kamide, Y., and
Kisabeth, J. L.: 1981, J. Geophys. Res. 86, 3389.)
I
210
v. KAMIDE
(a) Forward M e t h o d
This code has been developed by Kisabeth (1979). In this original modeling, a
spherical earth with a dipole magnetic field is considered, but it can be extended
for non-dipole fields. The high latitude ionosphere is first divided into a large
number of cells (typically - 1 5 0 ) , each of which is associated with two elementary
three-dimensional current systems. In the first current system, the current flows in
the east-west direction across a cell, and it is connected to field-aligned c u r r e n t s
at the eastern and western boundaries of the cell; also the field-aligned currents
are assumed to flow along dipole field lines and to close in the equatorial plane of
the magnetosphere. In the second current system, the current flows in the n o r t h south direction across the cell, and it is connected to the field-aligned currents at
the northern and southern boundaries; the field-aligned currents are assumed to
clock again in the equatorial plane. The computer code is designed to find a set of
the east-west (E-W) and north-south (N-S) currents for all cells and the associated
field-aligned currents and equatorial currents which reproduce best the input data.
The procedure is to calculate first the magnetic field at all observation points
due to a chosen set of unit current densities in each cell (the 'source cell'), 1 A m -1
for the E - W current and - 0 . 5 A m -1 for the N-S current in this particular example.
This choice of current densities is equivalent to utilizing a height-integrated Hall
to Pedersen conductivity ratio of 2.0, provided the ionospheric electric field has
only a north-south component. For practical purposes for the IMS Alaska chain
data, the high latitude ionosphere is divided into seven current cell rings with equal
latitudinal width of 3 ~ extending from 61 ~ to 82 ~ Note that all ionospheric currents
are thus assumed to be confined in this latitude range. Each current cell ring is
divided longitudinally into 24 cells, thus giving a total of 168 cells. The magnetic
field is then computed at 10 points along each MLT meridian, thus the total number
of observation 'points' is 240.
In this way, the relationship between the current intensity matrix P and the
corresponding magnetic field matrix B (the input data set) can be expressed by a
simple matrix equation
B = AP.
(3a)
Through linear inversion, it is possible to determine P ( E - W current intensities)
representing the model current system. That is to say, (3a) can be written as follows:
P = ( A T A ) -~ A T B ,
(3b)
where A T is the transpose matrix of A.
Field-aligned currents to and from the ionosphere can be estimated by calculating
the divergence of the horizontal current at the corners of the grid cells. It is also
possible from the ionospheric current i to derive the electric field E by assuming
the ionospheric conductivities (~:~,,~:H).
(b) K R M M e t h o d
The K R M method, which has been developed by Kamide, Richmond, and
Matsushita (1981), is different from the Forward method in terms of the principles
FIELD-ALIGNED CURRENTS AND AURORAL
ELECTRO JETS
211
and concepts involved. It requires, first of all, a given ionospheric equivalent current
system, which, as we shall define it, is a toroidal horizontal sheet current ir flowing
in a shell at 110 km altitude, for which the associated magnetic field agrees with
the external portion of the observed magnetic variation field which includes both
the external (namely, primary) field and the earth-induced field. The toroidal current
can be expressed in terms of an equivalent current function ~ as
ir = n~ x grad g,,
(4)
where nr is a unit radial vector. On the Earth's surface, the magnetic variation can
be expressed in terms of a magnetic potential V. Then the external portion of V
is uniquely related to tb by straightforward mathematical relations (see Chapman
and Bartels, p. 631, 1940). Fully automated derivations of V from instantaneous
magnetic field observations have been reported by Bostr6m (1971), Kamide et al.
(1976d), and Kroehl and Richmond (1980).
With a given equivalent current function 0 and a given conductivity distribution,
the ionospheric electric field, horizontal currents, and field-aligned currents can be
obtained under the assumption that geomagnetic field lines are radial. This assumption invalidates low latitude results, but is unlikely to affect substantially our high
latitude results.
The total height-integrated horizontal current i can be expressed as the sum of
the toroidal (equivalent) current and a 'potential' current i+. The potential component can be considered a closing current for field-aligned currents. The requirement that the three-dimensional current can be divergence free means that the
field-aligned current density ]11(positive downwards) satisfies the relation
]11= div i = div ir
(5)
since iT is by definition divergence free. Note that the current system represented
by ]11and ir together produces no ground magnetic variation under the assumption
of radial geomagnetic field lines, consistent with the implicit assumption that the
toroidal component of i is just the equivalent current (Kern, 1966).
The horizontal ionospheric current is related to the electric field E by Ohm's law:
i =.,~pE + ~ H E • nr,
(6)
where 2p and XH are the height-integrated Pedersen and Hall conductivities. The
electric field is derivable from an electrostatic potential q~ as
E = -grad 4.
(7)
A partial differential equation for ~O in terms of ~Ocan be obtained by taking the
curl of Equation (6). In spherical coordinates 0 (colatitude) and A (east longitude)
there results
02(I)
OC])
A~-~+B-~+C-~+
02(1) D 0 ~ = F
OA
'
(8)
Y. KAMIDE
212
where the coefficients are given by
A = .Yn sin 0
0
B = ~ (~Yn sin O) +
~2p
C = 2H/sin 0
oz
o( z . ~
D = - ~ p - O-h~s~nO)
O ( OtP
F = ~-~\~-~ sin
0)+
1
02~
sin 00A2
In solving Equation (8) we use the following boundary conditions on qb(0, h):
~(0, h) = 0 ,
)
00 \ 2 ' A -- 0 .
(9)
(10)
The particular nature of the equatorial boundary condition used is of little
significance to the solution of q~ in the high latitude regions of interest to us. The
form Equation (10) is used only for numerical convenience.
We solve Equation (8) numerically by a finite-difference scheme over a network
of points spaced 1~ in colatitude and 15 ~in longitude. Once the electrostatic potential
is obtained, we derive the electric field from Equation (7), the ionospheric current
from Equation (6), and the field-aligned current from Equation (5).
Akasofu et al. (1981a) compared the two advanced methods; some differences
are intrinsic to each method which has some advantages and disadvantages. In the
Forward method, the three-dimensional current system can be obtained from
ground magnetic records by assuming the ratio of the E - W and N-S ionospheric
current densities. Akasofu et aI. (1980) assured the ratio to be 2 : 1 everywhere.
