Cluster observations of the high-latitude magnetopause and cusp: initial results from the CIS ion instruments

Abstract

Launched on an elliptical high inclination orbit (apogee: 19.6 R_E ) since January 2001 the Cluster satellites have been conducting the first detailed three-dimensional studies of the high-latitude dayside magnetosphere, including the exterior cusp, neighbouring boundary layers and magnetopause regions. Cluster satellites carry the CIS ion spectrometers that provide high-precision, 3D distributions of low-energy (<35 keV/e) ions every 4 s. This paper presents the first two observations of the cusp and/or magnetopause behaviour made under different interplanetary magnetic field (IMF) conditions. Flow directions, 3D distribution functions, density profiles and ion composition profiles are analyzed to demonstrate the high variability of high-latitude regions. In the first crossing analyzed (26 January 2001, dusk side, IMF-BZ < 0), multiple, isolated boundary layer, magnetopause and magnetosheath encounters clearly occurred on a quasi-steady basis for ~ 2 hours. CIS ion instruments show systematic accelerated flows in the current layer and adjacent boundary layers on the Earthward side of the magnetopause. Multi-point analysis of the magnetopause, combining magnetic and plasma data from the four Cluster spacecraft, demonstrates that oscillatory outward-inward motions occur with a normal speed of the order of ± 40 km/s; the thickness of the high-latitude current layer is evaluated to be of the order of 900–1000 km. Alfvénic accelerated flows and D-shaped distributions are convincing signatures of a magnetic reconnection occurring equatorward of the Cluster satellites. Moreover, the internal magnetic and plasma structure of a flux transfer event (FTE) is analyzed in detail; its size along the magnetopause surface is ~ 12 000 km and it convects with a velocity of ~ 200 km/s. The second event analyzed (2 February 2001) corresponds to the first Cluster pass within the cusp when the IMF-BZ component was northward directed. The analysis of relevant CIS plasma data shows temporal cusp structures displaying a reverse energy-latitude "saw tooth" dispersion, typical for a bursty reconnection between the IMF and the lobe field lines. The observation of D-shaped distributions indicates that the Cluster satellites were located just a few R_E from the reconnection site.

Full text

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Annales Geophysicae (2001) 19: 1545–1566 c ? European Geophysical Society 2001
Annales
Geophysicae
Cluster observations of the high-latitude magnetopause and cusp:
initial results from the CIS ion instruments
J. M. Bosqued1, T. D. Phan2, I. Dandouras1, C. P. Escoubet3, H. R` eme1, A. Balogh4, M. W. Dunlop4, D. Alcayd´ e1,
E. Amata5, M.-B. Bavassano-Cattaneo5, R. Bruno5, C. Carlson2, A. M. DiLellis5, L. Eliasson6, V. Formisano5,
L. M. Kistler7, B. Klecker8, A. Korth9, H. Kucharek8, R. Lundin6, M. McCarthy10, J. P. McFadden2, E. M¨ obius7,
G. K. Parks2, and J.-A. Sauvaud1
1CESR, Toulouse, France
2University of California, Berkeley, CA, USA
3ESTEC/ESA, Noordwijk, the Netherlands
4Blackett Laboratory, Imperial College, London, UK
5IFSI, Rome, Italy
6SISP, Kiruna, Sweden
7University of New Hampshire, Durham, NH, USA
8MPE, Garching, Germany
9MPAe, Lindau, Germany
10University of Washington, Seattle, WA, USA
Received: 17 April 2001 – Revised: 13 July 2001 – Accepted: 19 July 2001
Abstract. Launched on an elliptical high inclination orbit
(apogee: 19.6RE) since January 2001 the Cluster satellites
have been conducting the first detailed three-dimensional
studies of the high-latitude dayside magnetosphere, includ-
ing the exterior cusp, neighboring boundary layers and mag-
netopause regions. Cluster satellites carry the CIS ion spec-
trometers that provide high-precision, 3D distributions of
low-energy (<35keV/e) ions every 4s. This paper presents
the first two observations of the cusp and/or magnetopause
behaviour made under different interplanetary magnetic field
(IMF) conditions. Flow directions, 3D distribution functions,
density profiles and ion composition profiles are analyzed
to demonstrate the high variability of high-latitude regions.
In the first crossing analyzed (26January2001, dusk side,
IMF-BZ < 0), multiple, isolated boundary layer, magne-
topause and magnetosheath encounters clearly occurred on
a quasi-steady basis for ∼2 hours. CIS ion instruments show
systematic accelerated flows in the current layer and adja-
cent boundary layers on the Earthward side of the magne-
topause. Multi-point analysis of the magnetopause, combin-
ing magnetic and plasma data from the four Cluster space-
craft, demonstrates that oscillatory outward-inward motions
occur with a normal speed of the order of ±40km/s; the
thickness of the high-latitude current layer is evaluated to
be of the order of 900–1000km. Alfv´ enic accelerated flows
and D-shaped distributions are convincing signatures of a
magnetic reconnection occurring equatorward of the Clus-
ter satellites. Moreover, the internal magnetic and plasma
structure of a flux transfer event (FTE) is analyzed in detail;
its size along the magnetopause surface is ∼12000km and it
Correspondence to: J. M. Bosqued (bosqued@cesr.fr)
convects with a velocity of ∼200km/s. The second event an-
alyzed (2February2001) corresponds to the first Cluster pass
within the cusp when the IMF-BZcomponent was northward
directed. The analysis of relevant CIS plasma data shows
temporal cusp structures displaying a reverse energy-latitude
“saw tooth” dispersion, typical for a bursty reconnection be-
tween the IMF and the lobe field lines. The observation of D-
shaped distributions indicates that the Cluster satellites were
located just a few REfrom the reconnection site.
Key words. Magnetospheric physics (magnetopause, cusp,
and boundary layers; magnetosheath) Space plasma physics
(magnetic reconnection)
1Introduction
Accelerated magnetosheath plasma flows within the bound-
ary layers adjacent to the magnetopause have long been con-
sideredasoneofthemostconvincingsignaturesofreconnec-
tion between the interplanetary (IMF) and magnetospheric
magnetic fields, as originally predicted by Dungey (1961).
Direct information on the particle penetration processes has
been obtained on the dayside magnetopause, at low-latitudes
(Paschmann et al., 1979; Sonnerup et al., 1981; Gosling et
al., 1990; Scurry et al., 1994). At medium and higher lat-
itudes, pioneering observations were carried out during the
1970s by Heos 2 in the plasma mantle (Rosenbauer et al.,
1975), in the entry layers (Paschmann et al., 1976), and by
Prognoz-7 in the same regions (Lundin, 1985; and references
therein). Indirect measurements in the mid and low altitude
cusps, such as the energy latitude dispersion (Reiff et al.,

