Download satellite observations of auroral acceleration processes

SATELLITE OBSERVATIONS OF AURORAL ACCELERATION PROCESSES
Akademisk avhandling
som med tillstånd av rektorsämbetet vid Umeå universitet för avläggande av filosofie
doktorsexamen framlägges till offentlig granskning vid Institutet för rymdfysik i Kiruna,
konferensrummet Aniara, fredagen den 16 september 1994, kl. 10.15
av
Lars Eliasson
fil kand
The thesis includes an introduction and the following papers:
I
Eliasson, L., L.-Å. Holmgren, K. Rönnmark, Pitch Angle and Energy Distributions of
Auroral Electrons Measured by the ESRO 4 Satellite, Planet. Space Sci., 27, 87-97,
1979.
II.
Eliasson, L., R. Lundin, and J.S. Murphree, Polar Cap Arcs Observed by the Viking
Satellite, Geophys. Res. Lett., 14,451-454, 1987.
in .
Eliasson, L., and R . Lundin, Acceleration/Heating Processes on Auroral Field Lines as
Observed by the Viking Spacecraft, Proceedings of the 21st ESLAB Symposium, ESA
SP-275, 87-91, 1987.
IV.
Lundin, R., and L. Eliasson, Auroral Energization Processes, Ann. Geophysicae, 9, 202223, 1991.
V.
Eliasson, L., M. André, R. Lundin, R. Pottelette, G. Marklund, and G. Holmgren,
Observations of Electron Conics by the Viking Satellite, submitted to J. Geophys. Res.,
1994.
VI.
Eliasson, L., M. André, A. Eriksson, P. Norqvist, O. Norberg, R. Lundin, B. Holback,
H. Koskinen, H. Borg, and M. Boehm, Freja Observations of Heating and Precipitation
of Positive Ions, Geophys. Res. Lett., in press, 1994.
Kiruna 1994
SATELLITE OBSERVATIONS OF AURORAL ACCELERATION PROCESSES
Lars Eliasson
Swedish Institute of Space Physics
P.O. Box 812, S-981 28 Kiruna, Sweden
Abstract
Measurements with satellite and sounding rocket borne instruments contain important
information on remote and local processes in regions containing matter in the plasma state.
The characteristic features of the particle distributions can be used to explain the morphology
and dynamics of the different plasma populations. Charged particles are lost from a region due
to precipitation into the atmosphere, charge exchange processes, or convection to open
magnetic field lines. The sources of the Earth’s magnetospheric plasma are mainly ionization
and extraction of upper atmosphere constituents, and entry of solar wind plasma. The intensity
and distribution of auroral precipitation is controlled in part by the conditions of the
interplanetary magnetic field causing different levels of auroral activity. Acceleration of
electrons and positive ions along auroral field lines play an important role in magnetospheric
physics. Electric fields that are quasi-steady during particle transit times, as well as fluctuating
fields, are important for our understanding of the behaviour of the plasma in the auroral region.
High-resolution data from the Swedish Viking and the Swedish/German Freja satellites have
increased our knowledge considerably about the interaction processes between different
particle populations and between particles and wave fields. This thesis describes acceleration
processes influencing both ions and electrons and is based on in-situ measurements in the
auroral acceleration/heating region, with special emphasis on; processes at very high latitudes,
the role of fluctuating electric fields in producing so called electron conics, and positive ion
heating transverse to the geomagnetic field lines.
Keywords: Aurora, particle acceleration, space plasma, magnetosphere, ionosphere, polar cap,
satellite observations.
IRF Scientific Report 217
Swedish Institute of Space Physics
Kiruna 1994
ISSN 0284-1703
ISBN 91-7174-926-8
pp 31+6 papers
Satellite Observations
of Auroral Acceleration Processes
by
Lars Eliasson
Swedish Institute of Space Physics
P.O. Box 812, S-98128 Kiruna, Sweden
IRF Scientific Report 217
August 1994
Printed in Sweden
Swedish Institutè of Space Physics
Kiruna 1994
ISSN 0284-1703
ISBN 91-7174-926-8
Satellite Observations of Auroral Acceleration
Processes
Lars Eliasson
Swedish Institute of Space Physics, P.O. Box 812, S-981 28 Kiruna, Sweden
Abstract Measurements with satellite and sounding rocket borne instruments
contain important information on remote and local processes in regions containing
matter in the plasma state. The characteristic features of the particle distributions
can be used to explain the morphology and dynamics of the different plasma
populations. Charged particles are lost from a region due to, for example,
precipitation into the atmosphere, charge exchange processes, or convection to
open magnetic field lines. The sources of the Earth’s magnetospheric plasma are
mainly ionization and extraction of upper atmosphere constituents, and entry of
solar wind plasma. The intensity and distribution of auroral precipitation is
controlled in part by the conditions of the interplanetary magnetic field causing
different levels of auroral activity. Acceleration of electrons and positive ions
along auroral field lines play an important role in magnetospheric physics.
Electric fields that are quasi-steady during particle transit times, as well as
fluctuating electric fields, are important for our understanding of the behaviour of
the plasma in the auroral region. High-resolution data from the Swedish Viking
and the Swedish/German Freja satellites have increased our knowledge
considerably about the interaction processes between different particle
populations and between particles and wave fields. This thesis describes
acceleration processes influencing both ions and electrons and is based on in-situ
measurements in the auroral acceleration/heating region, with special emphasis
on; processes at very high latitudes, the role of fluctuating electric fields in
producing so called electron conics, and positive ion heating transverse to the
geomagnetic field lines.
Keywords: Aurora, particle acceleration, space plasma, magnetosphere,
ionosphere, polar cap, satellite observations.
2
Contents
Introduction and Summary of Papers
Introduction
Electron Acceleration
Hot Plasma Observations in the Polar Cap
Ion Heating and Precipitation
Measurement Techniques
Viking
Freja
References
Publications
Acknowledgements
Summary of Papers
3
5
8
12
14
14
17
24
26
28
30
Included Papers
L
Eliasson, L., L.-Å. Holmgren, K. Rönnmark, Pitch Angle and Energy
Distributions of Auroral Electrons Measured by the ESRO 4 Satellite,
Planet. Space Sci., 27, 87-97,1979.
ü.
Eliasson, L., R. Lundin, and J. S. Murphree, Polar Cap Arcs Observed by
the Viking Satellite, Geophys. Res. Lett., 14,451-454,1987.
HI. Eliasson, L„ and R. Lundin, Acceleration/Heating Processes on Auroral
Field Lines as Observed by the Viking Spacecraft, Proceedings of the
21st ESLAB Symposium, ESA SP-275, 87-91,1987.
IV. Lundin, R., and L. Eliasson, Auroral Energization Processes, Ann.
Geophysicae, 9,202-223,1991.
V.
Eliasson, L., M. André, R. Lundin, R. Pottelette, G. Marklund, and G.
Holmgren, Observations of Electron Conics by the Viking Satellite,
submitted to J. Geophys. Res., 1994.
VI. Eliasson, L., M. André, A. Eriksson,.P. Norqvist, O.Norberg, R. Lundin,
B. Holback, H. Koskinen, H. Borg, and M. Boehm, Freja Observations of
Heating and Precipitation of Positive Ions, Geophys. Res. Lett., in press,
1994.
3
Introduction
The results of particle acceleration and precipitation at high latitudes in the
form of visual phenomena, such as the aurora, has for a long period of time
fascinated mankind. The scientific tradition in this field in Scandinavia is long. It
was thus very natural that the first Swedish satellite was devoted to studies of the
main auroral electron acceleration region. The orbit of Viking made it possible to
extensively study particle distributions on magnetic field lines connected to the
auroral oval as well as at latitudes equator- and poleward of the statistical oval at
altitudes above, in, and below the acceleration region. High resolution
measurements were made during the traversal of individual discrete auroral arcs.
The Viking observations were found to be much more variable and complex than
anticipated.
The importance of electrostatic acceleration, as well as time-varying
acceleration/heating mechanisms, are discussed in this report. High resolution
particle measurements can be used to determine the temporal behavior and spatial
extension of both remote and local acceleration regions. It is essential to have a
knowledge about the spatial extent and temporal development of the processes to
be studied. This seems to be very obvious but is sometimes neglected. Studies of
the coupling between the magnetosphere and the solar wind during low magnetic
activity, in the auroral oval, have recently given interesting results on auroral
phenomena in the region poleward of the oval, the region normally called the
polar cap, and in the dayside oval. Physical mechanisms related to the outflow of
ions from the ionosphere have been studied extensively lately and observations
have confirmed the ideas that the ionosphere can be a very important source of
magnetospheric plasma. Both Viking and Freja have made important
contributions to the understanding of ion heating processes.
The auroral energy comes from the Sun through the flow of solar wind plasma.
The solar wind - magnetosphere dynamo can generate a power of more than 1012
W. The potential across the magnetosphere amounts to about 50 kV and the
current dissipated in the polar ionosphere is of the order of million Amperes.
Chapman and Ferraro [1931] proposed that the Earth and its magnetic field are
confined, temporarily, in a cavity which is formed during the passage of hot gas
(called M-stream) from the Sun. Later, Biermann [e.g., 1957] proposed a
continuous flow of plasma from the sun, which means that the magnetosphere is a
permanent feature. The term magnetosphere was introduced by Gold [1950].
Piddington [1960] proposed that the magnetosphere has a long cylindrical tail
whose presence was confirmed by Ness [1965]. The concept of the convection
motion of the magnetospheric plasmas was originally proposed by Axford and
Hines [1961]. It is normally assumed that the magnetospheric dynamo has the
minimum efficiency, i.e. no energy will be dissipated in the polar ionosphere,
when the interplanetary magnetic field (IMF) has a large (> 5 gamma) northward
component for several hours. This is, however, only partly true as has been shown
by measurements in the polar cap [Zanetti et al., 1984],
The auroral oval was defined by Feldstein [1963]. A historical review on the
auroral oval can be found in Brekke [1984], Snyder and Akasofu [1974] defined
discrete aurora as a single, bright strand, separated from others by a dark space of
order of a few tens of km in width. When it is seen from the ground it has a
curtain-like structure. There are different characteristics in the electrodynamics of
the individual auroral arcs mainly depending on the closure of the current circuit,
the magnetospheric source (dynamo) and the action on (of) the ionosphere (load
4
or generator). One component, that plays a crucial role in the formation of auroral
structures is the convection of the ambient plasma. The diffuse aurora appears as a
broad band of luminosity with a width of at least several tens of km. It may not be
easily visible from the ground but can cover a large part of the sky. Sounding
rocket results published by Mcllwain [1960] showed that the visible auroral light
could be explained by an enhanced influx of electrons in the energy range 5-10
keV.
