JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, A12225, doi:10.1029/2012JA017896, 2012
Electron dynamics in the reconnection ion diffusion region
W.-L. Teh,1 R. Nakamura,1 M. Fujimoto,2 E. A. Kronberg,3 A. N. Fazakerley,4 P. W. Daly,3
and W. Baumjohann1
Received 2 May 2012; revised 18 October 2012; accepted 28 October 2012; published 22 December 2012.
[1] We investigate and compare Cluster observations of electron dynamics in different
locations of the ion diffusion region for magnetic reconnection in the Earth’s
magnetotail. On the basis of the 2-D reconstructed magnetic field map from Cluster 1
(C1), we pinpoint that the observed Hall field is 6000 km (9 ion inertial lengths)
away from the magnetic X-point, and reveal that C3 was the one in closest proximity
to the X-point at the time when the reconnection jet reversal was simultaneously seen
by three spacecraft, namely, C1, C3, and C4. No evidence is found for strong wave
emission and energetic electron enhancement near to the X-point, as compared to that
within the diffusion region. We find that (1) the Hall current loop is mainly carried by
the low-energy, field-aligned counterstreaming electrons; (2) a flat-top distribution in
phase space density is a common feature for Hall-related electrons; (3) an enhancement
of energetic electrons is observed together with the presence of the flat-top electrons;
and (4) electromagnetic wave emission is enhanced within the diffusion region. Two
different regions of field-aligned counterstreaming (FC) electrons are identified: one is
associated with the Hall current loop (i.e., the electron flow reversal) while another one
stays at the edge of the loop. Interestingly, observations show that at the transition
between the two FC regions, the waves seem to suppress the energetic electrons but to
promote the flat-top electrons.
Citation: Teh, W.-L., R. Nakamura, M. Fujimoto, E. A. Kronberg, A. N. Fazakerley, P. W. Daly, and W. Baumjohann (2012),
Electron dynamics in the reconnection ion diffusion region, J. Geophys. Res., 117, A12225, doi:10.1029/2012JA017896.
1. Introduction
[2] Magnetic reconnection is a process of breaking and
reconnecting of magnetic field lines in a localized diffusion
region. Through reconnection, the magnetic energy is converted into plasma kinetic energy in the form of particle
heating and acceleration. Therefore, diffusion region is
thought to be a prime region for studying particle acceleration and its physics. Figure 1 illustrates schematically the
Hall magnetic field BH and Hall current loop JH for the
earthward-directed reconnection jet at the northern separatrix of magnetotail reconnection. The Hall current loop is
induced by the decoupling motions of ions and electrons
[Sonnerup, 1979; Terasawa, 1983], and is mainly carried by
the electrons which velocities are expressed by the purple
1
Space Research Institute, Austrian Academy of Sciences, Graz,
Austria.
2
Institute of Space and Astronautical Science, JAXA, Sagamihara,
Japan.
3
Max-Planck Institute für Sonnensystemforschung, Katlenburg-Lindau,
Germany.
4
Mullard Space Science Laboratory, University College London,
Dorking, UK.
Corresponding author: W.-L. Teh, Space Research Institute, Austrian
Academy of Sciences, Schmiedlstr. 6, AT-8042 Graz, Austria.
(waileong.teh@gmail.com)
©2012. American Geophysical Union. All Rights Reserved.
0148-0227/12/2012JA017896
arrows in Figure 1, where the inflowing and outflowing
electrons are antiparallel and parallel to the magnetic field,
respectively. According to Ampère’s law, the out-of-plane
Hall magnetic field is thus generated by the Hall current
loop.
[3] According to the magnetic field orientation, the electron acceleration is categorized into the field-aligned and
perpendicular accelerations, for which the acceleration
mechanisms are expected to be different. Evidence for electron acceleration up to few keV in the diffusion region has
been reported in the Earth’s magnetotail [e.g., Nagai et al.,
2001; Asano et al., 2008], showing that the Hall-related
electrons are essentially accelerated in the field-aligned
direction and its phase space density exhibits a flat-top
distribution. Nagai and Fujimoto [2005] found that the
Hall current is mainly carried by the low-energy electrons
(<5 keV) flowing into the reconnection site. In addition to
that, Øieroset et al. [2002] found that in the diffusion region
the electron pitch angle distributions show a gradual evolution from field-aligned counterstreaming (<6 keV) to isotropy (>6 keV) with increasing energy. Inside the diffusion
region, they also found energetic electrons up to 300 keV
which are thought to be accelerated by the inductive electric
field at the X-type neutral line. Previous observations of
magnetotail reconnection showed that energetic electrons can
be generated not only in the X-type diffusion region [e.g.,
Asano et al., 2008], but also in the magnetic flux pileup
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Figure 1. Schematic of Hall magnetic field BH and Hall
current loop JH for the earthward-directed reconnection jet
Vout on the northern separatrix of the magnetotail reconnection. The (x, y, z) axes represent the GSM coordinates. The
Hall current is carried by the electrons which velocities are
shown by the purple arrows. The red dashed line illustrates
the current sheet crossing from Northern (Bx > 0) to Southern (Bx < 0) Hemisphere for the virtual spacecraft.