Their assumption is reasonable in view of recent observations that the ionospheric
electric field has mainly a north-south component in auroral latitudes (e.g.,
Heppner, 1977) and the Hall to Pedersen conductivity ratio is typically 2 (Brekke
et al., 1974). However, the assumption of a uniform value of this ratio for the
entire ionosphere provides a strong restriction to the results in the Forward model.
Although such an assumption can be removed for a large memory size computer,
the current ratio should not be given a priori as an input, but we must be solved
as an output such that the electric field, the conductivity, the ionospheric current,
the field-aligned currents, and the ground magnetic perturbations (which are the
input) are all consistent.
On the other hand, in the K R M method, the ionospheric conductivity must be
assumed. However, as indicated by Kamide etal. (1981), the computed field-aligned
current pattern does not seem to be very sensitive to the choice of the ionospheric
conductivity, although the electric field is determined primarily by the conductivity
distribution. The electric field, derived from its potential, as well as the ionospheric
and field-aligned currents can then be calculated such that all these quantities are
consistent with each other.
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
213
The Forward method assumes a spherical earth with a dipole field. On the other
hand, the KRM method assumes a horizontal ionosphere and straight field lines
which are perpendicular to the ionosphere. Thus, although the KRM method may
remove these assumptions, the present Forward method is more advantageous as
far as computation of the expected magnetic fields in low latitudes is concerned.
6.4.3. I M S A l a s k a Meridian Chain
As discussed in the previous section, the two computer codes for modeling take
the distribution of magnetic field vectors observed on the Earth's surface as the
input. The systematic magnetic observations along a close array in high-latitudes,
such as meridian chain observatories, are useful in constructing the distribution of
the magnetic perturbations. The IMS Alaska meridian chain of magnetic observatories provides an ideal data set for such a modeling; or more accurately, the
improved IMS Alaska chain was designed partly to present the data set for such
advanced codes.
Figure 40 shows the average horizontal magnetic variation vectors plotted at
every 1 hr interval in 'magnetic latitude and MLT' coordinates. These vectors are
obtained from 5-min average values of a 50-day interval from March 9 to April
27, 1978 during which most of the Alaska chain observatories were operated fairly
continuously. A comprehensive description of the original data and data process
techniques is given in Akasofu et al. (1980). It is evident from Figure 40 that the
magnetic vector distribution reveals systematic, large-scale features above 60 ~
magnetic latitude. In particular, it appears to reveal the so-called 'two-cell'
equivalent current system. This basic data set is the input for the following modeling.
12 MLT
9
200
ALASKA MERIDIAN CHAIN
0 0 0 0 - 2 3 0 0 UT
M A R C H 9 - A P R I L 27, 1978
M A G N E T I C V E C T O R A8
GAMMAS
0
Fig. 40.
Average magnetic variation vectors in magnetic latitude and local time coordinates. (Akasofu,
S.-I., Kamide, Y., and Kisabeth, J. L.: 1981, J. Geophys. Res. 86, 3389.)
~ig. 41.
8
0
12 MLT
METHOD
ALASKA
~_o~n
18
8 2fl/m
;
MERIDIAN CHAIN
UT
PRIL 27,
KRM
0
12 MLT
METHOD
;omparison for the calculated ionospheric current vectors between the Forward and KRM methods. (Akasofu, S.-I., Kamide, Y., and Kisabeth, J. L.:
1981, J. Geophys. Res. 86, 3389.)
:ORWARD
ONOSPHERIC CURRENT VECTORS
ba
FIELD-ALIGNED CURRENTS AND AURORAL
ELECTROJETS
215
Figure 41 compares the distribution of ionospheric currents by using the Forward
method on the left with that obtained by the K R M method on the right. One can
see that the large-scale patterns are quite similar. The westward electrojet in the
morning-midnight sector and the eastward electrojet in the afternoon-evening
sector are clearly brought up by the two methods. These two electrojets are known
to be the dominant features of polar substorms. It has long been feared by a number
of workers that owing to the non-uniqueness problem, entirely different current
patterns may result from the same input data set for different methods. However,
the results shown in Figure 41 assure us that with an appropriate data set, the two
independent codes have been developed to the point that the non-uniqueness of
the solution is no longer a serious problem, at least as far as the large-scale pattern
of ionospheric currents is concerned.
However, details of the two distributions are somewhat different. The reasons
for the differences arise mainly from the differences of details in modeling, which
are related partly to the practicality of both methods. On the other hand, such
differences often provide physical insight into some of the basic processes involved
in the polar current systems.
In the Forward cell method, the ionospheric currents are assumed to flow only
in the belt bounded by two latitude circles of 61 ~ and 82 ~ although the current
cells can be extended to the polar cap for a computer of a large memory size. Thus,
all ionospheric currents which reach the latitude circles of 82 ~ and 61 ~ are discharged
along geomagnetic field lines. On the other hand, the K R M method provides details
of the distribution of ionospheric currents in the polar cap. The most interesting
feature of the polar cap is a strong sunward component.
In the Forward model, the ratio of the E - W and N-S current densities is assigned
to be 2:1 everywhere, whereas this ratio is a calculated result, different at each
grid point, in the K R M method. According to the K R M method, the westward
currents flow nearly parallel to latitude circles in premidnight hours, while the
current is still forced to flow in the southwestward direction in the Forward model.
In the eastward electrojet region, the angle between the current vector and the
latitude circle tends to become larger with the advancement of magnetic local time
in the K R M method, while the current angle in the Forward method is constant
throughout the evening sector.
It is also noticeable that in the Forward model, the eastward and westward
electrojets have a similar intensity. However, in the K R M model, the westward
etectrojet is about three times as intense as the eastward electrojet. This is
mainly caused by the assumption made in the former model that the current
cannot flow below 61 ~ latitude. The fact that the former model assumed a uniform
conductivity, while the latter model took into account realistic day-night
differences of the ionospheric conductivity, may be partially responsible for
the relatively large eastward current intensity in the Forward model. Such a
difference of the conductivity could, in principle, be incorporated in the Forward
model.
