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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, A05215, doi:10.1029/2009JA014587, 2010
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Electron acceleration signatures in the magnetotail
associated with substorms
Y. Asano,1,2 I. Shinohara,3 A. Retinò,4 P. W. Daly,5 E. A. Kronberg,5 T. Takada,3
R. Nakamura,4 Y. V. Khotyaintsev,6 A. Vaivads,6 T. Nagai,7 W. Baumjohann,4
A. N. Fazakerley,8 C. J. Owen,8 Y. Miyashita,9 E. A. Lucek,10 and H. Rème11
Received 20 June 2009; revised 17 October 2009; accepted 2 December 2009; published 21 May 2010.
[1] We present Cluster multisatellite observations of accelerated electrons in the near‐
Earth magnetotail associated with substorms. We found that the hardest electron energy
spectra appear in the earliest stage of substorm expansion in the near‐Earth tail region and
that they gradually become softer during the events. Enhancement of the high‐energy
electron flux occurs generally associated with the bulk acceleration of ions (fast flow) and
electrons. It is also shown that the high‐energy electrons sometimes show preferential
perpendicular acceleration associated with the temporal enhancement of the normal
component of the magnetic field, and then the anisotropic distribution quickly becomes
isotropic. During the dipolarization interval, in which no convection signature is observed,
perpendicular flux drops to less than the initial value, and the parallel flux is more than
the perpendicular flux. The results suggest that the electron acceleration mechanism is
mostly consistent with adiabatic betatron acceleration, while Fermi acceleration is not clear
in the high‐energy part. The effect of the pitch angle scattering is also important. The
dispersive signature of the high‐energy electron flux indicates fast dawnward drift loss,
namely, the three‐dimensional effect of the limited plasma acceleration region.
Citation: Asano, Y., et al. (2010), Electron acceleration signatures in the magnetotail associated with substorms, J. Geophys.
Res., 115, A05215, doi:10.1029/2009JA014587.
1. Introduction
[2] The origin of high‐energy particles with energy beyond tens of kiloelectron volts in the magnetosphere has
been discussed for decades. While many of the particles
come directly from the cosmic or solar radiation penetrating
deep into the magnetotail and are trapped in the radiation
belts, there also exist high‐energy particles in the magnetotail,
supposedly accelerated inside the magnetosphere.
[3] Substorms are known to be the significant phenomena
during which drastic increase of high‐energy particle flux
1
Tokyo Institute of Technology, Tokyo, Japan.
Also at Japan Society for the Promotion of Science, Tokyo, Japan.
Institute of Space and Astronautical Science, Japan Aerospace
Exploration Agency, Kanagawa, Japan.
4
Space Research Institute, Austrian Academy of Sciences, Graz,
Austria.
5
Max Planck Institute for Solar System Research, Katlenburg‐Lindau,
Germany.
6
IRF Uppsala, Uppsala, Sweden.
7
Department of Earth and Planetary Sciences, Tokyo Institute of
Technology, Tokyo, Japan.
8
Mullard Space Science Laboratory, Surrey, UK.
9
Solar‐Terrestrial Environment Laboratory, Nagoya University, Aichi,
Japan.
10
Blackett Laboratory, Imperial College, London, UK.
11
CESR/CNRS, Toulouse, France.
2
3
Copyright 2010 by the American Geophysical Union.
0148‐0227/10/2009JA014587
often occur. Observations of sudden increase of the accelerated particle fluxes are usually initially observed near the
midnight around the geosynchronous orbit associated with
substorms. They are typically called injection [e.g., Arnoldy
and Chan, 1969; Lezniak and Winckler, 1970; DeForest and
McIlwain, 1971], and they may extend to the magnetotail 7–
8 RE from the Earth [e.g., Lopez et al., 1990]. The injection
is phenomenologically well known. Recent numerical simulations also refer to the importance of injected particles
showing that some of them are further accelerated to the
relativistic energy range [Summers et al., 1998; Horne et al.,
2003; Katoh and Omura, 2007]. However, the effect of the
radial diffusion by the ULF (Pc‐5) waves mostly inside the
inner magnetosphere are considered in storm‐time activities
[e.g., Li et al., 2005] and the signature of the azimuthal drift
path in the inner magnetosphere has been studied [e.g., Ejiri,
1978; Reeves et al., 1991; Takahashi et al., 1997]. Hence,
particle acceleration mechanisms and the three‐dimensional
particle flux transport in the magnetotail and its relation to the
substorm activities are not yet clear.
[4] Disruption of the tail electric current [e.g., Lui et al.,
1988; Lui, 1996] associated with substorms is considered
to be initiated in the magnetotail 7–8 RE from the Earth, and
some related acceleration mechanisms are theoretically considered [Lopez et al., 1990]. However, it is not clear if these
ideas are valid, since the acceleration in the current disruption
region has not yet been reported observationally. On the other
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Figure 1. Summary plot of an event on 27 October 2007 between 0900 and 0930 UT.
hand, Li et al. [1998, 2003] reproduced injection‐related
signatures such as dispersionless injections and drift echoes
by applying an earthward propagating electric field pulse.
Smets et al. [1999] and Wu et al. [2006] examined observational signatures of the high‐energy electron distributions and
discussed the relative efficiency of the adiabatic Fermi and
betatron accelerations in time and in space.
[5] Recent theoretical and numerical studies have revealed
that kinetic processes in magnetic reconnection in the magnetotail can also generate high‐energy particles. Near‐Earth
neutral lines are considered to form in the region 20–30 RE
down tail from the Earth [Nishida and Nagayama, 1973;
Hones and Schindler, 1979; Nagai et al., 1998; Baumjohann
et al., 1999; Miyashita et al., 2009] associated with substorm
onsets. Hoshino [2005] suggests that relativistic electrons can
be generated by a combination of the surfing acceleration in
the electric field potential structure around the X line and the
nonadiabatic gradient and curvature drifts in the piled‐up
outflow flux region. This prediction has been supported by
some observations [Imada et al., 2007]. A parallel electric
field in the presence of a guide magnetic field may also
accelerate electrons drastically [Pritchett and Coroniti, 2004;
Drake et al., 2005]. Formation of multiple flux rope structures
with multiple X lines may be able to generate relativistic
electrons by Fermi acceleration [Drake et al., 2006] associated with the coalescence of the island structures [Pritchett,
2008]. Chen et al. [2008] have reported that the island
structures of the flux ropes are related to the enhancement of
the high‐energy electron fluxes. Retinò et al. [2008] have
shown the electron acceleration signature within the thin
current sheet around an X line and in a flux rope structure.