region [e.g., Imada et al., 2007], and within the magnetic
island [e.g., Chen et al., 2008; Retinò et al., 2008].
[4] In the paper, we revisit a reconnection event in the
magnetotail seen by Cluster, in which the Hall effects and
the X-line crossing are confirmed by Eastwood et al. [2007].
Previous studies by Teh et al. [2011] have shown that the
regions of the Hall magnetic field are surrounded by electron-current loops, by use of the steady, 2-D Hall-MHD
reconstruction technique [Sonnerup and Teh, 2009]. The
purpose of the revisiting is to investigate and compare the
electron dynamics in different locations of the ion diffusion
region seen by different spacecraft. Further, we examine the
possible mechanisms for electron acceleration within the Xtype diffusion region. The layout of the paper is as follows.
In section 2, we give an overview of the magnetotail
reconnection event and show the Cluster observations of the
electron dynamics in different locations of the ion diffusion
region. In section 3, we give discussions of the event with
the reconstruction results from C1, and conclusions.
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[Eastwood et al., 2007; Teh et al., 2011]. During the event,
the four Cluster spacecraft, which form a tetrahedron configuration with an average interspacecraft separation of
2000 km (3 ion skin depths), were traversing the magnetotail current sheet from northern (Bx > 0) to southern
(Bx < 0) hemisphere around (18.7, 3.4, 1.1) RE in GSM.
The first encounter with the current sheet was by C3 at
09:42:19 UT, followed by C2, then by C1, and finally by C4
at 09:46:19 UT. During the traversal, an earthward-directed,
high-speed ion flow followed by a tailward flow was
simultaneously seen by C1, C3, and C4 at different locations
in space, as indicated by the gray dashed line in the bottom
panel of Figure 2. The weak tailward jet was suggested to be
due to the decrease of the reconnection rate [Eastwood et al.,
2007], which is evident because the Bz component becomes
smaller (see Figure 2c). But, the reconnection was still
ongoing because a strong tailward flow associated with a
magnetic island was seen at later time around 09:50 UT and
because there was a more or less stable tailward flow for the
time interval between the flow reversal around 09:44:28 UT
and the strong tailward flow around 09:50 UT (not shown).
At the time of the Vx reversal, C3 was at Bx < 0 but the other
three spacecraft were at Bx > 0, as shown in Figure 2a.
Evidently, a magnetic X-point, located on the tailward side
of the spacecraft, moves earthward through the Cluster tetrahedron [Eastwood et al., 2007; Teh et al., 2011]. During
2. Cluster Observations of Magnetotail
Reconnection
2.1. Overview of the Reconnection Event
[5] Figures 2a–2c show the GSM magnetic field components at 4 s from the FGM instrument [Balogh et al., 2001]
for all four Cluster spacecraft, in the time interval between
09:39:00 and 09:48:00 UT, on 22 August 2001. In
Figure 2d, the x component of the ion velocity from the
Cluster Ion Spectrometry (CIS) instrument [Rème et al.,
2001] is shown for C1 and C3 (HIA data) as well as for
C4 (CODIF data). Color codes for the spacecraft are: C1,
black; C2, red; C3, green; and C4, blue. Vector components
are displayed in GSM throughout the paper. For the event,
the GSM coordinates (x, y, z) approximately represent the
current sheet boundary coordinates (L, M, N), respectively
[Eastwood et al., 2007]. Note that the variance coordinates
derived from the minimum variance analysis [Sonnerup and
Scheible, 1998] are not far from the GSM coordinates
Figure 2. Time series plots of (a–c) the GSM magnetic
field components at 4 s and (d) the x component of the ion
velocity for C1 and C3 from Cluster Ion Spectrometry
(CIS)/HIA and for C4 from CODIF. The dashed line denotes
the reconnection jet reversal. Color codes for C1, C2, C3,
and C4 are black, red, green, and blue, respectively.