~
0
UPWARD
)
6
18
-
5~0 -~ A/m2 INTERVAL
ALASKA MERIDIAN CHAIN
UT
~RIL 2 7 , ]
CURRENTS
KRM
~
12 MLT
METHOD
~
UPWARD
RD
6
Comparison for the calculated field-aligned current distribution between the Forward and KRM methods. (Akasofu, S.-I., Kamide, Y., and Kisabeth,
J. L.: 1981, J. Geophys. Res. 86, 3389.)
<5xi0 -8
5 XIC]8~0-7
~ig. 42.
3
,8 -
METHOD
12 MLT
ORWARD
fIELD-ALIGNED
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
217
Figure 42 shows the distribution of the estimated field-aligned currents for the
two models. Again, both methods resulted in similar distributions of the region 1
and 2 field-aligned currents which were obtained by Iijima and Potemra (1976a),
by using TRIAD satellite data, although details are slightly different. The results
thus assure us that improvement of both methods would lead to a fairly accurate
distribution of the field-aligned currents by present or improved meridian chain
data.
The narrowness of the region 2 currents in both the morning and evening sectors
in the Forward method arises mainly from the sharp boundary of the current region.
Differences due to similar causes occur also at the poleward boundary of the current
region and in the polar cap, although the distribution of the downward currents in
the afternoon-evening sector above 75 ~ in latitude is surprisingly similar. A major
difference occurs in the morning-forenoon hours. In the Forward model, a weak
upward current is imbedded in the region 1 current region. Such a current is absent
in the results of the KRM method. In the Forward model there is also a belt of
medium intensity upward field-aligned currents approximately along the 80 ~latitude
circle in the evening, morning, and forenoon sectors. Such a current is also almost
absent in the results of the KRM method.
Figure 43 shows the electric field distribution obtained by the two methods. In
the KRM method, the electric field can also be obtained as one of the outputs. On
the other hand, the electric field is not one of the outputs derived from the Forward
model. However, as indicated in Figure 39, we can infer the electric field distribution
by using the calculated ionospheric current and by assuming the ionospheric
conductivity in the Forward model. Note that in this model, the assumption of the
ratio between the E - W and N-S current densities cannot be determined independently of the distribution of the ionospheric conductivity. However, since the
Forward method was not originally designed to solve self-consistently the electric
field distribution, uniform conductivity within the belt between the 61 ~ and 84 ~
latitudes is assumed; the conductivities, Xp = 5 mhos and Xn = 10 mhos. In the KRM
algorithm, the electric fields satisfy the relationship E - - - g r a d ~, whereas this
relationship cannot be satisfied by the former, in which only the north-south
component electric field is present.
Furthermore, the KRM method makes it possible to determine accurately a
self-consistent data set of the global distribution of the Pedersen and Hall currents
having different physical meanings, over the entire polar region as well as their
counterpart currents in the magnetosphere. The separation into the Pedersen and
Hall current components from the 'total' ionospheric current is of great importance
in understanding the deriving mechanisms of the three-dimensional current system.
This is because the Pedersen current must be driven externally and circuited to the
ionosphere by field-aligned currents, whereas the Hall current arises as a result of
an anisotropic ionospheric conductivity. In Figures 44(a) and (b) we show the
distribution of the Pedersen and Hall currents, respectively. The distribution of the
Pedersen current is characterized by a southward-directed component in the
5 mho
~ig. 43.
0
40 mV/m
~B~-~P~IL
~LASKA MERIDIAN CHAIN
~000-2300 UT
27,
FIELD
2 MLT
:RM METHOD
Comparison for the estimated electric field vectors between the .Forward
. . . . . . . . . and K R M methods. (Akasofu, S.-I., Kamide, Y., and Kisabeth, I. L.:
L98t, 3. Geophys. Res. 86, 3389.)
[H = IO mho
~p =
B
2 MLT
METHOD
/~7-,~ ? ~
:ORWARI]
-LECTRIC
5
,,)
t~
~-
8 ~r~
r-0
--4
~rn
i:l~m
m;:Q
~m
c~
e
e.,
r~
e'~ ~ .
0
\
9
~
--4
0
5
<
m
c3
O
E
6[~
$,L~tfO~l,i.3~fq~t
"~V~O~{FIV
GNV
S~LN~t~I~tI~3 G3NOI'3V-G'31~tI~
220
v. KAMIDE
latitudinal belt between 62 ~ and 72 ~ in the morning sector and a northward-directed
component in the same latitudinal belt in the afternoon sector.
Another prominent feature of the Pedersen current is divergence of the current
from about 76 ~ in latitude in the midmorning sector and convergence towards the
same latitude in the afternoon sector. Associated with this particular feature, there
is a fairly intense eastward component in the latitudinal belt between 70 ~ and 82 ~
in the day sector. The Harang discontinuity is manifested by a narrow belt in the
late evening sector along which a westward component flows and toward which a
northward component flows from lower latitudes and a southward component flows
from higher latitudes.
The distribution of the Hall current is characterized by an intense westward
component in the latitudinal belt between 62 ~ and 72 ~ in the morning sector and
an eastward component in the same latitudinal belt in the afternoon sector. Such
a distribution is expected from a large number of past studies, most recently by
Hughes and Rostoker (1979) and Rostoker (1980). However, its diverging trend
at about 75 ~ in latitude in the day sector and its converging trend in the evening
sector, together with two crescent flow patterns are determined accurately in the
modeling. It is quite obvious that also the Hall current does not close completely
in the ionosphere and is associated with field-aligned currents, a downward current
from the late morning sector and an upward current from the midnight sector. In
the Harang discontinuity region, the Hall current has a significant northward
component, produced by the westward electric field.
7. Concluding Remarks
This review paper was written for the purpose of highlighting and synthesizing the
recent progress in observations and simulation studies of the electric fields and
currents in high latitudes made during the last ten years. Some generally acceptable
interpretations have been found in a variety of observational features, leading to
an average model for current configuration, but there are still many problems which
are not satisfactorily settled in terms of basic physical concepts, or which hold a
considerable controversy. One of the difficulties in deducing a plausible view that
can consistently explain the observed features lies in the fact that all natural
phenomena occurring in the ionosphere-magnetosphere system are influenced by
many unknown conditions; we cannot repeat the same experiment under constant
conditions. Thus, one may often fail to find a crucial parameter that influences
basically the whole system of the ionosphere and magnetosphere. Nevertheless, it
may be useful to summarize the present status of our understanding in terms of
what we have learned and what we need to clarify in the future.
