JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, A11218, doi:10.1029/2005JA011149, 2005
On the generation of enhanced sunward
convection and transpolar aurora in the
high-latitude ionosphere by magnetic merging
S. Eriksson,1 J. B. H. Baker,2 S. M. Petrinec,3 H. Wang,4 F. J. Rich,5 M. Kuznetsova,6
M. W. Dunlop,7 H. Rème,8 R. A. Greenwald,2 H. U. Frey,9 H. Lühr,4 R. E. Ergun,1
A. Balogh,10 and C. W. Carlson9
Received 24 March 2005; revised 27 June 2005; accepted 18 July 2005; published 23 November 2005.
[1] The IMAGE Wideband Imaging Camera (WIC) instrument observed the duskside
development of an oval-aligned transpolar auroral arc (TPA) in the Northern Hemisphere
(NH) on 16 December 2001 during strong IMF jBj 18 nT and a generally steady 56
clock angle (positive IMF By and Bz). Observational evidence suggests that the dayside
part of the duskside TPA formed due to quasi-continuous merging between the IMF and
the lobe magnetic field tailward of the cusp while the nightside part is associated with
the Harang discontinuity. The low-altitude CHAMP satellite confirms an upward
northward IMF Bz (NBZ) field-aligned current (FAC) over the dayside TPA while
associating a downward NBZ current with the region of diminished WIC emissions in
between the auroral oval and the TPA. DMSP F14 suggests that the dayside region of the
downward NBZ current coincides with precipitating magnetosheath-like ions of reversed
energy-latitude dispersion consistent with high-latitude reconnection. SuperDARN
observes enhanced ionospheric sunward flows generally centered between the oppositely
directed NBZ currents. We associate these flows with a clockwise lobe convection vortex
and the dayside part of the TPA. The nightside TPA, however, is related to stagnant or
antisunward flow and the upward FAC region of the Harang discontinuity. Cluster
observations confirm the simultaneous presence of rotational discontinuities across the
duskside magnetopause with changes in the magnetosheath plasma velocity that indicate
an active merging region poleward of Cluster. A global MHD simulation generates
sunward flow between a pair of opposite FACs on either side of a lobe reconnection site
near (X, Y, Z)GSM = (4.7, 5.4, 10.2) RE thus conforming with Cluster and SuperDARN
expectations. The sense of these FACs agrees with the low-altitude NBZ observations.
Citation: Eriksson, S., et al. (2005), On the generation of enhanced sunward convection and transpolar aurora in the high-latitude
ionosphere by magnetic merging, J. Geophys. Res., 110, A11218, doi:10.1029/2005JA011149.
1. Introduction
[2] Theta aurora is a large-scale auroral configuration first
reported by Frank et al. [1982] that consists of the auroral
oval and a large-scale transpolar band of auroral arcs. Since
its discovery, it has been suggested that the transpolar arc
(TPA) is a region that coincides with sunward convection,
closed field lines and precipitating electrons of plasma sheet
or boundary plasma sheet origin [e.g., Peterson and Shelley,
1984; Frank et al., 1986]. Whether the entire TPA is related
1
Laboratory for Atmospheric and Space Physics, University of
Colorado, Boulder, Colorado, USA.
2
Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA.
3
Lockheed Martin Advanced Technology Center, Palo Alto, California,
USA.
Copyright 2005 by the American Geophysical Union.
0148-0227/05/2005JA011149$09.00
to sunward convection has been challenged, however, by
the coincident measurements of antisunward ionospheric
convection over the nightside section of a duskside TPA
by the STARE coherent radar system [Nielsen et al., 1990].
This report concluded that the overall nightside flow
pattern is in agreement with the expectations of the Harang
discontinuity (HD) [Erickson et al., 1991; Koskinen and
Pulkkinen, 1995].
[3] On the basis of statistical TPA observations [Cumnock
et al., 1997; Kullen et al., 2002, and references therein], it
4
GeoForschungsZentrum, Potsdam, Germany.
Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts, USA.
6
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
7
Space Sciences Division, Rutherford Appleton Laboratory, Chilton,
UK.
8
Centre d’Etude Spatiale des Rayonnements, Toulouse, France.
9
Space Sciences Laboratory, University of California, Berkeley,
Berkeley, California, USA.
10
The Blackett Laboratory, Imperial College, London, UK.
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has been concluded that a majority of TPAs exist during
northward interplanetary magnetic field (IMF), a strong
IMF magnitude, and high solar wind speed. Large-scale
polar arcs appear in a variety of different forms [Kullen et
al., 2002]. A clear distinction may be made, however,
between those TPA that move in the dawn-dusk direction
across the polar cap and oval-aligned TPAs that generally
stay on that side of the polar cap where they initially form
[Valladares et al., 1994]. Both dynamic and quasi-steady
TPA appear to be generated at the poleward edge of the
duskside (dawnside) auroral oval for positive (negative)
IMF By in the Northern Hemisphere (NH). The main
difference occurs when the IMF By changes its sign, leading
to a motion of the TPA across the NH polar cap in the
direction of the new IMF By [e.g. Valladares et al., 1994;
Cumnock et al., 2002; Kullen et al., 2002].
[4] Recent modeling efforts [Kullen and Janhunen, 2004;
Naehr and Toffoletto, 2004] provide results consistent with
the subsequent motion of the nightside TPA across the polar
cap under the assumption that a bifurcated nightside polar
cap corresponds to a TPA. It is unclear, however, whether
the dayside extension of the TPA is reproduced correctly by
these models. Despite a wealth of information on TPAs,
questions still remain concerning the magnetospheric source
region(s) and the electrodynamic mechanisms involved in
the initial appearance of the oval-aligned TPA, such as the
relation between the TPA, field-aligned current (FAC)
sheets, and the overall convection pattern.
[5] It is known that the magnetotail typically responds to
a strong positive (negative) IMF By by a counterclockwise
(clockwise) rotation of the plasma sheet (as viewed from the
Sun toward the Earth) [e.g., Siscoe and Sanchez, 1987;
Cowley et al., 1991; Kaymaz et al., 1994]. This rotation may
even be more pronounced for northward rather than southward IMF conditions [e.g., Owen et al., 1995; Maezawa et
al., 1997]. Naehr and Toffoletto [2004] focus on this IMF
By-dependent plasma sheet rotation mechanism for the
dawn-dusk motion of TPAs in both the Rice Field Model
and the BATSRUS magnetohydrodynamic (MHD) model in
response to a sign change of the IMF By. The TPA is
identified with a nightside section of the polar cap which is
bifurcated by a region of closed field lines in agreement
with Kullen and Janhunen [2004]. Naehr and Toffoletto
[2004] show that these field lines map to a bulge in the
plasma pressure of the tail plasma sheet. The pressure bulge
appears in an intermediate region of the tail where the
plasma sheet rotation reverses its direction between that of
the near-Earth and the downtail regions. However, since the
reversal region is expected to propagate downtail with the
changing sign of the magnetosheath By [Kullen et al., 2002,
and references therein], it would seem a less viable explanation for quasi-steady oval-aligned TPAs with a duration of
several hours. Moreover, assuming that the nightside TPA
corresponds to a region of bifurcated closed field lines, it
appears that some other mechanism is required to actively
extend the region of electron precipitation and the associated upward FAC to the dayside oval as the bifurcated
regions rarely appear to do so in simulations [Naehr and
Toffoletto, 2004]. In light of recent TPA observations of
proton aurora in the dayside region [Chang et al., 2004] and
MHD simulation analyses of current systems during northward IMF [Vennerstrøm et al., 2005], we suggest that this
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additional mechanism corresponds to IMF By-dependent
lobe reconnection tailward of the cusp [Dungey, 1963;
Maezawa, 1976; Luhmann et al., 1984] during northward
IMF conditions.
[6] Evidence of magnetic reconnection in terms of accelerated or decelerated plasma jets has been verified by
several spacecraft at various locations on the magnetopause
[e.g., Paschmann et al., 1979; Sonnerup et al., 1981;
Gosling et al., 1991; Phan et al., 2001; Eriksson et al.,
2004]. In situ measurements in favor of reconnection
tailward of the cusp [e.g., Kessel et al., 1996; Avanov et
al., 2001; Phan et al., 2003] have now confirmed the
general topological picture suggested by Dungey [1963]
that high-latitude lobe reconnection between the magnetic
fields of the magnetosheath and the magnetotail lobe is
likely to occur during northward IMF Bz conditions.
[7] Magnetic reconnection on the magnetopause is predicted to launch Alfvén waves toward the ionosphere and
eventually to set up a corresponding pair of FACs on either
side of the localized merging region [Glassmeier and
Stellmacher, 1996; Cowley, 2000, and references therein].
The FAC system consists of oppositely directed currents
with an earthward directed FAC on one side and an
antiearthward FAC on the opposite side of the merging
region. Southwood [1987] proposed that the stresses induced by such a system of FAC on the surrounding plasma
and magnetic field should cause an enhanced flow of
plasma in between the two FACs. Such ionospheric flow
channel events have been observed by the SuperDARN
radars in connection with flux transfer events at the magnetopause [Neudegg et al., 2000; Provan et al., 2005, and
references therein] and during quasi-steady merging tailward of the cusp for predominantly northward IMF [Chang
et al., 2004].
[8] The presence of an additional pair of FACs poleward
of the system of the region 1 (R1) and region 2 (R2)
currents during northward IMF was first reported by Iijima
et al. [1984]. These dayside FACs are referred to as
northward IMF Bz (NBZ) currents with the sense of their
direction being opposite to those of the more equatorward
R1 currents. Moreover, Iijima and Shibaji [1987] reported
that the NBZ currents exhibit a distinct dawn-dusk asymmetry for nonzero IMF By with a relocation of the NBZ
system toward the Southern Hemisphere (SH) dawnside
during positive IMF By. This should correspond to a similar
asymmetry with an NBZ system offset toward the NH
duskside polar cap during corresponding IMF as shown
by Vennerstrøm et al. [2005].
[9] Previous studies have suggested, directly or indirectly,
that NBZ currents may be related to high-latitude TPAs
[Iijima and Shibaji, 1987; Jankowska et al., 1990; Rich et
al., 1990; Zhu et al., 1997; Eriksson et al., 2003, and
references therein]. On the basis of extensive convection
and field-aligned current observations in connection with
MHD simulations, we propose that IMF By-dependent
magnetic reconnection tailward of the cusp [e.g., Dungey,
1963; Maezawa, 1976; Luhmann et al., 1984; Cowley et al.,
1991] provides a straightforward mechanism for dayside
oval-aligned TPA formation by means of generating a pair
of oppositely directed NBZ FAC sheets adjacent to the R1
current system during northward IMF and predominant IMF
By conditions [Iijima and Shibaji, 1987; Vennerstrøm et al.,
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2002, 2005]. We associate the dayside TPA with the upward
NBZ current. The two NBZ currents will typically confine a
region of sunward convection [Southwood, 1987] in the
duskside (dawnside) sector of the dayside NH polar region
for positive (negative) IMF By [see also Cowley, 2000, and
references therein]. High-latitude lobe reconnection
explains the initial formation of oval-aligned TPAs along
the dayside open/closed field line boundary [Meng, 1981;
Makita et al., 1991] and the quasi-steady situation prior to
any subsequent dawn-dusk motion of the TPA. Our observations, moreover, confirm that the nightside TPA coincides
with antisunward or stagnant flow and the upward FAC of
the HD in agreement with Nielsen et al. [1990].
[10] The NBZ currents of the dayside TPA sunward of the
high-latitude merging site are probably carried by magnetosheath particles to some extent due to the expected direct
coupling of the geomagnetic field and the IMF [e.g.,
Eriksson et al., 2002; Chang et al., 2004]. However, since
the plasma sheet rotation is governed by the direction of the
IMF By component [e.g., Siscoe and Sanchez, 1987], it is
suggested that the boundary plasma sheet (BPS) or the lowlatitude boundary layer (LLBL) may be more important
source regions for these dayside NBZ currents [e.g.,
Jankowska et al., 1990; Makita et al., 1991]. The IMF Bydependent rotation of the magnetotail and the plasma sheet
may also be an important controlling factor for the particle
precipitation into the nightside part of the TPA tailward of
the lobe reconnection merging line.
[11] In summary, we suggest that the entire optical TPA
phenomenon of theta aurora ought to be treated separately
in one dayside and one nightside entity which may be
independently driven by two separate processes; highlatitude lobe reconnection, and the rp-driven generation
of upward FACs [Vasyliunas, 1970] of the HD [Erickson et
al., 1991]. There is possibly some degree of coupling
between these two mechanisms since the dayside and
nightside sections of the TPA often occur as one apparently
continuous band of polar arcs across the polar region.
2. Instrumentation
[12] We present a case event of an oval-aligned TPA that
evolved in the duskside region of the Northern Hemisphere
after 0007 UT on 16 December 2001. The active TPA is
visually present during the 0141– 0444 UT interval. Data
are employed from the Imager for Magnetopause-to-Aurora
Global Exploration (IMAGE) satellite, the ground-based
Super Dual Auroral Radar Network (SuperDARN), the
low-altitude polar orbiting satellites Challenging Minisatellite Payload (CHAMP), the Defense Meteorological Satellite Program (DMSP) F14, and the Fast Auroral Snapshot
(FAST) satellite. The low-altitude satellites observe
the evolving ionospheric field-aligned current systems as
they traverse the TPA and provide information on magnetospheric plasma source regions. Plasma and magnetic field
data from the high-altitude Cluster s/c 1 (C1) provide
measurements from the magnetopause and the adjacent
magnetosheath regions on the NH duskside flank. These
C1 data are examined for quantitative signatures of magnetopause reconnection during this event.
