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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, A07S30, doi:10.1029/2007JA012770, 2008
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Multispacecraft observation of electron beam in reconnection region
A. Åsnes,1 M. G. G. T. Taylor,1 A. L. Borg,2 B. Lavraud,3 R. W. H. Friedel,3
C. P. Escoubet,1 H. Laakso,1 P. Daly,4 and A. N. Fazakerley5
Received 27 August 2007; revised 29 November 2007; accepted 8 January 2008; published 10 May 2008.
[1] On the 18th of August 2002, during a crossing of the near-Earth plasma sheet Cluster
observed an ion flow burst, caused by a near-Earth reconnection event. Cluster observed a
tailward bulk flow which reverse to earthward flow, indicating a close passage of the
diffusion region. We show that reversals in BZ and BY are consistent with reconnection.
During the event, a short duration burst of electrons in the range of a few keV up to
more than 100 keV are observed streaming away from the reconnection region. The
accelerated electrons were aligned with the magnetic field, streaming tailward, and were
observed simultaneously by all four spacecraft located on both the northern and
southern side of the current sheet. The four Cluster spacecraft, separated by 3700 km,
observe the electrons for a time period of 60 s, indicating the burst to be a temporal
rather than localized feature. A second burst of tailward accelerated electrons observed
for 40 s was observed by Cluster 1 and 2 upon entering the earthward outflow
region. The second beam thus appear to be directed toward the X-line. The flux levels of
the energetic electron bursts exceed those of the ambient plasma sheet by a factor 2–4.
In general, the highest energetic electron fluxes during this event were observed in the
earthward outflow region. Observations indicate that reconnection operates on closed
plasma sheet field lines in this event and does not reach lobe field lines.
Citation: Åsnes, A., M. G. G. T. Taylor, A. L. Borg, B. Lavraud, R. W. H. Friedel, C. P. Escoubet, H. Laakso, P. Daly, and
A. N. Fazakerley (2008), Multispacecraft observation of electron beam in reconnection region, J. Geophys. Res., 113, A07S30,
doi:10.1029/2007JA012770.
1. Introduction
[2] Bursts of energetic particles in the Earth’s magnetotail
have been reported since early satellite observations [Sarris
et al., 1976] and have often been associated with reconnection. Magnetic reconnection generally takes place in current
sheets separating field lines of opposite directionality,
leading to a change of the field line topology [Vasyliunas,
1975]. In this process magnetic energy is converted into
kinetic energy of the plasma, which can be manifested in
various ways. For reconnection to be allowed, the particles
must be decoupled from the magnetic field, within a
diffusion region. Owing to the larger mass and gyroradius,
ions will have a larger diffusion region compared to
electrons. This leads to differential motion of ions and
electrons which give rise to the Hall current system
[Sonnerup, 1979; Nagai et al., 2001]. Plasma jets of ion
outflow from the X-line in the current sheet (CS) plane
1
ESA/ESTEC, Nordwijk, Netherlands.
Norwegian Defence Research Establishment, Kjeller, Norway.
Space Science and Applications, Los Alamos National Laboratory, Los
Alamos, New Mexico, USA.
4
Max Planck Institute for Solar System Research, Katlenburg-Lindau,
Germany.
5
Mullard Space Science Laboratory, Dorking, UK.
2
3
have a velocity related to the inflow velocity of the plasma
into the X-line [Sonnerup et al., 1981]. Relatively strong
induced electric fields, in the dawn-dusk direction, in the
region surrounding the X-line are able to accelerate demagnetized particles during their meandering/Speiser motion
[Sato et al., 1982; Hoshino et al., 2001]. In addition, a
secondary acceleration in the magnetic field pile up region
of the reconnection outflow have been proposed [Hoshino
et al., 2001]. Particle in cell (PIC) simulations show that
electrons will gain energy in this region due to gradient and
nonadiabatic curvature drift antiparallel to the induced
electric field. Drake et al. [2005] presents an alternative
acceleration mechanism based on PIC simulations, where
electrons are trapped and accelerated by a Fermi mechanism within collapsing miniature islands of magnetic flux.
Observations of such a miniature island in a near-Earth
reconnection event was recently described by Eastwood
et al. [2007]. Another possible mechanism related to
reconnection is stochastic acceleration through turbulence
or wave-particle interaction [Ambrosiano et al., 1988;
Shinohara et al., 1998].
