Download Locations of night‐side precipitation boundaries relative to R2 and

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, A10233, doi:10.1029/2010JA015444, 2010
Locations of night‐side precipitation boundaries relative to R2
and R1 currents
S. Ohtani,1 S. Wing,1 P. T. Newell,1 and T. Higuchi2
Received 9 March 2010; revised 30 April 2010; accepted 7 June 2010; published 15 October 2010.
[1] The present study statistically examines the location of various precipitation
boundaries introduced by Newell et al. (1996) relative to R2 and R1 currents on the night
side. Results are summarized as follows: (1) The electron and ion zero‐energy boundaries,
b1e and b1i, are located mostly inside the R2 current at dusk‐to‐midnight and near the
equatorward boundary of the R2 current or farther equatorward at midnight‐to‐dawn.
(2) The maximum energy flux of ion precipitation (b2i) occurs inside the R2 current
irrespective of magnetic local time. (3) The occurrence distributions of the most
equatorward (b3a) and poleward (b3b) electron acceleration events indicate that mono‐
energetic electron precipitation is mostly confined in the upward R1 current at dusk‐to‐
midnight, whereas at midnight‐to‐dawn, it is more widely distributed including the
downward R1 current. (4) The transition between structured and unstructured electron
precipitation (b4s) tends to occur around the R2/R1 demarcation, but its occurrence
distribution has extending tails. (5) The distributions of the poleward boundaries of the
electron and ion auroral ovals, b5e and b5i, are centered around the poleward boundary
of the R1 current but have extending tails especially at midnight‐to‐dawn. Result 1
suggests that the overlap between the ring current and the plasmasphere is more significant
at dusk than at dawn. Result 2 indicates that the b2i boundary can be used as an identifier
of the R2 current. Results 3–5 suggest that the R1 current is more structured than the R2
current and that the field‐aligned current structure is more complex at midnight‐to‐dawn
than at dusk‐to‐midnight.
Citation: Ohtani, S., S. Wing, P. T. Newell, and T. Higuchi (2010), Locations of night‐side precipitation boundaries relative to
R2 and R1 currents, J. Geophys. Res., 115, A10233, doi:10.1029/2010JA015444.
1. Introduction
[2] Field‐aligned currents (FACs) and particle precipitation are two fundamental elements of the magnetosphere‐
ionosphere (M‐I) coupling, and their global distributions
have been studied for many years. The polar distribution of
FACs consists of a pair of zonal current systems, which are
known as region 1 (R1) and region 2 (R2) systems [Iijima
and Potemra, 1976]. The R1 current flows downward and
upward on the morning and evening sides, respectively,
whereas the R2 current is located on the equatorward side of
the R1 current and has the opposite polarity at a given local
time. Particle precipitation has also been addressed in terms
of two zonal regions, that is, CPS and BPS [Winningham et
al., 1975]. CPS is characterized by unstructured ion and
electron precipitation that extends to a high‐energy (>1 keV)
range. BPS is located adjacently poleward of CPS and is
characterized by structured and often mono‐energetic
1
Johns Hopkins University Applied Physics Laboratory, Laurel,
Maryland, USA.
2
Institute of Statistical Mathematics, Tokyo, Japan.
Copyright 2010 by the American Geophysical Union.
0148‐0227/10/2010JA015444
electron precipitation. It is now generally accepted that
CPS and BPS do not correspond one‐to‐one with the high‐
altitude central plasma sheet and plasma sheet boundary
layer.
[3] The size of the auroral oval significantly varies
depending on various conditions such as geomagnetic
activity [e.g., Feldstein and Starkov, 1967] and interplanetary magnetic field (IMF) orientations [e.g., Holzworth and
Meng, 1984]. The polar distributions of the FAC [e.g.,
Weimer, 2001; Papitashvili et al., 2002; Anderson et al.,
2008] and particle precipitation [Newell et al., 2004] constructed from large data sets also reveal clear dependence on
the IMF. Therefore, it should be reasonable to expect that the
spatial distributions of FAC and particle precipitation change
following each other. In other words, we expect that the
locations of various precipitation boundaries are organized by
the locations of large‐scale FACs (and vice versa).
[4] Here it is important to compare the locations of precipitation boundaries with those of FACs for each auroral
oval crossing and then to statistically examine their distributions rather than to compare the statistical distributions
of particle precipitation and FACs. The reason is twofold.
First, we believe that the variability of precipitation
boundaries relative to large‐scale FACs carries important
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Figure 1. The number of DMSP crossings of two (red),
three (blue), and four (green) FAC sheet structures, for which
particle precipitation boundaries are identified, at least for
electrons. The positive and negative numbers indicate those
of orbits with the most equatorward current flowing into
and out of the ionosphere, respectively.
information. Second, the locations of precipitation boundaries relative to large‐scale FACs may be different for different FAC structures (this is indeed the case for the b2i
boundary as will be shown later). Such relationships can be
addressed only if we compare FACs and particle precipitation for individual orbits.
[5] In the present study we statistically examine particle
precipitation boundaries relative to the R2 and R1 currents.
The locations of eight precipitation boundaries introduced
by Newell et al. [1996] are compared for each auroral oval
crossing with FAC boundaries identified by an automated
procedure developed by Higuchi and Ohtani [2000a,
2000b]. A focus is placed on the nightside (18 ≤ magnetic
local time, MLT < 06). Our accompanying paper, Wing et al.
[2010], reports a similar study but focusing on the dayside
with a different set of precipitation boundaries [e.g., Newell
and Meng, 1988].
[6] In section 2, we describe magnetic field and particle
measurements made by DMSP satellites, which we use in
this study. We also explain the event selection procedure in
section 2. In section 3 we examine two example events, one
at premidnigt (MLT ∼ 22.4) and another one at early
morning and address how electron and ion precipitation
signatures differ in different (R2 or R1) FAC systems or on
different sides of midnight. In section 4 we examine how the
latitudes of the FAC and precipitation boundaries change as
a function of MLT. Section 5 is the primary part of this
study, in which we statistically examine the locations of the
precipitation boundaries relative to the boundaries of R2 and
R1 currents. We discuss the results in terms of magnetospheric convection, structure, and the structuredness of
large‐scale FACs. We summarize the study in section 5.
2. Data Set and Event Selection
[7] In the present study we use magnetic field and particle
precipitation data acquired from DMSP‐F7 (1983–1988), ‐
F12 (1994–2002), ‐F13 (1995–2006), ‐F14 (1997–2001), ‐
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F15 (1999–2006), and ‐F16 (2003–2006) satellites. All
DMSP satellites are on Sun‐synchronous orbits (F13
approximately in a dawn‐dusk orientation and others in
prenoon‐premidnight orientations) at about 840 km in altitude with orbital inclinations of 98.7°. The orbital periods
are approximately 100 min.
[8] A triaxial fluxgate magnetometer is onboard each of
those DMSP satellites, which measures vector magnetic
fields with a time resolution of 1 s and a 1‐bit resolution of
2 nT [Rich et al., 1985]. The particle instrument package
(SSJ4) uses curved plate electrostatic analyzers to measure
ions and electrons from 32 to 30 keV in logarithmically
spaced steps [Hardy et al., 1984]. One complete 19‐point
electron and ion spectrum is obtained each second. The
satellites are three‐axis stabilized, and the detector apertures
always point toward local zenith. Since the magnetic field is
approximately vertical in the polar region, measured particles
are precipitating into the ionosphere.