However, it is considered that the accelerated high‐energy
electrons in this region might drift significantly dawnward
[Birn et al., 1998, 2004] and will not be transported to the
active region of substorms in the near‐Earth magnetotail.
Vogiatzis et al. [2005] reported an increase of the high‐
energy electron flux in the recovery phase of a substorm in
the magnetotail, and interpreted the observation by proposing
that the high‐energy electrons have drifted from the near‐
Earth duskward sector where the accelerated particles were
generated by the current disruption process.
[6] In order to understand global acceleration mechanisms
and particle flux transport in the magnetosphere, there is
now a good opportunity to study the physics of this region
using recent Cluster multisatellite observations around 10 RE
away from the Earth in the magnetotail. In this paper, we
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for the analysis presented here, we recalculate accurate pitch
angles for each angular bin using the high‐resolution magnetic field data together with the high‐quality ground calibration. The PAD comprises two data sets per sensor, each
sampled in ∼125 ms collected half a spin apart when the
sensor look directions contain the field direction.
[9] High‐energy electron data are obtained from the
Research with Adaptive Particle Imaging Detectors (RAPID)
instrument [Wilken et al., 2001] for the energy range 37.3–
406.5 keV. In this study we use omnidirectional electron data
(ESPCT6) with the time resolution of one spin (∼4 s), and the
three‐dimensional data (E3DD or L3DD) containing eight or
two energy samples, respectively.
[10] Ion moment data are obtained from the Comprehensive Ion Spectrometer (CIS) [Rème et al., 2001]. Proton data
obtained from the Composition and Distribution Function
analyser (CODIF) instrument are used for Satellite 4 (CL4)
with the time resolution of 8 s (two spins). Ion data without
mass spectrometry from CL1 and CL3 are obtained from the
Hot Ion Analyzer (HIA) instrument with the time resolution
of 12 s (three spins).
[11] Moment and magnetic field data are presented in the
Geocentric Solar Magnetospheric (GSM) coordinate system
if not specified otherwise.
Figure 2. Location of the satellites for 27 October 2007
event.
examine acceleration signatures of electrons and their temporal and spatial evolution during two substorms obtained by
satellites and ground magnetograms, and discuss the physical
processes.
2. Instrumentation
[7] In this paper, we use plasma, magnetic field and
electric field data obtained by the four Cluster satellites in
the magnetotail. Magnetic field data are obtained from the
Fluxgate Magnetometer (FGM) [Balogh et al., 2001]. High‐
resolution data are sampled at 22 or 67 Hz depending on the
telemetry mode. In order to adjust to the times of each
electron energy sweep, these data are linearly interpolated.
In the summary plots shown in this paper, they are integrated with the time resolution of about 4 s. Electric field
data are obtained from the Electric Field and Wave instrument
(EFW) [Gustafsson et al., 2001]. Two‐dimensional high‐
resolution data are sampled at 25 or 450 Hz in the spin plane
of the satellites (Despun System Inverted (DSI) coordinate
system, which is close to the GSE coordinate system). In the
summary plots, it is also averaged over the spin period of
about 4 s.
[8] Low‐ to mid‐energy electron data are obtained from
the Plasma Electron and Current Experiment (PEACE)
instruments [Johnstone et al., 1997]. They consist of two
instruments, High Energy Electron Analyser (HEEA) and
Low Energy Electron Analyser (LEEA) which are located
on opposite sides of the spin plane of the satellites. In combination their energy range is from 0.59 eV to 26.4 keV. We
use in this paper Pitch Angle Distribution (PAD) data. The
original PAD data are obtained with reference to magnetic
field vector sampled once per spin onboard the spacecraft, but
3. Observations
3.1. Event of 27 October 2007
[12] First, we show an event observed in the near‐Earth
magnetotail at Xgsm ∼ −10 RE. We present in Figure 1 a
summary plot of an event on 27 October 2007. This is a very
fortunate event in which the satellites were located just
around the neutral sheet at the timing of the onset. Total
intensity and three components of the magnetic field, x and
y components of the electric field in DSI coordinates, ion
(CL1) or proton (CL4) density and temperature and the x
component of the velocity are shown from top to bottom.
Black, red, and blue lines show the CL1, CL2, and CL4
data, respectively. At the bottom, position data labels for
CL4 is shown as well as the time (UT). The locations of
the three satellites during the event (0900–0930 UT) in xz
(Figure 2, top) and xy (Figure 2, bottom) planes are shown
in Figure 2. All the satellites were moving from the north
to the south and from dawn to dusk in the magnetotail near
the neutral sheet roughly in the same xy plane within the
separation of 10000 km. CL1 and CL2 were located at
[−8336, −194, −306] km and [−4793, 10170, 988] km
relative to CL4, respectively. Namely, CL4 was on the
earthward side of CL1, while CL2 was on the dusk side of
the other satellites. Note that CL3 (not shown) was very
close to CL4 during the interval within 40 km separation,
and did not show any typical difference from CL4 data in
the macroscopic view.
[13] A fast earthward flow was first detected by CL1 and
CL4 at 0906:30 UT, and it continued until 0909:30 UT. From
the timing difference of the associated temporal enhancement
of By and Bz among the satellites, we roughly estimate that
the earthward propagation speed about 600 km s−1. During
this interval there was a large amplitude oscillation of Bx,
followed by weak Bx. After that, continuous large Bz was
observed by CL4, namely, a dipolarization only in the near‐
Earth region. This dipolarization at CL4 location during the
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Figure 3. Ground magnetogram data of the 27 October 2007 event.
interval is not so steady, and becomes more dipolar later.
Smaller perturbation of Vx and the magnetic field were again
observed at around 0914 UT and 0921 UT. Tailward
expansion of the dipolarization region and the reconfiguration of the magnetotail during the event is further described
by Nakamura et al. [2009].
[14] In order to examine substorm activities during the
interval, Figure 3 shows ground magnetic field data for the
event. Figure 3 (top) shows the AL index. Following a
previous substorm event at about 0830 UT, the AL index
began to decrease again at 0906 UT. A Pi2 onset was
detected at CCNV (magnetic latitude = 45.35°, magnetic
longitude = 304.84°) at 0905 UT (Figure 3, bottom). No
clear enhancement of the positive bay was observed here,
associated with the onset (Figure 3, middle). It is also found
that the tail activity at 0914 and 0921 UT mentioned above
was associated with small and then large evolution of the
AL index, respectively. Azimuthal evolution of the substorm activity between 0904:18 and 0908:36 UT is examined using signatures of the auroral brightening observed by
the Polar UVI instrument shown in Figure 4. Local time is
indicated in MLT in the first panel (Figure 4, top left).