[3] While ions gain most of their energy with their bulk
outflow velocity, a flow velocity of 1000 km/s will
increase electron energy by only a few eV. However,
electrons might gain thermal energy more efficiently in
the diffusion region due to their low mass compared to the
heavier ions.
Copyright 2008 by the American Geophysical Union.
0148-0227/08/2007JA012770$09.00
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Figure 1. Positions of the Cluster spacecraft relative to Cluster 3 (17.7, 5.0, 3.4) RE GSM. Relative
position (a, b) in GSM-coordinates and (c, d) in current sheet coordinates. See text for details.
[4] Previous observations have shown that on the reconnection separatrices (separating reconnected from the prereconnected field lines) low-energy electrons are streaming
into the X-line while high-energy electrons accelerated in
the reconnection region are streaming away from the X-line
[Nagai et al., 2001; Manapat et al., 2006]. The low-energy
electrons are carrying the reconnection Hall current, while
the energetic part is accelerated electrons escaping along the
field. Detailed observations of low-energy electrons streaming into a near-Earth X-line on the plasma sheet boundary
layer have been shown by Asano et al. [2006] using four
spacecraft Cluster data. A strong beam of energetic electrons within the plasma sheet has also been reported Taylor
et al. [2006], which might be reconnection-related.
[5] This study is based on a list of 13 reconnection events
previously identified in the Cluster data [Borg, 2006], where
we have examined the energetic electron observations to
look for signatures of accelerated electron beams similar to
the event of Taylor et al. [2006]. Of the 13 events, none had
a beam of the intensity reported by Taylor et al. [2006].
However, all events where high-resolution electron data
were available did show evidence of unidirectional streaming energetic (suprathermal) electrons, which we will term
beams in this paper. These beams area identified as a flux
increase in the electrons along the magnetic field, directed
away from the reconnection region. We present one event
here, where all four spacecraft simultaneously observe an
electron beam. Of primary interest is that during this event
Cluster 3 is positioned on the opposite side of the current
sheet. We believe this to be the first report where highenergy electron beams are observed on opposite sides of the
current sheet simultaneously.
2. Instrumentation
[6] The Cluster spacecraft have a spin period of 4 s,
such that the particle detectors and plasma instruments
(RAPID, PEACE, and CIS) [Wilken et al., 2001; Johnstone
et al., 1997; Rème et al., 2001] will sample the entire 4p
sphere in 4 s. As we are interested particularly in the look
directions aligned with the magnetic field we note that each
of these two directions, parallel and antiparallel, is generally
sampled once per spin, and with half a spin period (2 s) in
between measurements in the two directions if the magnetic
field direction remains constant. In this event we have burst
mode (BM) data available, which provides us with good
resolution in both energy and look directions. For this event
we use full three-dimensional data from the PEACE HighEnergy Electron Analyzer (HEEA) sensor which was operating with a spatial resolution of 30° (polar angle), 11.25°
(azimuthal angle). In the case of RAPID IES (Imaging
Electron Spectrometer) has a resolution of up to 20° 22.5°. FGM magnetic field data [Balogh et al., 2001] are
presented with a resolution of five vectors per second. Ion
flow velocities are from the CIS instrument, using the Hot
Ion Analyzer on spacecraft 1 and 3, while Composition
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Figure 2. Overview of magnetic field and plasma bulk
flow. The data are presented in current sheet coordinates,
with Z 0 along the current sheet normal and X 0 XGSM
direction (see text for details). First, second, and third panels
show FGM data in directions of maximum, medium, and
minimum variation, respectively. Fourth and fifth panels
show CIS ion plasma flow in the X 0 direction and the total
flow perpendicular to the magnetic field, respectively. The
intervals of electron beams is indicated by boxes above the
first panel, color coded for time of observation at individual
spacecraft.
Distribution Function (CODIF) analyzer is used on spacecraft 4.
3. Cluster Observations
[7] We consider Cluster observations in the interval
1700 – 1720 UT on 18 August 2002 during which time
magnetospheric conditions were relatively quiet, with no
evidence of significant substorm or geomagnetic activity.