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1546J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp
1977, 1980; Escoubet et al., 1997a) and V-shaped energy-
pitch angle dispersions (Menietti and Burch, 1988) have all
been considered as arguments in favour of an ion injection
at a reconnection site. From ion precipitation signatures ob-
served by satellites crossing the low-altitude cusp and from
the ground, indirect arguments have been presented in favour
of patchy reconnection (Lockwood and Smith, 1992, 1994;
Escoubet et al., 1992).
When the IMF is southward pointing, much knowledge
about the processes acting at or near the magnetopause has
been accumulated (Russell, 1995, and references therein). In
spite of that, a number of related questions are still open:
why isn’t the predicted flow acceleration observed during
each magnetopause crossing? How unsteady is the recon-
nection rate at the magnetopause? What are the microscale
processes operating at the magnetopause? Does the diffusion
dominate in the transport processes at high-latitudes? What
are the detailed properties and the frequency of occurrence of
propagating flux transfer events (FTEs) discovered by Rus-
sell and Elphic (1978)? Are the FTEs connected to upstream
pressure pulses, or the signature of a sporadic reconnection?
On the other hand, evidence for reconnection when the
IMF is northward is more recent. Dungey (1963) first sug-
gested that a IMF pointing northward can lead to recon-
nection between the IMF and open field lines of the lobes,
poleward of the cusp regions, driving a localized sunward
flow. Indirect evidence of such a sunward convection flow
was given (a) in the dayside polar cap by ground magne-
tograms (Maezawa, 1976) and auroral emissions (Oeireset et
al., 1997), (b) at low- (Reiff, 1984; Escoubet and Bosqued,
1989) and mid- (Woch and Lundin, 1992) altitudes by satel-
lite observations of reverse energy-latitude dispersions. In
situ high-latitude observations for IMF-BZ > 0 are more
scarce and recent (Gosling et al., 1991; Kessel et al., 1996;
Chen et al., 1997; Fuselier et al., 2000).
Anotherevidenceofreconnection(eitherwhenIMF-BZ>
0 or <0), not completely explored until now, can be found in
the details of distribution functions measured at or near the
magnetopause current layer and in the adjacent low- or high-
latitude boundary layers. Only a limited part of an initial
magnetosheath distribution in velocity space can be trans-
mitted across the magnetopause (Cowley, 1982). Therefore,
distributions must exhibit a characteristic D-shape, but just a
few have been observed in the low-latitude boundary layers
(Gosling et al., 1991; Fuselier et al., 1991; Phan et al., 2001)
and very recently at high-latitudes by Polar (Fuselier et al.,
2000).
To summarize, the search for detailed reconnection signa-
tures near the magnetopause at high-latitudes remains one of
the major goals of magnetospheric research and constitutes
one of the most ambitious objectives of the multi-point Clus-
ter mission.
The four identical satellites of the ESA Cluster Mission
were launched in pairs on 15July and 9August2000. One of
the major goals of Cluster is to determine the local orienta-
tion and the state of motion of the small- and medium-scale
plasma structures, such as the magnetopause, and thereby
give, for the first time an unambiguous indication in order
to distinguish between spatial and, temporal variations (Es-
coubet et al., 1997b). The current polar orbit has a perigee of
∼4REand an apogee of ∼19.7RE, an inclination of 90◦, a
lineofapsidesaroundtheeclipticplane, andanorbitalperiod
of ∼58 hours. Orbital parameters of the four satellites dif-
fer slightly in order to achieve an almost perfect tetrahedral
configuration in the external cusp and magnetopause.
For this initial study, we use ion data provided by the CIS
instrument, supplemented by magnetic field measurements
made by the FGM instruments, to survey two earlier Clus-
ter passes near the magnetopause and in the exterior cusp,
on 26January and 2February2001. On 26January, the Clus-
ter spacecraft remained near the magnetopause for more than
2 hours, with several repeated encounters with the current
layer. On 2February, Cluster crossed the high-latitude exte-
rior cusp during a northward IMF. In both cases, our main
goals are to show: (a) how these new multi-point observa-
tions are valuable for studying the dynamics of the high-
latitude magnetopause and cusps and (b) what are the ob-
served signatures of the magnetic reconnection. With the
spacecraft separated by ∼600km in the earlier phase of the
mission, we are able to start defining the shape, structure, and
motion of the magnetopause and cusp region, the detailed
properties of FTEs, and the size and motion of the different
layers crossed near the magnetopause.
Our paper is organized as follows. In Sect. 2, we give
the useful information on the performance of the Cluster Ion
Spectrometry (CIS) instruments. A preliminary analysis of
the results obtained during a high-latitude, duskward pass
(26January 2001) is given in Sect. 3. The interplanetary
magnetic field (IMF) had a southward component, and clear
reconnection signatures (accelerated flows) can be seen at
the magnetopause; we perform a rigorous multi-spacecraft
analysis of one of the magnetopause inward-outward mo-
tions, as well as a detailed analysis of an FTE pulse observed
during the same pass. In Sect. 4, the example presented
(2February2001) is the first Cluster high-latitude pass for a
northward IMF. A structured cusp is evident and presumably
it results from reconnection between the IMF and lobe field
lines, poleward of the satellites.
2 Instrumentation
The Cluster Ion Spectrometer (CIS) experiment described in
greater detail in R` eme et al. (2001, this issue) provides full
3D simultaneous ion distribution functions up to ∼40keV/e
for the major ion species, with a time resolution of up to 4s
(i.e. 1 spin period). Two different instruments are used to
cover the needed energy, angle, dynamic flux ranges, and
mass requirements. A special feature of CIS is the dou-
ble sensitivity of the instruments that allow precise measure-
ments under extreme flux conditions.