Location of the Cluster satellites is indicated in the fifth
panel (Figure 4, bottom left). It is clearly seen that the initial
brightening was observed on the dusk side, between 20–
21 MLT at 0904:55 UT, which was also duskward of the
satellite locations. After that, we can also see that the second
brightening initiated at 0907:22 UT is between 21–22 MLT,
Figure 4. Auroral brightening observed by Polar UVI data
for 27 October 2007 event.
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Figure 5. Energy‐time spectra of ion (CL1) or proton (CL4) directed earthward and tailward; omnidirectional electrons are shown.
and this brightening included the foot point of the satellites.
Prior to the initial fast flow, CL2 observed a large magnetic
field oscillation at 0904 UT (no ion data for the satellite) as
is shown in Figure 1. Considering the facts that CL2 was
duskward of the other satellites, and that the initial auroral
brightening was observed duskward of the satellite location,
it is likely that only CL2 observed the initial, spatially
limited magnetospheric perturbation. Then the activity extended toward the midnight as indicated by the second
brightening, so that all the satellites observed magnetospheric disturbances. This result is consistent with the evolution of the geomagnetic activity shown in the work of
Nakamura et al. [2009, Figures 1 and 2].
[15] Figure 5 shows energy‐time spectra between 0904
and 0918 UT, specifically earthward and tailward ion fluxes
from CL1, proton fluxes from CL4, and omnidirectional
electron fluxes from the both satellites. The start time of the
fast flow is marked by a vertical dotted line. During the fast
earthward flow interval, the CIS ion data clearly show an
intensification and also an energization of the earthward
fluxes. At times the earthward fluxes are beyond the highest‐
energy range of the instruments, delimited by dashed vertical
lines. Despite the different duration of the fast flows measured at CL1 and CL4, both satellites clearly observed a
continuous earthward energetic component until 0909 UT.
Between 0907:50 and 0908:20 UT, average Ey at CL1 is
Figure 6. Electron pitch angle distribution of the phase space density during the accelerated interval.
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Figure 7. Electron fluxes of the same interval as in Figure 5. Low‐energy flux spectra were obtained by
PEACE, and high‐energy fluxes were obtained by RAPID in CL1, CL2, and CL4 satellites. High‐resolution
magnetic field and Vx are plotted from top to bottom.
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Figure 8. Energy spectra of high‐energy electrons on CL4 during the event are shown.
9.7 mV m−1, and Vx is estimated to be about 1990 km s−1
from Ey/Bz. During this time, we see that electrons are
accelerated simultaneously with the ions. In particular, after
0907:30 UT at CL1 and 0908:35 UT at CL4, electrons were
highly accelerated and ions were beyond the energy range.
An electron distribution of the phase space density (PSD)
at 0908:53 UT during the acceleration observed by CL4
is shown in Figure 6. The left half of Figure 6 (left) shows
data from the HEEA instrument. LEEA data (only low‐
energy part) looking at the opposite direction of the HEEA
is shown on the right half. Electron pitch angle data are
presented in a magnetic field coordinate system, in which
parallel to the local magnetic field is set to 0° (upward) and
180° for antiparallel direction. The perpendicular direction
to the magnetic field is set to 90° (horizontal). In Figure 6
(right), one‐dimensional cut of the PSD is presented. The
cut data are obtained from both instruments (HEEA/LEEA)
in the several pitch angles indicated at each line. Each label
shows the instrument (HEEA/LEEA) of the data and a pitch
angle. We can see that electrons are accelerated beyond the
PEACE instrumental energy range. It should be also noted
that the distribution is isotropic. Such isotropic acceleration
indicates either the acceleration is not caused by the adiabatic acceleration such as betatron acceleration or Fermi
acceleration, or the accelerated particles are immediately
pitch angle scattered. Such simultaneous acceleration was
observed again, between 0912:10 and 0912:45 UT and
between 0914:30 and 0915:50 UT by CL1, and between
0914:45 and 0915:35 UT by CL4.
[16] In Figure 7, in order to investigate signatures of high‐
energy electron acceleration in comparison with signatures
of ions, thermal electrons and magnetic field in the substorm,
we show the electron flux of both high‐ and low‐energy
components together with the high‐resolution magnetic
field data and Vx from top to bottom. Here, omnidirectional
high‐energy electron fluxes and low‐energy electron fluxes
with pitch angles of 0°, 90°, and 180° are plotted. After the
initial decrease of the high‐energy flux was observed by all
satellites at 0904 UT, the flux increased until 0906:40 UT at
CL1, until 0905:25 UT at CL2, and until 0906:50 UT at
CL4. This flux increase is observed before the appearance
of a fast flow at 0907 UT with a slightly dispersed signature.
The dispersion signature was less clearly observed by CL2.
Then, more drastic increase of the flux is observed at 0907
UT by CL1 and CL4. At this time, the fast earthward flow
accompanied by the acceleration of thermal electrons was
observed by CL1, CL3, and CL4. The peaks of the high‐
energy flux were detected between 0908:40 and 0908:50 UT.
CL2 on the dusk side the other satellites near the southern
plasma sheet lobe boundary did not observe significant
acceleration. After that, there were also several enhancements
between 0912 and 0915 UT associated with magnetic oscillations and earthward fast flows. In these flux enhancements,
thermal electrons observed by PEACE were more accelerated
than the initial enhancement. At 0912 UT, only CL1 and CL2
observed the acceleration but CL4 with enhanced Bz did not
observe such acceleration without any significant perturbation of the magnetic field. After these flux enhancements, the
magnetic field was relatively steady and higher‐energy flux
(>100 keV) preferentially decreased quickly. Figure 8 shows
the spectra of the omnidirectional PSD of the high‐energy
electrons for CL4. Relative to the initial state at 0904 UT
(thin solid line with plus marks, power law spectral index
g = −3.64), the high‐energy component (>100 keV) increased
in the course of the fast flow and showed harder spectra (thick
solid line with circles, g = −2.97). In the late phase of the
fast flow and after the fast flow, the lower‐energy component
(∼50 keV) gradually increased (thick dashed line with
crosses). After the fast flow (thin dashed line with triangles),
the high‐energy flux gradually decreased, and became even
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Figure 9. Energy spectra observed before (solid line with
plusses) and after (bold dashed lines with squares) the Bz
enhancement and spectra theoretically expected (dashed line
with plusses) from the betatron acceleration. Perpendicular
fluxes during both accelerated intervals and parallel flux
in Figure 9b are also shown.
less than the initial state. Thus, a significantly softer spectrum
(g = −4.45) was observed. Namely, the spectrum of the
high‐energy electrons varies significantly in the course of
substorms.