The solar wind conditions were calm at this time, with an
interval of weak southward interplanetary magnetic field
(IMF) BZ preceding the event. The Cluster spacecraft were
located at GSM X, Y, Z = [17.7, 5.0, 3.1] RE in the
downtail plasma sheet with an average separation of
3700 km (Figure 1).
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[8] Figure 2 shows an overview of the magnetic field and
ion flows (VX 0) and V? during the event. The data is
presented here in a current sheet coordinate system, where
Z 0 is the estimated current sheet normal, X 0 is in the
direction of maximum variation (XGSM) and Y 0 completes
a right-hand system. To obtain the orientation of the current
sheet plane, we use minimum variance analysis (MVA)
[Sonnerup and Cahill, 1967; Haaland et al., 2006] applied
to the FGM data of an interval just prior to the first tailward
flow burst, from 1658 to 1702 UT, while C1, C2, and C4 is
crossing the current sheet from south to north. We apply the
MVA-algorithm to the four spacecraft magnetic field data
ensemble, to obtain an average current sheet orientation.
The eigenvalues and vectors represented in GSM coordinates are given in Table 1. The coordinate transform is
necessary in this event, in order to see the reconnection
signatures, Hall BY and reversal of BZ, clearly, and we will
present data in the X 0, Y 0, Z 0 coordinate system throughout
this paper. We note that MVA-analysis applied to current
sheet crossings by C1 and C4 at 1707:40 – 1707:40 UT,
which would seem like a natural choice for this analysis,
appear to be significantly influenced by Hall-currents.
[9] At the beginning of the period, considering the X 0component of the magnetic field, C1 and C4 are north of the
current sheet (CS). C2 is located closer to but mainly north
of the CS and C3 is below the CS. Around 1703 UT a
tailward plasma flow is observed together with southward
BZ 0, interpreted to be caused by reconnection Earthward of
Cluster. At around 1707:30 UT C1 and C4 move south of
the NS and shortly after, 1708:30 UT, detect a flow and
BZ 0 reversal, suggesting that the X-line moves tailward of
Cluster. At C3 the tailward bulk flow cease around 1708 UT,
accompanied by a decrease in temperature and density (not
shown) but still remains in the plasma sheet. This indicates
that C3 either moved to the reconnection inflow region or
alternatively that the reconnection is rather localized or
filamentary in this case. We note that ion data is not
available from C2, but PEACE moment calculations of
electron bulk flow (not shown) indicate a lack of flow
reversal also at C2. After 1709:30 UT C1 and C4 observes
another brief interval of weak tailward plasma flow before
the flow turns earthward again until 1715 UT.
[10] We illustrate the BY 0 variations observed by C4 as
function of VX 0 and BX 0 in Figure 3. The signs of VX 0 and
BX 0 determine which quadrant of the Hall-current system is
observed, as illustrated in the top left inset. The core BY 0
field of 4.7 nT was estimated by the average BY 0 field prior
to the event. The BY 0 variations at C4 can be seen to match
well with predicted Hall effects, and this is also the case for
C1. At C3 the BY 0 deflections are not consistent with Hall
effects and also at C2 the initial positive BY 0 perturbations
does not appear consistent with Hall effects considering that
Table 1. Results of Minimum Variance Analysis of 4SC Magnetic
Field Data From 1658 to 1702 UT a
Eigenvalue
Maximum (X 0)
Medium (Y 0)
Minimum (Z 0)
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a
62.4
0.925
0.045
Eigenvector (X, Y, Z GSM)
0.939
0.340
0.049
Eigenvectors are expressed in GSM coordinates.
0.323
0.823
0.467
0.119
0.454
0.883
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Figure 3. Cluster 4, BX0 versus VX0 with BY0 represented as circles for the 1704– 1714 UT interval. Circle
size indicate dBY0 = BY0 4.7 nT, where 4.7 nT is the estimated core BY0 field. The inset figure in the top
left quadrant illustrate the reconnection geometry with Hall fields.
the flow here is almost certainly tailward from 1703 to
1707:30 UT. This indicates that C3 and initially also C2 is
located in a region away from the Hall currents.