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MHD reconstruction by using the C1 data (Figure 3b) [Teh
et al., 2011]; the electron energy-time spectrograms for
180 , 90 , and 0 pitch angles, respectively, from the
PEACE instrument (Figures 3c–3e); and the energetic electron fluxes from the RAPID instrument (Figure 3f) [Wilken
et al., 2001]. Figure 4 shows the time series measurements
of C1 for the same time interval and format as in Figure 3,
except that the ion velocity component Vx is obtained from
the CIS/HIA instrument [Rème et al., 2001], while the
electron velocity component Vex is derived from the HallMHD reconstruction [Teh et al., 2011].
[7] For the event, only C2 has available measurements of
the electron moment from PEACE with the resolution at
20 s. We note that C4 has similar PEACE spectrograms as
C1 but its overall flux is much lower than C1 (not shown),
and that the PEACE data from C3 are unfortunately contaminated by the byte-swap data corruption. In the paper,
knowledge of the electron velocity for the event is crucial in
the analyses of the PEACE data for C1 and C2. With resort
to the 2-D Hall-MHD reconstruction, we can estimate the
electron velocity for C1 for reference, which is useful in
analyzing the PEACE data. The reconstruction can also
provide the predicted ion velocity for C2 for reference,
Figure 3. Measurements from Cluster 2 of the magnetotail
reconnection in the time interval between 09:41 UT and
09:46 UT on 22 August 2001. From top to bottom, shown
are (a) the three components of the 22 Hz magnetic field from
FGM; (b) the x component of the ion (black) and electron
(red) velocity from the Hall-MHD reconstruction and
PEACE, respectively; the electron energy-time spectrograms
for (c) antiparallel, (d) perpendicular, and (e) parallel directions from PEACE; and (f) the energetic electron fluxes from
RAPID; Vector components are shown in GSM. The blue
horizontal lines denote the energies at 1 keV, 3 keV, and
8 keV. The Hall magnetic field region is enclosed by the
vertical dashed lines H1 and H3.
the event, the current sheet expands which is evident by
comparing the Bx profiles between C3 and the other three
spacecraft [Teh et al., 2011]. We can roughly estimate the
half thickness of the current sheet, which is 1800 km, the
relative distance in the zGSM direction between C3 and C4 at
the time when C3 is at the current sheet center but C4 is not
within it.
2.2. Electron Observations by C1 and C2
[6] Figure 3 shows the time series measurements of C2
between 09:41:00 and 09:46:00 UT. From top to bottom,
shown are the 22 Hz magnetic field data from the FGM
instrument (Figure 3a) [Balogh et al., 2001]; the electron
velocity component Vex (in red) from the PEACE instrument
[Johnstone et al., 1997], together with the ion velocity
component Vx (in black) derived from the steady, 2-D Hall-
Figure 4. Measurements from Cluster 1 for the same time
interval and format as in Figure 3, except that the x component of the ion (black) and electron (red) velocity are from
the CIS/HIA instrument and the 2-D Hall-MHD reconstruction, respectively. The Hall magnetic field region is enclosed
by the vertical dashed lines G1 and G3.
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Figure 5. One-dimensional cut of the C2 phase space density from PEACE for 0 (blue), 90 (black),
and 180 (red) pitch angles at different regions: (a and b) the FC1 region, (c and d) the FC2 region, and
(e) the plasma sheet. The dashed lines are the one-count level for the three pitch angles and the vertical
dashed line denotes the energy at 3 keV.
which can be used as a consistency check with the observations seen by the other three spacecraft.
[8] There was no significant By guide field in the tail lobe
during the reconnection. The positive By increase, enclosed
by the vertical dashed lines H1 and H3 in Figure 3a and also
by the vertical dashed lines G1 and G3 in Figure 4a, is
identified as one of the out-of-plane quadrupolar Hall magnetic fields at the northern separatrix for the earthwarddirected reconnection jet [Eastwood et al., 2007; Teh et al.,
2011], as illustrated in Figure 1. In Figures 3b and 4b, one
can see that the Hall magnetic fields correspond to the Vex
reversal, in which the negative (positive) Vex indicates the
electrons flowing into (out of) the reconnection region in the
direction antiparallel (parallel) to the magnetic field. Note
that the Vex can be seen as the x component of the parallel
electron velocity for the event. As the virtual spacecraft
trajectory in Figure 1 illustrates for the crossing from Bx > 0
to Bx < 0, this negative-then-positive Vex reversal is in
agreement with the flow pattern of the Hall electrons, as
predicted by the Hall reconnection physics [Sonnerup,
1979]. In the tail lobe, the Alfvén speed is 1726 km/s,
based on the ion density of 0.1 cm3 and the field strength of
25 nT. Therefore, the electron velocity Vex is super-Alfvénic.