[17] Next, we investigate pitch angle distributions of
electrons in order to discuss generation and loss processes
of electrons in the near‐Earth magnetotail. In Figure 9, we
display evolution of the electron PSD spectra during the
temporal Bz enhancement. In Figure 9a, we show the initial omnidirectional spectrum before the enhancement at
0906:28 UT observed by CL1 as a thin solid line with plus
marks. At this time, Bz was 3.17 nT. Then a fast earthward
flow was detected associated with an enhancement of the
magnetic field not only Bz but also By. After the passage of
the enhanced magnetic field structure, Bz was enhanced to
6.78 nT at 0907:55 UT. An omnidirectional spectrum at this
time is shown as a thick dashed line with open squares. The
expected spectrum assuming that the betatron acceleration
caused by the increase of Bz has occurred is shown as a thin
dashed line with plus marks. We found that the expected
and the observed spectra are similar. Thus, the initial
acceleration is supposed to be adiabatic. Note that the perpendicular component at this time (a thick solid line with
filled circles) is almost the same as the omnidirectional
PSD although we have only two data samples at energies
<100 keV. Therefore, it is not so simple as electrons have
not only been accelerated by betatron and Fermi acceleration, but experienced some additional process. Hence, we
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argue that the perpendicularly accelerated particles can be
considered to be pitch angle scattered immediately after
acceleration, and the distribution becomes isotropic. Figure 9b
shows a similar signature during the second enhancement
observed by CL4 from 0914:01 UT to 0914:34 UT when Bz
enhances from 12.61 nT to 31.73 nT. In this case, the clear
enhancement of only the perpendicular flux is observed.
On the other hand, the parallel component (thick solid line
with filled squares) or the omnidirectional PSD increase
only slightly. Since the enhanced perpendicular component
is comparable to the expected spectrum assuming betatron
acceleration, the acceleration mechanism of the high‐energy
electrons is considered to be consistent with the betatron
acceleration.
[18] In Figure 10, parallel (0°–20°, solid lines) and perpendicular (80°–100°, dashed lines) fluxes observed by CL1
and CL4 between 0904 and 0920 UT are shown in the two
available energy steps, 37.3–50.5 keV (thin lines) and 68.1–
94.5 keV (thick lines) in Figures 10b and 10g. They are
presented with Bx, Bz (Figures 10a and 10f), and Vx (Figures 10e
and 10j). Since the parallel and antiparallel fluxes were
mostly the same within the error in this interval, we present
only the parallel flux in the figure. We can see that the parallel
and perpendicular fluxes sometimes differ. There are several
intervals in which the perpendicular flux was more than the
parallel flux near the neutral sheet. Such flux anisotropy is
observed both by CL1 and CL4, and is indicated in solid
squares. It is seen that such preferential enhancements of the
perpendicular flux are associated with the rapid and significant increase of Bz. Such Bz increase usually occurs during the
earthward flow occurrence, but sometimes slightly after the
end of the flow. Associated with enhancement of the perpendicular flux, the temporal increase of the total flux is also
observed (Note that the short‐lived increase of Bz at CL1 at
0909 UT interval is shown in Figure 7). There is an exception:
CL4 observed an increase in Bz at 0909:50 UT without
enhancement of electron flux or flow velocity. It is also
interesting that CL4 observed an enhancement of the perpendicular flux at 0913:30 UT without any convection under
the dipolarized magnetic field configuration. At this time,
CL1 and CL2 were not in the dipolarized region on the tail/
dawn sides of CL4. The satellites observed enhancement of
both Bz and high‐energy flux with minor earthward flow
observed by CL1. Thus, the enhanced flux may be drifted
from the nondipolarized region to the dipolarized region
whose boundary may not be simply planar as is observed in
the initial activity (tailward expansion of the dipolarization
region is discussed also by Nakamura et al. [2009]). The
enhancement of the perpendicular flux immediately disappeared when the temporal Bz enhancement ceased. More
isotropic distribution of the energetic particle flux is observed
after such a short preferential perpendicular enhancement.
[19] On the other hand, the excess of the parallel flux
appears associated with the decrease of the perpendicular
flux observed between the enhancements of the flux, as is
indicated by dashed boxes. Excess of the parallel flux was
observed between 0918:00 and 0918:30 UT at CL1 and more
clearly between 0911 and 0912 UT and 0916 and 0919 UT
at CL4. This anisotropic signature is simultaneously observed
with bi‐streaming electrons in the low‐energy range. After
0909 UT, CL4 PEACE detected a bi‐streaming electron
distribution, as is shown in Figure 7. Figure 11a shows such a
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Figure 10. Flux anisotropy of the high‐energy flux between 0904 and 0920 UT in (a–e) CL1 and (f–j)
CL4. In Figures 10b and 10g, parallel and perpendicular flux to the magnetic field are plotted. Figures 10c
and 10h show 37.3–50.5 keV flux ratio of duskward to dawnward, and Figures 10d and 10i show the
68.1–94.5 keV flux ratio.
distribution in comparison with the almost simultaneous
isotropic distribution observed by CL1 (Figure 11b) in the
same format as Figure 6. At CL4, parallel (LEEA measurement up to 2 keV) and antiparallel fluxes exceeded the perpendicular flux by a factor of about ten. Such a difference is
also considered to be related to the spatial structure of the
magnetic field. CL4 was closer to the Earth than CL1 and was
in the dipolar Bz region. On the other hand, CL1 still detected
small Bz which indicates the stretched magnetic field.
[20] It is also noted that parallel flux in the low‐energy
electrons (less than a few keV) is sometimes more than the
perpendicular flux, even in the condition perpendicular flux
in the high‐energy electrons is more than the parallel flux.
Figure 11c shows this type of low‐energy electron distri-
bution observed by CL1 at 0913:31 UT, when the perpendicular high‐energy flux increased more than the parallel
flux. One can clearly see that while the low‐energy
(<40,000 km s−1, ∼5 keV) flux shows an anisotropy
which has more flux in the parallel direction, perpendicular flux becomes more pronounced in the energy range of
>40,000 km s−1. The result suggests that betatron acceleration is not always dominant in all energy levels.
[21] In order to examine the evolution of the pitch angle
distribution more clearly, we present time evolution of the
pitch angle distribution in PSD in Figure 12. The pitch angle
distributions are shown for two major enhancements of
the flux observed by CL4 at 0909 UT (dashed lines) and
0914 UT (solid lines) in comparison with the initial state at
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Figure 11. Electron pitch angle distributions of the phase space density during the quasi‐steady dipolarization interval.