The oval has been characterized as quiet when the magnetic activity in the
nightside is low. It seems, however, that there are different processes that control
the dayside compared to the nightside and it is obvious that the activity is
increased at high latitudes during “quiet” periods. The dayside auroral region is
connected to the low latitude boundary layer (LLBL). This region is characterized
by time-dependent magnetosheath plasma injection and strong plasma
acceleration. Another interesting region in the dayside magnetosphere is the polar
cusp. This is a region more or less directly connected to the solar wind plasma
that has penetrated the bow shock.
This report will concentrate on satellite observations of auroral acceleration
processes. Data from three satellites will be presented and two of the missions
will be described in some more detail. The discussion of the acceleration
processes is based mainly on hot plasma observations made with the Swedish
Viking and the Swedish/German Freja satellites except Paper 1. The ESRO 4 data
presented in Paper 1 have given ideas concerning the formation of electron conics
and information about electron acceleration and is therefore included in this
thesis. ESRO 4 had an orbit with apogee at 1177 km and perigee at 245 km.
Both Viking and Freja are very well suited for studies of auroral processes,
Viking, most of the time, being in or above the acceleration region and Freja
below. The possibilities to compare the two data sets offer a good opportunity to
learn more about the fascinating auroral processes that people at high latitudes can
enjoy. That detailed study of Freja data has only started when this is written.
The schematic on the next page (scale not correct) shows some of the
properties that are being studied with the two satellites. The magnetospheric
dynamo, powered by the solar wind, generates a current system in the
magnetosphere/ionosphere with upward currents on auroral arc field lines,
ionospheric Hall and Pedersen currents, and a downward return current close to
the arc. Parallel electric field acceleration, transverse and parallel ion
heating/acceleration, density depletions, wave generation and damping, and
plasma outflow are pronounced effects that are still not fully understood.
It is beyond the scope to here give an extensive reference to previous work
dealing with observations of auroral acceleration. Many excellent books and
review papers have been published recently. A short introduction to electron
acceleration, auroral activities at high latitudes, and ion heating is given in the
next sections.
5
Dynamo
Viking
2000- 13500 km
Upward election and ion beams
Electron and ion conics
AKRand broadband electrostatic noise
Plasma depletions
Freja
600-1750 km
Precipitating electrons and ions
Ion heating and evacuation
Vkve-particle interaction
Quasi-trapped elections
Aurora
! Magnetic field line
Earth
The orbits and instrumentation of the Viking and Freja spacecraft have been ideal
for extensive studies of auroral phenomena.
Electron Acceleration
Several auroral electron acceleration mechanisms have been suggested in the
past years to explain the energization of auroral electrons at altitudes from about
one thousand kilometers to 2 Re. Electrostatic shocks or bursts of intense electric
field reversals of various scale sizes, fields parallel to the ambient magnetic field
lines (e.g., double layers), solitary structures, resonant interaction with wave
fields, and other wave emissions may all be important. Viking particle
measurements have provided possibilities to study fine scale structures in the
distribution function of electrons and ions. Freja, mostly below the acceleration
region, observes mainly the effects on the cold ionospheric plasma, but the hot
precipitating magnetospheric plasma gives information on processes at higher
altitudes.
6
Monoenergetic electron distributions [e.g., Mclhvain, 1960; Albert, 1967;
Evans, 1968] and strongly field-aligned fluxes of electrons [Hoffman and Evans,
1968] became the first evidence that parallel electric fields could be established
along the magnetic field lines as proposed by Alfvén [1958]. Many signatures of
the hot plasma observed by satellites can be explained by the effects of
acceleration in quasi-static parallel electric fields. The electric fields can act as a
static field for some particles and as a fluctuating field for others.
Several modeling attempts were made during the 1970s [e.g., Knight, 1973;
Evans, 1974; Lennartsson, 1976]. One particular problem was early discovered in
the data [Albert and Lindström, 1970], namely that electrons were frequently
located in a part of phase space that should be a “forbidden region.” The ESRO 4
satellite, which was launched in 1972, did also observe a large amount of
electrons in the forbidden region in phase space. Paper 1 of this report
demonstrates a way to explain this type of observations. “Local trapping” of
particles between the parallel electric field above and the magnetic mirror force
below is frequently observed when the acceleration region is above the point of
observation.
Other features in the distribution function of electrons, that can not be
explained by static electric field acceleration, have been observed more recently
when higher resolution measurements have been performed and other regions of
space been studied. Low-frequency fluctuations of the electric field have been
found to play an important role in the auroral region. They can explain, e.g., the
observations of electron conics [André and Eliasson, 1992] and situations were
electrons and positive ions are accelerated in the same direction [Hultqvist et al.,
1988]. Electron conics were first discovered by Menietti and Burch, [1985].
Electron conics are electron distributions observed on auroral field lines with the
highest intensities at some angle away from the magnetic field. They are roughly
similar to ion conics but generated in a different way. Some electron conics may
be caused by a fluctuating electric field parallel to the Earth's magnetic field. An
electron accelerated through a certain potential drop when moving downward may
after magnetic mirroring find that the potential has changed, and it can escape and
gain an energy of maybe a few keV. This can be called a resonant effect, since the
electron travel time below the acceleration region and the frequency of the electric
field oscillations have to match. Some electron conics might also be caused by
wave energization mainly in the parallel direction or in the perpendicular direction
(e.g., by upper hybrid waves).
André and Eliasson [1994] made simulations including a dipole magnetic field
with a field strength varying as r 3, and an absorbing atmosphere at a geocentric
distance (r) of 6500 km. A time-varying electric field was assumed to be located
between 14500 km and 16500 km. The electric field was composed by a static
part and a fluctuating part. The static field was chosen as 1 mV/m which
corresponds to a total potential drop of 2 kV. Several different properties of the
time-varying electric field were tested. Fluctuations at a specific frequency as well
as broadband fluctuations were seen to give distributions similar to the observed
within certain limits (frequencies between 0.3 and 6 Hz, and a larger static than
fluctuating electric field amplitude) for the chosen altitude range and particle
characteristics. 50000 particles were randomly selected from a Maxwellian
distribution with a temperature of 100 eV, the temperature of the downgoing
population observed by Viking. The particle start times were spread over one
wave period and the particles were followed in small time steps until they were
either lost or they returned to the spacecraft. Similar electron conics can be
7
generated with the same static field without the time-varying part but including a
parallel diffusion coefficent equal to 1.5 x 1014 m2/s3. Perpendicular heating was
also tested using a static field of 1 mV/m and a perpendicular wave diffusion
coefficient equal to 1.5 x 1014 m2/s3 at geocentric distances between 7500 and
8500 km. This again gave electron conics in reasonable agreement with
observations. Lysak [1993] discusses the properties of Alfvén waves in a region
called the ionospheric Alfvén resonator. He shows that they have characteristics
that can explain the formation of electron conics in a way that can be described as
acceleration in a fluctuating parallel electric field, similar to the favored theory in
André and Eliasson [1992, 1994]. André and Eliasson [1992, 1994] assumed the
existence of the parallel electric field and did not discuss if, where, and when it
should appear.
The effects of acceleration below the satellite altitude can be seen both in the
ion data, upward beams, and in the electron data, widened and energy dependent
loss cone. Chiu and Schultz [1978] showed the effects of acceleration on
magnetospheric particles by plotting the distributions in velocity space. A
comparison, using Viking data, of the width of the electron loss cone with the
energy at maximum intensity of the ion beam shows a remarkably good
agreement, indicating that both electrons and positive ions are accelerated by the
same mechanism at relatively low altitudes. The figure below shows some data
that also were used by Block and Fälthammar [1990], who compared these
estimates with other ways to determine the parallel electric field. Reiff et al.
[1988] used Dynamics Explorer data for similar comparisons of signatures in the
electron and ion data.
Parallel electric field below Viking
101
E1
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ii
Hr p
<u
Ji
fi
'O
ä
1
cs
a.
10_i 1
10_1
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10 °
1
101
Acceleration from electron loss cone, keV
8
Hot Plasma Observations in the Polar Cap
There are a number of different ways to define the polar cap. These are based
on optical signatures, particle characteristics, or the magnetic field topology. The
auroral zone constitutes the statistical distribution of the maximum occurrence of
aurora while the auroral oval is the instantaneous distribution of aurora at any
specific time. The polar cap has earlier been defined as the area poleward of the
auroral zone but this was changed to be the area poleward of the auroral oval
[Akasofu, 1968]. This does not mean that the polar cap is completely void of
auroras. Discrete sun-aligned auroral features are frequently observed at polar cap
stations. Enhanced particle fluxes at high latitudes are observed, especially during
magnetically quiet conditions, that is, during northward interplanetary magnetic
field conditions. The polar cap is often completely void of visible auroras during
intense substorms, but often associated with weak uniform precipitation of lowenergy electrons, so called polar rain {Heikkila, 1972] during periods of
southward IMF.