In section 3, by using the 2-D Hall-MHD reconstruction
results we will show that the Vex reversal is associated with
the loops of the electron streamlines, which are called the
Hall current loops [Sonnerup, 1979].
[9] The electron energy-time spectrograms in Figures 3c–
3e demonstrate that the electrons are mainly accelerated in
the parallel and antiparallel directions within the Hall field
region. Also, the field-aligned electron acceleration is
accompanied with the enhancement of the energetic electrons as compared to the tail lobe and plasma sheet regions,
as shown in Figure 3f. After the time at the vertical dashed
line H3, the low-energy electrons (up to 3 keV) appear isotropic around the plasma sheet in which the suprathermal
electron flux is higher than that of the tail lobe. In the
energy-time spectrograms, a line cutting along the energy at
3 keV shows that two distinct energies of the field-aligned
counterstreaming electrons are formed inside the Hall field
region, enclosed by the vertical dashed lines H1 and H3.
Hereafter we label the two distinct regions of the fieldaligned counterstreaming electrons as FC1 and FC2. The
FC1 region, enclosed by the dashed lines H1 and H2, is
associated with the Vex reversal or the Hall current loop, with
the energy between 1 and 3 keV. By contrast, the FC2
region, enclosed by the dashed lines H2 and H3, is located
near the edge of the Hall current loop, with the energy
between 3 and 8 keV. Evidently, there is a dip of the energetic electron flux at the transition between the FC1 and FC2
regions, in which both regions are accompanied with the
higher fluxes of the energetic electrons. In contrast to what
C2 observed, more dynamic field-aligned counterstreaming
electrons were seen by C1, as shown in Figures 4c–4e,
where the Hall field region is enclosed by the dashed lines
G1 and G3. With the electron velocity estimated from the
reconstruction, we identify the FC1 and FC2 regions for C1,
in which the two regions are separated by the dip of the
energetic electron flux, the same feature as C2. The results
indicate that the FC1 region, which is associated with the
Hall current loops, consists of modest ingredients of the
electrons with the energy greater than 3 keV and, by contrast, that the FC2 region has more electrons below 3 keV. In
section 3, we will discuss more about the dip of the energetic
electron flux between the FC1 and FC2 regions.
[10] Figures 5 and 6 show the one-dimensional cut of the
phase space density from PEACE at 0 (blue), 90 (black),
and 180 (red) pitch angles for C2 and C1, respectively, at
different locations, namely, the FC1 region (Figures 5a, 6a,
5b, and 6b), the FC2 region (Figures 5c, 6c, 5d, and 6d), and
the plasma sheet (Figures 5e and 6e). The dashed lines are
the one-count level for the three pitch angles and the vertical
dashed line denotes the energy at 3 keV. One can find that
the flat-top distribution is established for the electrons within
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Figure 6. One-dimensional cut of the C1 phase space density from PEACE. The format is the same as in
Figure 5.
the FC1 and FC2 regions, for example, as shown in
Figures 5b, 5c, 6a, and 6c. For C2, within the FC1 region as
well as the plasma sheet, the phase space density for the
field-aligned pitch angles drops sharply for the energy
greater than 3 keV. But, it shows an increase within the FC2
region instead, as shown in Figure 5d. This result reveals
that the electrons exhibit further accelerations in the fieldaligned directions.
3. Discussions and Conclusions
[11] Figure 7 shows the 2-D Hall-MHD reconstruction
results from C1 for the time interval between 09:39 and
09:48 UT. The reconstruction assumes that the field and
plasma structures are governed by the steady, twodimensional, ideal Hall-MHD equations in which the electron pressure gradient and the resistive term are omitted in the
generalized Ohm’s law [Sonnerup and Teh, 2009]. The upper
panel shows the magnetic field line map, while the bottom
panel shows the ion streamlines (in gray), together with the
electron streamlines (in black) around the Hall field region.