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J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp1547
First, the Hot Ion Analyzer (HIA) sensor (CIS2 instru-
ment) selects ions according to their energy per charge, us-
ing a top hat, quadrispherical electrostatic analyzer with a
uniform 360◦× 5◦disc-shaped field-of-view. The instru-
ment has two 180◦sections with different sensitivities and
it lies parallel to the spin axis, called “high G” and “low g”.
The low g section, with a sensitivity attenuated by a factor of
∼24, analyses the high intensity solar wind with an angular
resolution of 5.6◦. To allow for precise measurements, the
sweep energy range can be automatically adjusted on each
spin. The high G sensitive section detects magnetospheric
ions over the full energy range from ∼5eV/e to ∼32keV/e.
A full (16θ × 32ϕ) angle’s pixels multiplied by 31 energy
steps yields a 3D distribution for every spin. Distribution
functions used in this paper are detected in the high G sec-
tion and are constructed by using (8θ × 16ϕ) angle pixels
(in reality, reduced to 88) and 31 energy steps; distributions
were on board and accumulated over 3spins (∼12s).
The CIS-1 instrument is the time-of-flight ion Compo-
sition and Distribution Function Analyzer (CODIF), which
is a high-sensitivity mass-resolving spectrometer capable
of providing full 3D distributions of the major ion species
(H+, He2+, He+, O+) in the energy range ∼20eV/e to
∼40keV/e; thisrangeisextendedbyanadditionalRetarding
Potential Analyzer (RPA) that covers the energies between
the spacecraft potential and about 25eV/e. In order to cover
the large anticipated flux dynamic range, the CIS-1/CODIF
instrument has also been divided into two 180 × 8◦fan-
shaped sections, the “High Side” (HS) and the “Low Side”
(LS) sections, that differ in sensitivity by a factor of ∼100;
in the solar wind, in general, the LS section is used. 3D full
phase space distributions for each ion species are transmit-
ted to the telemetry. Data used in this paper are 3D (88-angle
pixels multiplied by 31 or 16 energy steps) distributions from
the HS and/or the LS sections, accumulated over 3 or 4 spins.
Extensive onboard data processing allows for continuous
and systematic transmission every spin (∼4s) of the ba-
sic moments of the 3Ddistribution functions, measured by
the two CIS instruments, according to the section selected.
Transmitted moments are the ion density, Ni, of the major
ion species, the flow vector, Vi, the heat flux vector, Hi, and
the pressure tensor, P.
Due to an unidentified electronic failure, both CIS instru-
ments on Cluster spacecraft 2 were switch off following the
commissioning tests. Moreover, the CIS-2/HIA instrument
was also switched off on spacecraft 4 for the period presently
studied. To summarize, data presented in this paper come
from the CIS-1 instruments on three spacecraft (1, 3, and 4)
and from the CIS-2 instruments on two spacecraft (1 and 3).
The magnetic field measurements were obtained from the
FGM instrumentation (Balogh et al., 1997). It consists of
two tri-axial fluxgate magnetometers and an associated data-
processing unit that provides accurate, high time resolution
measurements of the magnetic field vector. Data used in
this paper do not have the full FGM resolution but are av-
eraged over the spin period (∼4s). Higher time resolution
data (∼1s) are used to determine time delays.
3 First case study: 26January2001, southward IMF
This example from 26January2001 was chosen as one of
the first to show a two-hour extended period (∼09:00–
11:00UT) of multiple encounters with the high-latitude
boundary layer/magnetopause interface in the northern dusk
sector (∼15:00LT). These multiple crossings are unambigu-
ously accompanied by accelerated flows within the current
layer and boundary layer, with a higher flow speed than in
the adjacent magnetosheath. Such signatures can indicate a
steady reconnection site not far from the Cluster location.
Figure 1 shows the interplanetary magnetic field (IMF)
and solar plasma measurements provided by the ACE satel-
lite located close to the L1 Lagrangian point, more than
220REupstream of the magnetosphere. From a comparison
with similar Geotail data (Geotail is located on the dawn-
side magnetosheath) and Wind data, the probable time de-
lay between ACE and the front magnetosphere can be eval-
uated to be around 65min. During the interval of interest
for Cluster (i.e. from ∼07:00UT to ∼15:00UT), the so-
lar wind dynamic pressure does not change drastically and
stays at ∼0.8nPa. The IMF is oriented duskward/southward
before 09:00UT (BX∼1.5nT, BY∼2nT, BZ∼−6nT at
08:00UT) and suddenly turns steadily dawnward/southward
(BY∼−6nT) with a small, positive BX component. Be-
tween 10:00UT and 11:00UT, the IMF is very steady
(+0.5,−6,−3nT) and, according to this direction, a pos-
sible site of reconnection is located on the duskward side of
the magnetopause (Crooker, 1979).
3.1Overview of CIS and FGM data
Magnetic field and ion plasma data provided by the FGM
and CIS instruments on board the Cluster spacecraft 3 are
combined and displayed in Fig. 2 for the interval 06:00–
12:00UT on 26January2001, for an outbound pass near
the cusp and magnetopause. Different regions can be eas-
ily identified along the Cluster orbit, starting from the al-
most empty northern magnetospheric lobe (connected to the
polar cap), and passing at ∼06:50UT into the low-energy
(∼100eV/e) plasma mantle characterized by a density that
slowly increases from 0.1 to 0.7cm−3at ∼08:05UT, and a
tailward/duskward-directedlow-speedflow(≤30km/s). The
magnetic field has an orientation (θ∼−45◦, ϕ∼−80◦) typi-
cal of the high-latitude open lobe field lines on the northern
dusk side. At ∼08:04UT, a new, hotter plasma regime ap-
pears, with successive (5–6) bursty structures at considerably
higher densities (reaching ∼2cm−3) and a continuously in-
creasing average ion energy with the temperature reaching
∼2keV at the boundary detected at ∼08:45UT. The field
line inclination is slowly rotating. From the plasma prop-
erties, we consider this part to be the exterior cusp and/or
the entry layer. The next region reached from ∼08:45 to
∼09:15UT is certainly the dayside extent of the plasma
sheet on closed field lines. When moving outward, the mag-
netic field rotates from θ∼−35◦to ϕ∼0◦at ∼09:00UT
(3.27, 6.56, 9.01REGSE) and finally, to +40◦, which is typ-

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1548J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp
Fig. 1. Upstream ACE interplanetary
data from 26January2001. From top to
bottom the panels display the total mag-
netic field amplitude (in nT), the three
components (BX, BY, BZ) of the inter-
planetary magnetic field (IMF) in GSE
coordinates, the solar wind velocity (in
km/s) and dynamic pressure (in nPa).
ical for closed field lines on the dusk side. A significant pop-
ulation of high energy (≥5keV) hot ions is evident on the
spectrogram, presumably of magnetospheric (plasma sheet)
origin. However, a number of narrow, high-speed flows (di-
rected along −VY) are apparent within this region.
Analysis of the next interval to about 11:04UT consti-
tutes the main topic of the following presentation. Clus-
ter satellites sequentially encounter the magnetopause and
boundary layer populations before exiting into the magne-
tosheath regime (Ni∼25cm−3, Ti∼102eV). The popula-
tions are identified there, as expected, for the post-noon sec-
tor at northern altitudes by a tailward/duskward/northward
plasma flow with VX∼−150km/s, VY∼VZ∼+50km/s.
The magnetosheath field is draped around the magne-
topause orientation (θ∼−20◦, ϕ∼ −60◦), but directed
sunward/duskward/southward as the IMF is measured by
ACE or WIND. It should be noted that the first bow shock
crossing occurs at ∼15:30UT (outside the figure).
convenience, this region is divided into two parts, 09:00–
10:00UT and 10:00–11:00UT.
For
Concentrating first on the ion spectrogram (bottom panel)
and the flow speed panel of this transition region, inter-
mittent, clearly accelerated plasma flows are observed in
the 09:00–10:00UT interval, according to a periodic se-

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J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp1549
Fig. 2. Magnetic field and ion plasma data from Cluster/spacecarft3, FGM and CIS instruments, for 06:00–12:00UT during the high-latitude
crossing on 26January2001. From top to bottom successive panels display: the 4s averaged FGM magnetic field strength (in nT), magnetic
field elevation θ and azimuth ϕ (in GSE coordinates), the 4s resolution on board-computed moments, i.e. full ion density (cm−3), the three
components of the flow speed (km/s), and the ion temperature (eV), and the 12s resolution colour-coded E-time spectrogram from ∼5eV
to ∼34keV/e, integrated over all azimuths and elevations (1D). Counts per second are colour-coded according to the logarithmic colour bar
shown on the right to emphasize the energy flux and presence of high energy ions and not the density . All ion data are provided by the
“C3–2” instrument (spacecraft Cluster 3 - CIS 2 instrument, without mass analysis).
quence.
than 350km/s, has a density ranging from 8 to 20cm−3,
slightly lower than the magnetosheath plasma density (af-
ter 11:10UT) but considerably higher than in the adjacent
plasma sheet density (before 09:10UT). It is worth noting
that all of these accelerated flows are superposed onto a non-
negligible background of hot (Ti≥3keV) magnetospheric
ions with a density of ∼0.1cm−3. The highest bulk flows
are directed Earthward along −X with an important +Z
component, which means that the flows are directed roughly
parallel to the magnetopause surface. The ion temperature,
1–3·102eV, is slightly warmer than in the magnetosheath.
A close examination of the magnetic field angles (top pan-
els) shows: (a) all of the accelerated flow events in this time
interval are correlated with small rotations of the magnetic
field direction from its magnetosphere-like configuration to
an intermediate direction, where periodic rotations of about
15◦in θ and ∼40◦in the angles are clearly seen, (b) except
for the notable short incursion around 09:15UT, these field
changes are insufficient to indicate successive magnetopause
crossings, such as those observed in the next time interval
Each accelerated plasma jet, with speed higher (10:20–11:20UT).
From the plasma and field observations, we may conclude
that for most of this time interval, the Cluster satellites, po-
sitioned almost tangentially along the magnetopause surface,
remain inside the magnetopause plasma layer, i.e. according
to the definitions introduced by Song (1995) for the high-
latitude plasma layer (HBL) and the current or “sheath tran-
sition” layer. Due to oscillations of the surface, with a pe-
riod of about 3min, the Cluster satellites skim a series of
accelerated flow events without completely crossing the cur-
rent layer. However, a detailed examination of high-speed
flows around the first current sheet magnetopause crossing
at 09:15UT, followed by the immediate return to the in-
ner boundary layer at ∼09:18UT, indicates that the highest
speed flow is well located near the Earthward edge of the
current layer where the density is the highest. Moreover, the
magnetospheric-like plasma extends outward from the cur-
rent layer, and disappears only around 09:17UT when the
spacecraft are located deeper in the magnetosheath.
Data for the 10:20–11:20UT interval are shown in Fig. 3,
where high-speed flows are highlighted by vertical lines.