Another definition of the polar cap is based on the hypothesis that some
geomagnetic field lines are connected to the interplanetary magnetic field. The
field lines having two footpoints at the Earth's surface are called closed and those
with only one, open. The regions with open field lines can be identified as the
polar caps. Regions at high latitudes with very low particle densities, thè lobes,
exist also in a totally closed magnetic field topology. These regions, with field
lines stretched to very far distances in the tail, might be considered the polar cap
as well. It is often very difficult to judge whether a magnetic field line is closed or
open from measurements. Observations of the particle angular characteristics can
be used, but in the polar cap the intensities are low and often close to or below the
threshold of the instruments. The results will also depend on which type of
particle and particle energy that is used as a tracer. High energy (>30 keV)
electrons have been used to determine if the field lines are open or closed [Frank
et al., 1986]. The similarity between spectra in the solar wind and the polar cap
was used by Fennell et al., [1975] as an indication of free access of solar wind
plasma to the polar cap region. The poleward limit of 1 keV electron precipitation
[Winningham et al., 1975] and differences in the ion precipitation [Troshichev and
Nishida, 1991] represent other possibilities to define the polar cap boundary. Low
energy (MeV) cosmic ray particles bombard the region bounded by approximately
the auroral oval [Stone, 1964] and is yet another way to determine this area.
The polar cap definition can also be based on the magnetospheric convection.
There is typically a region of antisunward convection at high latitudes. This is true
for the two-cell convection pattern theory [Axford and Hines, 1961] resulting
from a dawn to dusk electric field. Regions of sunward convection are located
equatorward of the polar cap according to this picture. Current systems are
persistent features in the magnetosphere. The poleward boundary of the region 1
current system can be used as a definition of the polar cap boundary [e.g., Coley,
1983; Mishin et al., 1992],
Some of the above mentioned methods to determine the polar cap area will not
give the same answer, but they can all often be used to give a crude estimate of
the size of the region more or less void of aurora at high latitudes. During
northward IMF the size of the polar cap decreases, which means that closed
magnetic field lines will exist at higher latitudes.
The relation of the magnetospheric particles to optical phenomena in the upper
atmosphere, as well as knowledge about their connection to different regions in
9
the outer magnetosphere, is essential for the understanding of the acceleration and
transport processes. Detailed hot plasma measurements give also clues to find the
sources of the particles.
Several attempts have been made to classify the auroral activity at high
latitudes. Lassen [1968] describes two main groups of polar cap emissions;
discrete polar cap auroras, and polar-glow aurora. Discrete polar cap arcs are
typically faint diffuse bands, draperies, or long quiet rays, where each ray may be
short-lived, but the total display is often present for several hours. The altitude
distribution showed that monoenergetic electrons with energies between 0.5 and
1.0 keV produced the aurora [Starkov, 1968]. The polar glow aurora, generally
subvisible, is associated with polar cap absorption, PCA, events and believed to
be generated by high energy protons and alpha particles at heights below 100 km.
Winningham and Heikkila [1974] described, besides polar rain, also “polar
showers” with structured precipitation of electrons with energies near 1 keV, and
a more intense and dramatic type of electron precipitation called “polar squall.”
Ismail and Meng [1982] classified, based on DMSP pictures, auroral arcs in the
polar cap into 3 categories: distinct sun-aligned arcs, morning/evening arcs
expanded from the oval, and hook-shaped arcs.
Sun-aligned arcs that connect both to the dayside and the nightside oval were
first observed by Frank et al. [1982] and described in more detail by Frank et al.
[1986]. The “theta aurora” was found to be associated with regions of, e.g.,
sunward convection in the polar cap, field-aligned electron acceleration on closed
magnetic field lines, and keV ions. The Dynamics Explorer observations of the
theta aurora started an interesting discussion on the topology of the
magnetosphere during these events. Observations by the DMSP and the ISIS
spacecraft have led to the conclusion that the polar cap arcs are not in the polar
cap but at an expanded poleward edge of the auroral oval [Meng, 1981; Murphree
et al., 1982]. Recent Viking results [e.g., Lundin et al., 1991] also confirm this
hypothesis. Structured fluxes of low energy electrons are often observed between
the polar cap arc and the diffuse auroral oval, in many cases together with a 1 - 10
keV plasma sheet like ion population, either in the morning half of the polar cap
or in the evening half. The particle characteristics together with information from
the imager experiment were used to determine if the polar cap arcs are located at
the poleward edge of the auroral oval or if the polar cap is divided by arcs into
two or more regions [Austin et al., 1993]. Their conclusion was that the high
latitude arcs are better described as the poleward edge of an expanded oval. The
UV images obtained by the University of Calgary instrument on Viking were
essential to understand the particle observations. It is very important to have a
knowledge about the spatial extent and temporal development of processes to be
studied.
Accelerated electrons of plasma sheet origin were found by Gorney et al.
[1986] at the edges of transpolar arcs. In the central region there was less or no
evidence for acceleration. Our observations frequently also show signs of accele­
ration in the center of polar cap arcs, but often that it occurs at higher altitudes at
the edges of the structures. This is seen as increased electron energies, intensities,
and often also downward field aligned angular distributions. Parallel electric field
acceleration is seen to occur below the satellite as concluded from the observation
of upward ion beams and widened electron loss cones. The ion beams studied
with Viking on high geomagnetic latitudes were often dominated by H+. There is
normally a region with downward field aligned currents close to the arc where the
electrons often appear as upward directed collimated fluxes and the ions as
10
conical shaped distributions. Possible particle sources are the solar wind, the
plasma sheet (boundary layer), and the upper atmosphere.
Polar cap crossings over auroral structures at high latitudes have shown that
acceleration processes below 2 Re are capable of accelerating electrons to
energies of 0.5 - 3 keV. Trapped “conical” electron distributions and the presence
of an isotropic population of 1 - 10 keV ions indicate, that the arcs occur on
closed magnetic field lines, most likely in the plasma sheet boundary layer. The
polar cap structures have dimensions similar to discrete auroral arcs, which means
that a satellite crossing is rather rapid and the complete distribution function of
electrons and ions difficult to measure. Peterson and Shelley [1984] also
concluded that polar arcs seem to be connected to regions with similar particle
distributions as in the distant plasma sheet boundary layer based on ion composi­
tion measurements on polar cap field lines. Hoffman et al. [1985] suggested that
the plasma sheet boundary layer was the source of sun-aligned arcs. They reported
no indication of an electrostatic field-aligned acceleration. However, the Viking
observations clearly demonstrate that downward acceleration of the electrons
seems to take place at altitudes below 2 Re. Obara et al. [1988] made
simultaneous observations of sun-aligned polar cap arcs in both hemispheres
using Viking and EXOS-C measurements. They concluded, that the arcs occurred
on closed magnetic field lines, and that they were conjugate.
The ESP5 instrument on board Viking was designed to measure with high
angular resolution and to give complementary information on short-lived (smallscale) phenomena. It was mounted on the opposite side of the spacecraft with
respect to the spectrometer normally used, ESPI. The advantage of this
arrangement is cleary demonstrated at the edges of the auroral structure shown
below where a field aligned flux of electrons can be seen in only one of the
detectors (at ~ 09.56.30 UT).
VIKING Orbit 1169
3
S. OOKEV'
JB
1
1
I
2
2
1I
I IT
M
I I I I rii I
M /W W ^
. 2 2 0 KEV
EV
0
UT 9.48
ILAT 83.7
ALT 9400
I 1
I iMabJllfclmMllill1 I
II I
li
I
ri^lL.JLiTJKiL-
9.55
88.0
8500
10.02
86.5
7600
Data from ESP5 (top panel) at energies around 5 keV and ESPI (two lower
panels) at energies o f 400 eV and 220 eV, respectively, obtained during a
traversal of an auroral arc at high latitude. Viking spent several minutes moving
along the structure. It is not clear that the satellite actually crosses the arc.
11
There is a region with very collimated electron fluxes in the downward current
region (09.58.30-09.59.30 UT), that gives a good example of the need for high
angular resolution measurements.
VIKING Orbit 1169 9.58.56 UT
300
X
3
<D
2
200
pitch angle
It is essential not only to have good resolution but also to cover the full pitch
angle range.
The auroral oval has been characterized as quiet when the magnetic activity in
the nightside is low, i e, when the Ae or Kp indices show low values. There are,
however, different processes that govern the dayside and the nightside and it is
obvious that the activity is increased at high latitudes during “quiet” conditions.
So, if a ground state of the magnetosphere exists we need a new reference [Lundin
et a i, 1991]. The mapping of auroral forms at high latitudes to regions in the
distant magnetosphere has given interesting new knowledge about the magnetic
field topology during different interplanetary magnetic field conditions [see, e.g.,
Elphinstone et al., 1994 and references therein]. However, the controversy still
exists in the definition of polar cap arcs. The Viking data indicate that the plasma
sheet boundary layer is a probable source and that these structures belong to a
poleward edge of an expanded oval.
Many types of wave phenomena are observed during polar arc events, e.g.
auroral hiss, lower hybrid waves, auroral kilometric radiation, and ion cyclotron
waves. The high frequency cutoff of the VLF emissions decreased when Viking
entered this arc region indicating a depletion in the plasma density (G. Holmgren,
personal communication). This conclusion was also verified by Langmuir probe
data showing a decrease of the density with a factor of ten. We have also observed
solitary structures of the type described by, e.g., Boström et al. [1989]. They are
density depletions of up to 50 % and moving upward with velocities of 10s of
km/s. Part of them have a net potential drop accelerating electrons downward and
positive ions upward. They are seen in the event during orbit 1169 most of the
time but with varying amplitudes. Power spectral density plots show well defined
12
peaks close to the hydrogen cyclotron frequency, in this case about 70 Hz. These
wave emissions are observed in the region of upward ion beams and indicate that
we are close to the generation region.
Positive Ion Heating and Precipitation
The Earth’s upper atmosphere is an important source of magnetospheric
positive ions [see, e.g., Chappell et al., 1987]. Energy transfer to atmospheric
particles takes place in the auroral region resulting in ionization, heating, accelera­
tion, and finally erosion of plasma. An important feature of the energization
region is that ions may become accelerated transverse as well as parallel to the
geomagnetic field. Ions heated perpendicular to the magnetic field will appear as
ion conics at higher altitudes due to the geomagnetic mirror force. Such accelera­
tion has been detected at virtually all altitudes above 400 km on auroral field lines
[e.g., Klumpar, 1986 and Chang et al., 1988]. No consensus has been developed
as to the fundamental mechanism for their acceleration. Many different
acceleration mechanisms are in principle possible. These mechanisms include
electrostatic shocks and electric field fluctuations well below the ion gyro
frequencies, broadband waves around the ion gyro frequencies, and waves above
the lower hybrid frequency. Recent sounding rocket measurements have shown
that transverse ion heating can occur in small-scale regions with large amplitude
bursts of monochromatic waves. These waves, above the lower hybrid frequency,
can occur in thin filamentary density cavities oriented along the geomagnetic field
lines [e.g., Vago et al., 1992].