0903:00 UT (solid line). In Figure 12, we show pitch angle
distributions of the electron PSD obtained from RAPID/
L3DD. The energy range is between 68.1 and 94.5 keV and
the pitch angle data is averaged in each 20° bin. It is
observed that the perpendicular flux was more enhanced in
the initial stages of the flux increase (pancake‐type), but it
rapidly disappeared within 1 min, even as the overall fluxes
increased, and became a parallel‐dominated (cigar‐type) distribution. In other words, the decrease of the perpendicular
flux is more significant than that of the parallel flux. The
perpendicular flux became even less than the initial state at the
lowest cases.
[22] We also menton the electron flux anisotropy in the
azimuthal direction. In Figure 10, the flux ratio of duskward
to dawnward electrons (±45° in ygsm and ±60° in zgsm)
averaged over 20 s are also plotted. The flux ratio in 37.3–
50.5 keV and 68.1–94.5 keV energy steps are shown in
Figures 10c and 10d for CL1 and in Figures 10h and 10i
for CL4, respectively. It is mostly close to 1.0, but a
slight excess of the dawnward component was observed
associated with the enhancement of the flux indicated by
error bars. It could be interpreted as either the existence of
the dawnward drift by the magnetic field gradient and the
curvature drift which are directed dawnward in the magnetotail configuration, or the radial density gradient in which
more electrons exist on the earthward side of the satellites.
3.2. Event of 3 September 2006
[23] Here we show another event on 3 September 2006,
which was observed at Xgsm = −15 RE. Figure 13 shows a
summary plot of the event between 2140 and 2220 UT. The
intensity and the three components of the magnetic field at
all four satellites, high‐energy and low‐energy electron flux,
ion flux at CL1, ion density, temperature, Vx and Vy,gse at
CL1 and CL3 are shown from top to bottom. Note that CL3
failed to get part of the ion data from high‐elevation (polar)
directions during the interval. However, we can still work
with the spin plane fast flow velocity, namely, Vx and Vy. As
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Figure 12. Pitch angle distribution of the high‐energy electron phase space density on CL4 during the event.
is shown in the ground magnetogram data Figure 14a, Pi2
onset was observed at 2148 UT and 2154 UT indicated by
two vertical lines. Then a positive bay with the intensity of
∼5 nT evolved until 2220 UT. During the event, satellites
were located in the postmidnight region of the near‐Earth
magnetotail Figure 15. CL3 was at [−15.2, −2.7, 0.7] RE in
GSM at 2200 UT. Relative to CL3, CL1 was on the
dawnward side, CL2 was on the northern side, and CL4 was
on the earthward as well as on the northern side of CL3 by
about 1 RE, forming a tetrahedron configuration. As is seen
from Bx values, CL3 was the only satellite which located on
the southern plasma sheet, and CL1 was the closest to the
neutral sheet while CL2 was the furthest. A fast earthward
flow accompanied by highly accelerated ions and electrons
was observed at 2155 UT. This fast flow would be associated with the second Pi2. A decrease of the Bx difference
(DBx) among the satellites and the enhancement of Bz more
than 10 nT are typical signatures of the dipolarization just
after the fast flow.
[24] While the thermal components of the electrons
(PEACE) did not show any significant variation until
2155 UT, the more energetic electrons (RAPID) first
showed a drop of the flux at 2146 UT. Then the dispersive
increase of the flux up to the highest energy channel was
observed at 2148 UT when the first Pi2 was observed.
After that, the repeated dispersive increase was observed at
2150 UT and 2153 UT. This signature is similar to the
previous event when the auroral brightening was observed
on the dusk side of the satellite location. Figure 14b displays
H and D components at three midlatitude ground magnetograms around the midnight: Borok (Geomagnetic Longitude = 38.23°), Uppsala (17.35°), and Black Forest (8.32°).
Foot points of the Cluster satellites were slightly on the
dawn side of Borok. Uppsala and Black Forest were well on
the dusk side of the foot points. All three stations observed a
clear positive bay. On the other hand, D components show
different polarities, namely, negative at Borok, initially
negative and changed to positive at Uppsala, initially weak
variation and then positive at Black Forest. The results
suggest that the auroral activity (current wedge) was initi-
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ated at around Black Forest, and then extended dawnward.
Therefore, the initial activity is considered to have been well
on the dusk side of the Cluster location.
[25] After 2156 UT, the increase of the high‐energy
electron flux became dispersionless associated with the
observation of local fast flows and magnetic disturbances.
The largest flux of the energetic electrons was observed at
2158 UT, which is at around the end time of the fast
earthward flow. After that, while the lower‐energy flux
(<50 keV) remained almost constant for more than 20 minutes,
the higher‐energy flux (>100 keV) decreased quickly. This
signature is also the same as that observed in the previous
event. The time evolution of the electron energy spectra
shown in Figure 16 is in the same format as Figure 8. Data
points at 2208:30 UT are slightly shifted to the left for
the better visibility of error bars which are also added to
2146:00 UT data. Relative to the initial state (g = −4.15, if the
data are fitted to the power law spectra) it is clear that the
increase of the higher‐energy (>100 keV) component was
significant in the initial phase of the increase and showed the
hard spectrum (g = −2.35). The PSD reached a maximum at
around the end of the fast flow, and subsequently changed to
softer spectrum (g = −5.90). We also show in Figure 17 the
flux anisotropy during the interval in the same format as
Figure 10. In this event, we use E3DD data set. For CL4, the
lowest‐energy channel data (37.3–50.5 keV) are not shown in
the parallel/perpendicular flux because of the nonnegligible
noise level. Again, one can see the temporal increase of
the perpendicular flux associated with the rapid increase of
Bz at 2156 UT and 2159 UT by CL1 and 2158 UT by CL4.
The excess of the perpendicular flux lasted for one to a few
minutes, otherwise, rather isotropic. It is also clearly seen that
the perpendicular flux dropped during the quasi‐steady
dipolarization intervals (2204–2212 UT for CL1 and 2207–
2212 UT for CL4) after taking several times from the
enhancement of Bz. Dawn/dusk asymmetry was small, but
dawnward flux sometimes exceeded the duskward flux
associated with the enhancement of the flux, as in the previous event.
[26] Figure 18 shows the high‐resolution magnetic field
(67 Hz) and the electric field (450 Hz) between 2155 and
2200 UT. Four colors in the magnetic fields (total intensity
and three components) and Vx show the same satellites as
those in the summary plot (Figure 13). Ex and Ey in the spin
plane (DSI coordinate system) from CL1 and CL3 are shown
separately. Red lines show the 450 Hz high‐resolution data
and the black lines show the spin‐averaged (4 s) electric
fields. At the bottom, wave intensity, degree of polarity
(DOP) and ellipticity of the wave activity derived from the
CL1 EFW data are also displayed. Here we mention the
significant bipolar Bz which was observed at 2156 UT during
the fast earthward flow detection. This bipolar Bz signature
was observed from CL2, CL3, CL1, and then CL4. Thus, the
structure is clearly found to propagate earthward with the
speed of about 400 km s−1 derived from the timing difference.