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1550J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp
Similar to the events detected between 09:00 and 10:00UT,
the first series of short-lived accelerated flows between 10:20
and 10:30UT is not associated with a complete crossing of
the current layer and the magnetopause. Between the ac-
celerated flows, Cluster remains on plasma sheet field lines,
characterized by a hot magnetospheric (>5keV energy)
plasma. Thefollowingmostsignificanteventsat∼10:30UT,
∼10:36UT, ∼10:46UT, ∼10:51UT, 10:55UT, 11:02UT
are periodic, transient, accelerated plasma flows observed
during the complete transition from the high-latitude bound-
ary layer (HBL) to the magnetosheath through the current
layer and magnetopause. Large B rotations, from its mag-
netosphere orientation (θ∼+40◦, ϕ∼−135◦) to its adjacent
magnetosheath orientation (θ∼−20◦, ϕ∼−45◦), are unam-
biguous signatures of magnetopause traversals, and are sys-
tematically associated with the presence of the accelerated
flows, as is evident in the general figure for this event. The
flow jump is of the order of 150km/s and is nearly in the tail-
ward northward direction (e.g. VX∼−400, vZ∼+200km/s
at 10:35UT), and is concentrated within a thin layer on
the Earthward side of the magnetopause itself.
and temperature properties remain identical to those already
mentioned for the previous events. It is important to note
that successive transitions towards the magnetosheath-like
plasma are evidenced by a flow speed dispersion, from the
accelerated flow down to the magnetosheath regime. More-
over, magnetospheric ions are still present within the mag-
netosheath on the sunward side of the magnetopause, with
their density reaching its lowest level after 11:18UT. Signif-
icant fluxes of high energy (>5keV) ions were observed in
the past at low- and medium-latitudes in the magnetosheath,
just outside the magnetopause (see, for instance, Kudela et
al., 1992). It seems likely that these ions escape via finite
Larmor radius effects; this ion leakage will be the topic of
future CIS studies.
Density
3.2 Multi-point measurements
The initial results from the Cluster multi-satellite mis-
sion demonstrate, unequivocally, that this mission will
be of great benefit for the separation space and time
variations, particularly in the attempt to interpret suc-
cessive encounters with propagating boundaries that sep-
arate different plasma regimes, such as the periodic
inward-outward motion of the entire boundary layer/current
layer/magnetopause/magnetosheath observed during this
26January2001 event.
Next, we discuss the use of the simultaneous observations
made by the CIS instruments from three Cluster satellites
to determine the inter-spacecraft propagation properties (ve-
locity, direction) and then to obtain the thickness of the dif-
ferent layers, surface propagation of disturbances along the
boundary, surfacewaves, possibledistortions, etc. Beforede-
scribing this first attempt to use measurements from CIS on
the Cluster satellites, it is important to recall that the inter-
spacecraft separation was only 600km during the first part
of the Cluster mission (until summer 2001), and that the best
time resolution for f(v) distributions is only 12s (3 spins)
for the events covered here. Therefore, the best available
CIS parameters for inter-spacecraft comparisons are the full
moments (Ni,Vi) computed on board every spin (∼4s). Of
course, magnetic field measurements allow for a much bet-
ter resolution, even if we only use in this paper 1s averaged
resolution data.
The Cluster tetrahedron geometry at ∼10:35UT is shown
in Fig. 4 and can be considered conserved for the entire pe-
riod of interest; at a quick glance, it appears that spacecraft 1
and 3 should belong to the magnetopause surface, and space-
craft 2 and 4 should, respectively, lead and end the tetrahe-
dron along the orbit. In particular, spacecraft 4 may remain
at ∼0.65 RE(420km) along the normal to the surface.
Two examples of simultaneous 3-spacecraft Cluster/CIS
measurements are presented, with the first one taken dur-
ing two incomplete traversals of the magnetopause (be-
tween 10:24 and 10:28UT), and the second for a complete
inward-outward double crossing of the magnetopause (be-
tween 10:33 and 10:38UT).
3.2.1Data interval: 10:24–10:28UT
Figure 5 shows simultaneous magnetic field orientations,
partial density (>5keV), and density and bulk flow profiles
for satellites 1, 3 and 4, for two accelerated flows separated
by ∼2min and lasting ∼1min; the three ion spectrograms
are given in the bottom panels. First, it is evident that space-
craft 1 and 3 measure very similar enhanced plasma densi-
ties and total bulk velocity profiles; however, a small (4s)
time delay was evident in the bulk velocity profile (space-
craft 3 leaving first) when the pair of satellites left the ac-
celerated plasma layer at ∼10:25:55UT to returned to the
less dense boundary layer, characterized by plasma mixing
with a majority of high energy trapped magnetospheric ions.
The same timing could be seen in the density and velocity de-
tails near 10:27:30UT. Second, the two major accelerated in-
creases in bulk flow and density are also identified by space-
craft 4, but are attenuated and nested within the spacecraft 1
and 3 profiles. The time delay between spacecraft 4 and 3
at the re-entry point into the magnetospheric-like boundary
layer(seeverticalmarksat∼10:25:55UT,atthespacecraft4
first re-entry) and exit (at ∼10:26:35UT, spacecraft 4 last
exit) is ∼8s. By neglecting the Cluster spacecraft motion
(<2km/s), we can interpret this latest delay as the result
from a back and forth motion of the magnetopause surface,
with a velocity unof ∼40km/s along its normal (roughly
aligned with the spacecraft 3–spacecraft 4 line). The time
delay deduced from the spacecraft 1 and 3 time differences
(≤4s from CIS, ≤2s from FGM) can be considered as a su-
perposed 90s period wavy oscillation propagating poleward
along the magnetopause surface at a velocity of ≥120km/s.
Apparently this velocity is of the order of magnitude of the
surface waves observed by the wave experiment on board
Cluster (Cornilleau et al., 2001). In summary, given these
measurements, the thickness of the plasma layer of accel-
erated flow can be crudely determined to be at ∼10:00–