[11] In Figure 4 we focus on the time interval of the beam
observation at all 4 Cluster spacecraft, using RAPID pitch
angle distributions from the lower-energy channel (41.7 –
52.7 keV). We note that although the signal is very close to
the background noise level, there are clearly distinguishable
beam signatures at all four spacecraft. The magnetic field
and ion flow signatures are consistent with the four spacecraft being located tailward of the reconnection X-line, with
C1, C2, and C4 on the north side, and C3 on the south side
of the current sheet. From 1705:45 to 1706:05 UT C1 and
C4 observe enhanced fluxes in the antiparallel direction. A
similar signature is seen at C2 over a longer period
(1705:20 – 1706:20 UT) with a more isotropic appearance
coinciding with the proximity of C2 to the current sheet. At
C3 the enhancement of flux (1705:15 – 1705:25 UT) is in
the opposite direction, parallel to the magnetic field, but due
to C3 position below the current sheet it is consistent with
tailward directed fluxes at all four spacecraft. Owing to the
alignment of these enhancements with the magnetic field we
will refer to them as beams and a more detailed view of
these beams is presented in Figure 6 where we show data
from Cluster 1 only.
[12] We use observations from Cluster 4 in Figure 5 to
provide an overview of the electron behavior during this
event. The observations at C1 are generally very similar to
those at C4, while C2 and C3 show differences resulting
from different location relative to the X-line geometry. The
top panel in Figure 5 shows spin averaged electron fluxes
from RAPID, for five energy channels (41.7 – 237.5 keV).
The second panel shows the PEACE energy-time spectrogram of energy flux. The third and fourth panels show
PEACE energy-time spectrograms of anisotropy and
j? jk
, and
streaming. Anisotropy is defined here as
j? þ jk
jka jkb
streaming as
where j is the measured flux and ka
jka þ jkb
and kb denote the parallel and antiparallel along the
magnetic field direction B, respectively. The time of the
electron beam at C4 (1705:52– 1706:05 UT) is marked with
an arrow in the RAPID data panel. The spacecraft is initially
located in the plasma sheet, characterized by several KeV
electron energies, but moves out into a colder and less dense
plasma in the middle of the interval. The third panel shows
the plasma during this interval to be mostly bidirectional
(blue color) at most energies. At the start of the plasma bulk
flow interval (1703 UT) the distribution is becoming more
strongly field aligned (deep blue) at energies up to a few
keV. The fourth panel shows a measure of the streaming of
the plasma sheet electrons; the balance of fluxes in the field
aligned and anti-field-aligned directions. Before and after
the event, the fluxes are well balanced in the two directions,
shown as black. Once the tailward flow initiates, one can
see that antiparallel electrons dominate (blue) as long as the
spacecraft is north of the current sheet (interval A – B).
When C4 moves south of the current sheet (marked with
vertical line B) this streaming turns field aligned (yellow)
for 30 s until C4 is moving into a colder, less dense
electron plasma. This electron streaming is not an observation of the plasma flow as the ion bulk flow velocity is
negligible compared to the electron thermal velocities.
[13] The beam (1705:52– 1706:05 UT) does not display
the most intense fluxes during the period. The electron
fluxes at the time of the current sheet crossing (1707:30,
line B) are comparable to the beam, while the most intense
fluxes observed by RAPID coincide with intervals of
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Figure 4. Electron beam observed by RAPID on all four spacecraft. Panels show pitch angle
distribution versus time from Cluster 1, 2, 3, and 4, at energy 41.7 – 52.7 keV.
earthward directed plasma flow (the intervals marked C– D
and E– F in Figure 5). An asymmetry between energetic
electron fluxes in the earthward and tailward outflow
regions have been discussed previously by Imada et al.
