1NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA. ... of uniform electron precipitation which produces diffuse aurora (LUI and ANGER, 1973).
J. Geomag. Geoelectr., 44,1239-1249,1992
Substorm-Associated
Changes
in the Particle Precipitation
Pattern
Rumi NAKAMURA1 and TatsundoYAMAMOTO2 1Natl . Inst.of PolarRes.,Kaga,Itabashi-ku,Tokyo173,Japan 1NASA/Goddard SpaceFlightCenter, Greenbelt, MD20771,USA. 2Insttof SpaceandAstr. Sci.,Sagamihara,Kanagawa229,Japan (Received January9,1992;AcceptedMay8, 1992) Relationshipsbetween averagecharacteristicsof particle precipitationand substorm activityare investigatedinthe low latituderegion as wellas inthe highlatituderegionwithin the dusk-to-midnightsector. "Low latitude" is defined here as the region below the equatorwardboundary of the most equatorward electron BPS, while "high latitude" is definedas the region higher than that boundary.DMSPparticle data are dividedaccording to the phase of the substormand accordingto the longitudewith respectto the onset region byreferringto ground-basedmagnetogramsand DMSPauroralimages.At highlatitude,not only electronprecipitationbut also ion precipitationsignificantlyintensifiesin accordance with substorm activity. The energizationof these particlespresumably takes place in the near-tailregionwhere an unstabletail currentdevelopsand locallydivergesassociatedwith substorm onset. At low latitude, temporal and spatial changes of energetic precipitation wouldbe causedby injected,driftingparticlesfrom the substormonsetregions,particularly in the case of electrons. 1. Introduction A number of studies have investigated the characteristics of electron precipitation in the night side aurora! oval (e.g. WINNINGHAM et al., 1975; LUI and ANGER,1973; FELDSTEIN and GALPERIN,1985and references therein). Most of these studies identified two major regions in the electron aurora! oval: BPS and CPS. BPS consists of structured electron precipitation which produces discrete aurora, while CPS is located at the lower latitude side of the BPS and consists of uniform electron precipitation which produces diffuse aurora (LUIand ANGER,1973). This simple BPS/CPS pattern is not always present. A multiple BPS/CPS pattern was, for example, observed during a particular magnetic storm (SANDAHL et al., 1990). It is not yet fully agreed upon as to the relationship between the CPS/BPS precipitation pattern and different regions in the magnetosphere (FELDSTEIN and GALPERIN,1985).One of the key points here is the response of the precipitation pattern to substorms. WINNINGHAM (1975) suggested that the BPS exhibits dynamical change in average energy and intensity, as well as in location, while the CPS is a relatively stable region that experiences uniform increase and decrease in intensity and average energy. LYONSand EvANS(1984) suggested that discrete aurora, and therefore BPS, might be mapped along the outer boundary of the plasma sheet into the tail current sheet. The dynamical change in BPS characteristics is in fact consistent with the discrete aurora! activation during substorms (e.g. AKASOFU,1964). However, particle signatures at the boundary of plasma sheet were observed to be less affected by substorm activity than at the central plasma sheet (BAUMJOHANN,1991). The CPS, on the other hand, 1239
1240
R. NAKAMURA and T. YAMAMOTO
was
suggested
change
in
to connect the
however,
plasma
is not
is consistent
all
from
parison reported
In this electron
that
region
into
BPS. source
and
low
spatial
and
as the
equatorward
is defined
latitude
characteristics
of
arc
changes
both
of
high
the
precipitation 1988).
From
expansion
ion
com-
These
pattern.
distribution
divided
the
boundary
of the
onset.
precipitation
We The
their
which
magnetosphere.
in latitudinal
boundary and
1988),
NAKAMURA et al. (1992) expanding aurora, and the
the
substorms.
latitude.
regions
the
et al.,
before in
localized et al.,
auroral-type
observations, region of
also
LUI
a relationship
in
discrete
aurora!
phases
as
(LYONS
particle at the
transient
of both
precipitation
between
most
the
equatorward
relationships
to the
two
electron
magnetospheric
region.