7.1.
EMPIRICAL
CURRENT MODELS
Perhaps one of the most fruitful efforts during the last several years is an attempt
to construct a possible configuration of the three-dimensional current system for
FIELD-ALIGNED
CURRENTS
AND
AURORAL
221
ELECTROJETS
the magnetospheric substorm by integrating the findings of the ground-based, rocket
and satellite observations of the electric fields and currents in high latitudes. Various
three-dimensional current systems associated with the magnetospheric substorm
have been proposed in the past. Several theoretical studies also been carried out
on the relationship between the field-aligned currents and the ionospheric electric
fields, and of their magnetic effects (e.g., Fukushima, 1976; Kawasaki and
Fukushima, 1974). Mechanisms for driving the field-aligned currents are discussed
by Bostr6m (1975), Sato (1976), Hasegawa and Sato (1980), and Sato and Iijima
(1980). Figure 45 shows a logic diagram displaying the relationship governing the
magnetosphere-ionosphere system in which various electrodynamic processes occur
(Bostr6m, 1975). A similar closed loop has been presented by Vasyliunas (1970),
Wolf (1975), and Harel et al. (1981) who have suggested a self-consistent calculation
of the entire process. In particular, recent extensive simulations by Harel and
colleagues (Harel et al., 1981) attempt to solve the closed system of equations for
given boundary conditions. In Section 6 we have presented major advances from
various simulation studies which have considered some links of the parameters
within the loop. The links which are dealt with in this review are marked in the
lower half in Figure 45.
Equation
Magnetospherie] of motion
perpendicular
~
electric field
Nomentum
]
Magnetospheric ]] balance ~IMagnetospheric|
plasma velocity~ "
~perpendicular ]
~and pressure |
I
current
|
I
Mapping of
potentials
Parallel
electric
field
I
[Instability[
I
Current
Plo o
1 2 arr ~
precipitating
iAccel~ation|particles
I
Mapping ofl
potentials
Ionization
I .....
continuity
mechanism
I
"
I ir ola d
I
I c.....
t
Heating
Ion drag
~~C ..... t
| continuity
~J~
Ionospheric
electric
field
Ionospheric
conductivity
I'onospheric1
winds
I
l
~
~
~ -~onospheric
~orizontal
~ ....nt ]
r
Ohm's l a w
Fig. 45. Interrelationshipsbetweenmagnetosphericand ionosphericparameters. (Bostr6m,R.: 1975,
in B. Hultqvist and L. Stenflo (eds.), Physics of the Hot Plasma in the Magnetosphere, Plenum Press,
New York,p. 341.)
Ionospheric
There are basically two possible current configurations, as pointed out by Bostr6m
(1964), type I and type II, as schematically illustrated in Figure 46. Type I was
originally proposed by Birkeland (1908) and includes an inflow of currents into
222
Y. K A M I D E
i
TYPE
I
TYPE
Fig. 46. Two possible configurations of field-aligned currents as pointed out by Bostr6m (1964).
(Bostr6m, R.: 1.964,J. Geophys. Res. 69, 4983.)
the morning half of the auroral oval and an outflow from the evening oval. Those
field-aligned currents are connected through the westward electrojet which flows
along the nightside auroral oval. In the type II current system, an east-west
electrojet, which is the Hall current, is generated between field-aligned current
sheets aligned in the east-west direction. Zmuda and Armstrong (1974b) considered
two pairs of such field-aligned currents, one in the morning sector and the other
(with the reversed current direction) in the evening sector. According to their
model, the field-aligned currents are closed via north-south Pedersen currents in
the ionosphere.
Sugiura (1975) has proposed a qualitative model of the field-aligned current
configuration in which two current systems ( C f a - 1 and C r a - 2) are connected to
different regions of the magnetosphere, as shown in Figure 47. Based on highaltitude observations of the magnetic field by O G O - 5 , he concluded that the polar
cap boundary can be identified by a sudden transition from a dipolar to a more
tail-like magnetic configuration, while the magnetic field in the region where the
lower-latitude, field-aligned current layer is situated is essentially meridional,
indicating that the equatorial current closure of the latter current must be via the
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
223
Cfa -1
---m~- Cfa -2
AURORAL
,?'" ..... ""~IL, BELT ,,/~lr'----~ ''~1
, .j
t
II / I /
\\
11
9
/
\\
\\
///
///
//
//
Fig. 47. Modelof field-alignedcurrent systems C#~ - 2 and Cf=- 2, which flowin the polar cap boundary
layer, and on the low-latitude side of the auroral belt, respectively. (Sugiura, M.: 1975, J. Geophys.
Res. 80, 2057.)
equatorial ring current. However, it was difficult from the observations to discuss
how these two current systems interact in the ionosphere.
A numerical calculation of the ionospheric current pattern m a d e by Yasuhara
et al. (1975) and subsequent numerical simulations can be used to check which
configuration (type I or II) of the ionospheric closures of the field-aligned currents
is consistent with the results of the recent field-aligned current observations. It was
suggested that the real situation appears to be a complicated combination of the
types I and II systems and that the relative importance of each of the closures
depends chiefly on the ionospheric conductivity distribution.
Rostoker and B o s t r f m (1976) have developed a mechanism in which a radial
closure of the currents in the magnetosphere (i.e., type II) is associated with a
convective motion of plasma in the magnetotail. It was demonstrated that the gross
field-aligned currents can be driven by energy supplied by the braking of this
convective motion of the plasma sheet particles as they drift toward the flanks of
224
Y. KAMIDE
the magnetosphere. Rostoker and Bostr6m (1976) have given the expected configuration of the electric field in the cross section of the magnetotail, which is consistent
with our current knowledge of the electric field in the ionosphere, and with the
assumption that magnetic field lines are nearly equipotential. Figure 48 shows the
basic character that away from the center of the tail, most of the electric field is
directed normal to the neutral sheet, indicating that toward the flanks of the tail
the dominant direction of convective flow is parallel to the neutral sheet and toward
the boundary between the magnetotail and magnetosheath. It was also discussed
that since the closure currents of the field-aligned current loops flow in directions
opposite to the electric field, the current region in the tail has the character of an
electric generator.
|
DUSK
:--
//
=--
DAWN
|
Fig. 48. Electric field configuration in the magnetotait projected on the cross-section area of the
magnetotail. (Rostoker, G.: 1980, in S.-I. Akasofu (ed.), Dynamics of the Magnetosphere, D. Reidel
Publ. Co., Dordrecht, Holland, p. 201.)