Page 7

J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp1551
Fig. 3. Magnetic field and plasma data
for the 10:20–11:20UT interval pro-
vided by the FGM and CIS-2 instru-
ments. From top to bottom, the mag-
netic field orientation θ and azimuth
ϕ angles (averaged over 4s), the par-
tial (5keV) and total ion densities Ni
(cm−3), the three components of the
bulk flow speed, the scalar ion tem-
perature, and the ion spectrogram are
plotted. Total ion density, flow speed
components, and temperature are com-
puted on board with a 4s time reso-
lution (see R` eme et al., 2001), while
the partial density is computed on the
ground from transmitted 3D distribu-
tions accumulated over 3 spins (i.e. 12s
resolution). Vertical lines correspond to
flows with |VX| > 350km/s.
20:00km. A more rigorous estimation of the magnetopause
motion and size will be performed below on the next time
segment, 10:33–10:38UT.
3.2.2Data interval 10:33–10:38 UT
The second inter-spacecraft comparison concerns the inter-
val 10:33–10:38UT during which the satellites make peri-
odic crossings of the magnetopause, moving back and forth
between the magnetosheath and the high-latitude boundary
layer (Fig. 6). At 10:33UT, the spacecraft are located within
the magnetosheath-like plasma, as indicated by the relatively
low density of energetic (>5keV) ions and the orientation of
the magnetic field. Afterwards, the magnetic field orientation
ϕBlmnin the (LM) plane tangent to the magnetopause starts
to rotate, while the normal component θBlmnremains approx-
imately constant. The four satellites enter the current layer
around ∼10:34:12UT for spacecraft 4 (FGM, red curve),
and slightly later, at ∼10:34:25 and at ∼10:34:32UT, for
spacecraft 3 and 2 (green and blue and lines) and space-
craft 1 (black line), respectively. The time lag between the
spacecraft 1 and 3 measurements is even less evident at the
shallow reentry in the magnetosheath around 10:36:50UT;
for its part, spacecraft 4 remains on the magnetospheric side
of the current layer for the rest of the interval. Concern-
ing the accelerated flows, the time delay is the same as
that deduced from the magnetic field. Due to the relative
outward motion of the magnetopause, the spacecraft 1 and
3 satellites detect a progressively accelerated flow just in-
side the current layer (or “sheath transition layer”) between
10:34:26 and 10:35:10UT (vertical black lines, region MP1)
which reaches >400km/s within the boundary layer, be-
tween ∼10:35:10 and 10:36:40UT. A gradient in the to-
tal density is not seen just at the boundary, but the density

Page 8

1552J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp
Fig. 4. Cluster spacecraft 3 orbit on 26January2001, from 06:00
to 14:00UT, projected onto the X − Y (bottom) and X − Z (top)
GSE planes. The relative configuration of the Cluster tetrahedron at
10:35:00UT, centered on spacecraft 3 is also mapped. For clarity,
the inter-spacecraft distances have been multiplied by a factor of 50.
The solid and dotted lines give cuts in the Y = 0 (top) and Z = 0
(bottom) planes of: (a) the model magnetopause location (Petrinec
and Russell, 1996) for IMF-BZ= −3nT and Psw= 0.8nT (solid
line); (b) the scaled magnetopause at spacecraft 3 (dotted line);
the scaling factor is 0.875. Magnetopause cuts at Y = Ysc3and
Z = Zsc3are also given (dashed lines). On the same diagrams are
also plotted for spacecraft 1, 3, and 4, the components of the mag-
netopause normal vectors nEpredicted by the Maximum variance
analysis of E = −V × B (see text for details); the nBnormal vec-
tor is plotted for spacecraft 2, the CIS instruments are switched off
on spacecraft 2.
of higher energy ions increases when the spacecraft move
Earthward relative to the boundary. Velocity gradients (and
gradients in the magnetic field rotation) are sharper when the
spacecraft reenter between ∼10:36:30 and 10:37UT (verti-
cal blue lines, region MP2) in the magnetosheath regime, and
do not show a measurable time lag in the CIS moments. At
that time, spacecraft 4 was deeper in the Earthward side of
the current layer.
3.3Evidence for reconnection at the magnetopause
All the periodic enhanced flows observed during the full
09:00–11:06UT period deserve special attention because
they could provide signatures of merging. To investigate
whether the field line merging process accounts for the ac-
celerated flow, we have analyzed all magnetopause cross-
ings by using the methods described in detail in Sonnerup
et al. (1987, 1990) and Khrabrov and Sonnerup (1998); here,
we present the results, summarized in Table 1, obtained for
the time interval 10:33–10:38UT, as shown in Fig. 6.
3.3.1Determination of normal vectors
As a first step, we have performed a variance analysis either
on the magnetic field data or on the convection electric field
Ec = −V × B (where V and B represent the flow veloc-
ity vector and the local magnetic field provided by CIS and
FGM, respectively), either for two separated or optimized
time intervals, marked M1 (shifted for spacecraft 4) and
M2 in the Fig. 6, for the full-time data segment, 10:34:30–
10:37:00UT. The direction of minimum of the variance of B
and nB, and the direction of maximum of the variance of Ec
and nE, are nearly identical; both are useful predictors of the
magnetopause normal vector, and their differences are small,
between 3.5 and 8◦(see Table 1). Normal directions are only
given for the full data interval, as they are roughly similar for
the individual magnetopause crossings.
nE(or nBfor spacecraft 2) normal directions, projected
onto the X − Y and the X − Z GSE planes, are plotted in
Fig. 4. Comparing directions obtained for each spacecraft it
may be noted differences of 9–14 degrees. Such differences
are not really surprising and cannot be eliminated by chang-
ing the segment of data; they could reflect the presence of
ripples on the magnetopause surface.
3.3.2Existence of a deHoffmann-Teller (DHT) frame
The second step consists of searching for the existence of a
dHT frame for each spacecraft, moving with a velocity VHT
along the presumed RD discontinuity, in which the electric
field vanishes (or nearly vanishes). Although not sufficient,
this existence is one means of determining reconnection at
the magnetopause. A constant VHTis determined by mini-
mizing the quantity:
????Vm− VHT
determination of VHTcan be improved by including a con-
stant acceleration aHT(VHT= VHT0+aHTt). The “quality”
D =
summed over the set of individual data (Vm,Bm); then the
?× Bm??2

Page 9

J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp1553
Fig. 5.
plasma data (partial density, total den-
sity, bulk speed) for the interval 10:24–
102:8UT, for three spacecraft (black:
spacecraft 1; blue: spacecraft 3; green:
spacecraft 4). Data for spacecraft 1 and
3 are provided by the CIS-2 instrument,
while data for spacecraft 4 are provided
by the CIS-1 (H+) instrument.
Magnetic field angles and
of the resulting dHT frame can be evaluated by fitting the
single scatter plot of each GSE X,Y,Z component of Ec
versus the corresponding three components of the dHT elec-
tric field, EHT = −VHT× B. From the results reported
in Table 1, it can concluded that: (a) a remarkably good
agreement is found between EHTand Ec, with fitting slopes
∼1 and a very small least-square residual “quality factor”
D/D0 ∼0.01; (b) for the three spacecraft, a good dHT frame
exists and moves with a VHTvelocity similar for spacecraft 1
and 3, but smaller for spacecraft 4; (c) an aHTacceleration
of the dHT frame can be noted, which introduces a small
change in the VHTvelocity between the beginning (VHT0)
and the end (VHT end) of the interval of interest.
3.3.3 Validity of the Wal´ en relation
Next, the most powerful experimental test is to validate the
Wal´ en relation predicted by MHD for rotational discontinu-
ities, i.e. the accelerated plasma flow is Alfv´ enic in the dHT
frame, V − VHT= ±VA. In spite of the simple conditions
assumed (lack of experimental errors, time-stationarity, 1D
structure, pure H+ion plasma), such a test works relatively
well in most magnetopause crossings (Sonnerup et al., 1990,
1995; Phan and Paschmann, 1996; Phan et al., 1996, 2001).
Scatter plots (not shown) between the individual X,Y,Z
(GSE) components of the plasma velocity (V − VHT) in the
dHT frame, and the corresponding components of the nomi-
nal Alfv´ en velocity
VA= B
?1 − α
µ0ρ
?1/2
(in SI units) during the same interval have been used to test
the agreement of the data with the Wal´ en relation. The pres-
sure anisotropy
α = (ρ ? −ρ ⊥)µ0
B2