The intensities at small pitch angles for ion beams sometimes show an
appreciable spread in energy, indicating the importance of transverse energization.
In fact the data sometimes show that the perpendicular heating is much higher
than the parallel. Ion conic energies reaching the upper threshold of the Viking
instrument (40 keV) have been observed.
Measurements with the Viking satellite established a relation between lowfrequency, broadband electrostatic noise (LEF) and ion energization [e.g., Lundin
et al., 1990]. Similar events have been found in the Freja data and will be investi­
gated in more detail. Transverse ion energization is often occurring in large-scale
plasma density depletion regions.
Energy dispersion signatures and anti-correlation between electron and ion
precipitation are pronounced effects in the Freja particle data. Another pro­
nounced feature in the F3H data is banded precipitation of positive ions. Ion
“bands” in the central plasma sheet associated with electron inverted-Vs at higher
latitudes have been described, e.g., using data from Dynamics Explorer
[Winningham et al., 1984], The source is most likely extended in longitude and
the ions appear at rather low latitude so the Freja orbit is excellent for the study of
this phenomenon. A wealth of examples have been collected and a detailed
investigation has started.
The Freja hot plasma data set looks very promising and interesting and it will
provide more detailed information on processes such as; heating of ions perpen­
dicular to the magnetic field lines, the source regions of magnetospheric plasma,
ion and electron precipitation, energy and mass dispersion features, plasma
injection in the dayside magnetosphere during magnetic disturbances, dynamics
of the low energy ring current population during magnetic storms and relation of
the hot plasma to the other parameters measured on board Freja or with ground
based instrumentation. We have thus reasons to believe that Freja will be a
13
continued success and contribute actively to an increased understanding of pheno­
mena in the lower part of the main acceleration region for auroral electrons.
Orbit:
FREJA F3H
1547 Date: 93-01-31 Esrange
3.1
h
Olili
2.5
I
'' » ly f . V ÿ W Ä
3 .1
2 .5
\A A A A A A /V W \M A A A A A A ^
^A A A A Ä Ä Ä Ä A M W
H (k .)
20.2«
07.5
70.3
-43.6
1766
09.20.40
07.7
69.8
-4 1 .7
1764
21.00
07.8
69.3
1762
09.21.20
07.9
68.7
-38.0
1761
vW
V ^
.21.40
Mem*
-3 6 .3
Summary plot showing Freja hot plasma data. The three upper panels show
energy-time spectrograms for oxygen, helium and hydrogen ions. The
fourth panel gives the pitch angle of the ion observations. Panel five and six
show electron data and the corresponding pitch angle. The data show
perpendicular, mainly oxygen, ion heating when Freja enters the auroral
oval from the poleward side at 09.20.30 UT and banded proton
precipitation after 09.21 UT. The MATE data indicate anisotropic and
variable electron distributions.
14
Measurement Techniques
The magnetosphere, and especially the auroral zone, can be tremendously rich
in particle and wave-particle interaction phenomena. Some have been described
above. The possibilities to do high resolution measurements have improved
during the era of in situ observations in space. One big challenge has been to
measure the complete particle distribution functions with high enough resolution
to resolve existing anisotropies. A large energy range must be covered and a high
resolution sampling of all components of the velocity vector is needed. It is
obvious, at least from ion observations, that full three-dimensional measurements
with high temporal and spatial resolution are essential. The manufacturing of
space instruments includes also other challenges, such as, minimal weight, size,
power consumption, and cost, but still enough radiation shielding and data output.
Particle distributions contain many characteristic features with information on
remote and local processes that need to be taken into account by theories on
auroral processes. The width of auroral structures inferred from in situ measure­
ments can be completely misinterpreted without knowledge about the angle of
traversal. It is statistically true that the aurora is elongated in the east-west
direction but segments of arcs can have any direction, which, of course, will
influence the conclusions. We have with Viking moved more or less parallel to
some auroral arcs at high latitudes. This gave us good oppurtunities to make
detailed studies of the characteristic features of these arcs. The particle distribu­
tions are often far from isotropic Maxwellians. Sharp gradients are often
observed. There is a possibility that we sometimes misinterpret data due to the
fact that Viking crosses narrow regions too fast, even though we are doing
measurements with high resolution. It is also obvious that a complete pitch angle
coverage is needed because peaks and minima can occur at all pitch angles. A
good pitch angle resolution is also needed, because the intensities can vary by
several order of magnitudes within a couple of degrees close to the magnetic field
direction. This makes estimates of currents and energy flux in the loss cone
doubtful in many cases during traversals of auroral forms.
A short description of the very successful Viking and Freja projects and the hot
plasma experiments on board these spacecraft is given below.
Viking
The orbit and instrumentation of the Swedish satellite Viking was well suited
for studies of phenomena in the high latitude region. Observations were primarilly
done in the acceleration regions at altitudes of 1 - 2 Re. Some of the orbit
parameters of Viking are given below.
Apogee altitude
13530 km
Perigee altitude
817 km
Inclination
98.8°
Orbital period
262 minutes
Spin period
20 seconds in cartwheel mode
Launch date
22 February 1986
15
The piggyback launch with the Spot spacecraft made Viking an extremely
good satellite for studies of dayside and high latitude phenomena, for example, in
die polar cusp and cleft region, and in the region traditionally called the polar cap,
although the prime objective was to study nightside auroral phenomena. The
following experiments are included in the Viking scientific payload:
VI
Electric Field Experiment
L. Block, Royal Institute of
Technology, Stockholm, Sweden
V2
Magnetic Field Experiment
T. Potemra, Applied Physics
Laboratory, Johns Hopkins University,
Laurel, USA
V3
Particle Experiment
R. Lundin, Swedish Institute of Space
Physics, Kiruna, Sweden
V4L
V4H
V5
Low-Frequency Wave
G. Gustafsson, Swedish Institute of
Experiment
Space Physics, Uppsala Division,
Sweden
High-Frequency Wave
A. Bahnsen, Danish Space Research
Experiment
Institute, Lyngby, Denmark
Auroral UV-Imaging
C. Anger /J.S. Murphree, University of
Calgary, Canada
Experiment
The Viking particle experiment, V3, included seven sensor units and two data
processing units. Six of the sensor units were built, tested, and calibrated at the
Swedish Institute of Space Physics in Kiruna.
Two units measured electrons. V3-1 contains three electron spectrometers,
ESPI-3, that use a toroidal electrostatic analyzer as energy filter. The energy
range 10 eV-40 keV was sampled in 32, 64, or 128 energy steps in 0.15, 0.3, or
0.6 s, respectively. The viewing direction of ESP1-3 are 90°, 70°, and 110°
relative to the satellite spin axis. The viewing directions were chosen so that the
magnetic field line direction was covered during most of the time when Viking
was in the auroral region. A schematic of theV3-l measurement principle is
shown below.V3-2 contains two spectrometers, ESP4-5 that use a magnetic
deflection system covering the energy range 0.1-200 keV in sixteen energy bands
(eight in each spectrometer, ESP4 covering the lower energies) with a relative
bandwidth of 0.5-1.0 and an angular resolution of about 2 x 2°. All energy levels
were sampled every 70 ms. The electrons are detected by a rectangular
microchannel plate. This type of detector provides simultaneous measurements of
several electron energies with very high angular resolution (2°) but with less good
energy resolution. The ESP 1 detector measured with higher energy resolution but
with not as high temporal and angular resolution.
16
Bild borttagen – se tryckt version
Image removed – see printed version
Schematic o f the V3-1 instrument [Sandahl et al,. 1985]
Positive ions were measured with 4 units, two with mass separation and two
without. V3-3 contains two positive ion spectrometers that cover the energy
ranges 40 eV-1.2 keV (PISP2) and 1.2-40 keV (PISP1), respectively. V3-4 is a
sectorized (8 sectors) positive ion spectrometer, SECPISP, that uses two toroidal
electrostatic analyzers with a total deflection of 180°. SECPISP is equipped with
two separate high voltage supplies for the energy ranges, 0.5-450 eV and 10 eV12 keV, respectively. The ion composition spectrometers contain toroidal
electrostatic analyzers and straight Wien velocity filters (crossed field analyzers).
V3-5 includes two spectrometers, ICS1-2 that cover the energy ranges 0.05-0.8
keV and 1.2-20 keV, respectively. V3-6 (ICS3) uses the same measurement
technique as ICS 1-2 but covers the energy range 0.001-70 keV.
V3-7 is a time-of-flight mass spectrometer for composition measurements of
positive ions in the energy range 10 keV- 10 MeV. The V3-7 instrument was not
manufactured in Kiruna.
Analysis programs were developed with various topics in mind. We attempted
to cover many aspects of the analysis in order to facilitate joint studies with other
Viking experimenters. Data summary files, DSF, were processed at Esrange. The
V3 experiment provided 16 point electron energy spectra, 16 point ion energy
spectra, and electron energy flux/number flux. The quick-look plots, QLP, utilized
data from the DSFs and are used for qualitative analysis. The V3 quick-look data
included: ion and electron energy-time spectrogram plots for: 0-30°, 75°-105°,
and 150°-180° pitch angle, electron/ion fluxes and mean energies versus time.
Data for scientific analysis are processed from the raw data tapes. Color-coded
energy-time spectrogram plots in a microfiche type format were distributed to co­
investigators. Other analysis programs produced time series of countrates/fluxes,
17
spectral plots of ions and electrons (flux, density etc.), pitch angle distributions of
electrons and ions, contour plots in velocity space or in energy/pitch angle,
computed moments of the distribution function, 3-D ion flow parameters, and ion
composition data. A large number of scientists from many countries have been
involved in the analysis of Viking hot plasma data.
Bild borttagen – se tryckt version
Image removed – see printed version
Schematic o f the V3-2 instrument [Sandahl et al., 1985].