[13] The IMAGE mission [Burch, 2000] records global
images of the plasma population in the inner magnetosphere
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using a variety of imaging techniques. The Far Ultraviolet
(FUV) instrument observes the global dynamic auroral
pattern at polar latitudes in three different wavelength
regions with one 10 s image exposure once every 2 min.
We employ images from the 140 – 180 nm passband of the
Wideband Imaging Camera (WIC) [Mende et al., 2000]
which is sensitive to precipitating electrons causing emissions primarily in the N2 Lyman-Birge-Hopfield (LBH)
band and the atomic nitrogen NI lines.
[14] SuperDARN presently consists of 15 high-latitude
HF radars (nine in the NH and six in the SH) that measure
coherent backscatter from decameter-scale density gradients
in the F region of the ionosphere [Greenwald et al., 1995;
Baker et al., 2004]. The Doppler shift caused by the
backscatter of the signal off these density irregularities is
proportional to the line-of-sight component of the highlatitude E B/B2 drift velocity in the scattering volume
[Villain et al., 1985; Ruohoniemi et al., 1987]. The SuperDARN HF radars collectively provide global plasma convection measurements in a significant section of the
duskside polar region on 16 December 2001 that coincides
with the observed oval-aligned TPA.
[15] The German CHAMP satellite [Reigber et al., 2002]
was launched on 15 July 2000 into a circular 87.3
inclination polar orbit at 456 km altitude. We here consider
measurements during three consecutive CHAMP orbits
from the triaxial fluxgate magnetometer which records in
situ data at a rate of 50 Hz and a resolution of 0.1 nT. There
are no plasma particle instruments on CHAMP.
[16] The FAST satellite was launched on 21 August 1996
into an 83 inclination elliptical orbit with perigee at
350 km, apogee at 4175 km altitude, and an orbital period
of 130 min [Carlson et al., 1998]. Among the various
onboard scientific instruments, the payload consists of a
top-hat electrostatic analyzer that measures the electron and
ion energy spectra and a fluxgate magnetometer instrument
to record the in situ magnetic field. One complete energypitch angle spectrum is constructed from the average of 32
individual 70-ms spectra (survey mode) and the employed
vector magnetic field data is gathered at 8 samples/s.
[17] Unlike the spinning FAST satellite, the DMSP F14
spacecraft (launched on 10 April 1997) is in a Sun-synchronous, three axis stabilized, and 102 min nearly circular
orbit at 850 km altitude with its ascending node at
19.9 magnetic local time (MLT). The precipitating energy
flux of electrons and ions are 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 SSJ/4
plasma instrument [Hardy et al., 1984]. Electrons and ions
are only measured in the loss cone since the particle
analyzers point toward satellite zenith. The magnetic field
is recorded by the SSM fluxgate magnetometer and the E B/B2 drift velocity is measured by the SSIES ion drift meter
[Rich and Hairston, 1994, and references therein].
[18] The four Cluster spacecraft [Escoubet et al., 2001] (4
by 19.6 RE polar orbit) have an identical set of instruments
to measure the magnetic field and plasma parameters such
as the plasma density and the velocity. The Fluxgate
Magnetometer (FGM) experiment [Balogh et al., 2001]
consists of two triaxial fluxgate magnetic field sensors.
The Cluster Ion Spectrometry (CIS) experiment [Rème et
al., 2001] measures full three-dimensional ion distributions
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of major magnetospheric species such as H+ and O+ from
thermal energies to 40 keV/e. The CIS instrument comprises two separate instruments; the top-hat electrostatic Hot
Ion Analyzer (HIA) sensor with a 5.6 angular resolution and
the time-of-flight mass-resolving ion Composition and Distribution Function (CODIF) spectrometer. The energy range
of the HIA sensor is approximately 5 eV/e to 32 keV/e.
3. Observations
[19] The extensive set of available ground-based, lowaltitude, and high-altitude observations during this 16 December 2001 event are presented in the separate subsections
below. A BATSRUS MHD simulation based on the upstream solar wind conditions recorded by ACE is provided
in section 4 and puts the joint low-altitude and high-altitude
observations into a global context.
3.1. IMAGE WIC Transpolar Aurora
[20] Figure 1 displays the auroral oval and the development of a duskside oval-aligned TPA in the NH for a
selected number of time frames from the IMAGE WIC
instrument between 0052:24 and 0315:49 UT on 16 December 2001. Figures 1a –1b, 1h, and 1j also display the
passes of the FAST, DMSP F14, and CHAMP satellites (see
section 3.4) as they traverse the TPA near the indicated
times of the IMAGE WIC observations.
[21] The first evidence of enhanced electron precipitation
poleward of the duskside oval occurs at 0007:19 UT and
lasts until 0015:31 UT (not shown) when the premidnight
oval activates in substorm-like fashion. There is considerable weakening of the duskside precipitation poleward
of the oval with only occasional intensifications until
0052:24 UT while the premidnight oval stays active.
Figure 1a shows a polar arc intensification above 70
geomagnetic latitude (MLat) in the 1800 – 2100 MLT evening sector at 0052:24 UT. The FAST satellite traverses this
arc in the 0049– 0051 UT interval. The polar arcs visually
merge with the diffuse postnoon oval at 0100:36 UT (not
shown). During most of the 0104:42 – 0141:35 UT period,
there are several instances of polar arcs flaring up and
fading away in the general vicinity of the TPA-like signature
(see Figure 1b). After 0141:35 UT, as shown by the
remaining images in Figure 1, there is a clear oval-aligned
TPA present in the duskside sector of the NH polar region
that displays an intriguing temporal evolution until the
IMAGE coverage is lost after 0443:49 UT (not shown).
[22] There are two major features concerning the connection of the TPA with the duskside oval that deserve a closer
inspection. Figure 1d illustrates a TPA at 0153:53 UT that
apparently fails to connect with a rather diffuse postnoon
oval. A distinct gap in dayside emissions is observed.
The opposite situation is suggested by 0238:57 UT (see
Figure 1e) with a TPA that is connected with the dayside
oval while clearly displaying a nightside gap in precipitation. These IMAGE WIC emission gaps either correspond
to regions of diminished energy fluxes of the precipitating
electrons or suggest an actual number flux decrease.
[23] Second, we observe a feature which is initiated at
0247:09 UT with an auroral configuration that seemingly
evolves from a double-oval with two parallel bands of polar
arcs developing side-by-side. This double-TPA evolution is
Figure 1. A series of IMAGE WIC images for a number
of time frames as indicated individually during the
development of a duskside oval-aligned TPA on
16 December 2001. The low-altitude FAST, DMSP F14,
and CHAMP satellite passes are shown as they traverse the
TPA at corresponding times. The satellite positions are
marked at 1-min intervals except for FAST which is shown
once every two minutes. IMAGE WIC is sensitive to
electron precipitation primarily in the N2 LBH band.
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Figure 2. The GSM position of both the Cluster C1 and the Geotail spacecraft are shown between 2200
UT on 15 December and 0700 UT on 16 December 2001 (thin lines). The initial positions at 2200 UT are
displayed as an open circle and a diamond symbol for C1 and Geotail, respectively. Highlighted segments
in bold correspond to the respective s/c locations in the 0110– 0330 UT interval on 16 December. The
predicted extreme locations of the Shue et al. [1998] model magnetopause are also shown during this
latter time period.
seen in a selected number of frames between 0249:12 and
0303:32 UT in Figures 1f – 1i. It appears that a nightside
polar arc evolves toward the dayside polar cap starting at
0247:09 UT (not shown) from the poleward part of a
double-oval. This evolution occurs poleward of a dayside
polar arc that is connected with an active postnoon oval.
The CHAMP satellite crosses this double-TPA feature
at right angles in the 0255 – 0258 UT time interval (see
Figure 1h). By 0309:41 UT, the two polar arcs are no longer
distinguishable from one another and there is again one
well-defined TPA.
[24] At 0313:46 UT, the oval-aligned TPA evolves into a
split-TPA feature with two clearly defined and very narrow
parallel polar arcs separated by 2 – 4 MLat (corresponding
to 220 – 440 km). This feature is well-resolved by the
IMAGE WIC instrument until 0348:35 UT. An example
of this phase is displayed at 0315:49 UT (see Figure 1j) and
it appears that the DMSP F14 satellite skims along the
equatorward arc between 0313 and 0316 UT as shown by
the indicated 1-min resolution satellite footpoints. Split-TPA
signatures have previously been observed during the Dynamics Explorer (DE) mission [e.g., Frank et al., 1986;
Craven et al., 1991].
3.2. Solar Wind Plasma and IMF
[25] Figure 2 shows the GSM positions of both the
Geotail and Cluster C1 spacecraft in the 0110 –0330 UT
interval on 16 December 2001 (highlighted sections in
bold). The predicted extreme GSM locations of the magnetopause during this time interval are also indicated by using
the solar wind dynamic pressure and the IMF Bz extrema
from Geotail as input to the Shue et al. [1998] empirical
model. Geotail is located between (X, Y, Z)GSM = (17.9,
15.6, 2.1) RE and (18.1, 13.4, 3.7) RE in the solar wind
during this 0110 – 0330 UT interval, while Cluster C1
moves into the magnetosheath on the NH duskside flank
behind the dawn-dusk terminator as further shown in
section 3.5 below.
[26] Figure 3 shows the time-shifted solar wind density,
velocity, and IMF data observed by the ACE monitor and
Geotail. The data have been shifted to match the actual
Cluster observations between 2200 UT (15 December) and
0700 UT (16 December). The vertical lines mark the time
period when IMAGE WIC clearly observes an intense ovalaligned TPA between 0141:35 and 0443:49 UT. The Geotail
data are shifted forward in time by a constant time delay
Dtc = 4 min 36 s to match the major features of the IMF
clock angle q = arctan(B y /B z ) at Geotail with the
corresponding clock angle of the magnetic field that Cluster
C1 obtained in the magnetosheath (see Figure 3f). The ACE
data are also shifted forward in time by a constant delay Dtp +
Dtc = 69 min 36 s, where Dtp = 65 min is the approximate solar
wind propagation from the ACE (X, Y, Z)GSE = (238.8, 28.1,
8.1) RE position to X = 10 RE based on the recorded Vx velocity
at ACE. The time shifted Geotail and ACE data in Figure 3
thus allow for a direct comparison between the solar wind and
the actual Cluster C1 and IMAGE WIC observations.
[27] The green line in Figure 3a corresponds to the
plasma density measured by the Cluster C1 HIA instrument
at the NH duskside flank of the Earth’s magnetopause. The
clear density increase observed by C1 after 0400 UT in the
magnetosheath matches rather well with a corresponding
density increase detected in the solar wind by both ACE
(black line) and Geotail (red line). The observed amplitude
difference is due to the compression of the solar wind
plasma across the upstream bow shock. Figures 3b and 3e
display the magnitudes of the solar wind velocity and the
IMF, respectively, with a near perfect agreement between
the ACE and Geotail measurements. The GSM components
of the solar wind velocity and the IMF measured by Geotail
are shown in Figures 3c – 3d with the X, Y, and Z components being color coded in black, green, and red, respectively. Figure 3g shows the solar wind dynamic pressure
Pd = rV2 which is derived on the basis of Geotail data and
by assuming a constant average solar wind He2+/H+ ratio of
2.3% from the ACE SWEPAM composition data in the
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propagated 2100 UT (15 December 2001) to 0800 UT
(16 December 2001) time interval.
[28] Figure 3h displays a comparison between the actual
radial distance of the Cluster C1 position and the predicted
radial distance to the Shue et al. [1998] model magnetopause rmp = r0(2/(1 + cos b))a using Geotail data. Here r0
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and a represent the magnetopause standoff distance and the
tail flaring, respectively. These empirical parameters only
depend on the solar wind dynamic pressure and the IMF Bz.
The Shue et al. magnetopause is, moreover, symmetric
about the XGSM axis where b is the angle between the XGSM
axis and the rmp radius vector. The black line is the standoff
distance r0 = xmp where b = 0 while the red line is the radial
magnetopause distance rmp using a b angle that corresponds
to the C1 position. This angle gradually decreases from b =
101.0 to b = 89.6 in the 0000 – 0700 UT period. On the
basis of the Shue et al. model, it is predicted that C1 should
cross the magnetopause during the time of the more intense
TPA (between the vertical lines) and enter the magnetosheath after 0340 UT or so when rC1 > rmp.