Page 10

1554J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp
Fig. 6. Multi-spacecraft magnetic field
and plasma data for the interval 10:33–
10:38UT. In the three first panels from
top, the magnetic field strength (in nT),
the elevation angle θBlmnand the mag-
netic field orientation ϕBlmnin the (LM)
plane tangent to the magnetopause (av-
eraged over 4s) are plotted. For each
spacecraft, the (LMN) frame used to
plot magnetic field data is derived from
the eigenvectors of the variance matrix
of B. In the next three central panels
the partial (≥ 5keV) ion densities, the
total ion densities Ni(cm−3), and the
total bulk flow speed are given. The 1D
energy-time spectrograms provided by
the CIS-2 instruments (spacecraft 1 and
3) and CIS-1 (spacecraft 4) are given in
the three bottoms panels.
and the total mass density, ρ, (ρ = Nimi, assuming 95% of
H+ions and 5% of He++ions) are averaged parameters eval-
uated over an adjacent interval, 10:37–10:38UT. The scat-
ter plots show an excellent agreement, and the magnitude of
the slopes of the regression fit lines are ∼1 for spacecraft 1
and 3, regardless of the accelerating term, 0 or aHT, intro-
duced in TVHT. In contrast, the quality of the fit is again
poor in the case of spacecraft 4, but some instrumental ef-
fects are suspected in the CIS-1 instrument (dead time effects
when the count rate is large), resulting in an underestima-
tion of the flow velocity. It can be concluded that the final
change in velocity (V − VHT) along the magnetic field di-
rection is positive, of the order of the local Alfv´ en velocity
VA, ∼+150km/s, as expected for a reconnection site lo-
cated equatorward of Cluster. The same tests for the exis-
tence of a good dHT frame were performed on a majority of
the accelerated flows evidenced during the 09:00–11:06UT
period, and agreed rather well with the Wal´ en relation.
3.3.4Magnetopause motion
We turn finally to an evaluation of the magnetopause mo-
tion along its normal, by using the most accurate normal
directions, nE, derived from the variance analysis of Ec,
and the components of the frame velocity, VHT. From the
plasma moments and the field orientation, we anticipate an
outward motion of the magnetopause (positive along nE)
starting at ∼10:34UT for spacecraft 4 (MP1 interval), fol-
lowedbyaninward(negative)motion(MP2interval)starting
at ∼10:36:40UT. For each crossing, and from each satel-
lite data set, different evaluations of the normal velocity,
VHT.nE, of the magnetopause are summarized in Table 1,
wih unand un averrepresenting the averaged normal veloc-
ities, in the case of constant and accelerated frame veloci-
ties, respectively. For the magnetosheath-to-magnetosphere

Page 11

J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp1555
Table 1. Derived Magnetopause Parameters on 26January2001, 10:34:20–10:37:01UT (See text for details)
spacecraft 1 spacecraft 3spacecraft 4
10:34:05–10:34:40 Parameter1
10:34:26-
10:35:10
Sh → Sp
10:36:40-
10:37:00
Sp → Sh
Magnetopause normals (from variance analysis)
(0.695 0.445 0.563)
(0.442 −0.003 0.896)
(0.470 0.140 0.871) (0.455 0.304 0.836)
10:34:26-
10:35:10
Sh → Sp
10:36:20-
10:37:00
Sp → ShMP crossing Sh → Sp2
n3
n4
nE= V × B
Angles, deg
(nB, nE)
(nEi, nEsc3)
B
(0.490 0.343 0.800) (0.351 0.469 0.810)
(0.353 0.520 0.777)
8.4◦
9.7◦
3.6◦
–
3.5◦
14.1◦
DeHoffmann-Teller frame and electric field correlation
(−277.43 133.5 121.85)
(0.185 −0.389 0.115)
(−296.17 165.73 114.34)
(−266.53 103.52 132.79)
(−281.35 134.62 123.56)
1.007
0.013505
VHT, km/s
aHT, kms−2
VHT0, km/s
VHT end, km/s
VHT aver, km/s
Slope Ec− EH
Ratio D(VHT)/D(0)
(−265.5 146.50 100.52)
(0.144 −0.720 0.177)
(−276.29 209.68 83.35)
(−253.24 94.40 111.71)
(−264.77 152.04 97.53)
1.003
0.008941
(−247.64 91.87 52.44)
(−2.425 5.634 −4.755)
(−183.86 −4.89 127.2)
(−261.6 175.8 −25.25)
(−222.75 85.45 51.00)
1.00213
0.007482
Wal´ en Relation and Magnetopause Motion along n
1.02546
1.04
−5.49
−7.45
−16.31
−8.34
4.95
−12.66
−5.68
-10.5028.32
Slope (V − VHT) vs. VA
Slope (V − VHT0- aHTt) vs.VA
un= VHT• ne, km/s
0.97
1.01
7.80
0.45841
0.45506
0.4420.22
−8.451.08
un0= VHT0• ne, km/s7.75
1.2711.63
−19.0131.35
VHT end• nE, km/s 6.91
11.6245.02
−5.46–20.52
un aver= VHT aver• nE, km/s7.33
6.45 –12.23 5.42
Magnetopause motion from Cluster Inter-spacecraft time lags
– 33.6
∼ − 40.6
un(spacecraft 4-spacecraft i), km/s
un(spacecraft 3-spacecraft i), km/s
27.9
24
–
–
–
–33.6
1Vectors are given in GSE coordinates
2Outward moving magnetopause at ∼10:37UT not detected by spacecraft 4
3(Petrinec and Russell, 1996)
4Magnetopause normal components at spacecraft 2 (from mvaB): (0.445 0.060 0.894)
crossing between 10:34:26 and 10:35:10UT, all estima-
tions of un (or un aver) are, as anticipated, positive; un
ranges between +0.44km/s (spacecraft 1) and +20.2km/s
(spacecraft 3), and the equivalent un aver ranges between
+5.42km/s (spacecraft 4) and +28.3km/s (spacecraft 3). It
must be noted that un estimations are extremely sensitive
to the associated normal direction. On the other hand, for
the traversal between 10:36:40 and 10:37:00UT, the normal
magnetopause velocity, which is negative, ranges between
−7.4 and −12.2km/s.
All of these single spacecraft evaluations of the magne-
topause motion along its normal have been compared with a
multi-spacecraft evaluation, by using the relative positions
and timings of spacecraft 1, 3 and 4, assuming the mag-
netopause is a planar surface (Schwartz, 1998). The inter-
spacecraft time lags lead to an averaged normal velocity, un,
of the order of +24 − +33km/s for the first MP1 crossing,
and −40km/s for the second crossing. These highly reliable
inter-spacecraft evaluations are somewhat larger than those
deduced from the single-spacecraft kinetic analysis; how-
ever, the outward velocity deduced from spacecraft 3 data
is, for instance, comparable, ∼ +28−+30km/s. It is known
that there are several pitfalls in the single spacecraft meth-
ods, which are particularly sensitive to the selected data in-
tervals (see, for more details, Khrabrov and Sonnerup, 1998).
Our velocity evaluations at high-latitudes by the time delay