[9] The low‐energy (30 eV to 1 keV) ion detector onboard
some DMSP satellites suffered from degradation immediately or sometime after their launch, which affects the
identification of two ion precipitation boundaries, b1i (the
zero‐energy ion precipitation boundary; see section 5.1) and
b5i (the poleward boundary of the ion auroral oval; see
section 5.5). For those two boundaries we use data obtained
only from DMSP‐F7 for its entire interval, F12 and F14 data
throughout 1998 and 2001, respectively, for which the effect
of degradation, if at all, is minimal. Another ion boundary,
b2i (the point of the maximum flux of ions above 3 keV; see
section 5.2), is determined only by measurements made by
the high‐energy ion detector, and therefore, it is not affected
by the degradation of the low‐energy ion detector.
[10] We use FAC boundaries identified for each DMSP
auroral‐oval crossing by an automated procedure developed
by Higuchi and Ohtani [2000a, 2000b], which has been
used for various statistical studies [Ohtani and Higuchi,
2000; Ohtani et al., 2000; 2005a, 2005b, 2009; Ueno et
al., 2007; Nakano et al., 2009; Wing et al., 2010]. This
procedure is basically the automation of the conventional
way we visually examine a plot of satellite magnetic field
data. It calculates the maximum‐variance component of
horizontal magnetic perturbations for each crossing of
auroral oval and fits line segments to its variation. The
structure of magnetic variations is quantified by several
parameters such as the ratio of the maximum‐ to minimum‐
variance component (a) and the ratio of the residual of the
line segment fitting to the magnetic change associated with
the most intense FAC (Rfit); for more details of these
parameters, see Higuchi and Ohtani [2000a, 2000b]. For
selecting DMSP orbits, we use the same criteria for those
parameters as Ohtani et al. [2009] used. For example, a = 2
and Rfit = 8%.
[11] Figure 1 shows as a function of MLT the number of
DMSP orbits for which both the structure of FACs and
particle precipitation boundaries, at least electron boundaries, were identified. Here the MLT of each orbit is defined
as that at the demarcation between the most equatorward
and second equatorward FACs. The positive and negative
numbers indicate those of orbits with the most equatorward
current flowing into and out of the ionosphere, respectively.
The different colors indicate different numbers of large‐
scale FACs crossed along an orbit: red, blue, and green for
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two‐, three‐, and four‐sheet structures, respectively. The
automated procedure occasionally identifies only one sheet
or more than four sheets along an orbit, but the number of
such orbits is even smaller than that of four‐sheet structures.
[12] Three features are clear. First, the MLT distribution
of orbits is far from uniform. There are ∼16,000 and
∼19,000 orbits at 18 ≤ MLT < 20 and 20 ≤ MLT < 22,
respectively, whereas there are only ∼1300 and ∼1100 orbits
at 00 ≤ MLT < 02 and 02 ≤ MLT < 04, respectively. This is
because the orbit of each DMSP satellite is Sun‐synchronous,
and it covers only a limited range of MLT. Second, the
polarity of the most equatorward current is predominantly
downward (into the ionosphere) at 18 ≤ MLT < 22, whereas
it is upward (out of the ionosphere) at 00 ≤ MLT < 06. The
transition appears to take place in the premidnight sector, 22 ≤
MLT < 24. Finally, for the overwhelming majority (83%) of
orbits, the FAC has two‐sheet structures. At 22 ≤ MLT < 24,
three‐sheet structures are often observed, but they are still a
minority (33%). The last two features are consistent with the
statistical spatial distribution of R2 and R1 currents [Iijima
and Potemra, 1976], and in the rest of this paper, we refer
to the most equatorward and second equatorward FACs as
R2 and R1 currents, respectively.
3. Example Events
[13] In this section we examine two auroral oval crossings, one in the dusk‐to‐midnight sector and another in the
midnight‐to‐dawn sector and address how electron and ion
precipitation differs between the two events. As for precipitation boundaries, we will return to these example events
when we examine their statistical distributions in section 5.
3.1. An Oval Crossing in the Dusk‐to‐Midnight Sector
[14] Figures 2a shows magnetic field and precipitation
measurements made by DMSP‐F7 on 12 March 1984. The
satellite crossed the auroral oval from poleward to equatorward at MLT = 22.3. The top panel shows the two horizontal
magnetic components, the minimum‐variance (BL: blue) and
maximum‐variance (BA: red) components as determined by
the principal axis analysis (section 2). The former can be
regarded as the latitudinal component (positive in the
direction of the satellite velocity, therefore equatorward for
this event) and the latter as the azimuthal component (positive
in the direction of the vector product between the vertically
downward axis and the satellite velocity, and therefore
westward for this event). The variations of BL are significantly smaller than those of BA, indicating that the FAC
distribution is locally well approximated by sheet structures.
The red dashed line represents the result of the line‐segment
fit (section 2), which traces the actual measurements very
well. The satellite crossed two large‐scale FAC sheets. The
FAC on the poleward side flows upward, whereas the one
on the equatorward side, which is weaker and is distributed
more widely in latitude, flows downward. The former can be
identified as a R1 current and the latter as a R2 current.
[15] The lower half of Figure 2a shows the energy flux
and average energy of precipitating electrons (black dots,
labeled on the left‐hand side) and ions (red dots, labeled on
the right‐hand side) and the E‐t diagrams of precipitating
electrons and ions. The vertical axis of the ion E‐t diagram
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is inverted. At first glance the characteristics of precipitation
are drastically different between the R2 and R1 currents.
Electron acceleration is most prominent in the R1 current,
whereas in the R2 current, there is no clear sign of the field‐
aligned acceleration of electrons, and the average energy as
well as the energy flux is noticeably lower than in the R1
current. The electron precipitation terminates rather abruptly
in the middle of the R2 current and so is the ion precipitation. Note, however, that the flux of the lowest energy
channels extends slightly equatorward for both electrons and
ions. The intense precipitation of energetic (∼10 keV) ions is
roughly confined between this termination and the R2/R1
demarcation. In the R1 current, in contrast, the ion precipitation is significantly weaker.
3.2. An Oval Crossing in the Midnight‐to‐Dawn Sector
[16] Figures 2b show an auroral oval crossing on 24 July
1997 in the same format as Figure 2a. The DMSP‐F13
satellite crossed the auroral oval from equatorward to
poleward in the Southern Hemisphere slightly before 0400
MLT. For this event BA and BL are positive westward and
poleward, respectively. The change of BL is far less significant than that of BA, indicating the sheet structure of FACs.
The polarities of the lower‐ (R2) and higher‐latitude (R1)
FACs are upward and downward, respectively, as expected
for the dawnside R2‐R1 pair.