The structure became more significant with larger amplitude
and shorter interval as the structure approached the Earth. The
bipolar structure is asymmetric. Bz became only a few nT in
the negative part, on the other hand, it became more than
20 nT in the positive part. The four Cluster satellites observed
large electric field just at the enhancement of the Bz. Such
large enhancement of the electric field reached 70 mV m−1.
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Figure 13. Summary plot of the 3 September 2006 event. Intensity and the three components of the
magnetic field for the four satellites, low‐ and high‐energy electron, and ion energy‐time spectra of
CL1, ion density, temperature, Vx, and Vy of CL1 and CL3 are plotted from top to bottom.
The wave activity consisted of mainly lower hybrid waves
(<10 Hz) and some whistler waves (DOP ∼1, ellipticity ∼1,
namely, 100–200 Hz activity with clear right‐handed circular
polarization).
4. Discussion
[27] The origin of the high‐energy particles in the magnetotail has been long discussed for decades. In this paper
we examined two events which were observed at Xgsm ∼
−10 RE and Xgsm ∼ −15 RE. The former event has been
supposed to be slightly outside or just around the region of
the injection boundary [e.g., Mauk and Meng, 1983], while
the latter event is supposed to be between the near‐Earth
neutral line region and the initial dipolarization (current
disruption) region [Miyashita et al., 2009]. We summarize
our observational results in Figure 19. Initially, the mag-
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Figure 15. Locations of the four Cluster satellites for
3 September 2006 event shown in xz and xy planes.
Figure 14. (a) H component and its Pi2 pulsation of the
ground magnetogram data obtained at Urumqi. Pi2 onset at
2148 UT and 2154 UT are indicated by vertical lines. (b) H
and D components of three midlatitude ground magnetograms.
netotail is stretched (black lines). Associated with initial
activities on the dusk side (local time with dashed lines),
dispersive flux enhancement of electrons (magenta dashed
arrow) are observed. In the −10 RE event, magnetic oscillations are also detected by the most dusk‐side satellite (red
one) associated with the dispersive flux enhancement. Then
earthward propagation of fast flows associated with temporal
enhancement of Bz (gray filled arrow) was observed associated with the second activities dawnward of the initial
activities (local time of the magnetic field lines shown in solid
lines). During the interval, both ions and electrons are highly
accelerated in the region in a gray solid oval. Enhancement of
the high‐energy flux is dispersionless and the spectra are very
hard with temporal preferential enhancement of the perpendicular flux. After the fast flow, the bulk plasma is more
thermalized than the initial state, and no convection signature
was observed. The quasi‐steady dipolar magnetic field (brown
lines) is observed more clearly by a satellite nearer to the
Earth (blue one), and the dipolarization region expands tailward. The highest‐energy electron flux decreases (magenta
solid arrow), but the flux of lower‐energy electrons remains
high. During the interval, the perpendicular flux of ∼100 keV
electrons decreases rapidly and parallel flux becomes relatively larger than the perpendicular one. Most of these signatures are similarly observed both in the first −10 RE event
and in the second −15 RE event, but more significantly
observed at the most earthward satellite of CL4 in the first
event.
[28] Two kinds of adiabatic acceleration may be associated with the global reconfiguration of the magnetotail:
betatron acceleration with the conservation of the first
adiabatic invariant for the perpendicular flux and Fermi
acceleration with the conservation of the second adiabatic
invariant for the parallel flux. Both our events exhibit a
temporal increase of the perpendicular flux followed by a
Figure 16. Evolution of the energy spectra during the
3 September 2006 event.
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Figure 17. Anisotropy of the electron flux shown in the same format as Figure 10 and in the same
interval as in Figure 13. In Figure 17g, the lowest channel data are not shown for CL4.
drop of the perpendicular flux to a level less than the parallel
flux. As the perpendicular flux increase is well correlated
with the enhancement of Bz as is shown in Figure 10 and
Figure 17, electron acceleration in the near‐Earth magnetotail is considered to be basically associated with the
betatron acceleration. Effect of the betatron acceleration was
reported previously [e.g., Kivelson et al., 1973]. However,
unlike a steady dipolarization at the geostationary orbit with
a steady enhancement of perpendicular electron fluxes in
their result, our observation results show that significant Bz
enhancements inside earthward fast flows do not last so
long, less than 1 minute. In addition, we also have to consider another additional acceleration mechanism and/or a
very fast pitch angle scattering mechanism. As Kivelson et
al. [1973] mentioned, the appearance of the whistler mode
wave can be closely related to the scattering of electrons.
Such whistler waves are clearly observed during the fast
flow interval as is shown in Figure 18 and the pitch angle
distribution during the corresponding interval becomes immediately isotropic (Figure 17). Such observation is also
reported in the first event on 27 October 2007 (A. Retinò,
personal communication, 2009). Thus, the whistler wave is
one of the most important candidates. While Birn et al.
[2004] showed the pitch angle distribution of the accelerated electron in the near‐Earth magnetotail in their numerical simulation, we have not yet resolved the effect of such
wave activities both in time and in space. It should be
studied further. The local Bz enhancement similar to the
local braking region of fast flows in the near‐Earth magnetotail [e.g., Shiokawa et al., 1997] discussed in this paper,
is frequently observed in the outflow region of magnetic
reconnection sites, where the Alfvénic outflow is deceler-
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Figure 18. High time resolution magnetic field and electric field data. Spin‐averaged electric field data
and ion velocity Vx are also shown. Electric field data are shown in DSI coordinate system. Wave intensity, degree of polarization, and ellipticity of EFW are displayed at the bottom.
ated outside the diffusion region by pile up of the ambient
plasma and magnetic field. In this region, Hoshino [2005]
proposed the existence of nonadiabatic curvature and gradient acceleration of electrons up to the relativistic energies
in his numerical simulation study. Imada et al. [2007] and
Asano et al. [2008] also show the enhancement of the high‐
energy electron flux in such a region. It may be possible that
electrons in the propagating leading edge of the fast flows
also undergo such a suprathermal acceleration process. It is
noted that the appearance of the net dawnward high‐energy
electron flux at the enhancement of the flux may indicate
that there are more electrons whose center of the gyration
locates on the earthward side of the satellites than electrons
whose center of the gyration is located on the tailward side
[Ohtani et al., 1992]. This would indicate that particles are
accelerated locally, not convected from the tail.