Kamide et al. (1976b) developed an empirical model of the three-dimensional
current system for the magnetospheric substorm which satisfies quite nicely the
recent new observations of the electric fields and currents in the ionosphere. In
Figure 49, we show a schematic illustration of their model, in which the shaded
area represents the region of the westward electro jet, that is the dominant feature
of the polar substorm. From nearly simultaneous observations of the field-aligned
FIELD-ALIGNED
CURRENTS
AND
AURORAL
ELECTROJETS
225
SUN
SUN
)
d'
a
,7
TOi~ Current
--,2:L
current
(b)
(a)
Fig. 49. (a) Model current system for the magnetospheric substorm. The dotted area represents the
region of the westward electrojet. (b) Schematic illustration of the three-dimensional current model
for the magnetospheric substorm. (Kamide, Y., Akasofu, S.-I., and Brekke, A.: 1976, Planetary Space
Sci. 24, 193.)
currents, the auroral electrojets, and the auroral distribution, Kamide and Rostoker
(1977) have considered the association of this current system with various types
of auroras in different local time sectors. According to their current configuration,
the westward electrojet flows approximately along the active westward traveling
surge in the evening sector, as well as along the entire diffuse aurora in the morning
sector. The latitudinal width of the westward electro jet is much larger in the morning
sector than that in the evening sector. The eastward electrojet flows equatorward
of the westward electrojet in the evening sector, namely along the diffuse aurora.
There is no significant ionospheric return current from the electrojets in the polar
cap and in mid-latitudes. Thus, the electro jet currents must mostly be supplied by
the field-aligned currents.
The westward electrojet in Figure 49 is fed by the field-aligned currents in the
way suggested earlier by Birkeland in 1908, but a significant part of the downward
field-aligned currents (represented by a and b) in the morning sector flows southwestward, and then flows out of the ionosphere as the upward currents (a' and b').
This is in agreement with the observations made by the Chatanika radar and the
T R I A D satellite. In particular, the radar measurements suggest that the westward
electrojet has a large southward deflection in the morning sector.
It can be seen that the current pattern is much more complicated in the evening
sector than in the morning sector. There are downward field-aligned currents (c'
and d') in the area of the eastward electrojet. The intense upward currents (c and
226
Y. KAMIDE
NOON
/
\
/
\
/
\
/
\
//
k
/
\
/
ii
%
/
I
/
I
I
\
/
\
\
I
/
_
DUSK
j
~ei-
~
- -~4
'
l
. .
~
DAWN
/
I
/
t
~
\
x
/
/
/
8O
/
/
\
//
\
\
\
\
,60
\
/
\\
50
\\
/ /
x x
Fig. 50(a).
Fig. 50. (a) Schematic diagrams of the three-dimensionalcurrent systems for steady-state conditions,
(b) Schematic diagrams of the three-dimensional current systems for substorms. (Hughes, T. J. and
Rostoker, G.: 1979, Geophys. J. Roy. Astron. Soc. $8, 525.)
d) are connected with the westward electrojet and also with the northward ionospheric current, which is eventually connected to the inward field-aligned currents
through the eastward electro jet. Thus, the total intensity of the upward field-aligned
currents is much more intense than that of the inward currents, as observed by the
T R I A D satellite (Yasuhara et al., 1975; Iijima and Potemra, 1976a). This current
configuration is also in good agreement with the radar observations which show
that the northward ionospheric current prevails in the evening sector, regardless
of the sign of the east-west ionospheric current. It is indicated that the eastward
electrojet flows north-eastward as it approaches the midnight sector and eventually
is connected to the westward electrojet.
Hughes and Rostoker (1979) have developed a comprehensive three-dimensional
model for moderate magnetic activity. Their model is based on the ground magnetometer data along the meridian array over Canada, and the average electric
field configuration, as the one discussed in Section 5 (Mozer and Lucht, 1974).
FIELD-ALIGNED
CURRENTS
AND
AURORAL
ELECTRO
227
JETS
NOON
\
/
N
/
X
/
\
I
I
\\
1
/
/
f
/
\
/
I
\
/
DUSK
I
I
l
\
\
I
\
J
I
8O
!
/
//////
/
\\
/I
/
\
/
\
/
/
\
\
-,
DAWN
",\
/
,60
/
/
//
Fig. 50(b).
Their model current system is shown in Figure 5 0a, which can reproduce successfully
the magnetic perturbation pattern observed at high latitudes. Only the net fieldaligned current is shown for simplicity; the balanced part of the field-aligned current
flows are not given, but are contained in the model. The essence of the model is
as follows:
(1) There is an intense downward field-aligned current near noon which diverges
into the ionosphere generating the eastward electrojet. Some of the electrojet
diverges up the field lines at the dusk terminator due to the conductivity discontinuity
there, while most of the current flows into the premidnight sector and eventually
becomes the upward field-aligned current near the Harang discontinuity.
(2) The downward current near noon and in the late morning sector diverges
into the ionosphere generating the westward electrojet.
(3) Because of the high conductivity in the dayside ionosphere, some of the
downward current diverging into the ionosphere flows equatorward in the pre-noon
228
u
KAMIDE
quadrant. This Hall current flows across the noon meridian at mid-latitudes and
flows back to the auroral oval in the afternoon hours.
(4) In the late morning sector, a weak eastward current flows equatorward of
the westward electrojet as reported by Rostoker and Hron (1975). Baumjohann
and Kamide (1981) have shown that such an eastward electrojet tends to often be
found in the recovery phase of substorms.
Rostoker and Hughes (1979) and Rostoker (1980) have modified the empirical
current model for steady-state conditions (Figure 50a) to include substorm activity.