[17] In the R1 current, electron precipitation is noticeably
weaker than in the R2 current, whereas the precipitation of
energetic ions is as intense as in the R2 current. In fact, the
ion precipitation appears to be continuous, but with the
center energy becoming higher, from the poleward edge of
the R1 current till about three fourth of the R2 current width,
where the overall energy of ion precipitation suddenly becomes lower. The electron precipitation is energetic (>1 keV)
but not very structured in the R2 current except near its
poleward boundary. The energetic electron precipitation
extends far equatorward of the R2 current and fades out. A
close inspection of the BA plot, however, suggests that a
very weak FAC extends farther equatorward of the R2
current (as identified by the line segment fit), which well
overlaps the region of this energetic electron precipitation.
Energetic but unstructured electron precipitation characterizes the midnight‐to‐dawn sector, which presumably
corresponds to diffuse aurora.
[18] It should be useful to compare the precipitation signatures of the present event with those of the 12 March 1984
event. First, the characteristics of both ion and electron
precipitation in the R1 current are significantly different
between the two events. In the downward R1 current of the
present event, the ion precipitation was intense and energetic, whereas the electron precipitation was rather weak. In
the 12 March 1984 event, in contrast, the upward R1 current
was collocated with mono‐energetic electron precipitation
along with very weak ion precipitation. These differences
can be explained at least qualitatively in terms of field‐
aligned electric fields with the same polarity as the R1
current. The electron precipitation was much more intense
and energetic in the upward R2 current of the present event
than in the downward R2 current of the 12 March 1984
event. However, the electron precipitation in the present
event is not mono‐energetic, and therefore, it is not likely
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Figure 2. (a) An auroral oval crossing in the dusk‐to‐midnight sector observed by DMSP‐F7 on
12 March 1984. (b) An auroral oval crossing in the midnight‐to‐dawn sector observed by DMSP‐F13 on
29 July 2004. See text for details.
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that the field‐aligned potential difference is the primary
cause of the difference between the two events. The ion
precipitation in the R2 current is intense and energetic in
both events. It is likely that both ion and electron precipitation in the R2 current largely reflects the magnetospheric
source population.
[19] In closing of this section, we would like to note that a
weak FAC often extends farther equatorward of the R2
current as we found for the 24 July 1997. Therefore, the
equatorward boundary of the R2 current identified by the
line segment fit for such events may be regarded as the most
poleward possible location of this boundary. In contrast, the
identification of the poleward boundary of the R1 current
and the demarcation of the R2 and R1 currents is far less
unambiguous in general.
4. MLT Dependence
[20] In the present study, unless we specify, we focus on
events in which DMSP observed a two‐sheet structure, a
pair of R2 and R1 currents, with their polarities consistent
with their statistical pattern [Iijima and Potemra, 1976].
That is, for an auroral oval crossing to be selected, we
require that the polarities of the observed R2 and R1 currents
are downward and upward, respectively, before midnight,
and upward and downward after midnight. The MLT of the
demarcation between the morning‐side and evening‐side R2
and R1 currents moves, for example, as a function of the
IMF B Y component [Weimer, 2001; Papitashvili et al.,
2002; Anderson et al., 2008]. However, the adoption of
this simple criterion can be justified for the following two
reasons. First, the events excluded by this criterion are a
very small fraction of our event set except for the premidnight (22 ≤ MLT < 24) sector (see Figure 1). Second, in the
premidnight sector, DMSP observed a number of two‐sheet
structure events with the opposite FAC polarities. However,
as we will address in section 5.2, some of such events can be
described as a three‐sheet structure without a R2 current
rather than a pair of R2 and R1 currents with the opposite
polarities, and the result of the present study would become
obscured if we include such events.
[21] Figure 3a shows the absolute values of the magnetic
latitudes, |MLat|, of the equatorward boundary of R2 currents
(blue), the demarcation between R2 and R1 currents (red),
and the poleward boundary of R1 currents (green) against
MLT. The median of |MLat| of each FAC boundary is plotted
every 2 h in MLT, and the error bars show ranges between
the 16 and 84 percentile points (±1 standard deviation from
the median value) for giving the idea of the scatter of data
points. The expected errors are actually the standard deviations divided by the square root of the number of events, and
because of an extremely large number of events (Figure 1),
they are significantly smaller than the error bars shown in
the figure and are very often not discernible from the plotted
lines.
[22] The equatorward boundary of R2 currents shifts
poleward, but only by 2°, from dusk to dawn. The latitude of
the poleward boundary of R1 currents is lowest around
midnight and becomes higher away from midnight. These
features can also be found in the initial study of R2 and R1
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currents [Iijima and Potemra, 1976]. One thing noticeable
about the R2/R1 demarcation is that its latitude is higher in
the postmidnight sector (00 ≤ MLT < 02) than in the surrounding sectors, which affects the statistical result of some
boundaries as will be shown in section 5. Here we would
like to make two points as possible reasons for this anomaly.
[23] First, DMSP occasionally observes at postmidnight a
pair of FACs that flow upward and downward at higher and
lower latitudes, respectively as expected for the R2 and R1
pair after midnight, but the equatorward upward current is
collocated with mono‐energetic electron precipitation as is
often observed for the upward R1 current before midnight
(section 5.3), and the precipitation of energetic ions, which
are presumably of ring‐current origin, peaks farther equatorward of this upward current. Such events are better
interpreted as a three‐sheet structure on the premidnight
side extending across midnight but with a R2 current absent
rather than as a conventional R2‐R1 pair at postmidnight.
Whereas the present study excludes such events if they
occur before midnight (because the FAC polarities are the
opposite), they are included if they occur after midnight as
far as other criteria are met. Since the (apparent) R2/R1
demarcation is higher in latitude in those events, the overall
distribution of the R2/R1 demarcation is shifted poleward.
Their impact on the distribution of the poleward boundary
of the R1 current may be less significant because the third
FAC, the current poleward of the R1 current, is usually
thinner than the R1 current. It is, however, still unclear why
the equatorward boundary of the R2 current is apparently
not affected.
[24] Second, in the postmidnight sector, the Sun‐
synchronous orbits of DMSP satellites, especially DMSP‐F7,
12, and 14–16, whose orbits are oriented in prenoon‐
premidnight directions, tend to skim the auroral oval. However, the identification of the FAC structure requires that the
satellite crosses through the R2 and R1 currents, for which the
contracted auroral oval is preferable. It is therefore possible
that the location of events at postmidnight are biased poleward. However, this idea alone does not explain why only the
R2‐R1 demarcation is higher in latitude but not the equatorward boundary of R2 or the poleward boundary of R1. It is
also unclear why the poleward shift can be found only for the
00 ≤ MLT < 02 sector but not, for example, for the 02 ≤ MLT
< 04 sector.
[25] Interestingly, some polar distributions of FACs
reported in the past show that the latitude of the R2‐R1 pair
(as defined from the FAC polarities) jumps around midnight
[Iijima and Potemra, 1976; Weimer, 2001; Papitashvili et al.,
2002]. We should therefore keep in mind that there is possibly
an unknown bias in our results for the 00 ≤ MLT < 02
sector, whether it is real or it originates from our event
selection.