[29] We observe that the solar wind is characterized by
enhanced 500– 600 km/s speeds and a strong 18– 20 nT
IMF magnitude during the intense TPA. The Geotail IMF is
characterized by a mostly negative IMF Bx, a strong and
fluctuating northward IMF Bz, and a generally stronger but
quasi-steady positive IMF By. This is further displayed in
Figure 3f by a steady IMF clock angle (black line) in the
range 45 < q < 70 with an average q = 56.4 between the
vertical lines. The red line in Figure 3f is the GSM magnetic
field clock angle measured by Cluster C1 further downstream (see Figure 2). The average magnetosheath clock
angle is q = 75.7 during the intense TPA suggesting a 20
duskward rotation of the IMF in the YZGSM plane. This
deflection may correspond to a draping effect of the IMF
near the magnetopause. The Geotail and C1 clock angle
measurements agree very well during most of the time
period in Figure 3 and it may be concluded that the temporal
uncertainty due to propagation delays in the corresponding
IMF and solar wind data during the TPA development has
been reduced to a minimum.
[30] Kullen et al. [2002] noted a good correlation
between the occurrence rates of large-scale polar arcs
and intervals of high solar wind speed and strong IMF
magnitudes during northward IMF. In accordance with the
Akasofu-Perreault -parameter [Akasofu, 1980] that provides a high correlation with ionospheric parameters such
as the auroral electrojets during southward IMF, Kullen et
al. [2002] suggested a corresponding energy coupling
Figure 3. Propagated solar wind plasma and IMF data
from ACE (Dt = 69 min 36 s) and Geotail (Dt = 4 min 36 s)
to Cluster C1 on 15– 16 December 2001. Panels from the
top display (a) plasma density, (b) solar wind speed, (c) solar
wind velocity (Vx, Vy, Vz)GSM components shown as black,
green, and red lines, (d) IMF (Bx, By, Bz)GSM (same color
code), (e) IMF magnitude, (f) magnetic field clock angle q =
arctan(By/Bz), (g) dynamic pressure Pd = rV2, and (h) the
predicted Shue et al. [1998] model magnetopause distance
rmp (red) in the radial direction of Cluster C1 and the
magnetopause standoff distance Xmp (black) using the
Geotail IMF Bz and dynamic pressure data. A solid line
shows the radial distance of the C1 position. (i) The
northward IMF *-parameter (see equation (1)). Blue
vertical lines indicate the time interval of the most active
and intense IMAGE WIC TPA in the 0141:35 – 0443:49 UT
period. Green markers in the bottom panel illustrate a set of
approximate times of TPA intensifications.
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parameter for northward IMF conditions that takes the
following form:
* ¼ jvjjBj2 cos4
2
q l0
;
2 m0
ð1Þ
where jvj and jBj are the magnitude of the solar wind
velocity and the IMF, respectively. The northward IMF
preference of TPAs is reflected by the cos4(q/2) clock angle
dependence and m0 is the permeability in vacuum. Kullen
et al. [2002] incorporated a constant spatial parameter l0 =
7 RE in order to provide * in units of watt. Here, l0
corresponds to the linear dimension of an approximate
geoeffective cross-sectional area which is given the same
value as for the southward IMF-dependent -parameter. The
absolute power input delivered by the *-parameter should
not be taken literally therefore, since the value of l0 is
indeed unknown. As illustrated in Figure 3i, however, it
seems that the relative * power based on the Geotail data
corresponds well with the sustained intense oval-aligned
TPA between 0141 and 0444 UT (vertical lines). Moreover,
many of the observed TPA intensifications (green bars in
Figure 3i) seem to coincide with enhanced * power input,
while the split-TPA feature in the 0314 –0348 UT interval
occurred during a period of reduced relative power. It
should be emphasized that Iijima and Shibaji [1987]
reported a rather similar IMF-dependence with NBZ current
intensities being well correlated with a solar wind parameter
of the functional form (B2y + B2z )0.5cos(q/2).
3.3. Ground-Based SuperDARN Convection
[31] Figures 4 and 5 display superimposed IMAGE WIC
data and the global plasma convection measured by the
SuperDARN radars during a selected number of eight 2-min
time intervals from 0132– 0134 UT to 0316 – 0318 UT on
16 December 2001. The right-hand side column show the
observed ionospheric convection and the derived equipotential pattern [Ruohoniemi and Baker, 1998]. The same
equipotentials are reproduced over the corresponding
IMAGE WIC data (see Figures 1b, 1d– 1j) in the left-hand
side column that also displays a grid of white dots at the
location of SuperDARN measurements.
[32] Figures 4 –5 indicate the initial times of each 2-min
SuperDARN integration time period and the times of the
IMAGE WIC data. The convection pattern that corresponds
to each individual IMAGE WIC observation trails the
IMAGE data by 7 – 63 s plus an additional 0 – 2 min. These
SuperDARN time periods were selected under the assumption that the precipitating electrons causing the high-latitude
auroral WIC emissions respond directly to any local
changes occurring at the magnetospheric source region
(e.g., the magnetopause) while the ionospheric convection
is locally excited after an approximate 1.2– 2.6 min delay
[Freeman et al., 1990, and references therein], that is, one or
two Alfvén-wave travel times later.
[33] The indicated tracks in Figures 4a, 5b, and 5d
at 0132, 0258, and 0316 UT correspond to the respective
1-min resolution satellite positions of the first DMSP F14
pass (0124 – 0136 UT), the third CHAMP pass (0250 – 0306
UT), and the second DMSP F14 pass (0306 – 0320 UT) on
16 December 2001 (see section 3.4). Large solid dots mark
A11218
the satellite positions at the central time of the SuperDARN
time integration periods.
[34] At 0132 UT (Figure 4a) we find the TPA spanning a
region of antisunward flow poleward of the center of a lowlatitude clockwise convection cell in the 1800 – 2100 MLT
sector. The clockwise convection cell likely corresponds to
converging electric fields and the presence of an upward
FAC in the central region of the vortex. This is fundamentally similar to the velocity shear zone of the HD [e.g.,
Erickson et al., 1991; Koskinen and Pulkkinen, 1995]
although shifted further westward in local time. The first
DMSP F14 pass traverses through the sunward half of this
convection cell (see section 3.4.1).
[ 35 ] The SuperDARN measurements at 0154 UT
(Figure 4b) also indicate a clockwise convection cell in
the 1800 – 2100 MLT sector. This vortex is characterized by
600– 700 m/s sunward return flows at lower latitudes that
turn poleward in the 1700– 1800 MLT sector. The dayside
gap of the TPA at 0153:53 UT occurs sunward of this
poleward turning flow, while the TPA itself is generally
found within the convection cell where the clockwise
vorticity suggests favorable conditions for an upward FAC
sheet. The westward 900– 1100 m/s high-speed flows near
midnight likely correspond to a region of earthward return
flows in the magnetotail which suggest an active tail reconnection region. These high-speed flows were first observed
in the 0140 – 0142 UT interval by the SuperDARN radars
and last until 0154 –0156 UT at this intensity and direction.
[36] Grocott et al. [2004] reported similar high-speed
flow bursts during nonsubstorm intervals in the midnight
sector. A close connection was proposed between the
azimuthal flow direction and the IMF By component with
eastward (westward) 1000 m/s bursts observed during
IMF By > 0 (By < 0) and Bz > 0 conditions. Milan et al.
[2005] proposed that the nightside formation of transpolar
arcs should occur at the western end of such strong 1000 m/s
eastward convection jets during By > 0 and Bz > 0. However,
the westward flow bursts seen in the 0140 – 0154 UT
interval during By > 0 is opposite to the suggested relationship. It is thus unclear whether this 16 December event may
be interpreted within the same framework.
[37] Figure 4c displays the state of convection at 0240 UT
which corresponds to the IMAGE WIC observation at
0238:57 UT of a TPA with an apparent nightside gap in
precipitation. The dayside TPA coincides with the 80 MLat
poleward boundary of sunward flow. A clockwise convection vortex reminiscent of a duskside high-latitude lobe cell
[see Burch et al., 1985; Eriksson et al., 2002, 2003, and
references therein] appears just poleward of the dayside
TPA. There are no IMAGE WIC emission signatures in the
vicinity of the 83.2 MLat and 16.4 MLT center of this cell,
however. The nightside emission gap coincides with a wide
region of stagnant convection that covers the entire 1800–
2100 MLT sector from 70 to 80 MLat. This emission gap
may either correspond to a region void of upward FAC
sheets or a region where the upward FAC density is
insufficiently high to produce a nightside extension of the
TPA.
[38] The SuperDARN convection at 0250 UT (Figure 4d)
corresponding to the IMAGE WIC image at 0249:12 UT
identifies the diffuse dayside TPA at 80 MLat with the
poleward boundary of sunward convection. The intense
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Figure 4. A selected number of 2-min resolution SuperDARN convection maps (right-hand side
column) are shown from 0132 to 0250 UT at the times of corresponding IMAGE WIC data in Figure 1
(left-hand side column). Thin solid lines show the derived ionospheric equipotential contour solutions as
a result of a weighted least squares fit to spherical harmonic expansions of associated Legendre functions
[Ruohoniemi and Baker, 1998]. The same equipotentials are reproduced over the IMAGE WIC data.
White dots indicate the SuperDARN coverage. (a) The trace indicates the 1-min resolution MLat and
MLT position of DMSP F14 in the 0124 – 0136 UT interval. A large solid dot marks the satellite
coordinate at 0133 UT.
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Figure 5. Another four SuperDARN convection maps and the derived equipotential contours are shown
from 0254 to 0316 UT in the right-hand side column. The same equipotentials are reproduced over the
corresponding IMAGE WIC observations on the left. A grid of white dots mark the locations of
SuperDARN information. (b) The 0250– 0306 UT CHAMP pass and (d) the 0306 – 0320 UT DMSP F14
pass. The MLat and MLT footpoints of both low-altitude passes are marked once every minute. Large
dots mark the s/c locations at the central times of the 2-min integration periods of SuperDARN.
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nightside section of the TPA is generally found at the
equatorward edge ahead of a region of enhanced 600 m/s
sunward flows centered in the 78.1 < MLat < 80.3 and
19.4 < MLT < 21.2 sector. The nightside TPA reaches
tailward from this region toward a stagnation region
equatorward of some rather weak but eastward HD-like
flows. The appearance of an additional but less intense
WIC signature above 80 MLat and tailward of 1800 MLT
coincides with the approximate 82.1 MLat and 20.3 MLT
center of a clockwise lobe cell vortex.
[39] The poleward part of the double-TPA feature intensifies at 0253:18 UT and the corresponding SuperDARN
observations at 0254 UT (Figure 5a) suggest that this is
directly related to the increased 700 m/s convection flows
in the westward and equatorward direction that emanates
from a 79.4 < MLat < 80.7 region in the 20.5 –21.5 MLT
range. It may be that the plasma pressure increases ahead of
these magnetospheric flows and that this indirectly generates rp-driven upward FACs [Vasyliunas, 1970] and the
corresponding electron precipitation. The convection flow
speeds increase to about 1000 m/s by 0256 UT (not shown)
and we associate them with a high-latitude clockwise lobe
cell centered at 83.2 MLat and 19.8 MLT. The most
dayside part of the poleward branch of the double-TPA is
again colocated with the central region of this lobe cell. The
equatorward part of the double-TPA is again seen to stretch
sunward from the stagnant convection region equatorward
of the nightside HD-like eastward flow toward the dayside
where the emissions become more diffuse and reminiscent
of the dayside emission gap at 0153:53 UT (see Figure 4b).
[40] As illustrated by the temporal development of the
global SuperDARN convection from 0258 to 0304 UT (see
Figures 5b– 5c), it is clear that the intense nightside part of
the TPA is colocated with the HD and the stagnant or
antisunward flow between the lower-latitude westward
return flow and the higher-latitude eastward flow. The
dayside extension of the TPA consists of two separate
branches with the equatorward section being directly driven
by the same processes that generate the enhanced 800 –
1100 m/s sunward flows between 0302 and 0312 UT (see
Figure 5c). The poleward branch of the dayside double-TPA
is still associated with the approximate center of the
clockwise lobe cell vortex which has moved toward 84.6
MLat and 14.3 MLT by 0304 UT. The indicated track of the
third CHAMP orbit in the 0250 – 0306 UT interval on this
day (see Figure 5b) traverses the sunward directed return
flows of the auroral oval between 0254 and 0255 UT while
crossing the enhanced high-latitude sunward lobe convection flow around 0256 UT.
[41] The final 0316 UT SuperDARN convection map
(Figure 5d) approximately coincides with the IMAGE
WIC observations at 0315:49 UT of two narrow parallel
polar arcs. The nightside part of the TPA stretches tailward
toward a region equatorward of the eastward turning flow of
the premidnight HD at 2100 MLT and 70 MLat. It may be
that the entire nightside section of the TPA is related to rpdriven upward FAC sheets ahead of the equatorward and
tailward directed convection flows. The high-latitude dayside arc appears on the poleward side of enhanced sunward
flows. The low-latitude arc of the split-TPA, however, is
found equatorward of the enhanced sunward flow and
suggests the presence of an additional upward FAC. There
A11218
Figure 6. The dusk-to-dawn orbits of the FAST satellite,
two consecutive DMSP F14, and three consecutive
CHAMP passes for the indicated time periods are shown
in the same MLat and MLT coordinate system in the
Northern Hemisphere on 16 December 2001. The mapped
5 min resolution positions of Cluster C1 between 0110 and
0140 UT using the Tsyganenko T01 model [Tsyganenko,
2002] are indicated as open circles. The mapped coordinates
of five MHD simulated magnetic field lines in the vicinity
of the high-latitude merging site at 0240 and 0300 UT (see
section 4) on this date are displayed as plus symbols.
is no obvious clue from the duskside SuperDARN convection that could explain the diminished emissions in between
the two narrow polar arcs. These parallel polar arcs (or
split-TPA) occurred during the 0313:46 – 0348:35 UT time
period. This interval coincides with a clear, but temporary,
reduction in solar wind power input as estimated by the
*-parameter (see Figure 3i). The second DMSP F14 pass
is also marked in Figure 5d and it appears that DMSP is
aligned with the sunward flow region between 0313 and
0316 UT (see section 3.4.2).
3.4. Low-Altitude Plasma and Field-Aligned Currents
[42] Although the global convection measurements by the
SuperDARN system and the IMAGE WIC observations
suggest the presence of upward FACs on the poleward side
of the sunward flows and within clockwise lobe cell
vortices, it is imperative to examine whether the TPA
emission regions correspond to upward FAC sheets by
comparing with low-altitude magnetic field measurements.