[29] On the other hand, it is not easy to confirm that the
excess of the parallel electron flux is caused by Fermi adiabatic acceleration. Theoretically, the acceleration mechanism should work as a result of the significant shortening of
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Figure 19. Schematic of the observed events.
the magnetic flux tube associated with earthward fast flows.
It can be also understood in terms of the dipolarization
process in the near‐tail region Xgsm > −10 RE, as has been
discussed by Hada et al. [1981]. In studies of ions, such
parallel acceleration has been further discussed as “bounce
phase–bunched” ions [e.g., Delcourt et al., 1990], although
the acceleration is not caused by the transport of the midtail
flux tube. However, during the interval when the drop of the
perpendicular flux was observed, Ey was close to zero, and
the formation of the cigar‐type distribution was not associated with local convection. Considering the result that the
drop of the perpendicular flux causes the excess of the
parallel flux, rather than an increase of the parallel flux, it is
more likely that the perpendicular high‐energy flux escapes
from the region where the measurements were made. The
observation of a dispersive signature associated with the
dusk‐side substorm activity also suggests that nonnegligible
dawnward drift motion is occurring in the regions shown in
our study, and supports the idea that the perpendicular flux
drifts as sketched in Figure 19.
[30] Bulk ions and electrons are thermalized during the
same interval, electrons measured in the lower‐energy
channel flux of the high‐energy particle detector may similarly be affected, such that the spectra becomes softer.
However, we do not want to deny the overall possibility of
the Fermi adiabatic acceleration. As the parallel velocity
is proportional to the inverse of the flux tube length, only
∼3 RE earthward motion to the 10 RE region can produce
an acceleration comparable to betatron acceleration by an
enhancement of Bz by factor 10. The Fermi acceleration
would occur at the same time as the betatron acceleration
during earthward fast flows, but may be less effective. Birn
et al. [2004] showed in their numerical simulation that both
betatron and Fermi accelerations are effective and betatron
acceleration is dominant for higher‐energy electrons. It is
interesting that only CL4, which is closer to the Earth,
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observed the additional parallel electron flux in the lower‐
energy range, sometimes simultaneously with the perpendicular acceleration in the high‐energy part. It may be
necessary to consider another source, such as an ionospheric
origin.
[31] Evolution of the pitch angle distributions from perpendicular (pancake‐type) to parallel (cigar‐type) has been
reported previously, by Smets et al. [1999] and Wu et al.
[2006]. Considering these two pitch angle distributions as
the betatron and Fermi acceleration, Smets et al. [1999]
interpreted the variation of the 1 keV flux pitch angle distribution in terms of a spatial structure observed due to the
satellite motion from midtail to the region nearer to the Earth
in the longer interval (3 hours) than in our results. Wu et al.
[2006] interpreted the same type of the pitch angle evolution
as a temporal variation lasting about 20 min. In their interpretation, earthward moving flux tubes nearer to the Earth,
namely, observed in the initial phase, are more affected by
the conservation of the first adiabatic invariant with larger
increase of Bz. On the other hand, flux tubes from the
midtail region which arrive at the observation point later are
more affected by the conservation of the second adiabatic
invariant due to the shrinking length of the flux tubes
themselves. Thus, the effect of the betatron acceleration is
predominantly observed in the initial phase and the effect of
the Fermi adiabatic acceleration is more clearly observed
later in this two‐dimensional picture. On the other hand, our
results suggest that the excess of the parallel flux is not
necessarily associated with the Fermi acceleration as we saw
no convection (no finite Ey) during the excess of the parallel
flux. And it is also clear with two similar observations at the
different radial distances that the transient signature is not
caused by the spatial difference. Here, we should consider a
three‐dimensional picture including the effect of the azimuthal drift. Although the exact drift paths of the particles in
the dynamic magnetosphere are complicated and have been
analyzed only under certain assumptions [e.g., Hamlin et al.,
1961; Ejiri, 1978; Reeves et al., 1991; Anderson et al., 1997;
Takahashi et al., 1997], it is clear that the highly energetic
particles in the near‐Earth magnetotail drift significantly. In
the three‐dimensional picture, the high‐energy particles will
drift azimuthally and will be lost at the magnetopause before
being trapped as ring current particles. In the 3 September
2006 event, the initial dispersive signatures of electrons
between 2148 and 2150 UT occur within about 10 s between
44.9 keV (center energy of the lowest energy channel) and
305.3 keV (that of the highest energy channel) with the Bz
gradient of the bipolar structure. Hence we can estimate
the magnetic field gradient between CL2 and CL4 near the
neutral sheet to be 0.012 nT km−1. As Reeves et al. [1991]
discussed, the magnetic field gradient drift is only weakly
correlated with the location of the particle along the bounce
motion paths, and we can derive the drift length to be about
1.1 × 104 km for particles with enhanced perpendicular
energy. Even if one considers the largest effect of the
electric field drift as large as 1000 km s−1 (factor 10 larger
than the normal speed), the azimuthal distance from the
source region to the satellite location can be roughly estimated to correspond to the distance to the dusk‐side auroral
brightening. It is already known that the auroral brightenings
are well correlated with the occurrence of fast plasma flows
in the magnetotail [e.g., Ieda et al., 2001; Nakamura et al.,
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2001], and we expect the existence of the temporal
enhancement of Bz and associated electron acceleration at
the same local time. On the other hand, the curvature drift
becomes larger only near the neutral sheet, and the averaged
drift velocity of particles with smaller pitch angles along the
longer bounce path becomes smaller, which may explain the
preferential drift loss of particles with ∼90° pitch angles.
Under the dipolar magnetic field configuration, the curvature radius even in the neutral sheet becomes larger, and
hence the azimuthal drift velocity should become much
smaller.
[32] While we argue that the existence of betatron acceleration is likely, in the above discussion, we have one more
important result about the variation of the spectral indices
during the activity. In the previous statistical results mostly
observed in the midtail region, Christon et al. [1991] concluded that electrons show somewhat harder spectra during
geomagnetically active intervals than in the quiet time, while
Åsnes et al. [2008] reported that the spectra are independent
of the geomagnetic activity (Kp indices). On the other hand,
Vogiatzis et al. [2005] have reported a midtail (Xgsm ∼ −19 RE)
observation of high‐energy electrons, and they found that the
spectra become softer during the substorm recovery phase
with a more dipolar magnetic field relative to the preonset
signature. Our result shows that the evolution of the spectral
signature is not simple. It becomes first harder and then softer
in the course of a substorm, and the statistical results reported
in the papers mentioned above may depend on the ratio of
the data set from the initial (early expansion) and the later
(recovery) phases as well as nonsubstorm intervals. Since
simple adiabatic acceleration alone cannot change the spectral index, the result also means that an additional acceleration
and/or thermalization process must be considered.
