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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, A11319, doi:10.1029/2006JA011976, 2006
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Electrodynamics of a split-transpolar aurora
S. Eriksson,1 G. Provan,2 F. J. Rich,3 M. Lester,2 S. E. Milan,2 S. Massetti,4 J. T. Gosling,1
M. W. Dunlop,5 and H. Rème6
Received 14 July 2006; accepted 21 September 2006; published 23 November 2006.
[1] We report new results on the relation between drift velocity shear, average inverted
V energy, and the separation of bifurcated high-latitude Sun-aligned polar arcs within a
global transpolar auroral arc system. The two parallel arcs appeared at the location of
converging v B electric field of two sunward flow channels that were separated by an
intermediate region of much weaker antisunward flow. The flow shear consisted of
differences mainly in the magnitude of the sunward drift velocity. The cross-track
disturbance magnetic field was in phase with the drift velocity fluctuations in such a way
that two upward field-aligned currents coincided with the peak average energy of the
electron precipitation that generated the two polar arcs while a downward field-aligned
current was observed in the intermediate region. The observed transverse arc separation is
consistent with a reported linear dependence between the average energy of the
precipitating electrons of the two arcs and the arc separation. We propose that this relation
mainly follows from a local Pedersen current closure requirement. The greater the upward
field-aligned current density of the two arcs, the wider the transverse region of the
intermediate downward field-aligned current needs to be in order to supply a sufficient
amount of current carriers to the bifurcated arc current circuit and thus to balance the
Pedersen current.
Citation: Eriksson, S., G. Provan, F. J. Rich, M. Lester, S. E. Milan, S. Massetti, J. T. Gosling, M. W. Dunlop, and H. Rème (2006),
Electrodynamics of a split-transpolar aurora, J. Geophys. Res., 111, A11319, doi:10.1029/2006JA011976.
1. Introduction
[2] Single and multiple auroral arcs are often observed at
very high latitudes during northward interplanetary magnetic field (IMF) conditions [Lassen and Danielsen, 1978;
Burke et al., 1982; Carlson et al., 1988; Valladares et al.,
1994]. Such arcs are generally aligned in the Sun-Earth
direction and they are typically caused by precipitating
electrons with 2 keV average energy. In the literature they
are referred to as Sun-aligned arcs, polar cap arcs, transpolar
arcs (TPA), or theta aurora. Burch et al. [1979] first reported
that polar arcs occur at flow shears where the corresponding
dawn-dusk convective electric field points toward the center
of the electron acceleration region. Subsequent reports have
confirmed that the arcs typically coincide with a localized
upward field-aligned current (FAC) sheet at a flow shear
with a converging electric field [Burke et al., 1982; Carlson
et al., 1988; Obara et al., 1993; Carlson and Cowley, 2005].
1
Laboratory for Atmospheric and Space Physics, University of
Colorado, Boulder, Colorado, USA.
2
Department of Physics and Astronomy, University of Leicester,
Leicester, UK.
3
Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts, USA.
4
Istituto di Fisica dello Spazio Interplanetario, Rome, Italy.
5
Space Sciences Division, Rutherford Appleton Laboratory, Chilton,
UK.
6
Centre d’Etude Spatiale des Rayonnements, Toulouse, France.
Copyright 2006 by the American Geophysical Union.
0148-0227/06/2006JA011976$09.00
[3] Obara et al. [1996] reported 17 events of pairs of
adjacent inverted V electron precipitation in the high-latitude
northern polar region using Akebono satellite measurements. Each inverted V pair was assumed to correspond
to a pair of polar arcs on the basis of the electron energy flux
alone. Akebono generally confirmed that the two arcs
coincided with upward FAC sheets within regions of
converging electric field that were separated by a downward
FAC. Obara et al. [1996] also found a linear relation
between the dawn-dusk transverse separation of the two
arcs (30 – 100 km at 120 km altitude) and the mean of the
average energy of the electron precipitation (0.3 – 1.1 keV).
A larger average energy was observed to result in a greater
separation. An indirect dependence is thus expected between the arc separation and the Hall-to-Pedersen SH/SP
conductance ratio, since this ratio depends on the average
energy [e.g., Spiro et al., 1982; Robinson et al., 1987].
This result was indeed reproduced in a magnetosphereionosphere (M-I) coupling simulation of multiple polar
arcs [Zhu et al., 1994; Obara et al., 1996]. However,
no satisfactory explanation has been proposed for the
original average energy dependence of the observed dualarc separation.