2.
Data
Analysis
We
used
particle
ground-based
observatories.
cipitating
electrons
Auroral
imagery
magnetic
field
1985
to
The
64
the
so that
the
local
divided
according
instead
of the AE
regardless
of
Figure coordinates satellite
pass
of
11
are
of the
is that
size
of the the
and
satellite
1(b)
north-south
to
because
11,
shows
satellites keV
board and
Campaign
(HARDY
the
and
measured
pre-
et al.,
satellites.
Scandinavia
region.
northern
the
1984).
The from
activity
the
respect
first
hemisphere
GADC
December
during
The
advantage at the
data the
analysis
GADC
in the
of using
substorms
of the
the
particle
satellite
to
part
observatories
determined. The
with For
ground-based be
of the
relationships
onset
substorm.
of the
sec
the
line.
(•`19•K).
zone
time
The
local
In this
zone
period
auroral
sets
zone
were
then
magnetogram
location
can
data
be determined
of the sector.
line,
whereas
we
concentrated
study,
aligned
in corrected
lines).
Each
represents
distribution
north-south
stations
(solid
number
premidnight
north-south
are
auroral
auroral
local
the the
they
on
Alaska
satellites
1988).
could
in the
of 380
along
radiometer
30
F7
Dynamics
DMSP and
and
substorm.
within
almost
eV
F6
Aurora!
the
Temporal
near
the phase
tracks
located
30
in the
distribution
a length
to the 06
phase
shows
are
region
to the
between
to the
orbits
activity
1(a)
aligned
slanted from
the
side
the
(1)
DMSP
Global
board
et al.,
respect
geomagnetic
Figure
them
F7 night
index
with
(hour).
All
with
the
Canada,
parts:
by
on
scanning
(OGuTI
relationships
traversed
the
in Northern
of two
DMSP
at
energies
by
taken 1986
obtained
obtained
with
provided
31,
data
experiments
consists
satellite
image
data
ions
were
January
selected
when
field
was data
Spatial
auroral
Particle and
analysis
(2)
and
magnetic
(GADC)
time
temporal at different
well
with
is expected
The 1984;
to identify
observation
at the
1977).
BAKER,
as
correlate
dependence
regions:
discuss
to
et al., (e.g.
altitude
satellite
precipitation
examine
regions
We
at low
found
(LUI
It is important
aurora and DMSP ion precipitation
precipitation
two
sheet substorm
CPS.
S3-3
ion
we
ion
precipitation
we
been
substorm
study,
and
a stable
polar-orbiting
energetic
suggest
plasma during
observations
has
the
of such
results
and
these
between ground-based enhanced energetic
absence
15,
with
precipitation
obtained
central
particles
consistent
with
Ion
to the sheet
and
same
passes later
two on
geomagnetic corresponds
its approximate
The the
line
shown
four
passes passes
located
and
such
as
within
in Fig.
passes 03
the
to
a
universal
from 05 the
are
1(a). 06
later
coverage
to
rather four, of
the
in
the
observatories. For northern within
the
second
part
hemisphere the
DMSP
of
from image
data.
the
analysis
January Particle
to
we
selected
March data
sets
1986 were
32 when divided
DMSP an
F6
and
expansion
according
F7
orbit
aurora to the
passes was
longitude
detected relative
Substorm-Associated
Fig.
1.
(a)
(•`19•K).
The
number
DMSP
passes
the
on January
expansion We
structured
electron that
of
most
the
intended
to
the
keV
electrons
slightly
from
between
two
such
the
image
and
to a satellite
of each
pass.
(b)
1241
satellite
tracks
pass
with
a length
Local
time
distribution
in the
of 380
sec
of the
data.
"low
latitude". and
the
observed
the
region
effect
where
CPS/BPS by
the
SANDAHL
with
such
energy
a field an
effect
al.