Page 12

1556J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp
technique are similar to those of ISEE 1/2 at lower latitudes
(< 35◦GSM), 20–80km/s (Berchem and Russell, 1982; Le
and Russell, 1994).
3.3.5Magnetopause thickness
We use the normal magnetopause velocities, un, and the du-
ration of the crossings to deduce the thickness of the differ-
ent layers (current layer/boundary layer) encountered by the
Cluster satellites during the outward-inward oscillatory mag-
netopause motion observed between 10:34:30 and 10:37UT.
For the MP1 crossing, the full magnetic field rotation across
the current layer (or sheath transition layer) occurred in
∼40s for spacecraft 1 and 3 (10:34:30–10:35:10UT), and
in ∼30s at spacecraft 4. Using the velocities given in Ta-
ble 1, one obtains a thickness of ∼40s×24km/s∼900km
(spacecraft 1 and 3) and 30s·33km/s∼1000km (space-
craft 4) for the MP current layer.
sal by spacecraft 1 and 3, the thickness is of the same
order, 24s×40km/s∼ 900km. Our evaluations at high-
latitudes, far from the subsolar region, are similar to the me-
dian thickness of the current sheet at the low-latitude magne-
topause, ∼800km, (Berchem and Russell, 1982), whereas
a thickness of 1500–2000km is predicted by extrapolat-
ing the increase in thickness with the latitude.
widths at low-latitudes were found for high β conditions
(Le and Russell, 1994), but the magnetosheath was very
low, ∼0.6, for this 26January2001 pass.
of the high-latitude boundary layer (HBL), defined as the
layer of enhanced plasma flow velocity, is more difficult
to evaluate. If we assume that the magnetopause motion
reverses at ∼10:35:50UT, the HBL crossing lasted ∼70–
80s (10:34:30–10:35:50UT and 10:35:50–1037UT). By us-
ing constant normal velocities, one obtains ∼70–80s·30–
40km/s∼2100–3200km for the HBL thickness, but this
evaluation is surely underestimated.
For the MP2 traver-
Thinner
The thickness
3.3.6 Velocity distributions
D-shaped distributions predicted by Cowley (1982) are one
of the most important kinetic signatures of reconnection. The
velocity filter effect, resulting from the convection of the re-
cently reconnected field line, introduces cutoffs in the veloc-
ity space that can be used to infer new information on the
topology and location of the reconnection site (Gosling et
al., 1991; Fuselier et al., 1991; Onsager and Fuselier, 1994).
Figure 7 displays four successive slices of the distribution
functions (time resolution: 12s) plotted in the (ν?− ν⊥)
frame, for the accelerated flows detected at ∼10:35UT by
the spacecraft 1 and 3. The distributions, which are basi-
cally identical for the two spacecraft, show the transmitted,
accelerated H+magnetosheath population flowing along the
magnetic field direction, but shifted in the E × B direc-
tion by VHT⊥. They have the characteristic D-shaped form,
with a low-energy cutoff at +VHT?along the +ν?direction.
It can be noted that vectors superposed on the distribution
functions in the (ν?−ν⊥) frame visualize the Wal´ en relation
V −VHT= +VA, where VA= 150−200km/s, is aligned
with B. Generally, in a steady case and at some distance
from the reconnection site, such distributions are accompa-
nied by a counterstreaming population of reflected magne-
tosheath ions, mirroring at lower altitudes in the ionosphere
and returning to the spacecraft after convection dispersion
(see Fuselier et al., 2000). Such a population is not present
inourexample, buttheboundaryismovingduringthiswhole
interval and precludes the detection of a reflected counterpart
of the distribution.
3.4Flux transfer event (FTE) observed by Cluster
Here we present an individual example of a FTE recorded on
the same pass in order to demonstrate how the combination
of plasma and magnetic field data provided by the Cluster
multi-spacecraft mission can be used to determine the in-
ternal structure of such FTE tubes, either their geometrical
(scale size, orientation) or their kinetic properties.
Detailed plots of magnetic field and plasma data for the
FTE encountered around 11:31UT are given in Fig. 8. The
satellites were outbound in the afternoon magnetosheath. For
the four spacecraft, the magnitude, the elevation, and the az-
imuthanglesofthemagneticfieldexpressedintheboundary-
normal coordinate system, (LMN), introduced by Russell
and Elphic (1979), and based on the Petrinec and Russell
(1996) model magnetopause, are plotted in the top three pan-
els. The density of energetic ions (>5keV), the total den-
sity, the magnitude of the plasma flow, and three energy-time
spectrograms for spacecraft 1,3, and 4 are given below. Be-
tween ∼11:30 and ∼11:32:20UT, for each spacecraft, the
positive-negative bipolar pulse typical of the FTE is clearly
identified in the profile of the elevation angle from the lo-
cal tangential magnetopause surface, although more complex
structures can be distinguished within the FTE tube. The
total magnetic field (and the magnetic pressure, not shown)
increases in the center of the event. The appearance of high-
speed flows, up to ∼350km/s, as well as the increase in the
fluxes of high-energy magnetospheric ions, demonstrate that
the four spacecraft are sampling the internal FTE open field
tube. Another significant feature apparent inside the FTE
is the deflection of the tangential magnetic field, measured
by the angle ϕBlmn, towards the magnetospheric direction.
This deflection does not occur at the center of the event, at
∼11:31:10UT, but is slightly time shifted.
3.4.1Flux tube orientation
To extract the basic kinetic and geometrical properties of the
FTE, we start by analyzing each set of spacecraft data, us-
ing the method adopted by Papamastorakis et al. (1989). the
results are summarized in Table 2. First, we determine the
orientation of the flux tube at each spacecraft, assuming this
orientation is well predicted by the minimum variance direc-
tion, kB, of the magnetic field (assuming the magnetic field
strength remains constant throughout the event). In spite of
the relatively large errors, the minimum variance analysis of

Page 13

J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp1557

SC1
SC3
VHT=[-265.5 146.5 100.5]GSE
VHT=[-277.4 133.5 121.8]GSE
Fig. 7. Four successive slices of the distribution functions (time resolution: 12s) obtained by the CIS-2 instruments on spacecraft 1 (bottom
row) and spacecraft 3 (top row) for the accelerated flows detected at ∼10:35UT. Distributions are plotted in the (ν?− ν⊥) frame, where ν?
is aligned with the local magnetic field and ν⊥is the (−V × B) × B direction. The bulk flow velocity (o, blue) and VHT(+, black) vectors
are also plotted, as well as the Sun direction (red line).
the magnetic field for the data interval 10:30–10:33UT gives
similar directions at each spacecraft, with the angle between
kB, for spacecraft 3 (reference spacecraft), and other three
spacecraft at only 4–6◦. kB vectors lie near the tangential
plane to the magnetopause, with their angle to the normal at
around 12–18◦. Moreover, the major component of the mag-
netic field, ∼28nT, is along kB.
3.4.2 dHt velocity
Second, the minimum variance analysis carried out on the
convection electric field data, Ec= −V × B at 4s resolu-
tion (V and B measured by the CIS and FGM instruments,
respectively), gives a very well-defined minimum-variance
direction, kE, demonstrating that a very good moving dHT
frame exists with a translating velocity vector, VHT, which
must be very close of the kEdirection. VHTis evaluated by
fitting the relationship between the three components of the
convection electric field, Ec, and the corresponding compo-
nents of the EHT= −VHT× B plot, similar to the previous
magnetopause crossings. The resulting VHTvectors (associ-
ated with the slope of the Ec− EHTregression line) and the
“quality factor”, D/D(0), are also given in Table 2. They are
practically similar for spacecraft 1 and 3 and, as anticipated,
are nearly aligned with kE. The obtained VHTvelocities
are rather similar to the VHTobtained for the magnetopause
crossings on this pass (see Table 1). For spacecraft 4 the VHT
vector direction remains the same, but differs in magnitude
under the combined effects of a slightly different magnetic
field and velocity profiles (see Fig. 8).
3.4.3 Wal´ en test
New information is gained by analyzing the Wal´ en relation-
ship, V − VHT = ±VA. The Wal´ en relation is approxi-
mately satisfied for spacecraft 1 (positive slope=0.87), but
this slope is somewhat less for spacecraft 3 (∼0.663), and
even ∼0.45 for spacecraft 4.
V − VHTis nearly parallel to the magnetic field B and the
magnitudes are positively related, as anticipated for a recon-
nection site equatorward of the satellites.
Nevertheless, in all cases,
3.4.4Motion of the FTE tube
Before discussing the motion and size of this event, we plot,
in Fig. 9, the projection of all the vectors (kB, VHT, magnetic