F reja
The Freja spacecraft was launched on October 6, 1992 at 6.20 UT, from the
Jiuquan Satellite Launch Center located at the Gobi desert in north-western China
with a Long March 2C vehicle. The orbit, with a perigee at 600 km, apogee at
1755 km and 63° inclination, is highly suitable for studies of a number of pheno­
mena in the auroral region. The inclination gives an orbit where the apogee can be
considered as fixed at the same latitude. This has some advantages when compar­
ing data from different local times and disturbance levels, A close to parallel
traversal of a part of the oval gives longer observation times and new perspectives
of some auroral structures than is obtained with higher inclination orbits. Freja
will not reach the polar cap during all orbits. There are four charged particle
instruments on board Freja: MATE, TESP, TICS, and CPA. The electron spectro­
meter MATE and the ion composition spectrometer TICS are included in the F3H
experiment, the cold plasma analyzer CPA in F3C, and the electron spectrometer
TESP in F7. The table below summarizes the experiments on board Freja.
18
FI
Electric Field Experiment
G. Marklund, Royal Institute of
Technology, Stockholm, Sweden
F2
Magnetic Field Experiment
L. Zanetti, Applied Physics Laboratory,
Johns Hopkins University, Laurel, USA
F3C
Cold Plasma Analyzer
B. Whalen, National Research Council
of Canada, Ottawa, Canada
F3H
Hot Plasma Experiment
L. Eliasson, Swedish Institute of Space
Physics, Kiruna, Sweden
F4
Wave Experiment
B. Holback, Swedish Institute of Space
Physics, Uppsala Division, Sweden
F5
Ultraviolet Imager
J.S. Murphree, Department of Physics
and Astronomy, University of Calgary,
Canada
F6
Electron Beam Experiment
G. Paschmann, Max-Planck-Institut für
extraterrestrische Physik, Garching,
Germany
F7
Correlator Experiment
M. Boehm, Max-Planck-Institut für
extraterrestrische Physik, Garching,
Germany
The Hot Plasma Experiment, F3H, on board Freja is designed to measure
auroral particle distribution functions with very high temporal and spatial resolu­
tion. The experiment consists of three different units; an electron spectrometer
designed to measure angular and energy distributions simultaneously, a twodimensional positive ion spectrometer using the spacecraft spin for threedimensional measurements, and a data processing unit. The main scientific
objective is to study positive ion heating perpendicular to the magnetic field lines
in the auroral region. Electron distributions in the energy range 0.1-100 keV and
positive ion distributions (with mass identification) in the energy range 0.5 eV - 5
keV (can be extended to 15 keV) are measured. A two Mbyte on-board memory is
used for intermediate data storage. Most of the hardware was designed and
manufactured at the Swedish Institute of Space Physics in Kiruna. The Finnish
Meteorological Institute in Helsinki has contributed to the DPU hardware. The
design of the spectrometers was checked with computer simulations of particle
trajectories in electric and magnetic fields and the instruments were tested and
calibrated at the Swedish Institute of Space Physics in Kiruna. A variety of tests to
verify the function of different parts of the instrument and the thermal properties
were performed as well as, e.g., calibrations with electron and ion sources, and
micro channel plate response functions.
19
The Freja Hot Plasma Experiment (F3H) consists of three units: an ion
composition spectrometer (TICS), an electron spectrometer (MATE),
and a data processing and experiment control unit (DPU).
The main objective with the magnetic imaging two-dimensional electron
spectrometer, MATE, is to measure the angular and energy distributions of
electrons with high temporal and spatial resolution. Previous missions have shown
that it is necessary to measure the electron distributions with high resolution, not
only in time and space, but also in energy and pitch angle. This is extremely
difficult with the measurement techniques presently available. A combination,
MATE and TESP, of different techniques is used on Freja. The TESP spectro­
meter complements MATE by measuring the energy spectrum with higher
resolution. MATE uses a 360° field of view sector magnet (permanent magnetic
field with a strength of 0.035 T) energy analyzer with 90° deflection angle for
simultaneous energy and pitch angle measurements. Only = 140° field of view is,
however, used on Freja. This gives almost full pitch angle and energy coverage
due to a high sampling rate if the spin axis is oriented in a favorable direction with
respect to die local magnetic field. A schematic of the instrument is shown below.
Schematic o f the MATE instrument showing the entrance apertures,
magnets, collimators and MCP package.
20
The sampling period of the full energy range is 10 ms, corresponding to a
spatial resolution of less than 100 m. A collimator system enables measurements
of the energy spectrum at 16 different energies with a reasonably high resolution,
ÀE/E ~ 30 %. Grids are used to prevent the cold plasma from entering into the
instrument. The spectrometer unit contains high voltage supplies for the
microchannel plate (MCP) assembly, detector logics, and registers for the energyangle matrix. MATE produces more data than can be transmitted by the telemetry
system in real time so the data output must normally be reduced. A summary of
the MATE characteristics is given below.
Energy range
O.lkeV-lOOkeV
Angular sectors
30
Energy levels
16
Field of view/sensor head
2° X 10°
Energy resolution
= 30 FWHM
Minimum sampling time
10 ms/energy-angle matrix
Maximum data rate
400 kbits/s (no data compression)
Normal data rate
= 20 kbits/s
Mass
2.84 kg
Power
3.7 W
Conversion factor/sector
IO-7 (cm2 sr keV/keV)
The three-dimensional ion composition spectrometer, TICS, measures the
“hot” magnetospheric and “cold” ionospheric ion distributions. The
heating/acceleration of ions perpendicular to the magnetic field lines is one of the
main scientific objectives. Measurements close to 90° pitch angle are therefore
very important. We have decided to primarily concentrate on lower energies, that
is below 5 keV. The major ion species are covered by the instrument. Data
reduction must be used to adjust to the allocated data rate. TICS measures
perpendicular to the spin plane thus giving 3-D measurements every 3 seconds.
The Freja spin period is close to 6 seconds.
TICS consists of a spherical “top hat” electrostatic analyzer with 360° field of
view followed by a cylindrical sector magnet momentum analyzer. The outer plate
of the electrostatic analyzer can be swept from 0 to -10 V for measurements of
low energy ions (0-60 eV) and the inner plate from 0 V to about 1.5 kV, which
covers the energy range 60 eV to 5 keV. This was changed after a couple of
months of operation. Now only the inner plate is swept covering energies from 0.5
eV to 5 keV. The dwell time on each energy level is 10 ms and 16 or 32 energy
steps are transmitted to ground. This means that one sweep of 16 energy levels
takes 160 ms plus an extra time interval of 40 ms that is used for adjusting high
voltages and data handling. The radius of the spherical analyzer is 45 mm and the
distance between the plates 3.5 mm.
21
Electrostatic
analyser
Magnets
MCPs
Sectorized anode
Schematic of the TICS spectrometer. The different ion masses hit the
MCP at different radial distances.
A summary of the TICS characteristics is given below.
Energy range
0.5 - 5000 eV/q
Mass range
1 -4 0 Amu/q
Angular sectors
30
Energy levels
32 (or 16)
Energy resolution
10 % FWHM
Field of view/sensor head
5x10°
Geometric factor
5 X 10'6 (cm sr keV/keV) per 11° opening
Time resolution
0.5 spin period (=3 s)
Maximum data rate
800 kbits/s (no data compression)
Normal data rate
= 20 kbits/s
Mass
3.55 kg
Power
4.0 W
22
A post-acceleration voltage is used to obtain the required mass range and
resolution. Four different post-acceleration levels can be used: 0, 300, 2500 and
4000 V. A low post-acceleration gives an opportunity to resolve the mass of
heavy ions but does not allow, the lightest ions to be measured. The circular
magnetic field strength is 0.13 T obtained with permanent magnets.
The particle “imaging” detector ( 360° field of view) is based on a large
diameter (10 cm) microchannel plate assembly with position sensitive anode. It
provides simultaneously the azimuth and mass per unit charge of the incident
ions. The anode system can resolve 1024 points and the logics can identify pulses
up to a frequency of 1 MHz. The mass detection technique provides the
possibility to measure ions within a broad mass range. The instrument can be
operated in several different modes.
The data processing and power conversion unit, DPU, comprises two 16 bitprocessors (Texas 9989), 2.25 Mbyte FIFO memory, telemetry interface, and the
experiment low-voltage power converters. The weight of the DPU is 2.83 kg and
it has a power consumption of about 10 W (a large fraction is DC/DC converter
losses). The DPU is used to control and command the experiment, as interface to
the Freja telemetry and power systems, for on-board data handling, and
processing of scientific data (e.g., computations of moments of the electron
distributions).
The ion composition spectrometer was successfully deployed on 14 October
1992, but MATE was stuck in the stowed position reducing the field of view from
360° to 140°. The experiment was switched on for the first time with nominal
high voltages on 20 October. Data are collected at two ground stations in the
northern hemisphere, Esrange in Sweden and Prince Albert in Canada. Some data
are also transmitted to a ground station at Syowa in the southern hemisphere. Two
major types of commands are used to operate the experiment; hardware decoded
commands to switch the +28 V on/off, (deployment on/off) and either of the two
microprocessors off, and normal commands that are used for status check, high
voltage adjustments, high voltage on/off, switching between different operational
modes, changing levels of deflection voltages, selecting mass calibration sectors,
changing processor programming etc.
A limited number of modes are used for the normal operation of the
instrument. In the normal modes a variety of combinations between electron and
ion data measurements are possible by loading programs to the on-board
computers. A calibration mode can be used to identify the mass response of the
TICS sensor. In this mode of operation all data from the TICS sensor are
transmitted to the ground. In the normal mode of operation, data are collected
from both the MATE and TICS sensor units. The highest possible time resolution
is 10 ms. During that time the complete data set from the spectrographs is
transferred to the DPU. Data are then normally integrated, or reduced, to meet the
size of the telemetry rate occupied by the experiment. An event mode can be
initiated by a triggering signal given by the Freja system. The Hot Plasma
Experiment can not generate that type of signal but some of the other experiments
have incorporated that possibility. A pre-programmed command is sent to the
experiment if a trig signal is received. F3H is then normally operated in one of the
pre-defined burst modes. High time resolution data from the MATE and TICS
sensors are stored in the DPU memory and later transmitted to ground. Only a
slight reduction of the data is made in the burst mode. The DPU memory is
normally used to store data during approximately 30 seconds in burst mode. It is
23
also possible to operate the instrument in an orbit summary mode where data are
collected with low resolution for a complete or large part of a Freja orbit.