"high
at
least
to
discrete
aligned is
as
potential negligible.
drop
BPS
the
boundary arc.
is
We
dominant
classification
we within
the
Latitude
auroral
This
example, being
latitude" all
1keV.
corresponds
For (1990)
at including
of
equatorward
classification. et
and
latitude
therefore
most of
latitude" zone
boundary
to
the
"low
auroral
peaks
The
hence
where the
as
spectral
BPS
of
at
latitude" with
from
as
time
Pattern
coordinates
corresponds
universal
to
"high
region
that
geomagnetic
line
characteristics
precipitation
separate
BPS
approximate
referring
identify
is called
corrected
Each
Precipitation
(a).
by
equatorward
in the Particle
in
lines).
precipitation
differs
region.
a few
in
stations
(solid the
shown
we
region
zone
1986
represents
aurora
Here
below
auroral 13,
examined
separately.
for
of the
zone
same
to
Distribution
auroral
Changes
included the
high
CPS latitude
1242
R. NAKAMURA and
3.
Temporal
We The
data
sector
Change
first
present
and
were
taken
from
(pass
auroral
06 pass
zone
from
top
to
bottom.
071
SUT
Barrow
(BRW)
activity.
The
Fig. 1)
a quiet
next and
took Pass
spectra during
the
boundary
of
of
and
high
latitude
latitude electron
for
the
flux
ions,
energetic
left
between
for
are
second
the
energy
and
the line,
with
structured are
can
precipitation
see
that
the
which
indicated
to the
YLK
flux
and
JN for
Pass
10
the
line
is indicated
by
commenced west
of
the
at
YLK
College
(COL)
occurred
during
region
number in each to
more
our
and
conventional
of
right. the
high
at lower
ions
indicates
definition.
BPS
to the intense
flux) panel
is at the
precipitation
be
one The
farther
and
five
lines.
onset
YLK
corresponds to
place the
according
with
boundary tends
took
latitude
electron
broken
orbit
before
dotted
consistent
by
for
western
at BRW.
region low
most
(SHM)
activity.
3. The
premidnight north)
the
satellite
place
activity
the
precipitation.
in the
Shamattawa
observatories
latitude
while
precipitation one
are
took
two
latitude
to
disturbance 06
in Fig.
low
station
and
of the
the
shown
of the
region
the
recovery
(JE
passes
DMSP nearest
Pass
during
eastern
Another
low
of DMSP (geomagnetic
(YLK)
0900UT.
the
place
total four
is at the
high
As
intense
the
diffuse
respectively. an
and
of
observatory
located
during
and
crossings
most
crossings
the
by
was
place
the
the
to •`1230UT.
11 took
region the
with
08,
from
at Yellowknife
recovered
pass,
Energy
region
from
latitude
zone
2 showsXMcomponents
zone
disturbance
electrons
examples
auroral
of high
auroral
plotted
from •`1030UT
period.
latitude
are
obtained
and
high
1). Figure
The
and
examples
11 in Fig,
A localized
around
four consecutive
They
trace
an arrow.
discuss
Pattern
four
stations.
magnetogram
(see
in Precipitation
T. YAMAMOTO
The
For
low
these
latitude and
CPS,
latitude
end
latitudes.
Fig. 2. XM components (geomagnetic north) of five auroral-zone magnetograms for the interval when the DMSP passes took place (indicate by broken lines). The magnetogram traces are plotted from the most eastern station to the most western one from top to bottom. PDB, Poste de la Baleine; SHM, Shamattawa; YLK, Yellowknife; COL, College; BRW, Barrow.
Substorm-Associated
Changes in the Particle Precipitation
Pattern
124:
Fig.3. Spectrumsandtotal flux (JEfor the energyflux and JNfor thenumberflux) of ions andelectrons. Thedotted line in each panel indicatesthe boundaryof high latitudeand low latituderegion(see text for its definition).