[2005, 2007] and can be interpreted as evidence for a twostep acceleration [Hoshino et al., 2001; Hoshino and Mukai,
2002]. The reason for the Earthward/tailward asymmetry
would be due to the intrinsic dipole field of the Earth which
lead to a stronger magnetic field pile-up effect on the
earthward side. The lack of an electron flux maxima
observed by C1 and C4 in the flow reversal region is
contrary to observations made by Wind in a diffusion region
at X = 60 RE discussed by Øieroset et al. [2002]. This
discrepancy is most likely due to Cluster not going through
the diffusion region itself but passing it on the southern side
from the tailward to earthward outflow region. This view is
supported from Figure 3, where BX 0 = 15 nT during the
time of flow reversals, whereas it should approach zero near
the X-line. In the plasma flow we do not observe expected
signatures of being in the plasma inflow direction, a
stagnation in the VX component accompanied by an inflow
of plasma (VZ). However, C1 and C4 observe a 15 s
interval of constant 100 km/s flow prior to the first
earthward flow (time C), which is of the same order as
the deduced tailward motion of the X-line, calculated from
the reversal signature in C1 and C4. From the PEACE
spectrogram in the second panel we notice that the electron
population is relatively cool and tenuous around the time of
flow reversals, possibly corresponding to a preaccelerated
plasma in the inflow region.
[14] From the perpendicular flow components in X 0 and
0
Y directions, shown as red traces in Figure 5, the fifth
and sixth panels, we note that the magnetic flux-carrying
flow is mostly in the Y 0 direction, except when C4 is
crossing the current sheet. Also, the perpendicular component is small during the earthward flow intervals (C– D
and E– F).
[15] In Figure 6 we show the full resolution electron data
for selected energy channels from RAPID (first and second
panels) and PEACE (third through fifth panels) together
with ion plasma flow in X 0 and Y 0 directions. The bottom
panel shows the X 0, Y 0 and Z 0 magnetic field components.
The electron panels show data by polar sectors versus time,
at subspin resolution. Contours of pitch angles sampled in
the look directions are superposed, with solid black, dashed
black, and solid white lines indicating parallel, 90° and
antiparallel pitch angles, respectively. The offset between
pitch angle contours of RAPID and PEACE results from the
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Figure 5. Overview of C4 electron population in reconnection interval. Top panel gives RAPID flux for
five energy channels (41.7 –237.5 keV). Second through fourth panels are from PEACE and show energy
time, anisotropy, and streaming spectrograms respectively (see text for details). Fifth and sixth panels
show the CIS plasma flow in X 0 and Y 0 directions, with perpendicular (to B) components in red. Seventh
panel shows X 0 (red), Y 0 (blue), and Z 0 (green) components of magnetic field.
locations the instruments are mounted on the spacecraft.
RAPID is mounted 90° in the direction of the rotation
from PEACE HEEA such that RAPID will look in the
(anti-)field-aligned direction 1/4 of a spin (1 s) prior to
PEACE HEEA.
[16] Starting from high-energy, RAPID (41.7 –70.3 keV)
shows that from one spin to the next, significant flux
suddenly is seen in the anti-field-aligned direction (inside
white contours). This enhancement is most intense initially
and lasts for only three spin periods. At PEACE we note
that the enhancement in the two upper channels (8.6– 12.2
and 3.6– 5.5 keV) last only two spins, where between the
antiparallel observation at RAPID (1706:01 – 1706:02 UT)
and the PEACE moving into that look direction, the beam
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Figure 6. Electron beam in RAPID and PEACE on Cluster 1. Polar ranges plotted against time at
subspin time resolution. Pitch angle contours are overlain; field-aligned (30° and 60°): black,
perpendicular (90°): dashed black, anti-field-aligned (120° and 150°): solid white. Bottom three panels
give FGM BX, BY and BZ components.
flux had significantly decreased. In the 0.9 – 1.5 keV
PEACE channel the main change during the beam is a flux
increase in the perpendicular direction, although the distribution remain field aligned.
[17] We are not able to discern any dispersion effects
here, as the beam appears simultaneously at all energies,
within the energy range of the two instruments. In the
lower-energy range (<1 keV) of PEACE, the fluxes are
bidirectional even before the beam occurs, and there appear
to be no significant change in this energy range.
[18] Concurrent with the electron beam there is also a
signature in the magnetic field, shown in the bottom three
panels of Figure 6. During a 6 s interval, corresponding well
to the beam interval as seen in PEACE, BY 0 is reduced by
more than 3 nT, while BZ 0 shows a negative deflection
before and possibly also after the negative BY 0 deflection.