[4] The Zhu et al. [1993] M-I coupling model of multiple
polar arcs assumes an initial magnetospheric flow velocity
shear [e.g., Lyons, 1980; Sojka et al., 1994] that generates
an initial upward FAC and a primary arc in the ionosphere.
Zhu et al. [1993] proposed that the adjacent downward FAC
and a second upward FAC (the secondary arc) would result
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on the average energy, a local self-consistent Pedersen
current closure requirement, and the Zhu et al. [1994,
1996] M-I coupling model results.
2. Observations
Figure 1. (a – b) Solar wind (ACE) and magnetosheath
(Cluster C1) IMF GSM components (X:black, Y:green,
Z:red) observed during the 2013 – 2140 UT period on
14 February 2003. (c) The ACE data were shifted by 45 min
35 s to match its IMF clock angle q = arctan(By/Bz) with
q measured at Cluster C1 (red line). (d) Alfvén Mach
number of the magnetosheath plasma flow measured by
Cluster C1. A clear TPA was observed by Polar UVI
during the time period marked in gray. The vertical red
line marks when DMSP F13 encountered this TPA.
due to the conductivity enhancement caused by the primary
arc as the M-I system approaches steady-state.
[5] Large-scale 200 – 400 km wide TPAs may occasionally also bifurcate along the nightside section or even along
their entire noon-midnight extension into two parallel polar
arcs separated by a region of much lower luminosity
[Craven et al., 1991; Eriksson et al., 2005]. This paper
examines the electrodynamics of such a split-TPA feature
observed by the Polar UVI instrument [Torr et al., 1995]
and recorded by the DMSP F13 satellite in the Northern
Hemisphere (NH) on 14 February 2003. We discuss a
possible physical explanation of the arc separation based
[6] Figure 1 displays the IMF conditions (GSM components and YZGSM clock angle) in the solar wind (ACE) and in
the magnetosheath (Cluster C1) during the 2013 – 2140 UT
interval on 14 February 2003. The X, Y, Z components are
color coded as black, green, and red, respectively, showing
that the IMF was northward after 2017 UT with IMF By > 0
and IMF Bx < 0. The bottom panel shows that the upstream
magnetosheath flow was mostly sub-Alfvénic along this
Cluster C1 trajectory from (X, Y, Z) = (4.4, 0.5, 9.1) to (6.3,
0.7, 9.4) RE. These conditions are favorable for antiparallel
lobe reconnection poleward and duskward of the NH cusp
and the generation of sunward E B convection [e.g.,
Gosling et al., 1991; Eriksson et al., 2005] in the vicinity of
the reconnection site. A flow shear region should develop
between such a sunward flow and the magnetosheath flow.
The gray area in Figure 1 corresponds to a period when the
Polar UVI instrument observed a clear signature of a duskside TPA in the NH that eventually bifurcated while the
vertical red bar at 2111 – 2112 UT marks when the DMSP
F13 satellite encountered this split-TPA.
[7] Figure 2 displays the Polar UVI observation of the
bifurcated TPA at 2110:44 UT (red color indicates higher
photon flux). The Polar s/c was at 1.88 RE altitude when the
field-of-view (FOV) of the UVI instrument captured this
section of the split-TPA over northern Greenland. The
auroral oval was not in the UVI FOV due to the low s/c
altitude. The dual-arc UVI observation was confirmed at
2110 UT by the ITACA2 all-sky camera in Daneborg,
northeastern Greenland (not shown).
[8] Figure 2 also shows the 2110– 2112 UT interval of the
global SuperDARN convection pattern displayed here as
equipotential contours [Ruohoniemi and Baker, 1998].
There was sufficient radar backscatter to confirm a sunward
E B drift over the split-TPA at this time. The DMSP F13
cross-track E B drift velocity vectors are also shown
along this 2107:08 – 2119:42 UT dusk-to-dawn satellite
trajectory. The SuperDARN and DMSP F13 drift information suggest that the split-TPA occurred at a structured
sunward flow section of a clockwise lobe cell that covered
most of the high-latitude polar cap. Colored along-track F13
segments correspond to the various magnetospheric source
regions as identified from the measured DMSP F13 particle
precipitation shown in Figure 3.