Raw data from the experiment are processed in Kiruna. Freja summary plots
(FSP) and special color summary plots (CSP) are available in an on-line data
base. This data base has also been copied to several of the co-investigators and is
regularly updated when more data have been collected with the experiment. The
CSPs are produced with two time scales, one covering a whole pass (30 minute
plots) and the other giving information with a higher time resolution (two minute
plots). They contain three panels showing energy-time spectrograms for oxygen,
helium, and hydrogen ions. The fourth panel gives the pitch angle of the ion
observations, panel five and six show electron data from MATE and the
corresponding pitch angle. Also TESP data are plotted in the regular CSPs.
More details on the Freja and Viking projects can be found in, e.g.:
Viking Scientific Aspects (the “Blue Book”), ed. K. Fredga, 1981.
The Viking Program, EOS 67,42,1986.
Viking and the Aurora, Geophys. Res. Lett., 14, 379-478,1987.
Scientific Results from the Swedish Viking Satellite: A 1988 Status Report,
B. Hultqvist, IRF Scientific Report 196,1988.
Viking Investigations of High-Latitude Plasma Processes, J. Geophys. R es.,
95, 5749-6131,1990.
- The Freja Scientific Satellite, ed. M. André, IRF Scientific Report 214,1993.
- The Freja Satellite Mission, EOS, 74, 29,1993.
Freja Special Issue, Geophys. Res. Lett., in press, 1994.
A series of papers with descriptions of the Freja experiments have been
submitted to the Space Science Reviews.
24
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observation of the high-latitude auroral activity on January 11, 1983, J. Geophys. Res., 91,
339, 1986.
Heikkila, J. D., Penetration of particles into the polar cap and auroral regions, in “Critical
Problems o f Magnetospheric Physics,” Ed. E. R. Dyer, IUCSTP Secretariat, Washington,
D.C., 1972.
25
Hoffman, R. A., and D. S. Evans, Field-aligned electron bursts at high latitude observed by OGO4, J. Geophys. Res., 73, 6201, 1968.
Hoffman, R. A., R. A. Heelis, and J. S. Prasad, A sun-aligned arc observed by DMSP and AE-C,
J. Geophys. Res., 90, 9697, 1985.
Hultqvist, B., R. Lundin, K. Stasiewicz, L. Block, P.-A. Lindqvist, G. Gustafsson, H. Koskinen,
A. Bahnsen, T. A. Potemra, and L. J. Zanetti, Simultaneous observation of upward moving
field-aligned energetic electrons and ions on auroral zone field lines, J. Geophys. Res., 93,
9765, 1988.
Ismail, S., and C.-I. Meng, A classification of polar cap auroral arcs, Planet. Space Sci., 30, 319,
1982.
Klumpar, D. M., A digest and comprehensive bibliography on transverse auroral acceleration, in
Ion Acceleration in the Magnetosphere and Ionosphere, Ed. T. Chang, AGU, Washington, D.
C., 389, 1986.
Knight, L., Parallel electric fields, Planet. Space Sci., 21, 741,1973.
Lassen, K., Polar cap emissions, Danish Meteorological Institute Geophysical Papers, 1968.
Lennartsson, W., On the magnetic mirroring as the basic cause of parallel electric fields, J.
Geophys. Res., 81, 5583, 1976.
Lundin, R., G. Gustafsson, A. I. Eriksson, and G. Marklund, On the importance of high-altitude
low-frequency electric fluctuations for the escape of ionospheric ions, J. Geophys. Res., 95,
5905, 1990.
Lundin R., L. Eliasson, and J. S. Muiphree, The quiet time aurora and the magnetospheric
configuration, in “Auroral Physics," Eds. C.-I. Meng, M.J. Rycroft, and L.A. Frank,
Cambridge UP, 177,1991.
Lysak, R. L„ Generalized model of the ionospheric Alfvén resonator, in “Auroral Plasma
Dynamics,” Ed. R. L. Lysak, Geophysical Monograph 80 American Geophysical Union, 121,
1993.
Mcllwain, C., Direct measurements of particles producing visible auroras, J. Geophys. Res., 65,
2727, 1960.
Meng, C.-I., Polar cap arcs and the plasma sheet,Geophys. Res. Lett., 8, 273, 1981.
Menietti, J. D„ and J. L. Burch, “Electron conic” signatures observed in the nightside auroral zone
and over the polar cup, J. Geophys. Res., 90, 5345, 1985.
Mishin, V. M., A. D. Bazarzhapov, T. I. Saifudinova, S. B. Lunyushkin, D. Sh. Shirapov, J.
Woch, L. Eliasson, H. Opgenoorth, and J. S. Murphree, Different methods to determine the
polar cap area, J. Geomag. Geoelectr., 44,1207,1992.
Murphree, J. S., C. D. Anger, and L. L. Cogger, The instantaneous relationship between polar cap
and oval auroras at time of northward interplanetary magnetic field, Can. J. Phys., 60, 349,
1982.
Ness, N. F., The earth's magnetic tail, J. Geophys. Res., 70, 2989, 1965.
Obara, T., M. Kitayama, T. Mukai, N. Kaya, J. S. Murphree, and L. L. Cogger, Simultaneous
observations of sun-aligned polar cap arcs in both hemispheres by Exos-C and Viking,
Geophys. Res. Lett., 15, 713, 1988.
Peterson, W. K., and E. G. Shelley, Origin of the plasma in a cross-polar cap auroral feature (theta
aurora), J. Geophys. Res., 89, 6729, 1984.
Piddington, J. H., Geomagnetic storm theory, J. Geophys. Res., 65, 93, 1960.
Reiff, P. H., H. L. Collin, J. D. Craven, J. L. Burch, J. D. Winningham, E. G. Shelley, L. A.
Frank, and M. A. Friedman, Determination of auroral electrostatic potentials using high- and
low-altitude particle distributions, J. Geophys. Res., 93, 7441, 1988.
Sandahl, I., R. Lundin, and L. Eliasson, The hot plasma spectrometers on Viking, KGI Technical
Report 035, 1985.
Snyder, A. L., and S.-I. Akasofu, Major auroral substorm features in the dark sector observed by a
USAF DMSP satellite, Planet. Space Sci., 22, 1511, 1974.
Starkov, G. V., Auroral heights in the polar cap, Geomag. Aeronomy, 36,1968.
Stone, E. C., Local time dependence of non-Störmer cutoff for 1.5-MeV protons in quiet
geomagnetic field, J. Geophys. Res., 69, 3577, 1964.
Troshichev, O. A., and A. Nishida, Pattern of electron and ion precipitation in the northern and
southern polar regions for northward interplanetary magnetic field conditions, J. Geophys. Res.,
97, 8337, 1992.
Vago, J. L., P. M. Kintner, S. W. Chesney, R. L. Amoldy, K. A. Lynch, T. E. Moore, and C. J.
Pollock, Transverse ion acceleration by localized lower hybrid waves in the topside auroral
ionosphere, J. Geophys. Res., 91, 16.935, 1992.
26
Winningham, J. D., and W. J. Heikkila, Polar cap auroral electron fluxes observed with ISIS 1, J.
Geophys. Res., 79, 949, 1974.
W inningham, J. D., F. Yasuhara, S.-I. Akasofu, and W. J. Heikkila, The latitudinal morphology
of 10-eV to 10-keV electron fluxes during magnetically quiet and disturbed times in the 21000300 MLT sector, /. Geophys. Res., 80 3148, 1975.
W inningham, J. D „ J. L. Burch, and R. A. Frahm, Bands of ions and angular V’s: A conjugate
manifestation of ionospheric ion acceleration, J. Geophys. Res., 89, 1749,1984.
Zanetti, L. J., T. A. Potemra, T. Iijima, W. Baumjohann, and P. F. Bythrow, Ionospheric and
Birkeland current distributions for northward interplanetary magnetic field: Inferred polar
convection, J. Geophys. Res., 89, 7453, 1984.
P ublications
The understanding of phenomena in the auroral region requires a broad
international collaboration and the use of different measurement techniques. It has
been a great pleasure for me to work together with colleagues in this field during
several years. The list of publications below is one proof that we have been fairly
successful. I will take the opportunity to thank all good friends and hope that we
will have as good co-operation in the future. It has been very stimulating to
participate in sounding rocket campaigns and the analysis of data from the Viking
and Freja satellites. Reports from most of the studies that I have participated in are
listed below:
Sounding rockets and ESRO 4
Eliasson, L„ L.-Å. Holmgren, K. Rönnmark, Pitch angle and energy distributions of auroral
electrons measured by the ESRO 4 satellite, Planet. Space Sci., 27, 87-97,1979.
Sandahl, I., L. Eliasson, and R. Lundin, Electron spectra over discrete auroras as measured by the
Substorm-GEOS rockets, Proceedings of the Vth ESA-PAC Symposium on European Rocket
and Balloon Programmes and Related Research, ESA SP-152,257-262,1980.
Eliasson, L., R. Lundin, and I. Sandahl, Some results from the particle experiment on board the
Barium-GEOS rocket, Proceedings of the 5th ESA-PAC Symposium on European Rocket and
Balloon, Programmes and Related Research, ESA SP-152,263-268,1980.
Sandahl, I., L. Eliasson, and R. Lundin, Rocket observations of precipitating electrons over a
pulsating aurora, Geophys. Res. Lett. 7, 309-312, 1980.
Opgenoorth, H., R. Pellinen, W. Baumjohann, E. Nielsen, G. Marklund, and L. Eliasson, Threedimensional current flow and particle precipitation in a westward travelling surge (observed
during the Barium-GEOS rocket experiment), J. Geophys. Res., 88, 3138-3152,1983.