[43] The three low-altitude satellites FAST, DMSP F14,
and CHAMP are in favorable positions during this event to
confirm whether these predictions are plausible. Additional
electron and ion precipitation data from the FAST and
DMSP F14 satellites help determine whether the TPA and
the FAC sheets are on definitely open or closed field lines.
The dusk to dawn passes of these satellites are summarized
in an MLat and MLT coordinate system in Figure 6 for the
indicated time periods that include two consecutive DMSP
F14 crossings and three consecutive CHAMP passes. Solid
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dots are displayed once a minute along these satellite
footpoints.
[44] Figure 6 also displays the mapped position of Cluster
C1 once every 5 min from 0110 to 0140 UT (open circles)
using the Tsyganenko T01 model [Tsyganenko, 2002]. The
T01 empirical model fails to trace the field earthward to the
polar regions from the Cluster C1 position after 0140 UT.
This suggests that C1 moves out into the magnetosheath
after 0140 UT in general agreement with the Shue et al.
[1998] model prediction (see Figure 3h). The Cluster data
presented in section 3.5 confirms this hypothesis.
[45] The pluses in Figure 6 illustrate the corresponding
footpoints of several simulated open magnetic field lines
that we traced from a high-latitude merging site in the MHD
simulation at 0240 UT (gray) and 0300 UT (black). These
lobe reconnected field lines (see section 4) coincide favorably with the sunward lobe convection flows recorded in
this region by SuperDARN (c.f. Figures 5c – 5d).
3.4.1. Initial Phase of the Nightside TPA
[46] Figure 7 shows the observed magnetic field along the
ascending part of a FAST orbit from 0047 to 0058 UT on 16
December 2001. This time period corresponds to the early
phase of the duskside TPA evolution (see Figure 1a). The
measured energy spectrum of the precipitating ions and
electrons within ±20 of the magnetic field are also shown.
[47] The top panel in Figure 7 displays the magnetic field
deviation dB = B BIGRF in a spacecraft coordinate
system. The gray line is the cross-track component of dB
(positive in the antisunward direction), while the black line
is the along-track component. Three large-scale gradients of
the cross-track magnetic field are observed in this duskside
sector. A negative (positive) gradient corresponds to a FAST
crossing of an upward (downward) FAC under the assumption that the current sheets extend in an east-west direction.
The midpoints of upward (downward) FACs are displayed
as arrows.
[48] The low-latitude R2 and part of the R1 duskside
current systems are unaccounted for prior to 0047 UT due to
a 3 min 20 s long data gap. The lower-latitude gradient
between 0047 and 0048 UT may therefore correspond to the
poleward part of an upward R1 system. As FAST moves
further poleward, we observe a downward FAC in the
0048 – 0050 UT period which is followed by an upward
FAC sheet at even higher latitudes between 0050 and
0051 UT. This large-scale high-latitude duskside FAC
system poleward of the R1 FAC is consistent with the sense
of an NBZ current system.
[49] The FAST ion and electron energy spectra from 0048
to 0051 UT suggest that the additional high-latitude pair
of FAC sheets coincide with precipitating particles of
mostly BPS origin while the equatorward R1 FAC corresponds with an LLBL-like plasma source [Newell et al.,
1991]. The LLBL-like and BPS regions are indicated in the
middle panel as dark and light gray horizontal bars, respectively. The increased ion energy fluxes near 0051 UT (black
and white bars) either indicates a central plasma sheet (CPS)
source or corresponds to increased BPS ion fluxes due to
the intense and small-scale downward FAC at 0050:50 UT.
The IMAGE WIC observations at 0052:24 UT suggest the
presence of high-latitude polar arcs (TPA-like emissions) as
FAST passes the most poleward and upward NBZ-like FAC
(see Figure 1a). These emissions are probably caused by the
A11218
Figure 7. The cross-track (gray) and along-track (black)
magnetic field data from 0047 to 0058 UT during FAST
orbit 21163 are shown in the top panel. Central locations of
upward and downward FAC sheets are indicated by arrows.
The precipitating ion and electron energy-time spectra are
also shown within ±20 of the magnetic field. Horizontal
bars in dark and light gray indicate possible intervals
corresponding to the LLBL and the BPS, respectively.
Intermediate bands in black and white either indicate
enhanced BPS ion precipitation due to a locally strong
upward FAC or a possible source in the distant CPS or the
LLBL (see text for details).
enhanced electron precipitation in this 0050 – 0051 UT
interval.
[50] FAST encounters the polar rain and the open field
line region of the polar cap after 0051:30 UT poleward of
the upward FAC and it may be argued that the duskside
regions equatorward of this boundary coincide with a
plasma source on closed rather than open field lines. The
dawnside part of this FAST pass displays a regular R1/R2
current system in the 0110– 0113 UT interval (not shown).
[51] The DMSP F14 satellite encounters the NH duskside
auroral oval at 0124:45 UT as shown in Figure 8 by the
precipitating electron and ion energy-time spectra obtained
between 0124 and 0136 UT. Note that the ordinate of the
ion spectrogram is reversed. Despite a rather significant
degradation of the high-energy electron SSJ/4 detector on
DMSP F14 by the time of this event, it is apparent from a
comparison between the energy-time spectra of both electrons and ions that FAST and DMSP F14 encountered very
similar plasma environments in the duskside sector. This is
clear when comparing the FAST ion energy-time spectra in
the 0047– 0052 UT interval with the corresponding DMSP
F14 ion energy spectra between 0126:50 and 0130:43 UT.
Note that DMSP F14 will sample a wider local time sector
of the plasma source regions than FAST, since FAST moves
due poleward and DMSP F14 is aligned in a more westward
trajectory. There are generally three separate plasma populations within both time periods.
[52] A low-energy (200 eV to 1 keV) ion population
with 107 eV/(cm2 s sr eV) energy fluxes and softer 200–
300 eV average electron energies typical of the LLBL are
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Figure 8. DMSP F14 observations of total ion density, magnetic field, precipitating electrons and ions,
and cross-track E B plasma drift (solid line) data in the 0124 – 0136 UT interval on 16 December 2001.
Note the reversed ordinate of the ion energy-time spectrogram. Arrows correspond to upward and
downward FAC sheets. Plasma source regions are indicated by horizontal bars as CPS (black), BPS (light
gray), LLBL (dark gray), and the LLBL or distant CPS (black and white bands).
seen at lower latitudes (dark gray horizontal bar). A highenergy (1 – 10 keV) ion population is observed at higher
latitudes toward the polar cap boundary (black and white
bars). The DMSP F14 plasma data suggest that this region
may be a signature of the distant CPS although the ion
energy fluxes are more intense than the distant CPS and
rather typical of the LLBL (P. T. Newell, personal communication, 2004). Separated in between these two plasma
regimes, both FAST and DMSP F14 observe an intermediate region of lower ion energy fluxes in the 1 – 10 keV
energy range (light gray bar) which appears characteristic of
a BPS-like ion population.
[53] Figure 8 also displays the measured magnetic field
perturbation (second panel from the top) and the cross-track
component of the E B/B2 drift velocity (solid line in the
bottom panel) measured by the ion drift meter. The alongtrack dBy (dotted line) and the cross-track dBz (solid line)
components of the magnetic field are shown in a spacecraft
field-aligned coordinate system where dBz > 0 in the
antisunward direction. A positive (negative) trend in dBz
is represented by a downward (upward) FAC sheet along the
trajectory if we assume azimuthally extended current sheets.
This assumption seems acceptable since the dBy and dBz
components are either completely in phase or out-of-phase
during the entire DMSP F14 pass. The midpoint locations
of upward and downward FAC sheets are indicated by the
corresponding arrows.
[ 5 4 ] A downward R2 system is observed until
0125:45 UT on the basis of an increasing trend in dBz
and the decreasing trend in dBy. This interpretation is
consistent with the energy-time particle spectra which is
characteristic of the plasma sheet (black bar) and a low-
latitude sunward 800 m/s return flow. These return flows
agree with the observed 600 – 800 m/s SuperDARN flows
(see Figure 4a) between the 0125 – 0126 UT footpoints of
DMSP F14. After 0125:45 UT, we observe an opposite
though more gradual trend in both dB components until
about 0128 UT after which these gradients become more
pronounced. This entire time period may either be related to
a latitudinally extended R1 current system or described in
terms of two spatially separated upward FAC sheets, each
being related to the two separate antisunward flow regions as
indicated by the DMSP F14 ion drift meter instrument.
[ 55 ] A representative IMAGE WIC auroral image
corresponding to this pass is shown at 0131:20 UT (see
Figure 4a). DMSP F14 crosses the more intense part of the
Sun-aligned TPA in the 0127:30 – 0129:00 UT interval
and moves out into the polar cap region after 0130:30 UT.
The SuperDARN observations at 0132 UT (see Figure 4a)
indicate that the TPA spans a region of mostly enhanced
antisunward convection poleward of the center of a clockwise vortex. This is consistent with the tailward DMSP F14
drift meter observations and enhanced electron precipitation
in the 0128 – 0129 UT interval and the higher-latitude
upward R1-type FAC from 0128:00 to 0128:34 UT that
coincides with a BPS-like plasma source (light gray horizontal bar).
[56] As DMSP F14 moves further poleward and sunward
of the TPA it observes a distinct downward FAC followed
by an upward FAC in agreement with the sense of the highlatitude FAST magnetic field data. This high-latitude NBZlike FAC pair coincides with the most poleward high-energy
ion population reminiscent of the LLBL or the distant CPS
(black and white horizontal bars). Note that FAST associates
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Figure 9. DMSP F14 observations from 0306 to 0320 UT on 16 December 2001. Same format as
Figure 8. Horizontal color bars indicate the CPS (red), the BPS (yellow), the LLBL (blue), the reversed
cusp (diagonal blue and white bands), and the regular cusp (opposite diagonal bands of blue and white).
the upward NBZ current with the TPA-like emission
enhancements while DMSP F14 suggests that the additional
upward FAC equatorward of the downward NBZ current
corresponds to the TPA emissions.
[57] The DMSP plasma drift meter instrument observes a
1200 m/s sunward plasma flow at about 0130 UT coincident with this most poleward pair of opposite NBZ-like
current sheets. This flow is found mainly in between these
two FACs in agreement with the electrodynamic response of
the ionosphere [Southwood, 1987] to high-latitude lobe
reconnection. Sunward flows are not observed in this
location by SuperDARN at 0132 UT nor at any earlier
times which may partly be due to the sparseness of the highlatitude radar coverage. There is, however, a region
of enhanced 800 – 1000 m/s sunward flow detected by
SuperDARN in the 2100 – 0000 MLT sector poleward of
80 MLat (c.f. Figure 4a) that may correspond to the
sunward flow region measured by DMSP.
[58] As DMSP F14 exits the prenoon auroral oval region,
it encounters an upward FAC sheet followed by a downward FAC. This system could be interpreted as an upward
region 0 (R0) current and a downward R1 FAC [Iijima and
Potemra, 1976; Ohtani et al., 1995]. It may also correspond
to a duskside R1/R2 system rotated westward due to the
strong IMF By > 0.
3.4.2. Main Phase of the Dayside TPA
[59] The subsequent DMSP F14 pass brings the satellite
in the immediate vicinity of the NH oval-aligned TPA
more than 3 hours MLT further sunward than during the
previous orbit (see Figures 1j and 6). Figure 9 displays the
observed magnetic field, precipitating electrons and ions,
and drift velocity in the 0306 –0320 UT time period except
for a 4-min long data gap. This DMSP F14 pass skims
along the equatorward section of the dayside split-TPA and
the enhanced sunward flow region in the 0313 –0316 UT
interval (see Figure 5d) before exiting the auroral oval
where it connects with the TPA. Since the ion drift meter
measurements in Figure 9 correspond to the cross-track
component of the drift velocity, it is possible that DMSP
F14 misses a major part of the sunward SuperDARN flow
during this pass.
[60] The 0312– 0314 UT time interval shows a rather
steady sunward convection in agreement with SuperDARN.