[33] We should also consider the context of large‐scale
substorm phenomena, such as magnetic reconnection and
current disruption. Recent numerical simulations have proposed several interesting mechanisms to create nonthermal
high‐energy electrons associated with magnetic reconnection caused by interacting multiple magnetic island structures [Drake et al., 2006] and the parallel electric field under
the existence of the off‐the‐plane magnetic field [Pritchett,
2006]. However, it is not likely that the energetic particles
are directly transported to the near‐Earth tail without any
drift loss signature as is estimated above. It is also important
that the observed flux in the 27 October 2007 event is a
factor 10 larger than that observed downstream of the
reconnection region reported by Asano et al. [2008], which
indicates the undersupply from the tail to account for the
observed flux in the near‐Earth magnetotail. On the other
hand, Li et al. [2003] have demonstrated using their
numerical simulation that an earthward propagating electromagnetic pulse which may be possibly created by the
earthward jet from the X line, can reproduce all the major
signatures of the particle injection very well, suggesting the
indirect effect of the magnetic reconnection far away and on
the earthward side of the X line itself. Therefore, it is more
probable that the particles are accelerated locally in the near‐
Earth tail region, even though the fast earthward flow originated by the near‐Earth neutral line is effective.
[34] It is also proposed that appearance of near‐Earth
electric field and the magnetic field turbulence are the origin
of the principal acceleration. Vogiatzis et al. [2005] reported
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a substorm event observed at Xgsm = −19 RE, and interpreted
the appearance of the high‐energy perpendicular flux as
drifting particles from the dusk flank, assuming the particle
acceleration was caused by the local current disruption
mechanism [e.g., Lopez et al., 1990]. Birn et al. [1998]
showed in their numerical simulation that electrons on the
dusk side can be further accelerated by the temporarily
enhanced electric field in the near‐Earth magnetotail. This
type of the high‐energy particle acceleration mechanism
may be one of the important candidates. However, it has not
yet been clarified. The relation to the well established phenomenological signatures of the injection boundary model
[e.g., Arnoldy and Chan, 1969; Russel and McPherron,
1973; McIlwain, 1974; Moore et al., 1981; Mauk and
Meng, 1983] is not clear, either. In addition to the large‐
scale current disruption model scenario, Retinò et al. [2007]
discussed the existence of effective particle acceleration due
to a turbulent magnetic field in the magnetosheath context.
As is shown in our results, an intense electric field was
observed in association with the arrival of the fast earthward
flows and enhancement of Bz. The electric field by 15–
20 mV m−1 during the flow and impulsive enhancement up
to 70 mV m−1 may accelerate electrons with nonadiabatic
motion much more effectively. Thus, effectiveness of such
nonadiabatic process for the electron acceleration just in the
thin layers of the flow braking region with dipolar magnetic
field may be important (A. Retinò, personal communication,
2009).
[35] Finally, we discuss the relation of the high‐energy
flux to midenergy components. Christon et al. [1991]
reported that the ion and electron temperatures vary highly
coherently not only in quiet times but also in active times.
Our result clearly shows that the enhancement of the electron acceleration (<30 keV) is well correlated to the ion
acceleration during fast plasma flows. On the other hand,
timings of the flux enhancement of the suprathermal electrons (>30 keV) are not exactly simultaneous. As is clearly
observed, some of the suprathermal electrons quickly drift
dawnward from another area and so observations of such
electrons do not always correspond to a local signature.
Otherwise, a close relationship between the enhancement of
the high‐energy electron flux and magnetic disturbance
indicates the existence of a local acceleration process in
addition to the adiabatic accelerations. Although we could
not examine high‐energy ion acceleration in this paper, it is
also important to understand both ion and electron acceleration mechanisms within the same frame of magnetospheric
dynamics. This topic should be examined in the future.
5. Conclusion
[36] Using Cluster multisatellite observations, accelerated
electrons in the near‐Earth magnetotail associated with substorms are examined. It is found that the hardest energy
spectrum is observed in the earliest stage, and that the spectrum becomes softer later on in the events. The peak values
of the high‐energy flux are observed at around the end of
the local fast earthward flow observations. The high‐energy
electrons sometimes show preferential perpendicular acceleration associated with the temporal enhancement of the
normal component of the magnetic field. The anisotropic
distribution quickly becomes isotropic. During the accelera-
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tion interval, large‐amplitude electric field and whistler wave
activity were observed. During the following steady dipolarization interval without any convection signature, on the
other hand, perpendicular flux drops to less than the initial
value, and the parallel flux exceeds. Such anisotropic distributions of electrons are consistent with adiabatic betatron
acceleration, but the effect of the pitch angle scattering is
supposed to be important. The effect of Fermi acceleration is
not so clearly identified in the high‐energy electrons. Furthermore, variation of the energy spectrum indicates that the
acceleration mechanism is not only betatron acceleration but
also nonadiabatic effect should be considered. Dispersive
signature associated with dusk‐side activities indicates dawnward drift loss.
[37] Acknowledgments. We thank H.‐U. Eichelberger for processing Cluster FGM data. Part of the data set was obtained from the Cluster
Active Archive. AL index and Urumqi geomagnetic data were obtained
from the World Data Center for Geomagnetism, Kyoto, Japan. We also
acknowledge NASA contract NAS5‐02099 and V. Angelopoulos for use
of data from the THEMIS mission, specifically, S. Mende and C. T. Russell
for use of the GMAG/CCNV data. G. Parks is acknowledged for use of
Polar UVI data, and we also thank K. Liou for processing Polar UVI data.
This work is supported by a Grant‐in‐Aid for Scientific Research for JSPS
Fellows 18.5427.
[38] Amitava Bhattacharjee thanks Joachim Birn and another reviewer
for their assistance in evaluating this manuscript.
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Y. Asano, Tokyo Institute of Technology, Ookayama 2‐12‐1, Meguro,
Tokyo 152‐8551, Japan. ([email protected])
W. Baumjohann, R. Nakamura, and A. Retinò, Space Research Institute,
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E. A. Lucek, Blackett Laboratory, Imperial College, London, SW7 2AZ,
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Y. Miyashita, Solar‐Terrestrial Environment Laboratory, Nagoya
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T. Nagai, Department of Earth and Planetary Sciences, Tokyo Institute of
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H. Rème, CESR/CNRS, Toulouse, France.
I. Shinohara and T. Takada, Institute of Space and Astronautical Science,
Japan Aerospace Exploration Agency, 3‐1‐1, Yoshino‐dai, Sagamihara,
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