Eliasson, L., I. Sandahl, and R., Lundin, Observations of auroral electrons with sounding rockets
and the GEOS 2 geostationary satellite, in “Results of the ARCAD 3 Project and of Recent
Programmes in Magnetospheric Physics Toulouse 84,” 797-802, 1985.
Sandahl, I., and L. Eliasson, Scattering of electrons in the equatorial plane during a pulsating
aurora, Proceedings of the 7th ESA-PAC Symposium on European Rocket and Balloon
Programmes and Related Research, ESA SP-229,103-109,1985.
Holmgren, G., G. Marklund, L. Eliasson, H. Opgenoorth, F. Söraas, F. Primdahl, G. Haerendel,
and P.M. Kintner, Ionospheric response to chemical releases in the high latitude E and F
regions, Adv. Space Res., 8, no 1, (1)79-(1)83, 1988.
Eliasson, L., R. Lundin, and G. Holmgren, Energetic electron enhancements due to the TOR
chemical release, Adv. Space Res., 8, no 1, (1)93-(1)97,1988.
Sandahl, I., L. Eliasson, and B. Holback, Investigation of precipitating electrons during an auroral
breakup, Physica Scripta, 37, 506-511, 1988.
Viking
Sandahl, I., R. Lundin, and L. Eliasson, The hot plasma spectrometers on Viking, KGI Technical
Report 035, 1985.
Eliasson, L., R. Lundin, and J. S. Murphree, Polar cap arcs observed by the Viking satellite,
Geophys. Res. Lett., 14, 451-454, 1987.
Lundin, R., L. Eliasson, B. Hultqvist, and K. Stasiewicz, Plasma energization on amoral field
lines as observed by the Viking spacecraft, Geophys. Res. Lett., 14,443-446,1987.
27
Eliasson, L., Characteristics of particle acceleration at high latitude, Proceedings of the 8th ESA
Symposium on European Rocket and Balloon Programmes and Related Research, ESA SP-270,
51-54, 1987.
Eliasson, L., and R. Lundin, Acceleration/heating processes on auroral field lines as observed by
the Viking spacecraft, Proceedings of the 21stESLAB Symposium, ESA SP-275, 87-91,1987.
Lundin, R., L. Eliasson, and I. Sandahl, First VIKING results: Hot plasma, Physica Scripta, 37,
482-490, 1988.
Pottelette, R., M. Malingre, A. Bahnsen, L. Eliasson, K. Staciewicz, R. E. Erlandson, and G.
Marklund, Viking observations of bursts of intense broadband noise in the source regions of
auroral kilometric radiation, Annales Geophysicae, 6, (5), 573-586,1988.
Bahnsen, A., B. M. Pedersen, M. Jespersen, E. Ungstrup, L. Eliasson, J. S. Murphree, R. D.
Elphinstone, L. Blomberg, G. Holmgren, and LJ . Zanetti, Viking observations at the source
region of auroral kilometric radiation, J. Geophys. Res., 94,6643-6654, 1989.
Kirkwood, S., L. Eliasson, H. Opgenoorth, and A. Pellinen-Wannberg, A study of .auroral electron
acceleration using the EISCAT radar and the Viking satellite, Adv. Space Res., 9,(5)49-(5)52,
1989.
Kirkwood, S., L. Eliasson, I. Häggström, and P. N. Collis, Enhanced electron density layers in
the high-latitude lower ionosphere, Proceedings of the 9th ESA Symposium on European
Rocket and Balloon Programmes and Related Research, ESA SP-291, 35-40,1989.
Opgenoorth, H. J., B. Bromage, D. Fontaine, C. La Hoz, A. Huuskonen, H. Kohl, U.-P.
Lpvhaug, G. Wannberg, G. Gustafsson, J. S. Murphree, L. Eliasson, G. Marklund, T. A.
Potemra, S. Kirkwood, E. Nielsen, and J.-E. Wahlund, Coordinated observations with EISCAT
and the Viking satellite - The decay of a westward travelling surge, Annales Geophysicae, 7,
N°5, 479-500, 1989.
Khrushchinskiy, A. A., A. A. Ostapenko, G. Gustafsson, L. Eliasson, and I. Sandahl, An
estimate of the position of the source of microbursts in the streams of precipitating electrons
from the Viking satellite data, Geomagnetism and Aeronomy, 29, No 6, 886-888,1989.
Kirkwood, S., and L. Eliasson, Energetic particle precipitation in the substorm growth phase
measured by EISCAT and Viking, J. Geophys. Res., 95, 6025-6038, 1990.
Brüning, K., L. P. Block, G. T. Marklund, L. Eliasson, R. Pottelette, J.S. Murphree, T.A.
Potemra, and S. Perraut, Viking observations above a postnoon aurora, J. Geophys. Res., 95,
6039-6050, 1990.
Sandahl, I., L. Eliasson, A. Pellinen-Wannberg, G. Rostoker, L. P. Block, R. E. Erlandson, E.
Friis-Christensen, B. Jacobsen, H. Lühr, and J. S. Murphree, Distribution of auroral
precipitation at midnight during a magnetic storm, J. Geophys. Res., 95, 6051-6072, 1990.
Lilensten, J., D. Fontaine, W. Kofman., L. Eliasson, C. Lathuillere, and E. S. Oran, Electron
energy budget in the high-latitude ionosphere during Viking/EISCAT coordinated measurements,
J. Geophvs. Res., 95, 6081-6092, 1990.
Lundin, R., and L. Eliasson, Auroràl energization processes, Ann. Geophysicae, 9, 202-223,
1991.
Lundin R., L. Eliasson, and J. S. Murphree, The quiet time aurora and the magnetospheric
configuration, in Auroral Physics eds. C.-I. Meng, M.J. Rycroft, and L.A. Frank, Cambridge
UP, 177-193, 1991.
André, M., and L. Eliasson, Electron acceleration by low frequency electric field fluctuations:
electron conics, Geophys. Res. Lett., 19, 1073-1076, 1992.
Pedersen, B. M., R. Pottelette, L. Eliasson, J. S. Murphree, R. D. Elphinstone, A. Bahnsen, and
M. Jespersen, Auroral kilometric radiation from transpolar arcs, J. Geophys. Res., 97, 1056710573, 1992.
Mishin, V., J. Woch, L. Eliasson, T. Saifudinova, A. Bazarzhapov, D. Shirapov, and S.
Lunyushkin, Substorm scenario with two active phases: A study of CDAW-9C events, in
Proceedings of the International Conference on Substorms (ICS-1) ESA-SP335, 383-389,1992.
Mishin, V. M., A. D. Bazarzhapov, T. I. Saifudinova, S. B. Lunyushkin, D. Sh. Shirapov, J.
Woch, L. Eliasson, H. Opgenoorth, and J.S. Murphree, Different methods to determine the
polar cap area, J. Geomag. Geoelectr., 44,1207-1214, 1992.
André, M., and L. Eliasson, Some electron conic generation mechanisms, in Proceedings of AGU
Chapman Conference on Micro- and Mesoscale Phenomena in Space Plasmas, Kauai, in press,
1994.
Menietti, J. D., D. R. Weimer, M. André, and L. Eliasson, DE-1 and Viking observations
associated with electron conical distributions, accepted for publication in J. Geophys. Res.,
1994.
28
Eliasson, L., M. André, R. Lundin, R. Pottelette, G. Marklund, and G. Holmgren, Observations
of electron conics by the Viking satellite., submitted to J. Geophys. Res., 1994.
Freja studies
Norberg, O., and L. Eliasson, The hot plasma spectrometers on Freja, Proceedings of the 10th
ESA Symposium on European Rocket and Balloon Programmes and Related Research, ESA
SP-317, 41-44, 1991.
Eliasson, L., F3H Hot Plasma Experiment, in The Freja Scientific Satellite, editor M. André, IRF
Scientific Report 214, 1993.
André, M., P. Norqvist, A. Vaivads, L. Eliasson, O. Norberg, A. Eriksson, and B. Holback,
Transverse ion energization and wave emissions observed by the Freja satellite, Geophys. Res.
Lett., in press, 1994.
Boehm, M. H., G. Paschmann, J. Clemmons, G. Haerendel, L. Eliasson, and R. Lundin, Freja
observations of narrow inverted-V electron precipitation by the two-dimensional electron
spectrometer, Geophys. Res. Lett., in press, 1994.
Eliasson, L., M. André, A. Eriksson, P. Norqvist, O. Norberg, R. Lundin, B. Holback, H.
Koskinen, H. Borg, and M. Boehm, Freja observations of heating and precipitation of positive
ions, Geophys. Res. Lett., in press, 1994.
Eriksson, A.I., B. Holback, P.-O. Dovner, R. Boström, G. Holmgren, M. André, L. Eliasson, and
P.M. Kintner, Freja observations of correlated small-scale density depletions and enhanced lower
hybrid waves, Geophys. Res. Lett., in press, 1994.
Erlandson, R. E., L. J. Zanetti, M.H. Acuna, A. Eriksson, L. Eliasson, M. H. Boehm, and L.
Blomberg, Freja observations of electromagnetic ion cyclotron (ELF) waves and transverse
oxygen ion acceleration on auroral field lines, Geophys. Res. Lett., in press, 1994.
Klumpar, D. M., L. Eliasson, O. Norberg, Freja observations of ion dispersion precipitation
signatures near the inner edge of the plasma sheet, submitted to Geophys. Res. Lett., 1993.
Lundin, R, L. Eliasson, G. Haerendel, M. Boehm, and B. Holback, Large-scale plasma density
cavities observed by Freja, Geophys. Res. Lett., in press, 1994.
Nilsson, H., S. Kirkwood, L. Eliasson, and O. Norberg, The ionospheric signature of the cusp A case study using Freja and the Sondrestrom radar, Geophys. Res. Lett., in press, 1994.
Norberg, O., M. Yamauchi, L. Eliasson, and R. Lundin, Freja observations of multiple injection
events in the cusp, Geophys. Res. Lett., in press, 1994.
Stasiewicz, G. Gustafsson, G. Holmgren, B. Holback, L. Eliasson, P. M. Kintner, and L. Zanetti,
Wave environment inside inverted-V electron events observed by Freja, Geophys. Res. Lett., in
press, 1994.