The electron and ion precipitation signatures in this period
are reminiscent of the BPS-like environment and the lowerlatitude part of the R1 system near 0126:30 UT during the
previous DMSP F14 pass. The ion population in the
0314– 0315 UT interval is characteristic of the plasma
sheet or the BPS (lower ion fluxes than typically seen in the
LLBL or in the cusp). This latter period also coincides with
inverted-V electron precipitation, mostly stagnant convection, and no large-scale current signatures.
[61] The first magnetic field signature suggestive of a
large-scale current (after the data gap) is observed between
0314:57 and 0315:33 UT. This upward FAC region coincides with an antisunward turning drift velocity and diminished BPS (yellow bar) or LLBL-like (blue bar) ion energy
fluxes. Another inverted-V electron precipitation feature is
observed as well. All of these features were also reported as
DMSP F14 traversed the nightside TPA during the 0128–
0129 UT interval (see Figure 8) that spanned a BPS-type
region of antisunward flows. The 0314 – 0315:30 UT
interval and the inverted-V electron signatures during this
period likely correspond to the equatorward part of the splitTPA in Figure 5d as DMSP F14 skims along this feature.
[62] At 0315:33 UT, DMSP F14 enters a region of mixed
magnetosheath and LLBL-like ion precipitation (dark blue
and white horizontal bar). A reversed energy-latitude ion
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dispersion signature typical for northward IMF conditions is
clearly seen until 0316:37 UT with gradually lower precipitation energies at lower latitudes [e.g., Woch and Lundin,
1992; Matsuoka et al., 1996] in both the low-energy ion
cutoff and the higher-energy ion populations. The core
region of the reversed cusp-energy dispersion occurs between 0315:47 and 0316:37 UT with a peak ion energy flux
near 1 keV. Immediately equatorward of this feature, there is
a typical cusp ion precipitation signature with gradually
higher energies instead being observed progressively further
southward [Reiff et al., 1977] until DMSP F14 exits the
auroral oval at 0317:20 UT (c.f. Figure 5d). The core region
of the reversed ion dispersion coincides with sunward drift
velocities, while antisunward flows coincide with the regular cusp dispersion as expected from a velocity filter effect
[e.g., Matsuoka et al., 1996].
[63] There is, moreover, evidence in terms of stressrelated FAC sheets on either side of the enhanced sunward
flow region. A downward NBZ-type FAC [Vennerstrøm et
al., 2002] is observed in the 0315:33 – 0316:37 UT interval
coincident with the reversed cusp dispersion, while a more
intense upward FAC is found between 0316:37 and
0316:50 UT. The regular cusp apparently coincides with
this intense upward R0-type FAC and a downward R1 FAC
at lower latitudes. The antisunward cusp flow is sandwiched
between this R0/R1 system. The two adjacent regions of
simultaneous ion cusp precipitation with opposite energy
versus latitude dispersion signatures suggest the presence of
two active merging regions [e.g., Fuselier et al., 2000a; Wing
et al., 2001], one at the high-latitude magnetopause and one
at the dayside low-latitude magnetopause, respectively.
3.4.3. CHAMP FAC Observations
[64] Three consecutive NH dayside polar passes by the
German CHAMP satellite on 16 December 2001 (see
Figure 6) provide important information of the evolving
duskside FAC system detected by FAST and DMSP F14.
The FAC sheets are calculated from the transverse components of the CHAMP magnetic field after low-pass filtering
with a cutoff period of 20 s and by assuming FAC sheets
perpendicular to the orbital track. This is a good approximation when considering the orientation of the auroral
features and the CHAMP trajectories as seen in Figure 1h.
For further details on the CHAMP FAC estimates, see Wang
et al. [2005] and references therein.
[65] Figure 10a shows the estimated along-track FAC
density during the second 0116– 0133 UT CHAMP orbit
on 16 December 2001 as it traverses the NH polar region
from dusk (14.4 MLT and 55.9 MLat) to the early dawnside region. A clear dawnside R2 and R1 current system is
present with current densities larger than 1 mA/m2, whereas
the duskside currents are much weaker. If we select a
current density magnitude of 0.2 mA/m2 as a conservative
criterion for the presence of a definitive FAC (dotted
horizontal lines in Figure 10), there seems to be four weak
current systems in the duskside sector. We interpret the most
equatorward downward (negative) FAC that peaks
at 0119:59 UT (jk = 0.22 mA/m2) and the adjacent
higher-latitude upward (positive) FAC at 0121:01 UT (jk =
0.24 mA/m2) as a corresponding R2/R1 system. On the
poleward side of this pair of FACs, we observe a downward
0.37 mA/m2 current at 0121:22 UT followed by an upward
0.23 mA/m2 FAC at 0122:02 UT.
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Figure 10. Derived field-aligned current data from the
magnetic field obtained by the triaxial fluxgate magnetometer along the dusk-to-dawn passes of three consecutive
CHAMP orbits on 16 December 2001 are shown for the
indicated time periods. Positive (negative) currents correspond to upward (downward) FACs. See text for details.
[66] The subsequent 0250 – 0306 UT CHAMP orbit is
shown in Figure 10b as it moves from dusk (14.7 MLT and
56.0 MLat) to dawn. The upward R2 current is clearly seen
on the dawnside with a magnitude of 1.2 mA/m2 while the
downward R1 FAC is now highly filamentary. The most
equatorward downward FAC and the adjacent upward FAC
in the dusk sector (at 0253:50 and 0254:44 UT) possibly
correspond to a R2/R1 system. As CHAMP moves further
dawnward toward higher latitudes, it first observes a strong
1.0 mA/m2 downward FAC that peaks around 0255:20 UT.
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Table 1. MLT and MLat Coordinate For the Peak of Each FAC
With Magnitudes Larger Than 0.2 mA/m2 Along Three CHAMP
Orbitsa
Peak Time, UT
jk, mA/m2
0119:59
0121:01
0121:22
0122:02
0129:10
0130:06
0.22
+0.24
0.37
+0.23
1.30
+1.09
0253:50
0254:44
0255:20
0255:41
0256:50
0301:07
0302:17
0302:42
0303:17
0303:46
0.31
+0.61
1.00
+0.52
+0.49
0.41
0.87
+0.29
0.33
+1.22
0427:38
0428:07
0429:01
0429:22
0435:35
0436:26
0437:01
0437:46
0.56
+0.91
0.29
+0.33
0.36
+0.29
0.78
+0.70
MLT, hour
CHAMP 2
13.97
13.72
13.59
13.28
3.69
3.54
CHAMP 3
14.23
13.99
13.75
13.56
12.49
4.58
4.18
4.07
3.96
3.88
CHAMP 4
14.73
14.62
14.31
14.12
4.56
4.37
4.29
4.18
MLat, deg
System
71.42
75.40
76.72
79.19
71.31
67.71
R2
R1
NBZ?
NBZ?
R1
R2
71.40
74.99
77.34
78.67
82.73
77.21
72.71
71.07
68.74
66.79
R2
R1
NBZ?
NBZ?
NBZ?
R1?
R1
R1?
R1?
R2
72.50
74.43
77.96
79.29
74.34
70.98
68.64
65.57
R2
R1
NBZ?
NBZ?
R1?
R1?
R1
R2
a
Region 2 and Region 1 Currents Are Referred to as R2 and R1.
This downward current is followed by two separate
0.5 mA/m2 upward FACs that peak near 0255:41 and
0256:50 UT, respectively.
[67] Figure 10c displays the observed FACs from 0424
UT (15.2 MLT and 57.8 MLat) to 0440 UT along the
final CHAMP orbit. It is clear that the most equatorward
duskside R2/R1 system at 0427:38 and 0428:07 UT,
respectively, has gained in strength in comparison with
the two previous CHAMP crossings. The higher-latitude
pair of currents poleward of the R1 FAC is still present
with a downward FAC at 0429:01 UT and an upward
FAC at 0429:22 UT. The MLT and MLat coordinates for
all currents with peak magnitudes larger than 0.2 mA/m2
during the three consecutive CHAMP orbits are listed in
Table 1.
[ 68 ] A detailed comparison between the dayside
IMAGE WIC emissions at 0257:23 and 0303:32 UT
and the CHAMP FAC signatures as it traverses the
dayside double-TPA feature in the duskside postnoon
sector is shown in Figures 11a – 11b. We observe that
the R2 current lies equatorward of the postnoon auroral
oval while the R1 current peaks in the center of the oval
as expected. The strong downward FAC coincides with
the sunward extension of the duskside IMAGE WIC
emission minimum. It is clear that the lower-latitude
upward FAC of the dual upward FAC system (current
peaks around 0255:40 UT) corresponds very well with
the location of the equatorward emissions of the dayside
double-TPA that connects to the auroral oval. The equally
strong higher-latitude upward FAC (current peaks near
0256:50 UT) would coincide rather well with the more
poleward emissions of the double-TPA if these were to
A11218
extend further sunward. It is possible that these signatures
are subvisual in the WIC wavelength band. The
corresponding SuperDARN convection and the CHAMP
FAC data are compared in Figures 11c – 11d. The most
poleward upward FAC corresponds with the central
region of the evolving lobe cell vortex while the adjacent
upward FAC coincides with the enhanced sunward flows.
[69] All observed FACs with peak magnitudes larger than
0.2 mA/m2 during the three consecutive CHAMP orbits (see
Table 1) and the approximate MLT and MLat coordinates
of the FAC sheets detected during FAST orbit 21163 (see
Figure 7) are summarized in Figure 12. The approximate
midpoint locations of the observed FAC sheets during the two
consecutive DMSP F14 orbits (see Figures 8 –9) are also
shown in Figure 12. Downward and upward FACs are
displayed in red and blue, respectively. It appears from the
duskside MLT and MLat distribution of these currents that the
CHAMP satellite likely traversed four spatially rather stable
dayside current sheets while FAST missed the duskside R2
system due to the data gap. The average location of the
poleward NBZ-like upward FACs from CHAMP (excluding
the lobe vortex-related upward FAC from CHAMP 3) are
found near 79.05 MLat. This location agrees very well with
the duskside IMAGE WIC observations of the TPA (see
Figures 1e – 1i). It is also clear from the IMAGE WIC
emissions and the locations of the downward NBZ-like
FAC on the basis of both FAST and CHAMP observations
that the minimum WIC emission region probably corresponds
to an extended downward NBZ current sheet in general
agreement with the MHD simulations of NBZ currents by
Vennerstrøm et al. [2005] (see their Figure 2 at 0110 UT).
3.5. High-Altitude Perspective by Cluster
[70] The SuperDARN and the low-altitude observations
suggest that reconnection of magnetic fields tailward of the
cusp with the IMF may be generating enhanced clockwise
and sunward convection at high latitudes. The central region
of this high-latitude lobe cell further coincides with an
upward FAC as expected and measured by the low-altitude
CHAMP satellite. An additional pair of a downward and
upward FAC is also found in the immediate vicinity of the
enhanced sunward flows as predicted by Southwood [1987]
with the downward FAC being located on the equatorward
side of the sunward convection channel.
[71] The Cluster spacecraft fortunately traverse the duskside NH flank magnetopause on their way out into
the magnetosheath at the time of these ground-based and
low-altitude measurements as previously suggested in
Figures 2 – 3. We presently focus on the aspect of showing
whether there is any quantitative evidence from Cluster in
favor of rotational discontinuities at the magnetopause.
[72] Figure 13 shows the 4-s spin resolution FGM magnetic field and HIA plasma data during the 0110– 0330 UT
period. Successively from the top down, the panels display
the HIA plasma density, the magnitude and the GSM
components of the ion velocity, the GSM components and
magnitude of thepmagnetic
ffiffiffiffiffiffiffiffiffiffiffiffiffi field, and the Alfvén Mach
number MA = V Nmp m0 /B. Here, V and B are the total
velocity and magnetic field, respectively, N is the HIA
proton density, and mp is the proton rest mass. The GSM
components are color coded with X, Y, and Z being shown
as black, green, and red lines. A number of magnetopause
15 of 27
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Figure 11. A comparison of IMAGE WIC observations at (a) 0257:23 UT and (b) 0303:32 UT with the
along-track CHAMP peak FAC data (see Figure 10b and Table 1). The CHAMP footpoints are displayed
once every minute during this 0250 – 0306 UT period. Upward (downward) FACs are shown as solid blue
(orange) dots. A corresponding comparison between the CHAMP peak FACs and the SuperDARN
convection data is also shown at (c) 0258 UT and (d) 0304 UT.
transitions into the magnetosheath are seen near 0125, 0140,
0157, and 0231 UT where the plasma density increases and
the magnetic field rotates from positive to negative Bx and
negative to positive By.
[73] The three intervals indicated between dashed-dotted
vertical lines and centered near 0203, 0227, and 0300 UT
suggest an increase of the Vy and a decrease of the Vz
velocity components as compared with the magnetosheath
velocity. The Vx velocity is occasionally decelerated with a
lower tailward speed as clearly seen during the latter two
time periods and we observe that the plasma density
decreases from about 15– 16 cm3 in the magnetosheath
to 9 – 10 cm3. The simultaneous magnetic field rotates
from a positive By toward a less positive or even negative By
while the Bz components generally increase. The negative
Bx field generally becomes stronger as well. The average
magnetosheath Alfvén Mach numbers prior to these
changes in the velocity and the magnetic field are MA =
0.78, 0.76, and 0.96, reflecting a generally sub-Alfvénic
magnetosheath throughout this event (see bottom panel in
Figure 13). There are many instances of similar out-ofphase changes in the velocity and the magnetic field
between 0144 and 0312 UT on this day although a plasma
density depletion is generally absent. The approximate
times of maximum change for seven such examples (including the three events in Figure 13) may be found at 0145,
0203, 0211, 0229, 0249, 0301, and 0311 UT.