[9] Figure 3 shows the cross-track magnetic field perturbation (DBz, solid line), particle precipitation, and crosstrack drift velocity (solid line) recorded by DMSP F13
along the duskside 2107 – 2114 UT interval. A positive
(negative) DBz slope along the direction of motion corresponds to a downward (upward) FAC sheet. The electron
and ion energy flux were measured by a low-energy (32 eV
to 1 keV) and a high-energy (1 keV to 32 keV) pair of
electrostatic particle analyzers of the DMSP F13 SSJ/4
plasma instrument [Hardy et al., 1984] whose entrance
aperture points toward local zenith. We note that the lowenergy ion detector has degraded over time and therefore
that low fluxes of low-energy ions may be undetected.
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scale FACs as a structured NBZ current system [Iijima and
Shibaji, 1987] offset toward the duskside of the polar cap in
agreement with the positive IMF By and northward IMF
[Eriksson et al., 2005, and references therein].
[12] Detailed electron precipitation observations during
the 2111:05 – 2112:05 UT interval are shown in Figure 4.
The average energy of the electrons is defined as E = JE/JNE
where JE is the energy flux (see Figure 4a) and JNE is the
number flux (not shown) of the electrons. The peak of the
average energy was E = 2.26 keV and E = 1.19 keV at
the 2111:24 and 2111:51 UT times of the two arcs, respectively, corresponding to an average E = 1.72 keV of this
bifurcated arc system. The intermediate region between the
two arcs was characterized by a low average Pedersen
conductance SP 0.9 mho on the basis of Spiro et al.
[1982]
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1:6 1012 pJE
SP ¼ 20E=4 þ E2
ð1Þ
while the Robinson et al. [1987] expression
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1:6 1012 pJE
SP ¼ 40E=16 þ E 2
Figure 2. The duskside oval-aligned split-TPA as observed by the Polar UVI instrument at 2110:44 UT is
rendered over the 2110– 2112 UT SuperDARN equipotential contours [Ruohoniemi and Baker, 1998] in geomagnetic
latitude and magnetic local time. The 2107 –2120 UT
DMSP F13 E B cross-track drift is also shown and the
along-track color-coded segments correspond to periods of
proposed magnetospheric source regions.
[10] Several plasma source populations are indicated
between the vertical dotted lines in Figure 3 such as a
central plasma sheet (CPS) and a boundary plasma sheet
(BPS). The most poleward region is identified as the polar
rain of the polar cap. DMSP F13 observed the center of two
1 – 3 keV inverted V electron precipitation regions at
2111:24 and 2111:51 UT that generated the two visual
polar arcs shown in Figure 2. The two arcs were separated by
201 km along the F13 orbit in a boundary region labeled
TPA between the poleward part of the BPS and a region that
we identify as a mantle population (MTL) on the basis of
low-energy 1 keV ion precipitation coincident with polar
rain-type electron precipitation [Newell et al., 1991].
[11] There was a clear correlation between the cross-track
E B drift velocity and DBz (solid lines). The two arcs
appeared on the poleward side of two distinct sunward flow
channels in a converging electric field region that was
dominated by a sunward flow shear. The corresponding
DBz indicated an individual upward FAC at each arc and a
downward FAC separated the arcs. This system of mesoscale upward and downward FACs related to the two polar
arcs occurred within a large-scale upward FAC region (dark
gray area in Figure 3) that reached poleward into the polar
rain. A large-scale downward FAC (light gray) was found
equatorward of the split-TPA. We identify this pair of large-
ð2Þ
resulted in an average SP 0.5 mho.
[13] Obara et al. [1996] illustrated a dependence between
the estimated dual-arc separation and the observed mean of
the average electron energy of the two inverted Vs. We
derived a linear fit
E ¼ 10:6y 83:7
ð3Þ
on the basis of their Figure 4, where y is the transverse
separation of the two arcs in km (mapped to 120 km
altitude) and E is the mean of the average energy of both
arcs in eV. The wider the arc separation, the greater the
mean average energy. This Akebono relation is consistent
with the observed mean E = 1.72 keV and the Dt = 27 s
along-track arc separation on 14 February 2003 which
corresponds to a 162 km perpendicular distance at 848 km
DMSP F13 altitude (vsc = 7.43 km/s) or 146 km at 120 km
altitude. We used an a = 36° angle of incidence between the
DMSP F13 trajectory and the TPA normal (see Figure 2).
The top panel of Figure 5 illustrates this comparison between
the DMSP F13 event (open circle) and the Akebono data
(black dots) that we reproduced from Figure 4 by Obara et
al. [1996]. The solid line corresponds to equation (3).