The first and the third panels are obtained during relatively quiet periods while the second and the fourth panels correspond to disturbed periods (see Fig. 2 for geomagnetic activity) . The energy flux of electrons both at high latitude and at low latitude significantly intensifies associated with substorms. The response of the ion precipitation to the substorm is somewhat different between the two latitude regions. The low latitude ions are quite stable and exhibit little change in energy flux level associated with substorms. On the other hand, the high latitude ions significantly enhance associated with substorm activity and the latitude where the maximum flux is observed correlates with that for the electrons. We further examined 64 DMSP events when the satellite passed near one of the GADC auroral zone stations in order to obtain more general relationships between the precipitation
1244
R. NAKAMURA andT. YAMAMOTO
pattern and the substorm phase. We defined six different categories of the geomagnetic activity levels as illustrated in Fig. 4. They are quiet time (q), less than one hour before the bay onset (b), bay increasing (i), maximum or prolonged activity (m), bay recovery (r), and less than one
Fig. 4. Illustration of a typical magnetogram trace (XM component) which represents the six categories of the geomagnetic activity level: quiet time (q), less than one hour before the bay onset (b), bay increasing (i), maximum or prolonged activity (m), bay recovery (r), and less than one hour after recovery (a). The number of the events for each category is given below.
Fig. 5. Frequency of occurrence when the energy flux exceeds certain values plotted as a function of the activity level for the electron precipitation at high latitude (upper left) and at low latitude (lower left) and for the ion precipitation at high latitude (upper right) and at low latitude (lower right).
Substorm-Associated
Changes
in the Particle
Precipitation
Pattern
1245
hour after recovery (a). The number of the events for each category is also given in the figure. We compared the intensity of the ion/electron precipitation at the high/low latitude among these defined activity levels. Figure 5 shows the frequency of occurrence when the energy flux exceeds certain values plotted as a function of the activity level defined in the previous figure. The two upper panels are for the high latitude precipitation, while the two bottom panels are for the low latitude precipitation. The electron precipitation in the high latitude region (upper left panel) is always more energetic than that in the low latitude region (lower left panel). In the high latitude region the possibility to get more energetic electron precipitation increases and decreases in accordance with substorm development and recovery. These electron signatures at the high latitude are consistent with the discrete auroral activity observed from the ground in the premidnight sector. That is, stable arcs exist even during very quiet time, while discrete aurora activate during substorms (e.g. AKASOFU,1964). The low latitude electrons, on the other hand, also energize in association with substorms. The intensification persists even after the recovery. The development and the decay of the low latitude electrons occurs with some time delay relative to substorm phases. In contrast to the electrons, the ions are always more energetic in the low latitude region (lower right panel) than in the high latitude region (upper right panel) for all activity levels. The low latitude ion precipitation becomes somewhat energetic prior to the onset and reduces already at the maximum of geomagnetic activity. Hence, this precipitation seems to be less affected with substorm activity. This is consistent with proton auroral signatures at the equatorward edge of the duskside auroral oval which were observed during various geomagnetic activity conditions (ONOet al., 1987). On the other hand, the high latitude ion precipitation significantly intensifies in accordance with the substorm activity as well as with the change in high latitude electron precipitation. 4. Precipitation Pattern Relative to the Expansion Aurora Associated with the substorm expansion onset, the aurora expand not only poleward but also westward (and somewhat eastward), and form a bulge (e.g. AKAS0FU,1964).A bulge (or an expansion aurora) generally consists of several different types of aurora as shown in Fig. 6(a), which is an image taken from DMSP 6. At the western edge of an expansion aurora there is a bright surge at the north of a discrete auroral arc, while the eastern part consists of more thin auroral structures (the eastward propagating aurora) and diffuse aurora at the low latitude side. Bright N-S-aligned aurora can be seen in between. The auroral break up develops from the region between the surge and the N-S aurora. Clearly, the precipitation characteristics are expected to change in the east-west direction relative to the onset region. In order to examine this spatial relationship of the precipitation characteristics and the expansion aurora, we selected DMSP events when the relative longitude of the pass could be determined from the auroral image. The data sets were divided into four longitude sectors as illustrated in Fig. 6(b). They are west of the surge head (denoted as WW), surge region (W), central part with N-S aurora (C), and eastern part of expansion aurora (E). Figure 7 shows the frequency of occurrence when the count level of 3keV- and 30keVdetectors for electrons/ions exceeds 1 count plotted for the four longitude sectors, which were defined in the previous figure. Here, the 3keV particles represent precipitating particles that have an energy comparable to the field aligned potential, whereas 30keV particles represent
1246
R. NAKAMURA and T. YAMAMOTO
Fig.6. (a)Anexampleofan expansion auroratakenfromDMSP6. (b)Illustrationoftheexpansionauroraandour definitionofthefourlongitudesectorsrelativetotheaurora;thesurgehead(denotedasWW),surgeregion(W), centralpartwithN-Saurora(C),andeasternpart of expansionaurora(E).