[74] A region of depleted plasma density in a boundary
layer earthward of the magnetopause with an intermediate
density between the magnetosheath and magnetosphere was
proposed by Sonnerup et al. [1981] in the vicinity of
magnetic reconnection sites. Whether the magnetic field
rotation and the corresponding change in the velocity that
Cluster C1 detected simultaneously with a plasma depletion
layer during the indicated intervals in Figure 13 are in
agreement with magnetic reconnection may be examined in
a quantitative sense by applying the shear stress balance test
which is also known as the Walén test.
[75] The magnetic field and plasma velocity across a
rotational discontinuity at the magnetopause may be stated
as
16 of 27
V VHT
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
B ð1 aÞ
¼ pffiffiffiffiffiffiffi ;
rm0
ð2Þ
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ERIKSSON ET AL.: ON MERGING AND DAYSIDE TRANSPOLAR AURORA
A11218
where DV = VBL Vs. VBL is the average ion velocity
measured in the immediate vicinity of the peak velocity
(maximum or minimum) that likely occurs within the
depleted boundary layer. The reference velocity Vs is
the average velocity evaluated in the magnetosheath. The
change DB = BBL Bs is the corresponding difference in
Figure 12. Spatial distribution of peak FAC density in
MLT and MLat coordinates encountered along the three
CHAMP orbits on 16 December 2001 (see Table 1). The
estimated midpoint coordinates of the peak FAC sheets are
also indicated along the FAST 21163 orbit (see Figure 7) and
the two consecutive DMSP F14 passes (see Figures 8– 9).
Blue (red) color corresponds to upward (downward) directed
FACs.
in the deHoffmann-Teller (HT) frame of reference
[deHoffmann and Teller, 1950; Khrabrov and Sonnerup,
1998] where V is the plasma velocity, VHT is the HT frame
velocity, B is the magnetic field, and a is the pressure
anisotropy factor defined as (pk p?)m0/B2. The plasma
pressures parallel and perpendicular to B are denoted pk and
p?, respectively. The above Walén relation thus predicts that
the plasma velocity V0 = V VHT in the HT frame is fieldaligned and that it should correspond to the Alfvén velocity
VA for reconnection events at the magnetopause [Phan et
al., 2001].
[76] Figure 14 displays a scatterplot between the plasma
velocity and VA in the HT frame during the three time
periods indicated in Figure 13 which includes a set of
measurements on either side of the magnetopause boundary
layer [Khrabrov and Sonnerup, 1998] as identified by the
different directions of the magnetic fields. The HT frame of
reference was retrieved following the procedure described
by Sonnerup et al. [1987] and we assume an approximate
isotropic pressure. The plasma is further assumed to consist
of 100% H+. The first interval suggests that the field-aligned
velocity is about 78% of the Alfvén velocity in the HT
frame, while the latter two intervals confirm that V0
reaches 99% and 95% of VA with high correlation
coefficients as expected for a rotational discontinuity
across the magnetopause.
[77] The Walén relation may also be applied in the inertial
frame of reference to test whether a rotational discontinuity
is present or not across the magnetopause. This more
qualitative form of the Walén test is stated as DV = ±DB/
pffiffiffiffiffiffiffiffi
rs m0 [e.g., Sonnerup et al., 1981, 1990; Phan et al., 2004]
Figure 13. Time series (4-s resolution) data from the
Cluster C1 HIA and FGM instruments are shown between
0110 and 0330 UT on 16 December 2001 as C1 traverses
the magnetopause into the magnetosheath at the NH
duskside flank. The panels from the top show the ion
density, magnitude and GSM components of the ion
velocity, the GSM magnetic field components, magnetic
field magnitude, and the Alfvén Mach number. Individual
X, Y, and Z components are displayed in black, green, and
red color. The Walén relation is applied during three
intervals as indicated between vertical dash-dot lines.
17 of 27
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ERIKSSON ET AL.: ON MERGING AND DAYSIDE TRANSPOLAR AURORA
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Figure 14. The Walén test is performed during three time intervals (c.f. Figure 13) in the deHoffmannTeller frame of reference. Correlation coefficients and regression slopes between the V VHT and VA
vectors are indicated separately as well as the deHoffmann-Teller velocity VHT.
the magnetic field. An average magnetosheath mass density
rs = Nsmp is used to convert DB to a change in the Alfvén
velocity. Table 2 displays Vs, Bs, the plasma density Ns, and
the Alfvén Mach number MA in the magnetosheath that
correspond to the seven events mentioned above. These
include the three intervals in Figure 14 as well. It is assumed
that the magnetosheath consists of 100% H+ and that the
plasma pressure is locally isotropic. The derived differences
DV, DB, and DVA relative to the magnetosheath are also
shown in Table 2. In displaying the individual GSM
components of DV versus DVA in Figure 15, we once again
obtain a negative Walén slope with a good correlation as
expected for a rotational discontinuity. The standard deviation of the magnetosheath observations provides a range of
error estimates to each data point in Figure 15a. These
estimates are reproduced in Figure 15b.
[78] On the basis of the derived DV changes for the seven
events in Figure 15, we may also find an approximate
direction to the reconnection X-line under the assumption
that the acceleration DV is primarily due to the j B force
generated at the reconnection site. We take this direction to
be the normalized GSM vector RX = DV/jDVj. Figure 16
displays the resulting directions relative to the Cluster C1
position at 0235:30 UT in the XYGSM, YZGSM, and XZGSM
planes, respectively. It is clear that these vectors systematically point in a poleward and dawnward direction relative
to C1 and generally somewhat tailward. An active highlatitude X-line in this direction is consistent with the
presence of an antiparallel lobe reconnection site [Luhmann
et al., 1984; Eriksson et al., 2004] poleward of Cluster for
IMF Bx < 0, By > 0, and Bz > 0 (see Figure 3).
[79] The quasi-steady long-term stability of this highlatitude merging region during the 0145– 0311 UT interval is also in agreement with the observed sub-Alfvénic
magnetosheath conditions (see Figure 13) [Fuselier et al.,
2000b]. Sub-Alfvénic magnetosheath flow is, moreover,
consistent with the generation of ionospheric sunward
convection [e.g., Gosling et al., 1991] as observed by
SuperDARN poleward of the mapped Cluster C1 position
using the T01 model (see Figures 6 and 11). It should be
Table 2. Average GSM Magnetosheath Velocity, Magnetic Field, Plasma Density, and Alfvén Mach Number Near Seven Magnetopause
Boundary Layers on 16 December 2001 and the Derived DV, DB, and DVA
Quantity
b
ta, UT
tb, UT
Vx, km/s
Vy, km/s
Vz, km/s
Bx, nT
By, nT
Bz, nT
Ns, cm3
MA
DVx, km/s
DVy, km/s
DVz, km/s
DBx, nT
DBy, nT
DBz, nT
DVAx, km/s
DVAy, km/s
DVAz, km/s
0145:05 UTa
0203:12 UT
0211:03 UT
0229:03 UT
0248:34 UT
0301:27 UT
0310:38 UT
0146:07
0146:37
286.52
88.30
76.22
32.30
44.73
5.75
21.69
1.19
109.15
136.79
115.00
10.01
34.74
19.68
46.86
162.73
92.18
0203:38
0204:19
193.41
196.67
13.70
47.92
28.85
12.28
12.38
0.78
2.13
111.67
59.02
1.36
21.63
10.22
8.44
134.07
63.38
0210:21
0210:46
198.21
143.89
18.30
51.99
37.15
12.19
10.55
0.56
34.61
75.46
31.05
0.93
11.21
14.24
6.25
75.28
95.58
0224:45
0226:28
237.56
91.90
63.90
43.14
43.32
5.24
14.97
0.76
45.72
172.99
89.19
6.81
35.50
12.43
38.37
200.15
70.09
0247:24
0248:05
244.23
118.19
41.22
43.95
35.74
10.93
17.54
0.91
36.94
137.22
99.69
1.09
34.05
25.32
5.70
177.36
131.86
0259:02
0259:39
249.77
102.08
83.52
38.18
37.49
2.57
15.87
0.96
84.26
141.69
112.48
14.72
20.31
18.28
80.61
111.20
100.07
0309:14
0310:09
250.55
96.41
78.98
40.13
36.78
3.21
13.29
0.86
20.01
113.74
99.27
6.92
23.19
27.65
41.39
138.77
165.46
a
These are the times of maximum change within each boundary layer.
The time intervals of magnetosheath averaging are given by ta and tb.
b
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Figure 15. (a) The qualitative Walén relationship DV =
pffiffiffiffiffiffiffiffi
±DB/ rs m0 is shown for seven events between DV and
DVA. Here, DV = VBL Vs is the difference between the
boundary layer and the magnetosheath velocity (see
Table 2). The X, Y, and Z GSM components of DV and
DVA are displayed as solid dots, crosses, and square
symbols, respectively. (b) The standard deviation of the
averaged magnetosheath quantities is incorporated. See text
for further details.
noted that this mapping could only be performed when
Cluster C1 was earthward of the estimated magnetopause
between 0110 and 0140 UT. This time period precedes the
observations of the DV changes by 5 min to 90 min.
4. MHD Simulation on 16 December 2001
[80] In order to put the joint low-altitude and the highaltitude Cluster observations into a global context, we
compare our in situ observations with a solar wind input
driven MHD simulation provided by the Block-AdaptiveTree-Solar wind-Roe-Upwind-Scheme (BATSRUS) MHD
code [Gombosi et al., 1998, and references therein] via the
Community Coordinated Modeling Center (CCMC) at
http://ccmc.gsfc.nasa.gov.
[81] The BATSRUS MHD model of the Earth’s magnetosphere solves the three-dimensional ideal MHD equations
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in finite volume form using numerical methods related to
Roe’s Approximate Riemann Solver on an adaptive cartesian grid which is composed of rectangular boxes of
variable size. The system spans 255 < XGSM < 33 RE,
jYGSMj 48 RE, and jZGSMj 48 RE. The inner boundary of
the magnetospheric solver is located at 3 RE.
[82] This real-time MHD simulation for the 16 December
2001 event employed the upstream solar wind input from
the ACE monitor with time-dependent inflow boundary
conditions during the 0200 – 0359 UT period. Enhanced
0.125 RE resolution is applied to the inner region near the
3 RE boundary. Figure 17 illustrates the simulation results at
0240 UT for three different quantities in the YZGSM plane at
XGSM = 1.3 RE. This plane corresponds to the momentary
(X, Y, Z)GSM = (1.3, 8.7, 9.3) RE position of Cluster C1 at
0240 UT (see Figure 13).
[83] Figure 17a shows the magnetic field magnitude. There
are two regions of suppressed field strengths at the NH and
the SH magnetopause, respectively, where jBj < 15 nT
(regions colored in black). These simulated features are
referred to as the ‘‘magnetospheric sash’’ [e.g., White et
al., 1998; Nishikawa, 1998; Maynard et al., 2001; Eriksson
et al., 2004] and correspond to regions where MHD
simulations predict antiparallel merging [Crooker, 1979;
Luhmann et al., 1984]. The resolution of the MHD grid
is increased to 0.25 RE in the vicinity of the northern highlatitude merging site. A dashed vertical line indicates the
duskside (Y, Z)GSM = (2.1, 9.3) RE center of the NH sash.
[84] Figure 17b displays the Vx component of the simulated
flow. Regions in white generally correspond to Vx < 70 km/s
and thus incorporate the magnetosheath and the solar wind
regimes. Enhanced sunward flows on the order of Vx = 70 km/s
are present near the NH sash. A clear 100 km/s peak
dawnward flow component (not shown) also coincides with
the focus of the maximum sunward flow near (Y, Z)GSM = (2.1,
8.6) RE (central white dot). It is evident that this flow occurs
over a wider region that reaches earthward to the NH polar cap
region. Figure 17c shows the corresponding field-aligned
current (Jpar) in the same plane. There are two distinct areas
of oppositely directed FACs on either side of the NH sash. An
antiparallel FAC (color coded in blue) stretches toward the
dawnside region of the noon-midnight meridian, while a
parallel FAC (red color) is found in the duskside magnetopause sector. The center of the FAC system is located near (Y,
Z)GSM = (2.1, 9.3) RE and thus coincides with the focus of the
sash and the peak sunward and dawnward flow. This FAC
system is consistent with an NBZ system.
[85] High-latitude lobe reconnection may generate sunward flows typically during sub-Alfvénic magnetosheath
conditions [e.g., Gosling et al., 1991; Matsuoka et al., 1996;
Phan et al., 2003] due to a sunward directed j B force that
initially acts to pull the kinked magnetic field sunward.
Figure 18a suggests that the simulated sunward flow is
indeed driven by such j B forces. A major part of this
process occurs on the earthward side of the magnetopause.