[14] The bottom panel of Figure 5 compares the arc separation and the SH/SP conductance ratio using equation (3) in
the Spiro et al. [1982] expression
SH =SP ¼ E5=8
ð4Þ
where E is in keV. The general nonlinear form of the
quantitative M-I coupling model result of Zhu et al. [1994]
is confirmed. We note, however, that the predicted range of
the conductance ratio using either equation (4) or the
Robinson et al. [1987] result
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SH =SP ¼ 0:45E 0:85
ð5Þ
ERIKSSON ET AL.: ELECTRODYNAMICS OF A SPLIT-TPA
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Figure 3. The panels from top to bottom display the 2107 – 2114 UT DMSP F13 along-track (dotted)
and cross-track (solid, positive in tailward direction) disturbance magnetic field (DB = B BIGRF),
energy-time spectra for precipitating electrons and ions, and the cross-track E B drift (solid line)
measured by an ion drift meter [Rich and Hairston, 1994]. Vertical dashed lines indicate the proposed
magnetospheric source regions while red vertical lines mark the TPA interval. The origin of the particle
precipitation within the interval marked ‘‘BPS?’’ is unclear and could be related to BPS or LLBL mixed
with polar rain. A downward (NBZ-D) and an upward (NBZ-U) NBZ current are suggested within light
gray and dark gray intervals, respectively.
where E is in keV, is lower than the 1 – 2 range predicted by
Zhu et al. [1994] for a corresponding 50– 100 km dual-arc
separation.
3. Discussion
[15] The two polar arcs that DMSP F13 intersected were
related to a flow shear corresponding to a converging
electric field directed toward the center of each upward
FAC which was carried by the earthward accelerated
electrons of the two inverted Vs [Burch et al., 1979; Burke
et al., 1982; Carlson and Cowley, 2005]. Moreover, the
Obara et al. [1996] relationship for the 0.3 – 1.1 keV
average energy double arcs appeared to hold for even higher
energy and greater separation. The question is how this
relation may be explained.
[16] We first examine whether the high-latitude bifurcated
arc system poleward of the duskside region 2 current is a
locally balanced current circuit as suggested by Zhu et al.
[1993]. We assume that the region 2 FAC density is
negligible as indicated by the DMSP F13 DBz observations
(see Figure 3). A closer inspection of the along-track FAC
budget between the CPS and the polar rain is summarized in
Table 1 using an infinite current sheet approximation. This
is a valid assumption, since the two horizontal disturbance
magnetic field components (see Figure 3) are either in phase
or out-of-phase. There were no bipolar along-track signatures which are expected at local current carrying flux tubes.
Table 1 lists the estimated time intervals Dt of each
significant FAC segment and the FAC densities using jk =
Dby/(m0Dx), where Dby is the difference in cross-track
magnetic field (by = DBz), Dx = vscDt, and vsc = 7.43 km/s
is the DMSP F13 speed.
[17] Current continuity [e.g., Lyons, 1980; Milan et al.,
2003] relates the height-integrated Pedersen current JP and jk
by
JP ¼ Z
x2
jk dx
ð6Þ
x1
where x is the DMSP F13 along-track distance (x2 > x1), j =
jk^z, and ^z is positive in the upward direction or antiparallel to
the NH magnetic field. JP > 0 corresponds to a Pedersen
current directed along the direction of motion ^
x. The alongtrack electric field Ex that drives this Pedersen current is Ex =
JP/SP while the cross-track drift velocity corresponding to
Ex is Vy = Ex/B0 under the assumption that the total field is
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[18] Assume now that the magnitude of jk increases at the
equatorward or primary 2111:24 UT arc and for simplicity
that nothing else changes. This will result in a nonbalanced
Pedersen current. It is well-known that density cavities
develop after some finite time period beneath downward
current regions. This is caused by a depletion of ions that
carry the Pedersen current away from the downward FAC
and the upward escaping electrons that carry the downward
FAC [Karlsson et al., 2005]. We therefore propose that the
intermediate downward current region between the two arcs
needs to expand in order to supply a sufficient amount of
current carriers to the bifurcated arc current circuit and thus
to balance the Pedersen current.
[19] Figure 7 demonstrates this fact on the basis of the
observed and derived electrodynamic quantities on
14 February 2003. There is a linear relation between the
Figure 4. DMSP F13 electron precipitation data from
2111:05 to 2112:05 UT. (a) Energy flux JE in eV/(cm2 s sr).
