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such as the SuperDARN network; Ruohoniemi and Greenwald
2005
) and based
on in situ measurements at ionospheric heights by satellites (e.g., Weimer
1995
,
2005
), but we use here statistical data obtained in the spacious area of the whole
magnetosphere by the Cluster satellites during the same time interval as the CHAMP
data acquisition.
The Cluster mission is an European Space Agency (ESA) project, launched in
2000, comprising four identical satellites flying in close formation around the Earth.
Cluster has a nearly 90
ı
inclination elliptical polar orbit, with initial perigee at
around 4 RE and apogee around 19 RE geocentric distance, and an orbital period
of approximately 57 h (Escoubet and Schmidt
1997
). Measurements of the plasma
convection (or drift) velocity have been obtained with the Electron Drift Instrument
(EDI) on board Cluster. The basis of the electron drift technique is the injection
of 2 weak beams of electrons and their detection after one or more gyrations in
the ambient magnetic field. Because of their cycloidal motion, beam electrons can
return to the associated detectors only when fired in directions uniquely determined
by the magnitude and direction of the plasma drift velocity. The drift velocity is
computed either from the direction of the beams (via triangulation) or from the
difference in their times-of-flight. An important advantage of EDI for high-latitude
convection measurements is its immunity from wake effects that can interfere with
the double-probe measurements under conditions of low plasma density that often
occur over the polar cap. Furthermore, the EDI measures the entire vector drift
velocity, which is equivalent to the transverse electric field when gradient drift
effects are small, as it is the case over the polar cap (for more details, see, e.g.,
Paschmann et al.
2001
).
Vector measurements of the EDI instrument, available for several years, have
been used to derive statistical maps of the high-latitude plasma convection, as shown
here in Fig.
4.3
for the Southern Hemisphere. The EDI measurements, obtained at
geocentric distances between 4 and 15 RE over both hemispheres, are mapped into
the polar ionosphere and sorted according to the clock angle of the IMF as measured
at ACE upstream in the solar wind (Forster et al.
2007
,
2009
; Haaland et al.
2007
).
The IMF clock angle considers the orientation of the IMF in the GSM
yz
-plane,
with zero angle for purely northward IMF and then counted clockwise in this plane,
so that we have
C
90
ı
for sector 2,
˙
180
ı
for sector 4 (purely southward IMF),
and so on.
Comparison with published statistical results based on Super Dual Auroral Radar
Network (SuperDARN) radar (e.g., Ruohoniemi and Greenwald
2005
)andlow-
altitude satellite measurements (e.g., Weimer
2005
) shows excellent agreement of
the average convection patterns, and in particular the incomplete mirror symmetry
between the effects of positive and negative IMF
B
y
, the appearance of a duskward
flow component for strongly southward IMF, and the general weakening of the
flows and potentials for northward IMF orientations (sector 0). This agreement lends
credence to the validity of the assumption underlying the mapping of the EDI data,
namely, that magnetic field lines are equipotentials.
Figure
4.4
gives an impression about the influence of the IMF orientation
and the differences between the hemispheres. The wind amplitudes are smallest