The color code is the out-of-plane magnetic field component
Bz′ . Note that the z′ axis is the invariant axis along which the
variation is negligible. In the bottom panel, the white arrows
at y′ = 0 are the reconstructed electron velocities along the C1
spacecraft trajectory, and those velocities are seen in the
moving frame with the structure. In the previous studies, Teh
et al. [2011] have systematically verified the reconstruction
results by optimizing the reconstruction axes (x′, y′, z′) and
the moving frame velocity V0 so as to obtain the high correlation coefficients between the predictions from the magnetic field map and the corresponding fields measured by the
other spacecraft not otherwise used in the reconstruction.
Moreover, they demonstrated that the key reconstruction
result namely that the regions of the Hall magnetic field are
surrounded by the loops of the electron streamlines, as shown
in the bottom panel, is a robust result. But, we should
remember that the map is only considered as a snapshot of the
Figure 7. Ideal Hall-MHD reconstruction results from C1.
(top) Map of the magnetic field lines with the out-of-plane
magnetic field Bz′ in color. (bottom) Map of the ion (gray)
and electron (white) streamlines with the Bz′ in color. The
reconstruction axes (^x ′, ^y ′, ^z ′) and the moving frame velocity
y′ ¼
V0 are in GSM: ^x ′ ¼ (0.896, 0.016, 0.445); ^
(0.444, 0.014, 0.896); ^z ′ ¼ (0.021, 1.000, 0.006); and
V0 = (29.4, 2.2, 5.4) km/s. The red plus symbol denotes
the magnetic X-point and the pink curve is the contour of
Bx′ ¼ 0 representing the current sheet center. In Figure 7
(top) the white arrows denote the magnetic field directions
on both sides of the current sheet. In Figure 7 (bottom) the
white arrows at y′ = 0 are the reconstructed electron velocities along the C1 spacecraft trajectory, crossing the current
sheet from left to right. The pink arrows represent the reconstructed ion flow velocities along the C3 and C4 spacecraft
trajectory. The white tetrahedron configuration denotes the
spacecraft locations at the time when the reconnection jet
reversal was seen. The cyan and blue bars denote the FC1
and FC2 regions, respectively, at C1 and C2. Tailward is
to right and northward is to the top.
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Figure 8. Cluster observations between 09:39:00 and
09:46:00 UT on 22 August 2001. Shown are (a) the magnetic
field component By in GSM for four spacecraft; (b) the ion
velocity component Vx for C1, C3, and C4,; and (c–f) the frequency-time spectrograms from STAFF for magnetic field
for four spacecraft, in order of the current sheet crossing.
The magenta dashed line denotes the Vx reversal; the three
gray lines denote the fast flow onset for C3, C1, and C4,
respectively; the green dashed line denotes the Hall-field
encounter for C4.
event, taken from the reconstruction for quasi-equilibrium in
the fixed moving frame, since there are time-dependent
effects resulting from the current sheet expansion and its
wavy motion [Teh et al., 2011]. Such time-aliased effect is
present in the ion and electron streamlines, for example, in
Figure 7 the outgoing electron flow in FC1 region is somewhat deviated outside of the separatrix.
[12] According to the field line map, we estimate the
distance between the magnetic X-point, denoted by the red
plus symbol, and the Hall magnetic field region, which is
6000 km (9 ion inertial lengths). In the map, the white
tetrahedron denotes the spacecraft locations at the time
when the reconnection jet reversal was seen at C1, and the
pink curve represents the current sheet center where Bx′ ¼
0. The map shows that C3 is on the right-hand side of the
current sheet, while the other three spacecraft are on the
opposite side. This result agrees with the observation that
C1, C2, and C4 are at Bx > 0 while C3 is at Bx < 0, as
shown in Figure 2a. Moreover, the map reveals that C3
was the one in closest proximity to the X-point at the time
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when the flow reversal was simultaneously seen by C1,
C3, and C4. Note that in the map, northward is to the top
(y′ > 0), and tailward is to the right (x′ > 0). Additionally,
Figure 7 shows that (1) the current sheet is curvy and slanted;
(2) the 2-D magnetic field and ion flow configurations near
the reconnection site are much more complicated than a
simple X-line configuration of a steady reconnection in a
planar current sheet; and (3) a tailward flow is simultaneously
observed by the three spacecraft (C1, C3, and C4) after the
flow reversal. We point out that the map cannot be sufficient
to describe the C3 flow behavior before the flow reversal.