(b) Average energy E = JE/JNE where JNE is the number flux
in cm2 s1 sr1. (c) Conductance ratios SH/SP using
equation (4) (dots) and equation (5) (crosses). (d) Spiro et
al. [1982] Pedersen conductance in mho (c.f. equation (1)).
vertical B = B0^z and B0 = 50 mT. A positive Vy corresponds
to a sunward drift velocity. The bottom panels of Figure 6
display the resulting along-track JP, Ex, and Vy [see also
Milan et al., 2003, Figure 4] during a 5-min section of the
DMSP F13 orbit from 2108:35 UT using the observed jk (see
Figure 6c and Table 1). An effective SfP = SP + SbP, where
SbP = 1 mho is an additional background conductance, was
employed in order to retrieve realistic Ex and Vy. We have
also assumed that the average energy E of the electron
precipitation (see Figure 6a) may be used directly in
equation (1) (see Figure 6b). We conclude two important
results from Figure 6. First, we find that the Pedersen current
J?
P = JP cos a transverse to the split-TPA (thin line) is
balanced to zeroth order. Second, it is clear that the enhanced
SP below the two polar arcs suppresses Ex = JP/SfP and that
this contributes to the structured sunward ionospheric drift
velocity Vy related to JP. The resulting drift Vy shown in
Figure 6f compares favorably with the observed cross-track
drift (c.f. Figure 3).
Figure 5. (a) Black dots reproduce the average inverted
V energy versus transverse arc separation of two parallel
polar arcs on the basis of Akebono satellite observations
[see Obara et al., 1996, Figure 4]. The bifurcated polar arc
event observed by DMSP F13 is shown as an open circle.
The best fit to the Akebono events is shown as a solid line.
(b) The arc separation versus SH/SP conductance ratio
using equation (3) in equations (4) – (5) [Spiro et al., 1982;
Robinson et al., 1987] is compared with the Zhu et al.
[1994] model results (their Figure 5). The data are shown
along the Spiro et al. formula.
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Table 1. Estimated DMSP F13 FAC Density for Each Time
Interval Poleward of the R2 Current System on 14 February 2003a
Start Time, UT
Stop Time, UT
jk, mA/m2
2108:35
2109:49
2110:09
Structured R1
2109:49
2110:09
2110:31
0.20
0.61
0.85
2110:31
NBZ Downward
2111:20
0.79
2111:20
2111:28
2111:44
2111:58
2112:25
NBZ Upward With Polar Arcs
2111:28
2111:44
2111:58
2112:10
2112:49
1.60
0.80
0.91
0.40
0.38
a
Positive values mark upward currents.
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magnitude of the primary upward FAC density and the
required transverse width of the intermediate downward
FAC in order to balance the bifurcated arc system by
Pedersen currents alone. Lyons [1980] showed in his numerical solution results of a single auroral arc (his Figure 4) that
an increased field-aligned potential drop fk typically
corresponds to an increased upward FAC density jk [Knight,
1973] as well as an enhanced precipitating electron energy
flux JE [Lundin and Sandahl, 1978], since both jk and JE
(see equation (3) and equation (10) as stated by Lyons
[1980]) are assumed to depend on fk in a similar form.
Figure 7 thus provides a possible explanation for the arc
separation dependence on the average energy E = JE/JNE
under the assumption that the upward FAC density jk is
proportional to the average energy E of the accelerated
electrons that carry the upward FAC. This scenario is in
agreement with the observed and modeled linear dependence between the average energy of the accelerated
electrons of a bifurcated arc system and the arc separation
[Zhu et al., 1994; Obara et al., 1996].
4. Summary and Conclusions
[20] The split-TPA event that was observed by DMSP
F13, SuperDARN, and Polar UVI on 14 February 2003
displayed a transverse dual-arc spacing of 146 km at 120 km
altitude with an average E = 1.72 keV inverted V energy
distributed between the two polar arcs. The bifurcation of
the original TPA is in agreement with the linear dependence
that Obara et al. [1996] reported between the average
energy and the arc separation. We propose that this relation
is caused primarily by a local Pedersen current closure
requirement and related to the dynamics of the intermediate
downward FAC and the density cavity that develops
beneath it. Our hypothesis is that the width of this
downward FAC region is determined by a need to supply
Figure 6. Electron precipitation data and estimated
electrodynamic quantities are shown along the 2108:35 –
2113:35 UT section of the DMSP F13 orbit. (a) Observed
average energy E. (b) SP on the basis of E and the Spiro et al.