particles whose energization would take place in the magnetospheric source region. At high latitude, the 3 keV electron precipitation is detectable throughout the longitude (see the upper left panel). This is expected from the discrete auroral activity. The 30keV electron precipitation is enhanced at the surge and at the central region of the bulge. This is consistent with previous observations of the electron precipitation near the surge (MENDet al., 1978). Most energetic electron precipitation at the high latitude, therefore, is located where the auroral breakup starts. The high latitude ion precipitation (upper right panel) significantly reduces to the west of the surge head for both 3keV and30 keV. From Figs. 5 and 7 it is suggested that the ion precipitation at the high latitude is also spatially as well as temporarily confined to the substorm activity region. At low latitude, on the other hand, the longitude of the breakup region is not where the most energetic precipitation occurs. The low latitude electron precipitation (lower left panel) tends to reduce west of the surge head and to enhance as one goes to the east of the expansion aurora. This tendency is particularly clear in the 30keV precipitation. As for
Substorm-Associated
Changes
in the Particle
Precipitation
Pattern
1247
Fig: 7. Frequency of occurrence when the count level of 3 keV- and 30 keV-detectors exceeds 1count in the electron precipitation at high latitude (upper left) and at low latitude (lower left) and in the ion precipitation at high latitude (upper right) and at low latitude (lower right). Differential flux level corresponding to 1count for each energy channel is also given in the figure.
the ions, the low latitude precipitation (lower right panel) can be seen in the entire region independent of the location relative to the expansion aurora. 5. Discussion and Conclusion We have shown that the enhancement of electron precipitation in the high latitude region during substorms is accompanied by enhancement of ion precipitation both in time (Figs. 3 and 5) and in space (Fig. 7). The region where maximum energy flux of ion precipitation is observed coincides with the region of the strongest electron precipitation. Such an energetic ion precipitation region was found to coincide with the poleward expanding auroral arc from simultaneous all-sky TV and satellite observations (NAKAMURA et al., 1992). That is, these ions are observed at a latitude region where a field aligned potential above the satellite causes acceleration of electrons but deceleration of ions. The acceleration of such ions therefore should occur either at the southern conjugate region or in the magnetotail region. We interpret the source region to be the magnetotail because the observed ions have about a factor of ten more energy than the usual field aligned potential drop of a few kV. LYONSet al. (1988) reported on a general association between the discrete aurora and
1248
R. NAKAMURA andT. YAMAMOTO
isotropic ion precipitation and suggested that these ions thread the tail current sheet at distances where ion motion violates the guiding center approximation. From the observed strong temporal relationships between substorm activity and energetic precipitation for both ions and electrons, we suggest that the energization of these particles is related to the substormassociated processes. In the late growth phase and early expansion phase an extremely thin current sheet was observed to develop close to the earth tail region where ions are scattered to become isotropic (BAKERand MCPHERRON,1990). We interpret the energization of the observed ions and electrons to take place in such