The substorm is characterized by a wider and more intense westward electrojet in
the evening sector and a westward traveling surge at the western edge of the
substorm-disturbed region. The local three-dimensional current system in the
vicinity of the surge has been proposed by Kisabeth and Rostoker (1973) and more
recently by Inhester et al. (1981). The modified current model for the presence of
substorm activity is shown in Figure 50b, where the latitudinally-confined,westward
electrojet penetrates into the poleward edge of the eastward electrojet to the
westward traveling surge.
7.2. MODEL CURRENT SYSTEM AND AURORAL DISTRIBUTION
Recent studies of the currrent configuration with respect to the distribution of
auroras, in particular, at their latitudinal boundaries have yielded the following
information:
(1) The eastward electrojet is confined within the diffuse auroral belt in the
evening sector. The equatorward portion of the eastward portion of the eastward
electrojet is fed by downward field-aligned currents, while the poleward edge of
the eastward electrojet region is associated with upward current flow. The region
poleward of the eastward electrojet is the site of upward field-aligned current fiow~
discrete auroral arcs, and westward ionospheric current flow.
(2) The westward electrojet in the morning sector is penetrated by downward
flowing field-aligned currents in the poleward portion and upward flowing fieldaligned currents in the equatorward portion. In the equatorward portion of the
westward electrojet, a type of diffuse aurora exists which is more luminous than
the evening diffuse aurora but less luminous than the evening discrete aurora.
(3) In the substorm-disturbed region in the evening sector the westward electrojet
penetrates toward the dusk sector along the poleward edge of the eastward electrojet. This eastward electrojet is considered to represent the convection electric field
but is enhanced significantly during substorms. The head of the westward electrojet
is associated with the well-known westward traveling surge. Field-aligned currents
are often downward at the poleward edge of the westward electrojet.
(4) The relative location of the auroral intensity and the auroral electrojets must
be discussed quantitatively, particularly in view of the fact that the ISIS-2 photometer is capable of sensing luminosity even less than lkR, while the threshold for
the DMSP satellites is 2-3kR. It is quite likely that although luminosities below
l k R are indicative of the presence of auroras, they do not imply the existence of
FIELD-ALIGNED
CURRENTS AND AURORAL
ELECTRO JETS
229
a significant ionospheric current, since the auroral intensity would have to rise
above some threshold before an adequate conductivity to support the electrojet
enhancement could be achieved.
There is no doubt that in the evening sector, the westward traveling surge is the
typical substorm feature (Meng et al., 1978), in which intense upward field-aligned
current is present. The upward current has been found to be carried by precipitating
keV electrons. The westward extremity of the main body of the surge often has
bright discrete arcs emanating attached to it which occupy the region to the west.
Each of these arcs is the site of intense upward current flow and occasionally there
will be downward current flow adjacent to the discrete arcs where the current is
carried by upward flowing thermal electrons of ionospheric origin. A schematic
diagram showing the field-aligned currents and the corresponding auroral forms is
given by Kamide and Rostoker (1977, see their Fig. 16). Kisabeth and Rostoker
(1973) and Rostoker and Hughes (1979) have found a localized, positive D component spike on the Earth's surface as one of the characteristics of the passage of
the westward traveling surge. The D deflection has been explained by an equatorward ionospheric current at the head of the traveling surge.
Kamide and Rostoker (1977) have indicated that a significant downward current
flow may occur in regions of low auroral luminosity, whereas upward current flow
appears to be related to intense auroral features such as discrete arcs. These features
are particularly noticeable in the morning sector westward electrojet region. On
the basis of these observations it is suggested that a downward flowing field-aligned
current is carried by ionospheric electrons moving upward into the magnetosphere,
while an upward flowing current is carried by the precipitating keV electrons
responsible for E region auroral luminosity. The suggestion that a downward
field-aligned current is carried by upward moving low-energy (thermal) ionospheric
electrons is in agreement with the results of Arnoldy and Choy (1973), who
observed, by a series of rocket detectors, the upward streaming of electrons of
energy less than a few hundred electron volts poleward of a main auroral luminosity.
Klumpar et al. (1976) also observed low-energy electrons having pitch angles near
180 ~ and with a sufficient flux to account for the downward field-aligned current
density in the morning sector measured simultaneously by the ISIS-2 magnetometer.
In evaluating the ability of energetic electrons to carry enough current to account
for the observed level of magnetic perturbations, we note that Klumpar et al. (1976)
and Shuman et al. (1981) indicated on the basis of simultaneous magnetic signatures
of the field-aligned currents and electron measurements (approximately 5 eV to
15 keV) that the precipitation of kilovolt electrons can account for the major portion
of the upward field-aligned current in the equatorward half of the morning auroral
belt. This suggestion also agrees with those of Evans et al. (1977) and Carlson and
Kelley (1977) in that the precipitating keV electrons constitute the upward fieldaligned current within bright auroral arcs.
Most of the rocket observations of the field-aligned current have indicated that
in the vicinity of individual auroral arcs the precipitating electrons with energies
230
Y. KAMIDE
between approximately 500 eV and 20 keV can carry a significant fraction of the
upward field-aligned current, whereas some rocket data (e.g., Pazich and Anderson,
1975; Spiger and Anderson, 1975) showed that although a total upward current
coincides with the main arc, the measured precipitating electrons (0.5-20 keV)
carry only a small fraction (< 15%) of the upward current necessary to produce
the simultaneously observed magnetic signatures. It may be that this apparent
discrepancy may be accounted for by taking into account the difference either in
altitude (i.e., 100-200 km for the rocket measurements) or in spatial resolution
(i.e., less than 5 km for rocket observations) or both.
Note, however, as Arnoldy (1977) and Theile and Wilhelm (1980) observed by
using data from rocket borne probes, the carriers of the upward field-aligned current
are sometimes not collocated with the discrete auroral arc but are located at its
boundaries. It is important to determine what types of auroral forms are associated
with the precipitating keV electrons at the edges of arcs, and to determine the
latitude and longitude of such auroras with respect to the substorm-disturbed region.