[26] The other frames of Figure 3 show the median of
∣MLat∣ of different precipitation boundaries. In each of
those frames, the FAC boundaries shown in Figure 3a are
plotted by gray lines. We will examine those precipitation
boundaries in section 5, but for now, we just like to point
out that the magnetic latitude of the precipitation boundaries
change smoothly with MLT except that the |MLat| of the
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Figure 3. (a) The absolute values of the magnetic latitudes, |MLat|, of the equatorward boundary of R2
currents (blue), the demarcation between R2 and R1 currents (red), and the poleward boundary of R1 currents (green) against MLT. |MLat| of the (b) b1e (red) and b1i (blue), (c) b2i, (d) b3a(red) and b3b (blue),
(e) b4s, and (f) b5e (red) and b5i (blue) boundaries. In Figures 3b−3f, the FAC boundaries shown in
Figure 3a are plotted by gray lines. The error bars show ranges between the 16 and 84 percentile points
(one standard deviation from the median value). The plots are slightly shifted horizontally for distinction.
b3b boundary is locally higher at 00 ≤ MLT < 02 and that in
general the latitudes of precipitation boundaries change
following the boundaries of large‐scale FACs.
5. Precipitiation Boundaries Relative to Large‐
Scale FACs
[27] In this section we examine the locations of different
precipitation boundaries relative to large‐scale FACs. For
the detailed definition of each precipitation boundary, see
Newell et al. [1996]. We introduce a normalized latitudinal
coordinate l, which is defined as follows. For each DMSP
crossing of the auroral oval, the origin of l (l = 0) is set to
be at the demarcation between R2 and R1 currents. l in-
creases poleward irrespective of the hemisphere. In the
positive (negative) territory, that is, on the poleward
(equatorward) side of l = 0, l is normalized by the width of
the R1 (R2) current sheet. That is, l = 1 (l = −1) at the
poleward (equatorward) boundary of the R1 (R2) current.
This normalization applies whether or not there is another
FAC sheet poleward of the R1 current. In the following we
examine the occurrence distribution of l for each boundary.
5.1. Zero‐Energy Precipitation Boundaries: b1e
and b1i
[28] “Zero‐energy” boundaries are defined separately for
ions and electrons and are denoted as b1i and b1e, respectively. For both species the boundaries are identified based
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Figure 4. The occurrence ratio of l (normalized MLat) of the (a and b) b1e and (c and d) b1i boundaries.
In Figures 4a and 4c (Figures 4b and 4d), the red, blue, and green lines plot the distributions for 18–20,
20–22, and 22–24 MLT (04–06, 02–04, and 00–02 MLT), respectively. The error bars show ranges
between the 16 and 84 percentile points (one standard deviation from the median value) divided by
the square root of the number of events in each bin.
on the cutoffs of the lowest‐energy channel (32–47 eV) flux.
Figure 4 shows the occurrence ratio of the normalized
magnetic latitude of the b1e (left) and b1i (right) boundaries
for 3 different 2 h wide MLT sectors before (top) and after
(bottom) midnight. The occurrence ratio p is defined as a
number of data points in each bin ni, divided by the total
number of data points n, that
is, p = ni/n. The error bar is
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
calculated for each bin as pð1 pÞ=n. Significantly less
numbers of orbits are available for the midnight‐to‐dawn
sector (Figure 1), which is especially the case for the b1i
boundary since the ion data set is smaller (section 2), and
accordingly the associated error bars are larger.
[29] At dusk‐to‐midnight the b1e boundary is located
inside the R2 current (−1 ≤ l < 0) for the majority of orbits
(Figure 4a). Note that in the dusk‐to‐midnight events we
selected, the R2 current flows into the ionosphere (section 4),
which is presumably carried by upward‐moving electrons. It
is therefore not surprising that the R2 current extends in the
region where there is no significant electron precipitation. At
earlier MLTs, the occurrence distribution is narrower with
its peak located more poleward (relative to the R2 system).
The b1i boundary is also very often located inside the R2
current (Figure 4b), but it tends to be equatorward of b1e
especially at earlier MLTs. These results can also be confirmed in the 12 March 1984 event (Figure 2a).
[30] At midnight‐to‐dawn the b1e boundary is located
noticeably more equatorward than at dusk‐to‐midnight and,
in most cases, equatorward of the R2 current (Figure 4b). Its
dependence on MLT, if at all, is not very clear (Figures 4b).
The result is similar for the b1i boundary (Figure 4d). In the
24 July 1997 event, the b1i boundary is 0.6° equatorward of
the equatorward boundary of the R2 current, and the b1e
boundary is 2.1° farther equatorward (Figure 2b). This event
is a good example of the zero‐energy (b1i and b1e)
boundaries located equatorward of the R2 current at dusk to
midnight.
[31] The overall results may be explained in terms of the
accessibility of zero‐energy particles to the inner magnetosphere. Zero‐energy charged particles, whether ions or
electrons, do not undergo magnetic gradient or curvature
drift. Accordingly, their convection paths in the magnetosphere follow equipotential contours and there is a certain
area around Earth they cannot reach. For a simple convection pattern with the corotation electric field superposed to a
uniform dawn‐to‐dusk electric field, this forbidden area is
tear drop shaped with pointed and round sides at dusk and
dawn, respectively, and it extends farther out in the dusk
sector than in the dawn sector [e.g., Chen, 1970]. It is
therefore expected that the outer boundary of this forbidden
region is magnetically mapped to the ionosphere more
poleward at dusk than at other MLTs, which can be confirmed in Figure 3b. In contrast, the equatorward boundary
of the R2 system tends to be located more equatorward at
early MLTs in the dusk‐to‐midnight sector (Figure 3a). As a
result, the normalized latitudes (l) of the b1e and b1i
boundaries tend to be highest at dusk.
5.2. Maximum Ion Energy Flux: b2i
[32] The b2i boundary is defined as the location where the
energy flux of ions above 3 keV has a maximum, and it well
agrees with the isotropic boundary of 30 keV ions [Newell
et al., 1998]. The isotopic boundary marks the most
equatorward point where the pitch‐angle distribution of
precipitating ions is isotropic. If the equatorial field line
curvature is sufficiently large (the curvature radius is suffi-
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Figure 5. The occurrence ratio of the normalized MLat l
of the b2i boundary for (a) dusk‐to‐midnight and (b) midnight‐to‐dawn sectors.
ciently small), ions undergo pitch‐angle scattering when they
cross the equatorial plane [Sergeev et al., 1993]; the pitch‐
angle scattering is effective if the curvature radius is less than
8 times the ion gyroradius for the equatorial magnetic field.
Roughly speaking, if the pitch angle distribution is isotropic,
the energy flux of ion precipitation increases along with the
equatorial ion energy density, which increases with
decreasing radial distance in the magnetotail. It is therefore
expected that the maximum ion energy flux, that is, the b2i
boundary, takes place at, or slightly equatorward of, the
isotropic boundary.
[33] In both 12 March 1984 and 24 July 1997 events, the
b2i boundary was located inside the R2 current (Figures 2a
and 2b). We found that these events are typical in terms of
the b2i location. Figure 5a shows the occurrence ratio of l
of the b2i boundary for three different MLT sectors at dusk
to midnight. For the predominant majority of orbits, the b2i
boundary is located inside the R2 current (−1 ≤ l < 0) and it
is rarely found in the R1 current (0 ≤ l < 1). The distribution
slightly shifts equatorward (relative to large‐scale FACs)
and becomes wider closer to midnight.