This magnetic signature observed both by C1 and C4
indicates that there is a significant current related to the
event. We have not been able to estimate the current
configuration or intensity, as only two spacecraft observe
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Figure 7. Second electron beam event, observed only at C1 and C2. First though third panels show C1
PEACE and RAPID data, while fourth through sixth panels show C2 data. First and fourth panels show
PEACE energy spectrum. Second and fifth panels show RAPID pitch angle data. Third and sixth panels
are streaming spectrogram as in Figure 5. Seventh and eighth panels show the BX0 and BZ0 component for
C1 (black) and C2 (red). Bottom panel shows the C1 VX0 component of ion plasma flow.
it, and also the magnetic signature cannot be interpreted as
being caused by a 1-D current sheet. At C3 the electron
beam is observed without a similar magnetic signature,
while at C2 the magnetic field variations are more complex.
[19] A second electron beam was observed by C2 and C1
in the time interval 1708:27 – 1708:58 UT, presented in
Figure 7. At 1708:27 – 1708:40 UT C2 observes the beam,
in the field-aligned (tailward) direction. Directly after the
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Figure 8. Electron spectra from PEACE and RAPID during the beams, ordered by start time. Spectra
are ordered by pitch angle, with 0 – 30° in red, 70– 110° in black, and 150– 180° PA in blue.
flow reversal from tailward to earthward, C1 also observe a
field-aligned electron beam (tailward) lasting for 20 s,
before the flux intensifies in all directions. As C1 is
observing both a BZ 0 and flow reversal prior to the beam
observation, it is located earthward of an X-line during the
beam. This implies that the high-energy electron beam is
directed toward the X-line, contrary to the first beam where
all observations showed the beam directed away from the
X-line. At C2 we do not have the ion flow information, but
the reversal in BZ 0 occurs during the beam. The electron
plasma at C2 does not show an increase in temperature and
density similar to what is seen by C1 during the earthward
flow, indicating that C2 does in fact not enter the earthward
flow at this time. C4 enters the earthward flow only 20 s
after C1 but does not observe a similar beam.
[20] The electron spectra measured during the beams,
observed by PEACE and RAPID, are shown in Figure 8,
ordered by their time of observation. In the first beam,
shown in Figures 8a to 8d, the difference between parallel
and antiparallel fluxes is only apparent starting from a few
keV, above the peak energy of the distribution. In the lowerenergy part, the flux is balanced between the two direction.
It should be noted that the most intense beam in the RAPID
energy range is seen by C3. C3 is also isotropic in the
PEACE range around the peak energy, unlike the other
spacecraft where the distribution is field aligned at peak
energy. In the second beam, in Figures 8e and 8f, the beam
is observed in a lower density, cooler plasma. However, the
flux intensity in the RAPID energy range is nearly the same
as seen in the first beam (at C1, C2, and C4). It should also
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be pointed out that from the six beam observations presented here, all of them are directed tailward, with an
increase in the parallel (antiparallel) direction south (north)
of the current sheet.
4. Discussion and Summary
[21] We have described a reconnection event from the
point of view of energetic and accelerated electrons. The
reconnection event was identified from ion plasma flow
reversal, with corresponding BZ reversal and Hall magnetic
field signatures. The first electron beam event is unique as
the beam is not only observed by four spacecraft but also
with C3 on the opposite side of the current sheet compared
to the other spacecraft.
[22] The four Cluster spacecraft are initially tailward of
the X-line, the existence of which is first indicated at 1703
UT when a tailward plasma flow is observed. At this time
C1, C2, and C4 are north of the current sheet, and C3 is on
the south side. Around 1707 UT, after the first beam, C2
followed by C1 and C4 moves south of the current sheet.
After 1708 UT C1 and C4 enters into the earthward
outflow region, while C3, due to the absence of flow at
this time, is presumably outside of this region but still
within the plasma sheet observing a colder, less dense
plasma.
[23] We highlight the following characteristics of these
observations: (1) a persistent outflow away from the X-line
of field-aligned electrons at energies higher than the nominal plasma sheet energy (>2 keV) observed by PEACE,
(2) a short-lived (1705:15 – 1706:10 UT) accelerated electron beam, directed away from a X-line earthward of the
tetrahedron, observed by all four spacecraft. (3) An additional beam feature observed only at C1 and C2, interpreted
to be in the direction towards the X-line at C1, right after C1
moves into the earthward flow region (1708:30– 1709:00
UT). (4) An absence of low-energy electrons counterstreaming against the high-energy beam. (5) No peak of
accelerated electrons associated with the time of flow
reversal. Therefore, we suggest that the diffusion region is
not directly sampled. (6) High fluxes of energetic electrons
coincide with earthward flow intervals.