This is partly because of the time-dependent effects on the C1
reconstruction results as mentioned previously.
[13] Figure 8 shows from top to bottom: the By field
component for all four spacecraft(Figure 8a); the ion velocity component Vx for C1, C3, and C4 (Figure 8b); and the
frequency-time spectrograms from STAFF [CornilleauWehrlin et al., 2003] for the magnetic field for all four spacecraft, in order of the current sheet crossing (Figures 8c–8f).
In Figure 8, the magenta dashed line denotes the Vx reversal;
the three gray lines for the fast flow onset for C1, C3, and
C4; and the green dashed line for the Hall-field encounter
for C4. We see that no indication is found for strong wave
emission at the time of the Vx reversal for C3. But, as compared to the tail lobe and plasma sheet regions, the wave
emissions are strongly enhanced within the Hall field regions
seen by C3 and also by the other three spacecraft. We point
out that C3, which is the spacecraft closest to the X-point,
observed a decrease in the energetic electron flux at the time
of the Vx reversal (not shown). Similar results have also been
obtained by Fujimoto et al. [2011] and Nagai et al. [2011]
when studying a reconnection event in the magnetotail
seen by Geotail. In addition, Figure 8 shows that for C1 and
C3 the wave onset is well correlated not only with the fast
flow onset, but also with the Hall-field encounter. For C4,
the wave onset is prior to the fast flow onset but is still well
correlated with the Hall-field encounter. These results demonstrate that the wave enhancement is attributed to spatial
effects. Note that the wave frequency falls within the lowerhybrid frequency range.
[14] Because the suprathermal electrons are seen inside the
diffusion region together with the presence of the flat-top
electrons, we do not consider the two-step acceleration
mechanism proposed by Hoshino et al. [2001] for which the
suprathermal electrons are accelerated in the pileup region.
During the current sheet crossing, the observations show that
the slope of the Bx field profile first seen by C3 is steeper
than that observed by the other three spacecraft at the later
different times (see Figure 2a). In other words, the current
sheet expands during the event. As a result, this particular
event suggests that the electron acceleration does not occur
under current sheet thinning conditions [e.g., Imada et al.,
2007, 2011]. In addition to the reconnection electric field,
there are possible mechanisms that have been proven for
electron acceleration near the X-type diffusion region, for
example, (1) formation of an acceleration potential [Egedal
et al., 2010a, 2010b]; (2) surfing acceleration [Hoshino,
2005]; and (3) wave particle interaction with obliquely
propagating lower-hybrid waves [Shinohara et al., 1998].
Yet we are not sure which mechanism finally plays the key
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Figure 9. (left) Cluster 2 and (right) Cluster 1 observations of (a) the energetic electron fluxes from
RAPID, the PEACE spectrograms for (b) antiparallel and (c) parallel directions, and (d) the STAFF spectrogram for electric field.
role in the electron acceleration for this event. Nevertheless,
the condition of this event seems to favor the third mechanism because we have seen the lower-hybrid wave
enhancement within the diffusion region.
[15] In the present study, we have identified two FC
regions within the diffusion region. The FC1 region, which
is associated with the Hall current loop, has been identified
by the previous studies [e.g., Nagai et al., 2001; Nagai and
Fujimoto, 2005]. What is the FC2 region? Yet we do not
have a clear answer for that, but we conjecture that the FC2
region could be the consequence that the electrons are heated
through the resonance with the obliquely propagating lowerhybrid waves [Shinohara et al., 1998].
[16] Figure 9 shows for C1 and C2 the observations of
(Figure 9a) the energetic electron fluxes, (Figures 9b and 9c)
the PEACE spectrograms for antiparallel and parallel directions, and (Figure 9d) the STAFF spectrogram for electric
field. One can find that (1) the overall wave activity in the
FC1 region is stronger than the FC2 region; (2) in the FC2
region the wave emission for C1 is much declined than C2.
The figure also reveals that, at the transition between FC1
and FC2 regions, as denoted by the dashed lines H2 and G2,
the wave emission is enhanced together with a decrease of
the energetic electron flux, but with an increase of the energy
flux of the flat-top electron. This relationship between the
wave and the flat-top and energetic electrons is, however,
not unambiguously evident at the other locations. For
instance, for C2 the energetic electron flux decreases with a
weak wave emission around 09:43 UT; for C1 and C2 the
wave emission increases with no increase of the energy flux
of the flat-top electron around 09:42 UT. Yet, it is unclear to
us why the waves behave quite differently at different
locations of the diffusion region. The result seems to suggest
that there are different mechanisms responsible for the
electron acceleration and heating.