[1982] equation (1). (c) Derived jk (see Table 1). (d) Alongtrack Pedersen current JP (thick line, equation (6)) and its
component transverse to the TPA (thin line) using an a =
36° angle of incidence of the F13 trajectory. (e) Along-track
electric field Ex = JP/SPf. (f) Cross-track drift velocity Vy =
Ex/B0 where Vy > 0 is a mostly sunward drift. The horizontal
bar in Figure 6b indicates the dual-arc separation.
Figure 7. The relation between the magnitude of the
primary upward FAC and the transverse width of the
intermediate downward FAC of the 14 February 2003 event
using a balanced transverse Pedersen current requirement at
DMSP F13 altitude. All other FAC density magnitudes and
widths were held constant (see Table 1), including the
secondary upward FAC (jk = 0.91 mA/m2) of the bifurcated
arc system.
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a sufficient ion source population to balance the upward
FAC system of the two adjacent polar arcs via Pedersen
currents.
[21] The split-TPA feature also appears to be consistent
with the physics contained in the Zhu et al. [1993] M-I
coupling model of multiple polar arcs. However, the main
difference between this model and the 14 February 2003
split-TPA event is related to the initial state and the
subsequent evolution of the dual-arc system. The splitTPA and the mesoscale structure observed by DMSP F13
evolved as a bifurcation of a preexisting large-scale TPA
[see also Craven et al., 1991] and the transverse width of
the split-TPA was comparable to the width of the original
TPA. The Zhu et al. [1993] model, however, gradually
evolves from one mesoscale primary arc into multiple arcs
distributed over an increasingly wider region until a steady
state width is achieved. In summary, although a local
Pedersen current closure requirement probably determines
the width of the intermediate downward current region, we
still need to understand what initiates the development of a
downward current at the center of the primary TPA in the
first place if we assume that the TPA coincided with an
initial large-scale upward FAC.
[22] The two arcs of the split-TPA presented here occurred
at a drift velocity shear which was dominated by a sunward
component with a narrow intermediate region of weak
antisunward flow. It may be argued that the enhanced
Pedersen conductance SP (caused by the downward accelerated electrons of the two arcs) eventually contributed to the
deceleration of the sunward flow at the two arcs and resulted
in a more severe and localized sunward flow shear in the
ionosphere than what was perhaps originally imposed on it
from the magnetosphere. We suspect that the sunward flow
at the arcs and the corresponding high-latitude convection
pattern were generated at a magnetopause source region in
the vicinity of a lobe reconnection site [e.g., Eriksson et al.,
2005] that resulted in the original flow shear [e.g., Lyons,
1980] during these northward IMF conditions.
[23] Acknowledgments. We thank Steve Petrinec for making us
aware of the Obara et al. [1996] reference and Stan Cowley for useful
comments on the manuscript. Polar UVI data were obtained via CSDS UVI
Online Search Tool at http://csds.uah.edu/uvi-ost/index.asp and ACE Level 2
data were retrieved from http://www.srl.caltech.edu/ACE/ASC/level2/. This
work was supported by NASA grant NNG04GH82G at the University of
Colorado at Boulder.
[24] Wolfgang Baumjohann thanks Lie Zhu and another reviewer for
their assistance in evaluating this paper.
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M. W. Dunlop, Space Sciences Division, Space Science and Technology
Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire,
OX11 0QX, UK.
S. Eriksson and J. T. Gosling, Laboratory for Atmospheric and Space
Physics, University of Colorado, 1234 Innovation Drive, Boulder, CO
80303-7814, USA. (eriksson@lasp.colorado.edu)
M. Lester, S. E. Milan, and G. Provan, Department of Physics and
Astronomy, University of Leicester, University Road, Leicester, Leicestershire,
LE1 7RH, UK.
S. Massetti, Istituto di Fisica dello Spazio Interplanetario, Via del Fosso
del Cavaliere, 100, I-00133 Roma, Italy.
H. Rème, Centre d’Etude Spatiale des Rayonnements, 9 Avenue du
Colonel Roche, B.P. 4346, F-31028 Toulouse, France.
F. J. Rich, Air Force Research Laboratory, AFRL/VSBXP, 29 Randolph
Road, Hanscom AFB, MA 01731-3010, USA.
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