Arrows illustrate the velocity component in the XZGSM
plane at YGSM = 2.1 RE and the XGSM component of the
j B force is color-coded from blue (tailward) to red
(sunward). The topology of the magnetic field is also
illustrated with open, closed, and IMF fields indicated in
black, red, and blue, respectively. Figure 18b shows how the
magnetic field on either side of the magnetopause in this
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Figure 16. The DV velocity changes (see Figure 15 and Table 2) between the boundary layer and the
magnetosheath are used to define a normalized GSM direction RX = DV/jDVj from the Cluster C1
location at 0235:30 UT to the probable X-lines. It is assumed that DV is proportional to the j B force
due to the generated kink of the merged magnetic fields.
plane are pulled toward a common center near the highlatitude lobe reconnection site by the j B force (arrows)
due to the lobe magnetopause current.
[ 86 ] The simulated MHD quantities presented in
Figures 17 – 18 are all in agreement with the expected
high-altitude global signatures of lobe reconnection tailward
of the cusp for northward IMF Bz, positive IMF By, and
negative IMF Bx. It is also suggested that the generated pair
of FAC on either side of the merging site communicates the
induced magnetic field stresses to the ionosphere to support
the sunward convection observed there by SuperDARN
[Southwood, 1987].
[87] In order to compare the merging scenario in the
MHD simulation with the observed sunward flows from
SuperDARN, we select seven GSM coordinates (x0, y0, z0)
in the vicinity of the enhanced high-altitude sunward flow
region at 0240 and 0300 UT and trace the field lines that
pass through these coordinates in both directions along the
MHD magnetic field. Figure 19 illustrates the resulting
flux tubes from three different perspectives at 0240 UT.
One field line is closed and appears to be anchored in the
vicinity of the merging regions in both hemispheres (see
Figure 17a). Four of the field lines are open and connected
to the NH polar cap at one end with the other end being
connected to the draped IMF. One field line has recently
become disconnected from the Earth and another is a typical
IMF field line. These flux tubes all pass in the proximity of
(X, Y, Z)GSM = (4.7, 5.4, 10.2) RE at 0240 UT which we
interpret as the approximate location of the NH lobe
reconnection site between the draped IMF and the open tail
lobe field on the basis of the MHD magnetic field topology
at this time. One of these open lobe field lines is not yet
affected by this merging site, however, and may be considered as an old field line previously opened by merging on
the dayside magnetopause. This field line is only shown in
Figures 19a –19b for clarity.
[88] The corresponding inner magnetosphere coordinates
(x1, y1, z1) at 3 RE that result from the field line tracing from
(x0, y0, z0) are mapped further earthward along the dipole
field to an altitude of 1 RE by using the Tsyganenko T01
model [Tsyganenko, 2002]. These GSM positions were
finally transformed to corresponding MLat and MLT coor-
dinates and displayed as plus symbols in Figure 6. Table 3
shows the selected magnetospheric GSM coordinates, their
inner boundary coordinates at 3 RE, and the corresponding
MLat and MLT for the field lines passing in the immediate
proximity of the high-latitude merging sites at 0240 and
0300 UT. All field lines that pass through the given
coordinates (x0, y0, z0) are open at the magnetospheric end
except for field lines I which are closed at both ends. All
open field lines at 0240 UT (0300 UT) map to a region near
78.31 (78.27) MLat and 14.09 (12.91) MLT which is very
close to the enhanced 800– 1100 m/s sunward flows that the
SuperDARN radars observe in the 0302– 0312 UT time
interval (see Figures 5 – 6).
[89] Figure 20 shows the radial component (Jr) at 1 RE
of the mapped field-aligned current (Jpar) from the inner
3 RE boundary at 0240 and 0300 UT. A jagged black
circular line indicates the open/closed boundary on the
basis of the simulated field line topology. The downward
FAC on the dawnside (in white) and the upward FAC on
the duskside (light blue) in the 70 – 75 MLat range
correspond to a R1 current system. There is some
indication of a R2 system equatorward of 70 MLat in
the nightside sector. Realistic R2 systems are generally
not reflected with much confidence by MHD codes,
however, due to insufficient model resolution and missing
gradient and curvature drift physics for ions and electrons
[Ridley et al., 2001].
[90] As suggested by this MHD simulation, there is a
relatively strong upward FAC poleward of the open/closed
boundary and inside the polar cap. There is also some
indication of a weaker downward FAC (dark blue regions
in Figure 20) in the 1300– 1800 MLT duskside sector. This
high-latitude pair of simulated FAC sheets likely corresponds to an NBZ current system in general agreement
with the duskside FAC observations recorded by CHAMP,
DMSP F14, and FAST (see Figure 12). Similar results of the
simulated NBZ system are obtained by other MHD codes
[e.g., Vennerstrøm et al., 2005]. The region between the two
NBZ currents coincides with the open/closed boundary.
Note that there are no indications of a bifurcated polar
cap in the NH on the basis of this MHD simulation but
rather a poleward expansion of the duskside open/closed
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field line boundary in agreement with our low-altitude
observations and the results reported by Meng [1981] and
Makita et al. [1991] on the formation of oval-aligned TPAs.
5. Discussion
[91] On the basis of this unprecedented set of observations and the supporting MHD simulations, we propose the
following electrodynamic scenario for the oval-aligned TPA
on 16 December 2001 which is discussed in further detail in
this section. The apparent continuous band of the duskside
TPA is divided into one dayside and one nightside entity
that may be driven by two seemingly independent physical
processes. The dayside part of the duskside TPA is associated with sunward lobe convection and an upward NBZ
current as shown by the SuperDARN and the CHAMP
observations (see Figures 10– 11). We identify the highlatitude dayside branch of the double-TPA poleward of the
NBZ currents with an upward FAC near the center of this
evolving clockwise lobe cell (see Figures 1f– 1i and Figure 5).
The nightside part of the TPA generally tailward of the dawndusk meridian is related to an upward FAC associated either
with clockwise convection cells or the velocity shear zone of
the HD [e.g., Koskinen and Pulkkinen, 1995].
[92] The sunward convection and the NBZ currents are
directly driven by quasi-continuous high-latitude lobe re-
Figure 17. A real-time MHD simulation is performed via
the CCMC using a propagated solar wind input from the
ACE upstream monitor between 0200 and 0359 UT on
16 December 2001. The results at 0240 UT are shown in the
YZGSM plane (X = 1.3 RE) for (a) the magnetic field
magnitude, (b) the Vx component of the MHD flow, and
(c) the field-aligned component of the current. Dashed
vertical lines mark the duskside center of the MHD sash at
YGSM = 2.1 RE in this plane. Large-scale Vx regions in white
correspond to speeds Vx < 70 km/s except at (Y, Z)GSM =
(2.1, 8.6) RE where Vx > 70 km/s. The results of the MHD
simulation is available via the CCMC at http://ccmc.gsfc.
nasa.gov.
Figure 18. MHD simulation results of the magnetic field
topology, velocity, and the j B force near the lobe
reconnection site are shown projected into the XZGSM plane
at Y = 2.1 RE. (a) Vector field shows the projected velocity
and the background color indicates the (j B)x component.
(b) Vector field shows the projected j B force and the
background color displays the Vx velocity.
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caused by the kink in the open lobe reconnected magnetic
fields. The active high-latitude merging site also generates a
pair of NBZ currents [Iijima and Shibaji, 1987; Cowley,
2000] that communicates the induced magnetic field stresses
to the ionospheric plasma environment with enhanced
sunward convection being driven in between these NBZ
field-aligned currents [Southwood, 1987].
[93] The MHD simulation suggests that the enhanced
sunward convection occurs on open lobe field lines near
the open/closed field line boundary while the downward
NBZ current is found on closed field lines equatorward of
this boundary (see Figures 6, 11– 12, 17– 20). The lowaltitude particle precipitation observations tailward of the
simulated lobe reconnection site suggest that the NBZ
currents are colocated with LLBL or BPS-like magnetospheric source regions (see Figures 7 – 8) along the polar cap
boundary. The downward NBZ system and the adjacent
low-latitude regions are thus likely to coincide with closed
rather than open field lines. This appears to be the case also
for the upward NBZ current. Only the most sunward NBZ
region corresponds with a cusp/magnetosheath region (see
Figure 9) that at least suggests a prior history of plasmas on
open field lines.
[94] The strong positive IMF By during this northward
IMF event induces a torque on the magnetosphere that
likely brings the LLBL, BPS, and the auroral oval to higher
latitudes on the duskside [Meng, 1981; Siscoe and Sanchez,
1987; Cowley et al., 1991; Makita et al., 1991]. The
apparent open field line region equatorward of the dayside
TPA should therefore rather be interpreted as the minimum
IMAGE WIC emission region due to a downward directed
NBZ current generally on closed field lines [Vennerstrøm et
al., 2005]. This is consistent with the low-altitude plasma
Table 3. Mapping of Five MHD GSM Coordinates Near the
Simulated Merging Sites at 0240 and 0300 UT on 16 December
2001 to the Inner Magnetosphere and a Transformation to
Geomagnetic Latitude and Local Time Using the CCMC Utility
and the Tsyganenko T01 Modela
Figure 19. Seven magnetic field lines are traced from
seven (x0, y0, z0) locations in the proximity of the enhanced
sunward MHD flows at 0240 UT. All field lines also pass
near (X, Y, Z)GSM = (4.67, 5.38, 10.19) RE which is the
approximate location of a lobe reconnection site between
the draped IMF and the tail lobe field. See text and Table 3
for more details.
connection between the IMF and the lobe magnetic field
tailward of the cusp [e.g., Dungey, 1963; Maezawa, 1976].
These generally antiparallel magnetic fields are forced toward one another by converging j B forces (see Figure 18b)
on either side of the magnetopause. The sunward convection
flows develop inside the magnetopause by the j B force
Coordinate
I
x0
y0
z0
x1
y1
z1
MLat, deg
MLT, hour
Topographyb
1.300
2.290
9.290
0.737
0.405
2.833
77.10
13.37
0
x0
y0
z0
x1
y1
z1
MLat, deg
MLT, hour
Topography
1.200
1.380
7.800
0.805
0.248
2.843
77.82
12.87
0
II
III
Mapping at 0240 UT
2.125
2.135
2.875
2.885
9.875
9.875
0.868
0.865
0.520
0.517
2.784
2.790
78.29
78.29
14.14
14.14
1
1
Mapping at 0300 UT
1.200
4.000
1.380
4.000
9.120
10.000
0.725
0.850
0.149
0.265
2.867
2.824
77.25
78.35
12.48
12.90
1
1
IV
V
2.175
2.825
9.825
0.823
0.466
2.789
77.90
13.86
1
5.000
5.000
10.100
0.901
0.516
2.765
78.76
14.22
1
4.250
4.500
10.000
0.852
0.272
2.806
78.22
13.08
1
6.000
5.000
10.000
0.921
0.286
2.805
79.26
13.17
1
a
The resulting MLat and MLT for all five field lines are displayed in
Figure 6 as gray (black) plus symbols at 0240 UT (0300 UT).
b
CCMC magnetic field topology status with 0 being ‘‘closed’’ and 1
being a ‘‘Polar Cap/open’’ field line.
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Figure 20. The resulting ionospheric radial components
(Jr) at 1 RE of the mapped field-aligned current (Jpar) from
3 RE are shown at (a) 0240 UT and (b) at 0300 UT. A
jagged black circular line indicates the open/closed fieldline boundary.
observations and the MHD simulation. Whether the particle
precipitation of the upward NBZ current along the polar cap
boundary is on mostly closed field lines on the basis of BPS
and LLBL-like energy fluxes or on open field lines as
suggested by the MHD simulation remains an open issue.
[95] Several explanations have been proposed concerning
the electrodynamic nature of the occasional band of auroral
arcs that stretches across the polar regions from midnight
toward the dayside section of the auroral oval since its
discovery by Frank et al. [1982]. Examples include the
bifurcation of the polar cap by closed field lines [Frank et
al., 1986], an IMF By-dependent plasma sheet rotation
model [Siscoe and Sanchez, 1987; Cowley et al., 1991] that
causes an asymmetrically expanded auroral oval [Meng,
1981; Jankowska et al., 1990; Makita et al., 1991], and an
IMF By-dependent antiparallel merging model [e.g., Chang
et al., 1998, 2004; Eriksson et al., 2003].
[96] Frank et al. [1986] proposed that the entire TPA of
theta aurora correspond to sunward convecting plasma on
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closed field lines. Plasma convection was only observed
along a fraction of the entire TPA, however, as the lowaltitude DE-2 satellite crossed the TPA. Antisunward
plasma flows, assumed to be on open field lines, were
detected on either side of it. Nielsen et al. [1990] reported a
case of an oval-aligned TPA which appeared on the duskside
in the NH for similar IMF conditions as those observed
during 16 December 2001 and contested the notion of
sunward convection over the entire TPA. Evidence of antisunward convection by the STARE radar within the nightside
section of the TPA confirmed an association with the HD.
Nielsen et al. [1990] moreover claimed that the antisunward
flows on the equatorward side of the duskside TPA coincided
with the LLBL on closed field lines rather than open field
lines. It should be noted that the events reported by Frank et
al. and Nielsen et al. likely corresponded to different snapshot
events of the evolving TPA feature across the polar region.