[34] Figure 5b shows the occurrence distributions of the
b2i boundary at midnight to dawn. The b2i boundary is
located mostly inside the R2 current (−1 ≤ l < 0) at 02 ≤
MLT < 04 and 04 ≤ MLT < 06 as was found for dusk to
midnight. At 00 ≤ MLT < 02, in contrast, l tends to be
more equatorward, and it is < −1 for 48% of the events in
this 2 h sector. However, this tendency is probably overemphasized because the R2/R1 demarcation, the reference
of l, in this MLT sector is located more poleward than in
the surrounding sectors as we found in section 4.
[35] Here we briefly examine the occurrence distribution
of the b2i boundary for different FAC structures. Figure 6
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shows the occurrence distribution of the b2i boundary for
two‐sheet (gray) and three‐sheet (red) events at 20 ≤ MLT <
22 (Figure 6a) and at 22 ≤ MLT < 24 (Figure 6b). The
distributions for the two sheets were already shown in
Figure 5a. In the selected three‐sheet events as well as in the
two‐sheet events the most equatorward FAC flows downward, that is, in the same direction as the eveningside R2
current. The second equatorward current flows upward, to
which we have been referring as R1 current for the two‐
sheet structure, but its identity is less clear for the three‐
sheet structure. For the three‐sheet structure, there is an
additional current farther poleward, at l ≥ 1, which flows
downward. Note that three‐sheet structures are often
observed in the premidnight sector [Iijima and Potemra,
1976] (see also Figure 1 for the occurrence frequency).
[36] The occurrence distribution of the b2i boundary is
almost identical for the two‐sheet and three‐sheet events for
each MLT sector. In the predominant majority of the events,
the boundary is located either inside the R2 current (−1 ≤
l < 0) or farther equatorward (l < −1), and it is rarely
located poleward of the R2 current (l ≥ 0). Since this is
also the case for the midnight‐to‐dawn sector (Figures 3c
and 5), the b2i boundary may be used for identifying the
R2 current. The present result also suggests that the middle
upward current of the three‐sheet structure in the dusk‐to‐
midnight sector is mapped farther down the tail than the
upward R2 current in the midnight‐to‐dawn sector although
these currents may appear to be a continuum in the polar
distribution [e.g., Iijima and Potemra, 1976; Weimer, 2001;
Papitashvili et al., 2002; Anderson et al., 2008].
[37] We also examined events with different FAC polarities. In Figure 7 the red lines show for 20 ≤ MLT < 22
Figure 6. The occurrence ratio of the normalized MLat l
of the b2i boundary for (a) 20–22 and (b) 22–24 in MLT
for the two‐sheet (gray) and three‐sheet (red) events. In both
event sets the polarity of the R2 current is downward.
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Figure 7. The occurrence ratio of the normalized MLat l
of the b2i boundary for (a) 20–22 and (b) 22–24 in MLT
for the two‐sheet events with the standard (gray) and opposite (red) FAC polarities.
(Figure 7a) and 22 ≤ MLT < 24 (Figure 7b) the occurrence
distributions of the b2i boundary for the two‐sheet events
with upward and downward FACs at lower and higher latitudes, respectively, that is, with the FAC polarities opposite to what are expected from the R2 and R1 pair in the
premidnight sector. The gray lines show, as a reference, the
distributions for the two‐sheet events with the standard R2
and R1 polarities. For each MLT sector the distribution for
the opposite polarity events is significantly different from
that for the standard polarity events, and the b2i boundary is
located more often at l < −1 than at l ≥ −1, that is, farther
equatorward of the upward FAC.
[38] This result can be explained by assuming that the
lower‐latitude upward FAC in the opposite polarity events is
actually the R1 current and that the R2 current is missing for
some reason. In such a case the b2i boundary, which would
be most likely inside or equatorward of the R2 current, is
expected to be farther equatorward of this upward FAC as
is shown in Figure 7. Note that the normalization of latitude
is different for the equatorward and poleward sides of the
R2‐R1 demarcation, and therefore, even if the R2 current is
missing, the distribution for the opposite‐polarity events
should not be like the distribution for the standard‐polarity
events shifted by −1. The present result strengthens the idea
that the b2i boundary can be used for identifying the R2
current.
5.3. Most Equatorward and Poleward Electron
Acceleration: b3a and b3b
[39] The b3a and b3b boundaries are identified as the most
equatorward and most poleward electron acceleration
events, in which the electron energy spectrum has a mono‐
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energetic peak indicating the existence of upward field‐
aligned electric fields. The field‐aligned acceleration has to
be 3–4 times the thermal energy of the source electrons and
the energy flux has to be above 0.25 [ergs/cm2 s].
[40] Figure 8a shows the occurrence distributions of the
b3a and b3b boundaries for the two‐sheet events with the
standard R2 and R1 polarities in the dusk‐to‐midnight
sector. The distribution of the b3a boundary has a sharp
peak at l = 0.1 (the adjacently equatorward data point is at
l = −0.1), whereas the distribution of the b3b boundary
has a sharp peak at l = 0.9. This result suggests that at dusk
to midnight, electron acceleration is very often confined in
the upward R1 current (0 ≤ l < 1). This is reasonable
because the upward electric field, which accelerates electrons downward, is associated with upward FACs.
[41] The difference in MLT is rather small, and the only
noticeable MLT dependence is that closer to midnight the
distribution of b3a tends to extend slightly more equatorward; l is < 0 for 22, 27, and 42% of events at 18 ≤ MLT <
20, 20 ≤ MLT < 22, and 22 ≤ MLT < 24, respectively. The
fact that l is < 0 for some b3a boundaries indicates that
field‐aligned electron acceleration occasionally takes place
inside the downward R2 current, which presumably corresponds to small‐scale upward FACs embedded. We found
that the b3a boundary is more often located in the downward
R2 current as geomagnetic activity becomes higher (as
measured by Kp; not shown). Electron acceleration can also
take place poleward of the upward R1 current as indicated
by the occurrence distribution of b3b extending to l ≥ 1.
The 12 March 1984 event is a good example, in which the
b3b boundary is located outside of the horizontal range of
Figure 2a, at 73.8° in geomagnetic latitude, corresponding to
an isolated mono‐energetic precipitation in the polar cap,
Figure 8. The occurrence ratio of the normalized MLat l
of the b3a and b3b boundaries for (a) dusk‐to‐midnight
and (b) midnight‐to‐dawn sectors.
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5.4. Structured/Unstructured Boundary: b4s
[44] The b4s boundary marks the transition of electron
precipitation from a spatially unstructured (on a scale of ≥5–
10 km) region on its equatorward side to a highly structured
one on its poleward side. This boundary is defined based on
correlation of electron energy spectra of some consecutive
data points and is considered to separate CPS from BPS
precipitation [Winningham et al., 1975], which used to be
visually identified. In both 12 March 1984 and 24 July 1997
events the b4s boundary is located in the middle of the R2
current (Figure 2).
[45] Figure 9 shows the occurrence distributions of this
boundary in the same format as Figure 5. At dusk to midnight (Figure 9a), the distributions are centered around l =
0, the R2/R1 demarcation, and are almost confined within
−1 ≤ l < 1. The occurrence distributions at 02 ≤ MLT < 04
and 04 ≤ MLT < 06 are narrower and centered slightly more
equatorward than the duskside counterparts. The distribution
at 00 ≤ MLT < 02 is significantly different with a tail
extending equatorward, which might be again attributed to
the complication of the identification of large‐scale FACs in
this MLT sector (section 4). However, the overall result
about the b4s boundary suggests that electron precipitation
is more structured in the R1 current than in the R2 current.