a substorm-associated near-earth current sheet. Such a thin current sheet is unstable and leads to its local disruption (MITCHELLet al., 1990). A field aligned current system is produced and the electron precipitation, causing auroral breakup, is also expected to take place in that region. The energetic particle precipitation, which we observed mostly at the breakup region is therefore consistent with this interpretation. After the substorm expansion onset when the field is changing from a tail-like to a Bipolar configuration both ions and electrons are further accelerated due to the inductive electric field (HEIKKILAand PELLINEN,1977; WILLIAMSet al., 1990), causing probably further enhancement in energetic particle precipitation. At low latitude, the electron precipitation dynamically changes associated with substorm activity. The CPS therefore seems not to be a stable region which merely shifts its latitude as was suggested by WINNINGHAM et al. (1975). Important characteristics are that the enhanced precipitation lasts until after the recovery (Fig. 5) and the region of the electron precipitation, particularly that of energetic particles, is predominantly to the east of the expansion aurora (Fig. 7). We interpret these precipitation characteristics as follows: the observed electrons at the low latitude consists of electrons which are injected at the substorm onset into the inner magnetosphere. This region would expand due to the earthward and eastward drift of the injected electrons. Furthermore, during multiple expansion events, successive expansion which might predominantly tend to occur west of the previous ones would also cause the expansion of this precipitation region. This eastward extension of the energetic electrons from westward moving source regions can explain the observed temporal and spatial relationship of the low latitude electron precipitation to the substorm activity. The characteristics of the low latitude ion precipitation can only be partly explained by the injected and westward drifting ions from a westward moving source region as discussed for the electron precipitation. The ions seem to have more permanent structure where they tend to be more energetic with decreasing latitude, independent of the substorm phase (Fig. 3). This is consistent with an adiabatic compression of the plasma sheet (HARDYet al., 1989). The low latitude ion precipitation would also be controlled by such a mechanism which has a longer time scale than the substorm expansion. These different behaviors of electron and ion become crucial when we infer from the precipitation pattern its conjugate area of the onset region in the magnetosphere. Wegratefullyacknowledgeuseful discussionswith T. Hirasawaand D. N. Baker.We are greatly indebtedto those members of the GeophysicsResearch Laboratory,University of Tokyo who were involvedinconstructingthe instrumentsand collectingthemagneticdata.The DMSPdatawas provided by WDC-Afor Solar TerrestrialPhysics through WDC-C2for Aurora (National Institutefor Polar Research).We would like to thank H. Nakajimafor his help in constructing computerprogramsfor DMSPdata plot. The work of R. Nakamurawas supportedby the fellowshipsof the JapanSocietyfor the Promotionof Sciencefor JapaneseJunior Scientists.
Substorm-Associated Changes in the Particle Precipitation Pattern
1249