Page 14

1558J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp
Fig. 8.
and magnetic field measurements dur-
ing the flux transfer event detected
around 11:31UT on 26January2001.
The magnetic field data are shown in
the top three panels: magnetic field
strength B (nT), magnetic field ele-
vation and azimuth angles, θblmnand
ϕBlmn, in (LMN) boundary normal co-
ordinates.The GSE components of
the normal direction are (0.716 0.452
0.530) and were calculated using the
Petrinec and Russell (1995) model. In
the following panels the number density
ofenergetic ions (Ei> 5keV), the total
ion density Ni(cm−3), and the plasma
bulk velocity Vi(km/s) are shown. 1D
energy-time ion spectrograms obtained
by the CIS-2 (spacecraft 1 and 3) or
the CIS-1 (spacecraft 4) instruments are
given in the bottom panels, with the
GSE coordinates of spacecraft 1.
Overview of Cluster plasma
field, plasma flow velocities) in the (LM) plane tangential to
the magnetopause. The kBdirection, anticipated as a reason-
ably good predictor of the FTE axis direction (Walthour and
Sonnerup, 1995), does not coincide with the magnetosheath
magnetic field direction and the direction taken by the mag-
netic field within the tube. Indicated by the ϕBrotation in
Fig. 8, this tilt can be interpreted as a field twisting about the
tube axis (Saunders et al., 1984). From the diagram, it can
be also concluded that the FTE flux tube is convecting north-
eastward (in the LM Plane) with a velocity VHTevaluated to
be about ∼190–220km/s for spacecraft 1 and 3. This veloc-
ity is certainly higher than the perpendicular component of
the ambient flow, so that the FTE flux tube is not exactly con-
vecting with the ambient flow. The same diagram, where the
(LM) positions of the four Cluster spacecraft (spacecraft 3
is centered on the origin) are also represented, can be very
useful in determing the reliability of the predictions of the ki-
netic analysis concerning the tube axis direction and its mo-
tion, compared to the direct inter-spacecraft measurements.
From Fig. 7, it can be seen that at around 11:31:04UT, the
sequence of entry is first spacecraft 3-spacecraft 2 (almost
simultaneously at the magnetic field data resolution of 1s),
then spacecraft 4 (with a delay <1s.), then spacecraft 1, with
a delay of 2–3s. From the plasma velocity profiles, the delay
between spacecraft 3 and 1 is of the order of the CIS time res-
olution, i.e. 4s. By knowing the satellite relative positions,
it can be concluded that the exact FTE axis is nearly aligned
with the spacecraft 3–spacecraft 2 line and does not coincide
with the kBdirections found by the minimum variance anal-
ysis. Suchadifferenceof∼5–10degreesisanticipatedinthe
case of the encounter with a 2D structure and could be eval-
uated by using more sophisticated methods (Walthour and

Page 15

J. M. Bosqued et al.: Ion observations of the high-latitude magnetopause and cusp1559
Table 2. Derived FTE Parameters on 26January2001, 11:30–11:33UT
spacecraft 1
11:30–11:33UT
spacecraft 2
11:30–11:33UT
spacecraft 3
11:30–11:33UT
spacecraft 4
11:30–11:33UTParameter
npr96
(0.716 0.452 0.530)
MVA analysis : FTE axis and VHTdirection
(0.225 −0.974 −0.01)
106.5
4.09
kB
Angle (npr96,kB)
Angle (kBSC3,kBSCi)
kE
(0.240 −0.970 0.035)
104.0
3.74
(−0.82 0.429 0.372)
(0.293 −0.955 0)
102.8
–
(−0.835 0.434 0.337)
(0.236 −0.967 −0.09)
108.3
6.05
(−0.854 0.452 0.254)
DeHoffmann-Teller frame and electric field correlation
(−252.8 142.3 108.7)
101.0
0.79
1.013
0.042
VHT, km/s
Angle (VHT,npr96)
Angle (VHTSC3, VHTSCi)
Slope Ec− EH
Ratio (DVHT)/D(0)
(−233.7 130.3 96.2)
101.6
–
1.031
0.044
(−207.0 106.46 50.6)
107.9
7.74
1.07
0.040
Wal´ en Relation
Slope (V − VHT) vs. VA
0.871 0.663 0.442
FTE Motion
VHT, km/s
Ulm(SC1/SC3), km/s
220.3187.7 138.5
600/3∼ 200
Sonnerup, 1995). Moreover, the tube convecting velocity,
ulm, evaluated from the time lags, is of the order of 200km/s
at the entry, comparable to the VHTcomponent.
3.4.5 Size of the FTE tube
Finally,
allel to the magnetopause can be estimated to be
80s×200km/s∼16000km,
for instance, by Saunders et al. (1984). Within this tube, the
diameter of the region where the magnetic field rotates is
concentrated to 20 × 20 = 8000km, presumably due to a
core central current, and the region of accelerated flow has
a diameter of ∼12000km. Finally, we notice that the FTE
internal structure is certainly more complicated, as revealed
by the shifted observations made by spacecraft 4 located
∼500km from the (LM) plane, or by a low-energy plasma
jetting observed by spacecraft 3 in the center of the tube (see
Fig. 7, spectrogram panel C3). Moreover, magnetopause
motions along the normal are not negligible; an inward
motion of the magnetopause of the order of ∼50–70km/s
can be deduced from the dHT analysis and could result in a
sharp increase in the field tilting that is observed later after
the polarity change of the normal field.
thetotaldiameterof theFTEtubepar-
ofthe orderpublished,
4Second case study: 2February2001, Northward IMF
This Cluster outbound pass in the northern afternoon side of
the magnetosphere (Fig. 10) is of special interest as it is the
first to occur under northward IMF-BZconditions. For this
event, another paper in the present issue of Annales Geo-
physicae is devoted to ground-based radar signatures (Pitout
et al., 2001) and convection cells at low altitudes.
We determined from ACE observations upstream of the
bow shock near the L1 point at ∼220RE (Fig. 11), and
WIND observations on the duskside of the magnetosphere
but far away (YGSE ∼200RE), that the IMF during this
event was fluctuating, directed primarily northward (IMF-
BZ∼3–4nT) for a large interval (09:05–15:00UT). This
includes the Cluster cusp observations for 2February2001,
except for a short IMF-BZ < 0 incursion around ∼11:30
and 12:20UT. Strongly directed duskward before 09:10UT,
IMF-BY started 40-min periodic dusk-dawn fluctuations
(4nT peak-to-peak), with most being positive (BY ∼2–
3nT) between ∼11:00 and 12:00UT. Therefore, from these
data, we expect a high magnetic shear near the duskward
northern magnetopause around 10:00–10:30UT, when Clus-
ter crossed the cusp during its duskward outbound pass; thus,
conditions are very favourable for reconnection in this region
(Crooker, 1979; Luhmann et al., 1984). On the other hand,
the solar wind dynamic pressure, given in the bottom panel
of Fig. 11, remained relatively steady and near its nominal
value, around 1–1.2nPa for the 08:00–12:00UT interval.
4.1CIS and FGM data
Figure 12 shows an overview of the relevant FGM mag-
netic field data and CIS ion data gathered during the 08:00–
16:00UT interval on 2February2001, in the form of mag-