Figure 9. The occurrence ratio of the normalized MLat l
of the b4s boundary for (a) dusk‐to‐midnight and (b) midnight‐to‐dawn sectors.
whereas the b3a boundary is located just poleward of the
R2/R1 demarcation.
[42] Figure 8b shows the occurrence distributions of b3a
and b3b for the midnight‐to‐dawn sector, where the R2 and
R1 currents (as we selected) flow upward and downward,
respectively. The overall distributions are significantly different from those in the dusk‐to‐midnight sector. The b3a
boundary is located mostly inside or equatorward of the R2
current, that is, at l < 0. However, caution needs to be
exercised in interpreting the b3a boundary at midnight to
dawn in the context of field‐aligned acceleration because
some data points in diffuse electron precipitation apparently
satisfy the criteria for mono‐energetic electron events (as
defined by Newell et al. [1996]). In the 24 July 1997 event the
b3a boundary was located in the middle of diffuse electron
precipitation (Figure 2b). Such events may be quite common
as we will discuss in section 6.3.
[43] The occurrence of the b3b boundary is peaked around
l = 1, i.e., the poleward boundary of the R1 current, as we
found for the dusk‐to‐dawn sector, but its distribution
spreads far more widely. The b3b boundary poleward of the
R1 current can be attributed most likely to isolated electron
acceleration inside the polar cap. For example, in the 24 July
1997 event the b3b boundary was at −71.7° in geomagnetic
latitude (outside of the horizontal range in Figure 2b), far
poleward of the R1 current. In contrast, the b3b boundary
inside the R1 current probably corresponds to the internal
structure of the R1 current. That is, even though the R1
current (as we selected) flows downward at midnight‐to‐
dawn, upward field‐aligned electric fields may often be
formed along with a local upward current in association with
its internal structure.
5.5. Poleward Boundary of the Auroral Oval: b5e
and b5i
[46] The last boundaries we examine are the b5e and b5i
boundaries, which can be regarded as the poleward
boundaries of the auroral oval as determined by an abrupt
drop (by a factor of at least 4) in the electron and ion energy
flux, respectively. Although statistically the b5e and b5i
boundaries agree well with the poleward boundary of the R1
current (Figure 3f), in individual events they do not have to
agree. For example, downward FACs can be carried by
upward‐moving electrons and may not be collocated with any
significant electron precipitation. It is also possible that
intense electron precipitation is not accompanied by any
noticeable FAC if the collocated upward electron flux is also
large. For example, in the 24 July 1997 event, the b5e
boundary is located more than 5° poleward of the poleward
boundary of the R1 current (Figure 2b), whereas in the
12 March 1984 event, it is located very close to the poleward
boundary of the R1 current.
[47] Figures 10a and 10b show the occurrence distributions of b5e for the dusk‐to‐midnight and midnight‐to‐dawn
sectors, respectively. The occurrence ratio peaks around, but
slightly equatorward of, the poleward boundary of the R1
current (l = 1) on both sides of midnight. However, the
occurrence distribution for the dusk‐to‐midnight sector has
more extending tails on the poleward side. These features are
similar to what we found for the b3b boundary (Figure 8), but
a careful inspection reveals that the distributions of b5e
slightly shift poleward of b3b as expected from the definitions
of these boundaries.
[48] The occurrence ratio of the b5i boundary is also
centered around l = 1 both before and after midnight
(Figures 10c and 10d). Whereas at midnight to dawn, the
b5e and b5i distributions are similar, the occurrence of the
b5i boundary is more widely distributed than that of the b5e
boundary at dusk to midnight. Here intriguing questions are
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Figure 10. The occurrence ratio of the normalized MLat l of the (a and b) b5e and (c and d) b5i boundaries
for the dusk‐to‐midnight (Figures 10a and 10c) and midnight‐to‐dawn (Figures 10b and 10d) sectors.
why at dusk to midnight, the b5e boundary is often located
inside the upward R1 current, which is carried mainly by
downward electrons, and why at midnight to dawn the b5i
boundary is often located inside the downward R1 current,
which is presumably mapped to the plasma sheet. We will
address these questions at section 6.3.
6. Discussion
6.1. Zero‐Energy Precipitation Boundaries: b1e
and b1i
[49] The locations of the b1e and b1i boundaries relative
to the R2 current provides an important clue about the
overlap between the ring current and the plasmasphere,
which is crucial for better understanding the dynamics of
each plasma population [e.g., Fok et al., 2005; Goldstein et
al., 2005]. If the magnetospheric convection is steady for a
sufficient time for refilling the plasmasphere, the locations
of the b1e and b1i boundaries should agree with the location
of the plasmapause. On the other hand, it is generally
accepted that the R2 current originate in the ring current,
although its source region may extend to the near‐Earth
plasma sheet outside of geosynchronous orbit [Iijima et al.,
1990]. We found in section 5.1 that at dusk to midnight,
both b1e and b1i boundaries are often, more often than not,
located inside the R2 current, which suggests that the
plasmapause is very often located inside the ring current in
this MLT sector. At midnight to dawn, in contrast, those
boundaries are very often located equatorward of the R2
current and occasionally inside but near the equatorward
boundary, of the R2 current. Therefore, the spatial overlap
between the plasmasphere and the ring current may be more
limited in this MLT sector although caution needs to be
exercised because in principle the ring current can extend
closer to Earth than the source region of the R2 current.
6.2. Maximum Ion Energy Flux: b2i
[50] We found in section 5.2 that both before and after
midnight, the b2i boundary is very often located inside the
R2 current. Our result even suggests that the b2i boundary
can be used as an identifier of the R2 current. On the
poleward side of this boundary the pitch‐angle scattering at
the equator is efficient enough to isotropize ions. Thus, a
significant poleward portion of the R2 current should be
described based on the assumption of isotropic ions rather
than, for example, the conservation of the first and second
invariants.
[51] A similar remark applies to the formation of the
Harang discontinuity, which characterizes the ionospheric
convection pattern in the premidnight sector [Maynard,
1974]. The convection flow is directed eastward on its
poleward side and westward on its equatorward side. The
discontinuity originally referred to the reversal of horizontal
ground magnetic deviations, but now the term is used
interchangeably with the convection, therefore electric field,
reversal. The convection electric field converges at the
Harang discontinuity, and therefore, the discontinuity is
collocated with an upward FAC. It is most reasonable to
consider that this upward current consists in the upward R1
current or the middle upward current in case there are three
FAC sheets, which may also be considered to be a R1
current based on the characteristics of particle precipitation
(section 5.2).
[52] The formation of the Harang discontinuity has been
explained in terms of the transport of energetic ions to the
inner magnetosphere and the subsequent magnetic drift
[Erickson et al., 1991]. The b2i boundary is rarely in the R1
current but it is located farther equatorward in the predominant majority of events. It is therefore suggested that in
the magnetotail area corresponding to the Harang discontinuity, the equatorial curvature is large enough to isotropize
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the local ions. For a typical range of the b2i latitude in the
midnight sector (>64°; see Figure 3c), the b2i boundary is
mapped to the equatorial plane outside of geosynchronous
orbit (Plate 1 of Wing and Newell [1998]), and therefore, the
Harang discontinuity is mapped farther outside.