REFERENCES AKASOFU,S.-I., The development of auroral substorms, Planet. Space Sci.,12, 273-282, 1964. BAKER,D. N., Particle and field signatures of substorms in the near magnetotail, in Magnetic Reconnection in Space and Laboratory Plasmas, Geophysical Monograph 30, edited by E. N. Hones, 193 pp., American Geophysical Union, 1984. BAKER,D. N. and R. L. MCPHERRON, Extreme energetic particle decreases near geostationary orbit: a manifestation of current diversion within the inner plasma sheet, J. Geophys. Res., 95, 6591-6599,1990. BAUMJOHANN, W., Heating and fast flows in the near-earth tail, in Magnetospheric Substorms, Geophysical Monograph 64, edited by J. R. Kan, T. A. Potemra, S. Kokubun, and T. Iijima,141 pp., American Geophysical Union, 1991. FELDSTEIN, Y. I. and Tu. I. GALPERIN, The auroral luminosity structure in the high-latitude upper atmosphere: Its dynamics and relationship to the large-scale structure of the Earth's magnetosphere, Rev. Geophys., 23, 217275,1985. FRANK,L. A. and K. L. ACKERSON, Observations of charged particle precipitation into the auroral zone, J. Geophys. Res., 76, 3612-3643, 1971. HARDY,D. A., L. K. SCHMITT, M. S. GUSSENHOVEN, F. J. MARSHALL, H. C. YEH, T. L. SHUMAKER, A. RUBE,and J. PANTAZIS, 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, Bedford, Mass, 1984. HARDY,D. A., M. S. GUSSENHOVEN, and D. BRAUTIGAM, A statistical model of auroral ion precipitation, J. Geophys. Res., 94, 370-392,1989. HEIKKILA, W. J. and R. J. PELLINEN, Localized induced electric field within the magnetotail, J. Geophys. Res., 82, 1610-1614, 1977. LUI,A. T. Y. and C. D. ANGER,A uniform belt of diffuse auroral emission seen by the ISIS-2 scanning photometer, Planet. Space Sci., 21, 799-809, 1973. LUI,A. T. Y., D. VENKATESAN, C. D. ANGER,S.-I. AKASOFU, W. J. HEIKKILA, J. D. WINNINGHAM, and J. R. BURROWS, Simultaneous observations of particle precipitation and auroral emissions by the ISIS 2 satellite in the 19-24 MLT sector, J. Geophys. Res., 82, 2210-2226, 1977. LUI,A. T. Y., R. E. LOPEZ,S. M. KRIMIGIS, R. W. MCENTIRE,L. J. ZANETTI,and T. A. POTEMRA, A case study of magnetotail current sheet disruption and diversion, Geophys. Res. Lett.,15, 721-724, 1988. LYONS,L. R. and D. S. EVANS,An association between discrete aurora and energetic particle boundaries, J. Geophys. Res., 89, 2395-2400, 1984. LYONS,L. R., J. F. FENNELL,and A. L. VAMPOLA, A general association between discrete auroras and ion precipitation from the tail, J. Geophys. Res., 93, 12932-12940,1988. MENG,C.-I., A. L. SNYDER, Jr., and H. W. KROEHL, Observations of auroral westward traveling surges and electron precipitations, J. Geophys. Res., 83, 575-585, 1978. MITCHELL, D. G., D. J. WILLIAMS, C. Y. HUANG,L. A. FRANK,and C. T. RUSSELL, Current carriers in the near-earth cross-tail current sheet during substorm growth phase, Geophys. Res. Lett.,17, 583-586, 1990. NAKAMURA, R., GADC group, Precipitation pattern of electrons and protons near the onset of auroral substorms, Proc. NIPR Symp. Upper Atmos. Phys., 5, 60-69,1992. OGUTI,T., T. KITAMURA, and T. WATANABE, Global aurora dynamics campaign,1985-1986, J. Geomag. Geoelectr., 40, 485-504,1988. ONo, T., T. HIRASAWA, and C.-I. MENG,Proton auroras observed at the equatorward edge of the duskside auroral oval, Geophys. Res. Lett.,14, 660-663, 1987. SANDAHL, I., L. ELIASSON,A. PELLINEN-WANNBERG, G. ROSTOKER, L. P. BLOCK,R. E. ERLANDSON, E. FRIISCHRISTENSEN, B. JACOBSEN, H. LUEHR,and J. S. MURPHREE, Distribution of auroral precipitation at midnight during a magnetic storm, J. Geophys. Res., 95, 6051-6072,1990. WILLIAMS, D. J., D. G. MITCHELL, C. Y. HUANG,L. A. FRANK,and C. T. RUSSELL, Particle acceleration during substorm growth and onset, Geophys. Res. Lett.,17, 587-590,1990. WINNINGHAM, J. D., F. YASUHARA, S.-I. AKASOFU, and W. J. HEIKKILA, The latitudinal morphology of 10-eV to 10keV electron fluxes during magnetically quiet and disturbed times in the 2100-0300 MLT sector, J. Geophys. Res., 80, 3148-3171, 1975.