The diffuse aurora in evening hours delineating the equatorward half of the
auroral oval is the persistent feature. Compared with the structured discrete aurora
in the poleward half of the auroral oval, the diffuse aurora is less structured and
relatively stable. It has been found that the downward field-aligned current is
collocated with the diffuse aurora. This correspondence may not be unexpected, if
we combine several independent observations that the downward field-aligned
current flows in the latitudinal regime within the eastward electrojet (Kamide and
Akasofu, 1976b), that the diffuse radar aurora is associated with the eastward
electrojet and is confined in the region of the downward field-aligned current
(Tsunoda et al., 1976a, b), and that the upward field-aligned current is located in
the discrete aurora region (Armstrong et al., 1975; Kamide and Akasofu, 1976a).
Since it is difficult to identify the boundaries of the diffuse aurora in all-sky
camera records, a direct comparison of the diffuse aurora and the downward
field-aligned current has long been desired. However, it is important to note that
discrete auroral arcs may be immersed also in the diffuse aurora and in the region
of the northward electric field, particularly in the poleward portion of the diffuse
auroral oval (Wallis et al., 1976; de la Beaujardi6re et al., 1977). Furthermore,
recent simultaneous measurements of magnetic fields and soft particle distributions
by ISIS-2 have shown that the region of the downward field-aligned currents extends
on the average 2.4 ~ equatorward of the low-latitude boundary of 1 keV electron
precipitation in the evening sector (Klumpar, 1979).
Insofar as the current carrier in the diffuse auroral region in the evening sector
is concerned, a plausible candidate for both the downward field-aligned current
and for the production of the diffuse aurora might be precipitating positive ions.
However, there is some evidence that although both precipitating electrons and
protons are observed in the diffuse aurora region, the 5 eV to 15 keV protons carry
only 0.1-0.01 of the field-aligned current carried by the electrons over the same
energy range (Winningham et aI., 1975, Lui et al., 1977). Thus, the precipitating
F I E L D - A L I G N E D CURRENTS AND A U R O R A L ELECTROJETS
231
protons are not suitable for the downward current, and this leads us to speculate
that upward flowing thermal electrons are the most likely current carriers. Klumpar
(1976) suggested that isotropic keV electrons originating in the plasma sheet give
rise to the diffuse aurora. It was shown by Meng (1976) that the energy spectrum
of the diffuse aurora during quiet periods is characterized by nearly constant
differential fluxes from 0.2 to about 8 keV with a sharp cutoff above 8 keV and
that the energy flux is about 0.1 ergcm-2s -1 sr -1. The electron characteristics
observed by Kamide and Rostoker (1977) are essentially the same as Meng's
observation, except that the energy density during substorms increases by a factor
of 5 or more.
A remark is given on Ps-6 magnetic variations, which are quasi-periodic perturbations observed generally in the morning sector during substorms (Saito, 1978). The
Ps-6 variations have a time scale of 10-15 rain and appear mainly in the D and Z
components with the amplitude reaching several hundred nanotesla (nT) in the
latitudinal center of the westward electrojet. To explain these characteristics, Saito
(1978) presented the 'snake' model in which Bostr6m's type I field-aligned current
system which has the east-west electrojet current in the ionosphere 'wriggles' in
the north-south direction like a moving snake. Since several such 'wriggles' are
generated during a substorm in the dawn sector, the irregular Ps-6 variations tend
to be observed usually in succession. Kawasaki and Rostoker (1979) determined
the eastward drift velocity of Ps-6 to be of the order of 0.8-2 km s-~ with auroral
12 bands and proposed that the equatorward flowing ionospheric current in the
westward electrojet, i.e., Bostr6m's type II current system is responsible. Rostoker
and B arichello (1980) observed that Ps-6 activity has a peak in occurrence frequency
around local dawn, indicating that the southeast-oriented electric field configuration,
not electron precipitation, is the most crucial factor in the generation of Ps-6
magnetic disturbances.
7.3.
FUTURE
PROBLEMS
The review presented here suggests that recent observations by means of several
new techniques have made it possible to begin to unfold some of the complicated
phenomena occurring in the polar ionosphere and to unveil gradually the cause of
the ionospheric and 'magnetospheric processes. However, we are still far from
providing acceptable answers to many problems of the relationship of the current
configuration and other parameters, and there even emerge some new basic questions in the recent new findings. We list some of these questions:
(1) What are the sources of the ionospheric electric fields during both quiet and
disturbed times? Recent, reliable measurements by means of several different
techniques have revealed the existence of the characteristic diurnal variations of
the large-scale auroral zone electric field. It is directed northward and southward
in the evening and morning sectors, respectively. The predominance of the westward
field in between has also been detected near the midnight sector. These features
are consistent with the two-cell convection pattern. However, there is no agreement
232
u KAMIDE
in various data sets as to the significance and nature of the small-scale electric
fields, in particular, in the east-west component associated with complicated auroral
forms. For the determination of the electric field behavior in understanding the
sources of its spatial and temporal structure in auroral latitudes, it would be
necessary to make a direct comparison of the electric field with other ionospheric
quantities, such as the electron density, field-aligned currents, and ionospheric
conductivities, as well as the magnetic field and auroral distribution.
(2) What are the roles of magnetospheric convection and field-aligned currents in
the course of the magnetospheric substorm? If one assumes that magnetic field lines
are equipotential, it is possible to use the observed electric field topology to study
the large-scale convection in the magnetosphere. There is no doubt that various
ionospheric and magnetospheric phenomena depend upon how the magnetospheric
plasma convection varies during magnetospheric substorms. In the midnight sector,
the westward electric field drives the magnetotail plasma inward toward the earth;
this motion in the ring current and plasma sheet regions appears to be closely
related to a drastic change of the magnetospheric configuration observed during
substorms. Thus, by examining the electric field variations during a substorm, it
may be possible to deduce the role of the convection that is the major process in
the magnetosphere. The field-aligned currents that connect the ionospheric and
magnetospheric currents also play a dominant role. It may be required to combine
recent observations on the distribution and substorm changes of the large-scale
electric field configuration, field-aligned currents and ionospheric conductivities in
order to interpret the observed complicated processes and expected phenomena
in terms of theoretical consequences of the particle distribution resulting from the
electric potential distribution in the magnetosphere. Recent high-quality data from
the IMS meridian networks, together with sophisticated computer modeling codes,
are useful in providing us with self-consistent information on these quantities.