[ 97 ] The global plasma convection provided by
SuperDARN in coincidence with the entire duskside TPA
on 16 December 2001 and its temporal evolution suggest a
very similar division of the TPA in one dayside and one
nightside part. The dayside part generally corresponds to the
sunward flows on the duskward side of a lobe convection
cell [e.g., Burch et al., 1985]. These lobe cell flows are
occasionally enhanced above 1000 m/s. The nightside
portion of the TPA, however, generally spans the region
of tailward convection and/or the stagnation region on its
equatorward side consistent with the velocity shear zone of
the HD in agreement with Nielsen et al. [1990].
[98] TPA emissions are often reported to stretch across the
entire polar cap from the nightside to the dayside oval. This is
consistent with the simultaneous occurrence of a duskside
lobe cell in the dayside polar region and a premidnight
velocity shear region (see Figure 5c). There are also instances
of either clear nightside or dayside gaps in the IMAGE WIC
emissions (see Figures 1d – 1e) which we showed to be
directly related to much weaker or absent nightside velocity
shear zones and lobe cell vortices (see Figures 4b – 4c),
respectively. This suggests that the two proposed mechanisms
of lobe reconnection and the tail electrodynamics of the HD in
general are independent from one another. Once a strong lobe
convection cell is generated in the dayside, however, a welldefined nightside HD shear flow velocity region also develops as seen during the evolution of the double-TPA (see
Figures 4d– 5c). We propose that the clockwise lobe vortex
indirectly supports the stable formation of the near-Earth HD
during times of relatively stronger duskside lobe convection
and positive IMF By.
[99] How may these two generally independent processes
interact? The open flux tubes of the clockwise lobe cell are
likely swept downtail along the dawnside flank magnetopause after an initial sunward displacement [e.g., Maezawa,
1976; Cowley, 2000; Hasegawa et al., 2002] and must
eventually return westward and sunward across the magnetotail lobe toward the duskside high-latitude lobe reconnection site. A sketch outlining the schematic features of lobe
convection for IMF By > 0 and northward IMF is seen, for
example, in Figure 7 by Cowley [2000]. The upward FAC of
the HD, on the other hand, is believed to be generated as the
source of energetic ions of the dawnside plasma sheet are
depleted by westward gradient/curvature drifts [Erickson et
al., 1991]. In this sense, it may be that the enhanced
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westward E B drift of the returning magnetotail lobe cell
plasma could promote the formation of the HD by coupling
with the westward gradient drift. Another possibility is that
the plasma pressure increases ahead of the magnetospheric
plasma and magnetic field circulating in the lobe cell that
indirectly could generate some fraction of the rp-driven
FACs of the HD [Vasyliunas, 1970; Erickson et al., 1991].
The nightside section of the duskside oval-aligned TPA is
indeed indirectly driven by lobe reconnection if this mechanism is able to stabilize or even promote the formation of
the HD.
[100] Quantitative evidence in favor of high-latitude lobe
reconnection is provided by Cluster C1 as it traverses the
magnetopause on the NH duskside flank. Cluster C1 confirms the frequent presence of rotational discontinuities
throughout most of the interval between 0142 and
0312 UT on 16 December 2001 when IMAGE WIC detects
the intense duskside oval-aligned TPA. The evidence consists of changes in the velocity DV and the magnetic field
DB that satisfy the Walén relation. These velocity changes
systematically suggest a merging site poleward and dawnward of the Cluster C1 location (see Figure 16) in agreement with the MHD simulation (see Figures 17– 19) and the
predicted lobe reconnection site at (X, Y, Z)GSM = (4.7, 5.4,
10.2) RE for these quasi-steady directions of positive IMF Bz
and dominant positive IMF By [Crooker, 1979; Luhmann et
al., 1984]. A merging site poleward of Cluster C1 is also
consistent with the mapped C1 position [Tsyganenko, 2002]
relative to the sunward high-speed flow channel that the
SuperDARN radars observe (see Figures 6 and 11). Since
C1 steadily moves away from the magnetopause further into
the magnetosheath as time progresses after 0312 UT, it is
likely that it ceased to detect the effects of the reconnection
region although IMAGE WIC continued the observation of
the dayside TPA until it lost the polar region coverage after
0443:49 UT. Additional evidence in favor of lobe reconnection is provided by the DMSP F14 observation of
reversed cusp ion energy-latitude dispersion (see Figure 9)
as it moved along the dayside part of the oval-aligned TPA.
[101] The dayside part of the quasi-steady duskside
TPA in the IMAGE data coincides with the high-latitude
upward FAC in the low-altitude CHAMP and FAST data
near the poleward boundary of the sunward flows (see
Figures 11 – 12). A downward FAC is observed on the
equatorward side of the enhanced sunward flows and
generally appears in a region where the auroral luminosity
due to electron precipitation experiences a clear minimum.
The high-latitude sunward flows sandwiched between the
upward and downward FAC sheets are consistent with the
ionospheric signatures of magnetic merging as predicted by
Southwood [1987]. The duskside offset and the sense of this
high-latitude FAC pair are consistent with an NBZ current
system during the strong prevailing northward IMF Bz and
positive IMF By conditions [Iijima and Shibaji, 1987;
Cowley, 2000; Vennerstrøm et al., 2002, 2005] as observed
by Geotail and ACE during this event.
[102] Although the observations from the first DMSP F14
pass and the FAST satellite were recorded about 1 hour prior
to the appearance of the intense TPA, it is likely that the
most poleward pair of oppositely directed FACs correspond
to the same evolving dayside NBZ current system that
CHAMP later observed in connection with the dayside
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TPA. The FAST data associate the TPA with a BPS source
and the upward NBZ current. The nightside TPA encountered by DMSP F14 (see Figure 8) is related to an upward
FAC equatorward of the NBZ system, however, that we
interpret as the upward FAC of the HD. The interpretation
that the entire TPA is driven by two different mechanisms in
the dayside and the nightside regions are therefore supported by the global SuperDARN convection as well as the
low-altitude FAC systems during this event. The energy
spectra of the particle precipitation related to the nightside
section of the TPA clearly suggests a plasma source in the
BPS (rather than the magnetosheath) [e.g., Peterson and
Shelley, 1984]. A TPA source in either the BPS or the LLBL
is consistent with a poleward expansion of the auroral oval
[Meng, 1981; Makita et al., 1991] due to the IMF Bydependent rotation of the magnetotail and the plasma sheet
[Siscoe and Sanchez, 1987; Cowley et al., 1991] since there
are no regions equatorward of the oval-aligned TPA consistent with a source on open field lines. A similar BPS and/
or LLBL-like source is observed over the upward current of
the split-TPA. The dayside region thus experiences a similar
poleward expansion of the LLBL and the BPS as the
nightside.
[103] The particle measurements from the two DMSP F14
and the FAST passes indicate the presence of a high-latitude
merging site sunward of the 18 MLT TPA crossing by
FAST but tailward of the DMSP F14 crossing of the dayside
NBZ system on the basis of the reversed ion energy-latitude
dispersion signature over the downward NBZ current. This
is also in agreement with the MHD simulation and the
general location of the mapped lobe reconnection merging
site relative to the low-altitude observations (see Figure 6).
[104] There is no evidence of opposing plasma sheet
twists or pressure bulges in the simulated BATSRUS MHD
plasma pressure from 0200 to 0359 UT on 16 December 2001
(c.f. MHD simulation Stefan_Eriksson_081704_1 at http://
ccmc.gsfc.nasa.gov). Such signatures were reported by
Naehr and Toffoletto [2004] at times of IMF By direction
changes with closed field lines connecting the nightside
region of the assumed TPA with a plasma pressure bulge
in the tail. No pressure bulge signatures appeared [Naehr
and Toffoletto, 2004] when the IMF By was held constant, however, in agreement with the MHD simulation
on 16 December. As suggested by the consistently
positive IMF By in the 0200 – 0359 UT interval (see
Figure 3d), it is evident that such steady IMF intervals
will not generate any regions of opposite plasma sheet
twists in the tail.
[105] It has been proposed [e.g., Kullen et al., 2002;
Cumnock et al., 2002] that regions of opposite plasma
sheet rotations can be ascribed as causing the TPA
formation and its dawn-dusk motion from one side of
the polar cap to the other with closed field lines bifurcating the otherwise open polar cap region. This may be
true for the nightside portion of the TPA, although it
appears that the physics of the HD needs to be incorporated in any such model of the nightside TPA formation.
By tracing field lines toward the ionosphere from the
vicinity of the simulated merging region in connection
with sunward MHD flows (see Figures 17 – 20) it is
shown that these sunward convecting field lines are open
rather than closed and map to the boundary between
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closed and open field lines. What then generates the
quasi-steady oval-aligned TPA during steady IMF, especially the dayside portion, once such a pressure bulge/
bifurcation has exited the magnetotail? As previously
indicated by Naehr and Toffoletto [2004], some other
mechanism must actively extend the polar arcs to the
dayside oval into a complete theta aurora. On the basis of
the collective set of observations and the supporting
MHD simulations presented during this fortuitous event,
it is clear that this additional dayside mechanism corresponds to high-latitude lobe reconnection.
6. Summary and Conclusions
[106] We have presented an extensive set of observations
during a duskside oval-aligned TPA in the Northern Hemisphere on 16 December 2001 that includes measurements
from IMAGE WIC, SuperDARN, the low-altitude CHAMP,
DMSP F14, and FAST satellites, as well as high-altitude
Cluster data. This data set provide information on the state
of convection and FAC sheets in the TPA region complemented by high-altitude plasma and field measurements in
agreement with a high-latitude merging site.
[107] A quasi-steady duskside oval-aligned TPA in the
IMAGE WIC data coincides with a dayside pair of opposite
FACs in the low-altitude CHAMP and FAST data along the
open/closed field line boundary. This dayside high-latitude
FAC system is consistent with an NBZ current system
[Iijima and Shibaji, 1987; Vennerstrøm et al., 2002, 2005]
offset toward the duskside sector. The high-latitude upward
NBZ FAC is identified with the dayside TPA while the lowlatitude downward NBZ current probably coincides with the
minimum IMAGE WIC emission region between the duskside auroral oval and the TPA.
[108] SuperDARN confirms the presence of a sunward
ionospheric convection channel which is generally located
between the oppositely directed NBZ currents in connection
with the dayside part of the TPA. The enhanced sunward
flows are associated with a duskside lobe cell and the highlatitude dayside branch of the double-TPA coincides with an
upward FAC within this clockwise lobe cell. The nightside
part of the TPA is related to an upward FAC and tailward
convection and/or a stagnation region in a velocity shear
zone. This shear convection region is typically consistent
with the HD [Nielsen et al., 1990; Erickson et al., 1991].
[109] The Walén test and the analysis of plasma acceleration as Cluster C1 traverses the magnetopause during
this event quantitatively confirm the presence of an active
high-latitude lobe reconnection region poleward of Cluster
consistent with the prevailing duskward and northward
IMF [e.g., Luhmann et al., 1984]. The BATSRUS MHD
simulation suggests that this is indeed the case with
NBZ-type opposite FAC currents being generated on either
side of a lobe reconnection site at (X, Y, Z)GSM = (4.7, 5.4,
10.2) RE. This location is consistent with the Cluster C1
DV observations (see Figure 16) and the C1 position at
0240 UT [see also Eriksson et al., 2004]. The MHD
simulation also indicates that sunward convection flows
are driven inside the magnetopause by the j B force
caused by the kink in the open lobe reconnected magnetic
fields. The simulated NBZ system is expected to transfer
the generated magnetic field stresses to the ionospheric
A11218
plasma which is accelerated in the sunward direction in
between these opposite FACs [e.g., Southwood, 1987;
Gosling et al., 1991; Cowley, 2000]. This is confirmed
by the simultaneous SuperDARN measurements of enhanced sunward convection channels sandwiched between
the opposite NBZ currents as observed by the low-altitude
CHAMP satellite. Further evidence in favor of highlatitude lobe reconnection is provided by the reversed
ion energy-latitude cusp dispersion signature observed by
the DMSP F14 satellite just postnoon and poleward of a
R0 FAC system.
[110] We propose that quasi-continuous lobe reconnection
sustains the dayside oval-aligned TPA. The nightside part of
the duskside TPA is related to the upward FAC and the
electrodynamics of the HD, however, which is likely to be
affected by lobe cell convection. Any dynamic model of the
subsequent dawn-dusk motion of TPA will need to incorporate the physics of the HD and the changing high-latitude
merging sites in response to the changing IMF when
explaining the separate nightside and dayside parts of the
apparent continuous band of the TPA.
[ 111 ] Acknowledgments. We thank S. Vennerstrøm and both
reviewers for their useful comments. We are also indebted to the ACE
MAG and SWEPAM instrument teams for providing the Level 2 solar wind
data via http://www.srl.caltech.edu/ACE/ASC/level2/. We also acknowledge the use of Geotail level K0 magnetic field MGF data (courtesy of S.
Kokubun, STELAB, Nagoya University, Japan) and the use of Geotail level
H0 plasma CPI instrument data (courtesy of L. Frank, University of Iowa,
USA) made available to us via the CDAWeb pages of Japan. This work is
supported by NASA grant NNG04GH82G at the University of Colorado at
Boulder.
[112] Arthur Richmond thanks James Burch and Stanley Cowley for
their assistance in evaluating this paper.
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