6.3. Other Precipitation Boundaries: b3a, b3b, b4s,
b5e, and b5i
[53] The results of the other boundaries (i.e., b3a, b3b,
b4s, b5e, and b5i) clearly show the dawn‐dusk asymmetries
of FAC and precipitation signatures. At dusk to midnight,
the occurrence distributions of the b3a and b3b boundaries
have their peaks just poleward of the R2/R1 demarcation
and just equatorward of the poleward boundary of the R1
current, respectively, and the b4s boundary is located around
the R2/R1 demarcation. It is therefore suggested that the
upward R1 current is structured and is characterized by
mono‐energetic electron precipitation. The structuredness of
the R1 current may also explain why the b5e boundary is
often, as often as not, located inside the R1 current. In
general, an upward FAC is collocated with electron precipitation, but this does not mean that the energy flux of
electron precipitation is uniform in latitude. If, for example,
small‐scale downward FACs are embedded, the energy flux
can drop significantly, which may be identified as the b5e
boundary. Our results also suggest that in the R2 current at
dusk to midnight, the spectrum of precipitating electrons
changes gradually in space, and field‐aligned electron
acceleration is far less common than in the R1 current. The
overall characteristics are consistent with what one may
expect from the FAC polarities, that is, upward for the R1
current and downward for the R2 current.
[54] The correspondence between FACs and precipitation
is more complex in the midnight‐to‐dawn sector. There are
two points to note. First, the occurrence distribution of the
b3a boundary is peaked in the equatorward half of the
upward R2 current and has extending tails (Figure 8b).
However, we need to be cautious in interpreting this result
in terms of the occurrence of auroral electron acceleration.
The occurrence distribution of the b4s boundary has its peak
in the poleward half of the R2 current, and at 02 ≤ MLT <
04 and 04 ≤ MLT < 06 it is almost confined poleward of the
equatorward boundary of the R2 current (Figure 9b). Thus,
we can conclude that the b3a boundary is often located
equatorward of the b4s boundary, that is, in the middle of
unstructured electron precipitation. We can confirm this
tendency by comparing the |MLat|s of those boundaries at
midnight to dawn (see Figures 3d and 3e). It is therefore
likely that the data point identified as the b3a boundary at
midnight to dawn is often embedded in diffuse electron
precipitation as we found in the 24 July 1997 event.
[55] Second, at midnight to dawn, the b3b boundary is
located most frequently around the poleward boundary of
the downward R1 current, l = 1 (Figure 8b), although its
occurrence distribution has extending tails. That is, mono‐
energetic electron precipitation often takes place inside the
R1 current as well as farther poleward as we saw in the
24 July 1997 event (Figure 2b). Since mono‐energetic
electron precipitation should be collocated with upward
FACs, this result suggests that the downward R1 current at
midnight to dawn is very often structured with upward FACs
embedded in it.
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[56] The structuredness of the R1 current may also be the
reason why the b5i boundary is often located inside the
downward R1 current rather than farther poleward at midnight to dawn. Poleward of the b2i boundary, the magnetic
field lines are considered to be stretched enough to effectively fill the loss cone at the equator. The latitudinal profile
of the precipitating ion energy flux therefore reflects its
equatorial distribution unless precipitating ions are blocked
or decelerated by field‐aligned potential differences. If the
ion energy flux abruptly drops in the middle of the
downward R1 current (as indicated by the b5i boundary), it
suggests either that there exists a sharp spatial gradient of
the ion energy density in the plasma sheet or an upward
field‐aligned electric field prevents magnetospheric ions
from precipitating into the ionosphere. For the former we
cannot think of any good candidate. If the latter is the case
the spatial scale of the upward electric field should be
much smaller than the scale of the downward R1 current,
which is consistent with the above discussion about the b3b
boundary.
7. Summary
[57] In this study we statistically examined the location of
various precipitation boundaries relative to R2 and R1 currents on the night side. For each DMSP pass, the precipitation boundaries were identified based on the definitions
proposed by Newell et al. [1996] and the FAC structure was
identified by the automated procedure developed by Higuchi
and Ohtani [2000a, 2000b]. Except in the analysis of the b2i
boundary, we focused on two‐sheet FAC structure events
with downward R2 and upward R1 currents at dusk to
midnight and with upward R2 and downward R1 currents at
midnight to dawn. The following is the summary of the
results:
[58] 1. The b1e and b1i boundaries (electron and ion zero‐
energy boundaries) are located mostly inside the R2 current
at dusk to midnight and near the equatorward boundary of
the R2 current or farther equatorward at midnight to dawn.
This result suggests that the overlap between the ring current
and the plasmasphere is more significant at dusk than at
dawn.
[59] 2. The b2i boundary (the maximum energy flux of
ion precipitation) occurs in the R2 current both before and
after midnight. This is also the case for three‐sheet structure
events in the premidnight sector. For premidnight events
with the opposite R2 and R1 polarities, the b2i boundary
tends to occur equatorward of the R2 current. Such two‐
sheet structure events may be interpreted as a poleward part
of the three‐sheet structure with a R2 current missing. These
results suggest that the b2i boundary can be used as an
identifier of the R2 current.
[60] 3. The occurrence distributions of the b3a and b3b
boundaries (the most equatorward and poleward electron
acceleration events) indicate that mono‐energetic electron
precipitation is mostly confined in the upward R1 current
at dusk to midnight, whereas at midnight to dawn, it occurs
in a wider latitudinal range including the downward R1
current.
[61] 4. The b4s boundary (the transition between structured and unstructured electron precipitation) tends to occur
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around the R2/R1 demarcation, but its occurrence distribution has extending tails.
[62] 5. The distributions of the b5e and b5i boundaries
(the poleward boundaries of the electron and ion auroral
oval) are centered around the poleward boundary of the R1
current, but they have very extending tails especially at
midnight to dawn.
[63] Results 3–5 suggest that the R1 current is more
structured than the R2 current and that the FAC structure is
more complex at midnight to dawn than at dusk to midnight.
[64] Acknowledgments. Magnetometer and particle data from the
DMSP satellites were provided by F. J. Rich, G. Wilson and the Air Force
Research Laboratory. We thank G. Ueno for his assistance in processing
the DMSP magnetometer data. Work at APL was supported by NSF grants
ATM‐0503065 and ATM‐0538513. The second author (S.W.) acknowledges additional support from NSF grants ATM‐0703445 and ATM‐
0802715 and NASA grant NNX06AB87G.
[65] Robert Lysak thanks the reviewers for their assistance in evaluating this paper.
References
Anderson, B. J., H. Korth, C. L. Waters, D. L. Green, and P. Stauning
(2008), Statistical Birkeland current distributions from magnetic field
observations by the Iridium constellation, Ann. Geophys., 26, 671–687.
Chen, A. J. (1970), Penetration of low‐energy protons deep into the magnetosphere, J. Geophys. Res., 75(13), 2458–2467, doi:10.1029/
JA075i013p02458.