[17] In conclusion, we have investigated and compared the
electron dynamics in different locations of the Hall field
region seen by different spacecraft. From the reconstruction
map, we pinpoint that the observed Hall field is 6000 km
(9 ion inertial lengths) away from the X-point, and reveal
that no enhancements of the energetic electron and the wave
emission are found near to the X-point, consistent with the
result by Fujimoto et al. [2011] and Nagai et al. [2011]. In
the study, the flat-top distribution in the phase space density
is found to be a common feature for the Hall-related electrons, a result that had been reported by Asano et al. [2008].
According to the energy-time spectrograms from C2, the
Hall current is mainly carried by the low-energy (1–3 keV),
field-aligned counterstreaming electrons, but sufficiently by
the inflowing ones. This result is consistent with the previous finding by Nagai and Fujimoto [2005]. Interestingly, we
find another field-aligned counterstreaming electron, with
the energy between 3 and 8 keV, which is formed near the
edge of the Hall current loop. Such structure of the flat-top
electrons seen by C2 is, however, less evident in C1. Instead,
more dynamic field-aligned counterstreaming electrons were
seen by C1, in which the high-energy electrons (3–8 keV) are
less obviously separated from the low-energy ones. Within
the diffusion region, the energetic electron is observed
together with the presence of the flat-top electrons, and the
electromagnetic wave emission is enhanced. Finally, it is
interesting to note that at the transition between the FC1 and
FC2 regions the waves seem to suppress the suprathermal
electrons but to promote the flat-top electrons.
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[18] Acknowledgments. This work is supported by the Austrian
Science Fund (FWF I429-N16). We thank the ESA Cluster Active Archive
for providing data and the PEACE team for diagnosing the Cluster 3 data.
[19] Philippa Browning thanks the reviewers for their assistance in
evaluating this paper.
References
Asano, Y., et al. (2008), Electron flat-top distributions around the magnetic
reconnection region, J. Geophys. Res., 113, A01207, doi:10.1029/
2007JA012461.
Balogh, A., et al. (2001), The Cluster magnetic field investigation: Overview of in-flight performance and initial results, Ann. Geophys., 19,
1207–1217, doi:10.5194/angeo-19-1207-2001.
Chen, L. J., et al. (2008), Observation of energetic electrons within magnetic islands, Nat. Phys., 4(1), 19–23, doi:10.1038/nphys777.
Cornilleau-Wehrlin, N., et al. (2003), First results obtained by the Cluster STAFF experiment, Ann. Geophys., 21, 437–456, doi:10.5194/
angeo-21-437-2003.
Eastwood, J. P., T.-D. Phan, F. S. Mozer, M. A. Shay, M. Fujimoto, A.
Retinò, M. Hesse, A. Balogh, E. A. Lucek, and I. Dandouras (2007),
Multi-point observations of the Hall electromagnetic field and secondary
island formation during magnetic reconnection, J. Geophys. Res., 112,
A06235, doi:10.1029/2006JA012158.
Egedal, J., A. Lê, N. Katz, L.-J. Chen, B. Lefebvre, W. Daughton, and A.
Fazakerley (2010a), Cluster observations of bidirectional beams caused
by electron trapping during antiparallel reconnection, J. Geophys. Res.,
115, A03214, doi:10.1029/2009JA014650.
Egedal, J., A. Lê, Y. Zhu, W. Daughton, M. Øieroset, T. Phan, R. P. Lin,
and J. P. Eastwood (2010b), Cause of super-thermal electron heating
during magnetotail reconnection, Geophys. Res. Lett., 37, L10102,
doi:10.1029/2010GL043487.
Fujimoto, M., I. Shinohara, and H. Kojima (2011), Reconnection and
waves: A review with a perspective, Space Sci. Rev., 160, 123–143,
doi:10.1007/s11214-011-9807-7.
Hoshino, M. (2005), Electron surfing acceleration in magnetic reconnection, J. Geophys. Res., 110, A10215, doi:10.1029/2005JA011229.
Hoshino, M., T. Mukai, T. Terasawa, and I. Shinohara (2001), Suprathermal electron acceleration in magnetic reconnection, J. Geophys. Res.,
106, 25,979–25,998, doi:10.1029/2001JA900052.