(3) Where do auroral paticles originate and what processes accelerate these
particles ? Although there seems to be a general agreement on the energy distribution
of precipitating auroral particles for the discrete aurora, it is unclear in which part
of the plasma sheet (near its inner edge, central part, or high-latitude boundary?)
these particles originate and what acceleration mechanisms are effective to produce
the abrupt population of the auroral particles at the onset of a substorm. It is
important to see how the tail current disruption occurs, resulting in a large, induced
electric field. Although parallel electric fields are beyond the scope of the present
review, it might be emphasized that the most exciting observation during the last
several years is the discovery of these 'parallel' fields in the altitude range below
10 000 km (see Mozer et al., 1980). Presumably, the acceleration of auroral electrons takes place by the field-aligned potential drop. Using $3-3 satellite records,
Sharp et al. (1977, 1979) found energetic ions streaming away from the ionosphere
above the acceleration region. However, it remains to be examined as to the relative
location of discrete auroral arcs and the parallel fields.
FIELD-ALIGNED
CURRENTS
AND AURORAL
ELECTROJETS
233
(4) Discrete and diffuse auroras? One of the important findings in auroral physics
during the last decade on the basis of satellite-viewed auroral imagery is the
identification of discrete and diffuse auroras. Akasofu (1977) summarized some of
the differences in morphological features between the discrete and diffuse auroras.
In essence, a discrete aurora appears as a single, bright strand, well separated from
other auroras by a dark space with a latitudinal width of, at least, several tens of
km. This distinction of the two types of auroras is quite important, since it is now
believed that the first indication of an auroral substorm, viz., the auroral breakup,
occurs near an arc located between the discrete and diffuse auroras in the midnight
sector. However, such a distinction between the two main auroral types cannot
always be made unambiguously both in ground-based and satellite observations.
We note furthermore that the classification into the two types of auroras is only
the first step to understand physical processes in the polar ionosphere occurring in
association with auroral particle precipitation.
(5) What is an auroral arc? The term 'auroral arc' had originally been used for
a long time to describe a special class of band-like forms which appear as a simple,
curving arch for visual observations. However, this term has recently been used to
denote a discrete curtain-like form of auroral display, departing from the classical
definition. The width of auroral arcs ranges from a minimum of 50 m (a few electron
gyro-radii) to a maximum order of 10 km (Davis, 1978). The distribution of visible
auroras can now be observed by ground-based, all-sky cameras and TV camera
systems, airborne all-sky cameras, and satellite scanners from above the poles. It
may well be that the discrete auroras can be divided into the auroral arc and the
inverted-V aurora on the basis of the latitudinal structure in particle flux, electric
field, and auroral luminosity measurements. The discrete aurora associated with
the inverted-V events is typically hundreds of km in latitudinal width, while the
auroral arc is typically much less than that, perhaps hundreds of meters. Note that
the auroral luminosity obtained by the auroral scanner photometers on board the
recent satellites represent only groups of auroral arcs. The relationship between
the small-scale current systems observed by sounding rockets near individual auroral
forms and the large-scale current systems extending over the entire auroral oval
scanned by polar-orbiting satellites should be clarified urgently. It is perhaps
necessary to design either satellites or longer-range rockets which can scan the
entire auroral belt with high data rates supported by optical observations of auroral
fine structures.
(6) What is the Harang discontinuity? There is some degree of confusion as to
the spatial structure of the Harang discontinuity. Although a study of satellite
electric field data indicates that the Harang discontinuity has a finite thickness, it
is important to determine how fine structures within the Harang discontinuity
'region' relate to ionospheric processes in conjunction with the growth of a substorm.
At present, even the characteristic features of particle precipitation, optical auroras
and field-aligned currents in the Harang discontinuity region are unclear.
234
Y. K A M I D E
(7) What are morning sector features of the auroral electrofet and fieM-aligned
currents? Most previous studies of field-aligned currents and their spatial relation
to auroral forms and the auroral electrojets have been made only for the evening
and midnight sectors. As far as the present author is aware, there are only five
published papers which have examined empirically such a relationship for morning
hours (Klumpar et al., 1976; Kamide et al., 1976; de la Beaujardi6re et al., 1977;
Sulzbacher et al., 1978; Hughes and Rostoker, 1979) in contrast to a large number
of papers for the evening sector. Since it has been shown that characteristics of
auroras in the morning sector are quite different from those in the evening sector
(see Oguti, 1976) and that the direction of the field-aligned currents is reversed
between morning and evening sides of the auroral oval, it is important to extend
such correlation studies to the morning sector.
(8) How are field-aligned currents closed in the magnetosphere ? Although in this
paper we have dealt only with the current closure in the ionosphere, the connection
of field-aligned currents with the auroral electrojet cannot be completely discussed
without clarifying the closure of the current circuit in the magnetosphere. Sato and
Iijima (1979) have reviewed primary generation mechanisms of the field-aligned
currents in the magnetosphere. Sato (1974) and Rostoker and Bostr6m (1976)
presented ionosphere-magnetosphere current circuits in which magnetospheric
generators associated with plasma convection are involved. More recently, Lyons
(1980) has suggested the importance of discontinuities in the magnetospheric
convection field E, such that div E < 0, in generating the large-scale field-aligned
currents with associated parallel electric potential difference and electron precipitation. In spite of the recent suggestion that the Hall electrojet currents have as large
a divergence as the Pedersen currents (Akasofu et al., 1981b) there has been little
work on the Hall current circuits, which do not result in energy dissipation processes.
Acknowledgements
I would like to thank S.-I. Akasofu for his continuing encouragement and informative discussions during the preparation of this paper. Comments on 'a' final version
received from A. D. Richmond are helpful in improving the presentation and are
greatly appreciated. My special thanks go to several institutes for their hospitality
where portions of the manuscript were written and revised. These are detailed as
follows: Geophysical Institute of the University of Alaska, National Geophysical
and Solar-Terrestrial Data Center of NOAA, High Altitude Observatory of the
National Center for Atmospheric Research, and the Department of Physics of the
University of Alberta. The work was brought essentially to its present form while
I was visiting the NOAA Space Environment Laboratory.
This work was supported in part by a grant of the National Institute of Polar
Research, and also in part by the Ministry of Education in Japan under a grant
554105. The author is supported by an NRC (U.S. National Research Council)
Resident Research Associateship.
FIELD-ALIGNED CURRENTS AND AURORAL ELECTROJETS
235
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