Erickson, G. M., R. W. Spiro, and R. A. Wolf (1991), The physics of the
Harang Discontinuity, J. Geophys. Res., 96(A2), 1633–1645,
doi:10.1029/90JA02344.
Feldstein, Y. I., and G. V. Starkov (1967), Dynamics of auroral belt and
polar geomagnetic disturbances, Planet. Space Sci., 15, 209.
Fok, M.‐C., Y. Ebihara, T. E. Moore, D. M. Ober, and K. A. Keller (2005),
Geospace storm processes coupling the ring current, radiation belt and
plasmasphere, in Inner Magnetosphere Interactions: New Perspective
from Imaging, AGU Monogr., 159, 207.
Goldstein, J., J. L. Burch, B. R. Sandel, S. B. Mende, P. C. son Brandt, and
M. R. Hairston (2005), Coupled response of the inner magnetosphere and
ionosphere on 17 April 2002, J. Geophys. Res., 110, A03205,
doi:10.1029/2004JA010712.
Hardy, D. A., L. K. Schmitt, M. S. Gussenhoven, F. J. Marshall, H. C. Yeh,
T. L. Shumaker, A. Hube, and J. Pantazis (1984), Precipitating electron
and ion detectors (SSJ/4) for the block 5D/flights 6‐10 DMSP satellites:
Calibration and data presentation, Rep. AFGL‐TR‐84‐0317, Air Force
Geophys. Lab., Hanscom Air Force Base, Mass.
Higuchi, T., and S. Ohtani (2000a), Automatic identification of large‐scale
field‐aligned current structures and its application to night‐side FAC systems, in Magnetospheric Current Systems, edited by S. Ohtani, R. Fujii,
R. L. Lysak, and M. Hesse, p. 389, AGU, Washington, D. C.
Higuchi, T., and S. Ohtani (2000b), Automatic identification of large‐scale
field‐aligned current structures, J. Geophys. Res., 105(A11), 25,305,
doi:10.1029/2000JA900073.
Holzworth, R. H., and C.‐I. Meng (1984), Auroral boundary variations and
the interplanetary magnetic field, Planet. Space Sci., 32, 25.
Iijima, T., and T. A. Potemra (1976), The amplitude distribution of field‐
aligned currents at northern high latitudes observed by TRIAD, J. Geophys. Res., 81(13), 2165, doi:10.1029/JA081i013p02165.
Iijima, T., T. A. Potemra, and L. J. Zanetti (1990), Large‐scale characteristics of magnetospheric equatorial currents, J. Geophys. Res., 95(A2),
991, doi:10.1029/JA095iA02p00991.
A10233
Maynard, N. (1974), Electric field measurements across the Harang discontinuity, J. Geophys. Res., 79(31), 4620, doi:10.1029/JA079i031p04620.
Nakano, S., G. Ueno, S. Ohtani, and T. Higuchi (2009), Impact of the solar
wind dynamic pressure on the Region 2 field‐aligned currents, J. Geophys. Res., 114, A02221, doi:10.1029/2008JA013674.
Newell, P. T., and C.‐I. Meng (1988), The cusp and the cleft/boundary
layer: Low‐altitude identification and statistical local time variation,
J. Geophys. Res., 93(A12), 14,549, doi:10.1029/JA093iA12p14549.
Newell, P. T., Y. I. Feldstein, Y. I. Galperin, and C.‐I. Meng (1996), Morphology of nightside precipitation, J. Geophys. Res., 101(A5),
doi:10.1029/95JA03516.
Newell, P. T., V. A. Sergeev, G. R. Bikkuzina, and S. Wing (1998), Characterizing the state of the magnetosphere: Testing the ion precipitation
maxima latitude (b2i) and the ion isotropy boundary, J. Geophys. Res.,
103(A3), 4739, doi:10.1029/97JA03622.
Newell, P. T., J. M. Ruohoniemi, and C.‐I. Meng (2004), Maps of precipitation by source region, binned by IMF, with inertial convection streamlines, J. Geophys. Res., 109, A10206, doi:10.1029/2004JA010499.
Ohtani, S., and T. Higuchi (2000), Four‐sheet structures of dayside field‐
aligned currents: Statistical study, J. Geophys. Res., 105(A11), 25,317,
doi:10.1029/2000JA900074.
Ohtani, S., G. Ueno, T. Higuchi, and H. Kawano (2005a), Annual and
semiannual variations of the location and intensity of large‐scale field‐
aligned currents, J. Geophys. Res., 110, A01216, doi:10.1029/
2004JA010634.
Ohtani, S., G. Ueno, and T. Higuchi (2005b), Comparison of large‐scale
field‐aligned currents under sunlit and dark ionospheric conditions,
J. Geophys. Res., 110, A09230, doi:10.1029/2005JA011057.
Ohtani, S., S. Wing, G. Ueno, and T. Higuchi (2009), Dependence of premidnight field‐aligned currents and particle precipitation on solar illumination, J. Geophys. Res., 114, A12205, doi:10.1029/2009JA014115.
Papitashvili, V. O., F. Christiansen, and T. Neubert (2002), A new model of
field‐aligned currents derived from high‐precision satellite magnetic field
data, Geophys. Res. Lett., 29(14), 1683, doi:10.1029/2001GL014207.
Rich, F. J., D. A. Hardy, and M. S. Gussenhoven (1985), Enhanced ionosphere‐magnetosphere data from the DMSP satellites, EOS, 66, 513.
Sergeev, V. A., M. Malkov, and K. Mursula (1993), Testing the isotropic
boundary algorithm method to evaluate the magnetic field configuration
in the tail, J. Geophys. Res., 98(A5), 7609, doi:10.1029/92JA02587.
Ueno, G., T. Higuchi, S. Ohtani, and P. T. Newell (2007), Particle precipitation characteristics in the dayside four‐sheet field‐aligned current structure, J. Geophys. Res., 112, A06242, doi:10.1029/2006JA012036.
Weimer, D. R. (2001), Maps of ionospheric field‐aligned currents as a
function of the interplanetary magnetic field derived from Dynamics
Explorer 2 data, J. Geophys. Res., 106(A7), 12,889, doi:10.1029/
2000JA000295.
Wing, S., and P. Newell (1998), Central plasma sheet ion properties as
inferred from ionospheric observations, J. Geophys. Res., 103(A4),
6785, doi:10.1029/97JA02994.
Wing, S., S. Ohtani, P. T. Newell, T. Higuchi, G. Ueno, and J. Weygand
(2010), Dayside field‐aligned current source regions, J. Geophys. Res.,
doi:10.1029/2010JA015837, in press.
Winningham, J. D., F. Yasuhara, S.‐I. Akasofu, and W. J. Heikkila (1975),
The latitudinal morphology of 10‐eV to 10‐keV electron fluxes during
magnetically quiet and disturbed times in the 2100‐0300 MLT sector,
J. Geophys. Res., 80(22), 3148, doi:10.1029/JA080i022p03148.
T. Higuchi, The Institute of Statistical Mathematics, Tokyo 106‐8569,
Japan.
P. T. Newell, S. Ohtani, and S. Wing, Johns Hopkins University Applied
Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA.
([email protected])
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