Imada, S., R. Nakamura, P. W. Daly, M. Hoshino, W. Baumjohann, S.
Mühlbachler, A. Balogh, and H. Rème (2007), Energetic electron acceleration in the downstream reconnection outflow region, J. Geophys. Res.,
112, A03202, doi:10.1029/2006JA011847.
Imada, S., M. Hirai, M. Hoshino, and T. Mukai (2011), Favorable conditions for energetic electron acceleration during magnetic reconnection
in the Earth’s magnetotail, J. Geophys. Res., 116, A08217, doi:10.1029/
2011JA016576.
Johnstone, A. D., et al. (1997), PEACE: A plasma electron and current experiment, Space Sci. Rev., 79, 351–398, doi:10.1023/A:1004938001388.
A12225
Nagai, T., and M. Fujimoto (2005), The Hall current system for magnetic
reconnection in the magnetotail, in Particle Acceleration in Astrophysical
Plasmas: Geospace and Beyond, Geophys. Monogr. Ser., vol. 156,
edited by D. Gallagher et al., pp. 149–160, AGU, Washington, D. C.,
doi:10.1029/156GM18.
Nagai, T., I. Shinohara, M. Fujimoto, M. Hoshino, Y. Saito, S. Machida,
and T. Mukai (2001), Geotail observations of the Hall current system:
Evidence of magnetic reconnection in the magnetotail, J. Geophys.
Res., 106, 25,929–25,949, doi:10.1029/2001JA900038.
Nagai, T., I. Shinohara, M. Fujimoto, A. Matsuoka, Y. Saito, and T. Mukai
(2011), Construction of magnetic reconnection in the near-Earth magnetotail with Geotail, J. Geophys. Res., 116, A04222, doi:10.1029/
2010JA016283.
Øieroset, M., R. P. Lin, T. D. Phan, D. E. Larson, and S. D. Bale (2002),
Evidence for electron acceleration up to 300 keV in the magnetic reconnection diffusion region of Earth’s magnetotail, Phys. Rev. Lett., 89,
195001, doi:10.1103/PhysRevLett.89.195001.
Rème, H., et al. (2001), First multispacecraft ion measurements in and
near the Earth’s magnetosphere with identical Cluster ion spectrometry
(CIS) experiment, Ann. Geophys., 19, 1303–1354, doi:10.5194/angeo19-1303-2001.
Retinò, A., et al. (2008), Cluster observations of energetic electrons and
electromagnetic fields within a reconnecting thin current sheet in the
Earth’s magnetotail, J. Geophys. Res., 113, A12215, doi:10.1029/
2008JA013511.
Shinohara, I., T. Nagai, M. Fujimoto, T. Terasawa, T. Mukai, K. Tsuruda,
and T. Yamamoto (1998), Low-frequency electromagnetic turbulence observed near the substorm onset site, J. Geophys. Res., 103,
20,365–20,388, doi:10.1029/98JA01104.
Sonnerup, B. U. Ö. (1979), Magnetic field reconnection, in Solar System
Plasma Physics, edited by L. J. Lanzerotti, C. Kennel, and E. Parker,
pp. 45–108, North-Holland, New York.
Sonnerup, B. U. Ö., and M. Scheible (1998), Minimum and maximum variance analysis, in Analysis Methods for Multi-Spacecraft Data, edited by
G. Paschmann and P. W. Daly, pp. 185–220, Int. Space Sci. Inst., Bern.
Sonnerup, B. U. Ö., and W.-L. Teh (2009), Reconstruction of twodimensional coherent structures in ideal and resistive Hall MHD: The
theory, J. Geophys. Res., 114, A04206, doi:10.1029/2008JA013897.
Teh, W.-L., R. Nakamura, B. U. Ö. Sonnerup, J. P. Eastwood, M. Volwerk,
A. N. Fazakerley, and W. Baumjohann (2011), Evidence of the origin of
the Hall magnetic field for reconnection: Hall MHD reconstruction results
from Cluster observations, J. Geophys. Res., 116, A11218, doi:10.1029/
2011JA016991.
Terasawa, T. (1983), Hall current effect on tearing mode instability,
Geophys. Res. Lett., 10, 475–478, doi:10.1029/GL010i006p00475.
Wilken, B., et al. (2001), First results from the RAPID imaging energetic
particle spectrometer on board Cluster, Ann. Geophys., 19, 1355–1366,
doi:10.5194/angeo-19-1355-2001.
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