Lundin, R., L. Eliasson, O. Norberg, G. Marklund, L. Zanetti, B. Whalen, B. Holback, J.S.
Murphree, G. Haerendel, M. Boehm, and G. Paschmann, First high resolution measurements by
the Freja satellite, accepted for publication in Proceedings from the Yosemite Meeting, 1994.
Boehm, M., G.Paschmann, J. Clemmons, H. Hoefner, R. Frenzel, M. Erti, G. Haerendel, P. Hill,
H. Lauche, L. Eliasson, R. Lundin, The TESP electron spectrometer and correlator (F7) on
Freja, accepted for publication in Space Sei. Rev., 1994.
Eliasson, L., O. Norberg, R. Lundin, K. Lundin, S. Olsen, H. Borg, M. André, H. Koskinen, P.
Riihelä, M. Boehm, and B. Whalen, The Freja Hot Plasma Experiment - Instrument and first
results, accepted for publication in Space Sei. Rev., 1994.
Whalen, B. A., D. J. Knudsen, A. W. Yau, A. M. Pilon, T. A. Cameron, J.F . Sebesta, D.J .
McEwen, J. A. Koehler, N. D. Lloyd, G. Pocobelli, J. G. Laframboise, W. Li, R. Lundin, L.
Eliasson, S. Watanabe, G. S. Cambell, The Freja F3C Cold Plasma Analyzer, accepted for
publication in Space Sei. Rev., 1994.
A cknow ledgem ents
There are many that have made significant contributions in different ways to this
work. I wish to thank you all for assistance and making my time at the Institute
exciting, stimulating, and interesting.
Kjell Rönnmark introduced me to the activities at the Institute a long time ago.
From Lars-Åke Holmgren and Hans Borg I learnt how satellite experiments behave
in real life. Thank you for all support during my time in Umeå and after I moved to
Kiruna.
29
It is a pleasure to acknowledge the support that I have received from Rickard
Lundin and Ingrid Sandahl. Their experience in sounding rocket as well as satellite
project activities has been of outmost importance for this work.
Kjell Lundin has been the technical manager for most of the projects that I have
been involved in. I highly appreciate his very dedicated work, always with tight
schedules and impossible demands. The experienced and skillful engineering and
technical staff at IRF, headed by Sven Olsen and Kjell Lundin, have made it
possible for us to receive excellent scientific data. It was a hard time to meet the
deadlines for delivery of the Freja plasma instrument, special thanks to the above
mentioned and Joakim Gimholt, Jonas Olsen, and Pekka Riihelä and of course,
Olle Norberg who made very important contributions to the Freja project.
Programmers are essential for the success of a project. I still do not understand
how Hans Borg managed to get the Freja microprocessors operational Many others
have contributed with software for data analysis. Arne Moström has done most of it
for the results presented here.
The co-operation and many hours of stimulating discussions with Mats André
have been very valuable. This isirue also for the recent co-operation with Anders
Eriksson. Bengt Hultqvist has always been encouraging and willing to share his
experiences. Many thanks also to Joachim Woch, Gunnar Holmgren, Bengt
Holback, Georg Gustafsson, Göran Marklund, Hannu Koskinen and the Viking
and Freja Science and Engineering Teams for numerous discussions on essential
questions about space physics and life on earth.
The Viking and Freja projects have been managed by the Swedish Space
Corporation under contract from the Swedish National Space Board. I wish to
thank SSC staff at Esrange and in Solna for excellent support during the many
projects that I have participated in. This research was financially supported by the
Swedish National Space Board.
Special thanks also to Birgitta Määttä, Stig Björklund, Torbjörn Lövgren and the
administrative staff at IRF.
Most important of all for this work is my wife Maj-Lis. She has spent many
lonely evenings during instrument calibrations, sounding rocket and satellite
campaigns, conferences, etc., throughout the years. This would not have become a
reality without her support and patience. Finally I am highly indepted to Johan,
Erik, and David for not complaining too much about extra work when I have been
away from home.
30
Summary of Papers
Paper 1 “Pitch Angle and Energy Distributions of Auroral Electrons Measured
by the ESRO 4 Satellite” is based on data from particle instruments on board the
ESRO 4 satellite. The satellite was launched on 22 November 1972 and collected
data until re-entry in April 1974. The main objectives with the mission were studies
of the neutral atmosphere, the ionosphere, and auroral particles. Particle data from
sounding rockets and satellites had shown that a forbidden region in phase space
was frequently filled with particles. In this paper we demonstrate a scenario that
could explain that type of distribution, a population trapped by the magnetic mirror
force below and a parallel electric field above the satellite. Our hypothesis was that
the auroral electrons are accelerated by a quasi-static parallel electric field above the
ESRO 4 altitude (perigeum 245 and apogeum 1.177 km). This electric field can in
principle be considered as constant on time scales comparable to electron transit
times. Previous explanations of these locally trapped electrons included
backscattered and secondary particles, but without a plausible explanation for how
they could reach this region of phase space. The parallel electric field may,
however, be time dependent. If the time it takes to reach maximum potential is long
compared to the time it takes for electrons to move from the acceleration region to
the mirror point and back to the acceleration region, then the particle parallel energy
will be smaller than the parallel electric field component. The electron will become
trapped in a region were no particles should exist according to the simple picture of
injecting a particle population from above, neglecting particles below and the time
history of the intensity of the electric field. Comparing theoretical calculations with
the observed angular and energy distributions gave a very good agreement.
Paper 2 “Polar Cap Arcs Observed by the Viking Satellite” discusses some
preliminary Viking results on auroral structures at high geomagnetic latitudes. Sunaligned arcs and/or theta aurora had been observed with other-spacecraft and from
ground based observatories but the magnetic field line topology during this type of
events was not settled. The paper discusses acceleration mechanisms leading to the
precipitation of electrons into the high latitude atmosphere, at latitudes poleward of
the statistical oval. It was found that the particle characteristics were in principle
similar to those observed in the night-time auroral arcs at more normal auroral
latitudes. The acceleration voltage was, however, found to be considerably lower,
of the order of a few keV. Another conclusion was that these arcs are located on
closed magnetic field lines. This fact was based on the shape of the angular
distribution of electrons, the presence of plasma sheet-type positive ion
populations, and structured low intensity electron fluxes in the region between the
ordinary auroral oval and the high latitude arc.
Paper 3 “ Acceleration/Heating Processes on Auroral Field Lines as Observed by
the Viking Spacecraft” shows some characteristics of the particle distributions
observed by the Viking particle experiment. It demonstrates the necessity to
perform measurements with full pitch angle coverage and high temporal and spatial
resolution. The particle distributions show a large variability with signatures of
acceleration both above and below the Viking altitude. A possible generation
mechanism of electron conics as well as observational evidence of electron
acceleration by electric fields parallel to the geomagnetic field lines are briefly
discussed.
Paper 4 “Auroral Energization Processes” gives a fairly extended review of
auroral acceleration and heating mechanisms in the altitude region from about 1000
km up to 2 Re. Viking results are discussed after a short historical resume and
discussion of existing theories and status of observational results. Signatures in the
particle distribution function such as size and shape of the loss cone and “monoenergetic” energy distributions with different degrees of field alignment, contain
information on processes that have influenced the particles. The importance and
31
proof of the existence of quasi-static parallel electric fields are discussed. It was
found that so called static acceleration can indeed explain major features in the
electron and ion distributions. The static parallel electric is, however, not capable of
explaining all features. Electron conics and perpendicular heated positive ions are
created in a different way. Proposed theories are reviewed and conclusions given
on what fit the Viking observations. The importance of low frequency fluctuating
electric fields is stressed and discussed. It is, e.g., observed that the ion conics are
correlated with large amplitude fluctuations in regions of low plasma density. A
good correlation is found between the ion conic temperature and the power spectral
density of low frequency fluctuations.
Paper 5 “Observations of Electron Conics by the Viking Satellite” gives some
statistics on almost 200 electron conic events observed by Viking. The particle data
are used together with information on wave activity at low and high frequencies. It
is found that the electron conics are more frequently observed in the dusk sector
compared to the dawn sector. The electron conics are always observed together
with ion beams or elevated conics, indicating the presence of parallel acceleration
below the Viking altitude. Possible heating/acceleration mechanisms were tested by
computer simulations in papers by André and Eliasson [1992,1994] showing that
several mechanisms are in principle able to explain the electron observations. Some
mechanisms are, however, less likely if one takes into account also the ion
observations. We favour the theory that low frequency electric field fluctuations in
the parallel component are in “resonance” with the electron travel time below the
parallel electric field. Broadband hiss and upper hybrid waves are often seen in the
electron conic region but it is not obvious in what way they influence the electrons
or if they are generated by the electron conic anisotropy.
Paper 6 “Freja Observations of Heating and Precipitation of Positive Ions”
summarizes the first results on transverse ion acceleration at altitudes around 1.700
km obtained with the Freja Hot Plasma Experiment. The Freja orbit was chosen to
cover one of the regions where cold ionospheric ions are heated perpendicular to the
geomagnetic field lines. This paper concentrates on two different types of ion
heating events. Firstly, an event with high intensity and field-aligned electron fluxes
that are believed to generate waves, both broadband low-frequency waves and
waves above the lower hybrid frequency. Secondly, an ion heating event
accompanied with fairly intense proton precipitation. There is no obvious free
energy available in the super-thermal electron distribution during the second event.
The precipitating protons were, however, believed to be capable of generating
waves in the lower hybrid frequency regime. The proton distribution can be
characterized both as a loss-cone and a ring type distribution. The waves can
account for the observed ion heating. The main conclusions were that several
mechanisms play an important role in the heating of ions at low altitudes. Waves
above the lower hybrid frequency were present during both events but they were,
most likely, generated in different ways. Two other papers that are partly based on
results from the same events and with discussions related to perpendicular heating
of positive ions will appear in the same issue of Geophysical Research Letters. First
authors of these papers are André (Transverse ion energization and wave emissions
observed by the Freja satellite), and Eriksson (Freja observations of correlated
small-scale density depletions and enhanced lower hybrid waves).