[24] The continuous weak streaming of the electrons
above 2 keV away from the reconnection region may be
interpreted as ongoing acceleration in a wide region around
the X-line. Following the acceleration, electrons subsequently escape along the field line to reach the spacecraft.
As the spacecraft is presumably on closed field lines based
on the observed trapped population, this implies that some
part of the electrons must be lost before being either
magnetically mirrored or have bounced all the way around
the closed field line structure (plasmoid). The observation is
consistent with acceleration in a magnetic field pile up
region surrounding the diffusion region [Hoshino et al.,
2001].
[25] The electron beam at 1705 – 1706 UT, observed by
all four spacecraft, may be interpreted to originate in the
X-line itself, which means that it should be observed close
to the reconnection separatrix. In order for the beam to be
seen more or less at the same time at all spacecraft
separated by thousands of kilometers, this would imply a
rapid burst of reconnection. Subsequently, the separatrix
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would expand rapidly and move over each spacecraft.
However, Cluster does not show other clear indications
of crossing a separatrix at this time. In particular we note
an absence of low-energy electrons streaming into the
X-line during these beams, contrary to what is commonly
observed [Nagai et al., 2001].
[26] Another possible interpretation is that the beam is
caused by a temporary intensification of the ongoing
acceleration taking place in a wider region around the
X-line, e.g., by rapid intensification of inductive electric
fields. For either of these explanations, it seems that the
reconnection process is bursty at this time and cannot be
considered to be a steady state process. Overall, the
reconnection event is interpreted to last for the duration
of the flow burst (1703 – 1715 UT).
[27] The second beam observed only by C1 and C2 does
not easily fit in the current reconnection picture, as the beam
at C1 is directed toward the X-line. The beam extends to
such high energies (>70 keV) that it can not easily be
explained by a field-aligned potential, a mechanism which
is generally invoked for the low-energy electrons streaming
toward the X-line. It is possible that the beam at C1 has
been magnetically mirrored at low altitudes, only to return
in the opposite direction of the initial acceleration. Another
possibility is that there are more than one X-line, with the
electron beam originating from a X-line on the earthward
side. The case of more than one X-line would also allow for
acceleration in contracting magnetic islands as suggested by
Drake et al. [2006].
[28] Although we do observe reconnection accelerated
electrons in this event, the observed intensity is far from
overwhelming. The energetic electron flux in the beams
exceeds the previous plasma sheet fluxes by a factor 2 – 3.
The overall highest fluxes in the event occur in the
earthward outflow region, where the energetic electron
fluxes are isotropic. Our observations generally resembles
the simulation results of Hoshino et al. [2001, Plate 3].
[29] The geomagnetic activity during this event is not
significant, and the near-Earth reconnection event does not
appear to occur with a substorm. The observation that C3
(and probably C2) is exiting the reconnection outflow
region, while remaining on closed field lines, indicate that
the reconnection X-line does not reach the lobe field lines in
this event. Reconnection of lobe field lines is expected to
proceed much faster and release more energy than reconnection of plasma sheet field lines, which may explain why
there is no substorm related to this event [Baker et al.,
1996]. The degree of particle acceleration may also be
different in a plasma sheet reconnection event compared
to the more rapid lobe reconnection events.
[30] Finally, we note that after the reconnection event
there has been no significant increase in plasma sheet
temperature or energetic electron fluxes compared to the
initial state. This indicates that an isolated reconnection
event such as described here only provides very localized
acceleration and that a substorm is required to heat the
entire plasma sheet, such as is commonly seen.
[31] Acknowledgments. Most plots in this paper were produced
using the PaPCo software package. We thank H. Rème and E. Lucek for
providing CIS and FGM data, respectively, through the CAA. A. Åsnes is
grateful for helpful discussions with S. E. Haaland and M. Dunlop.
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A07S30
ÅSNES ET AL.: RECONNECTION RELATED ELECTRON BEAMS
[32] Wolfgang Baumjohann thanks Marit Øieroset and another reviewer
for their assistance in evaluating this paper.
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