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Ann. Geophys., 22, 2451-2461, 2004
www.ann-geophys.net/22/2451/2004/
© European Geosciences Union 2004
Plasma convection across the polar cap, plasma mantle and cusp: Cluster EDI observations
H. Vaith1, G. Paschmann1, J. M. Quinn2, M. Förster1, E. Georgescu1, S. E. Haaland1, B. Klecker1, C. A. Kletzing3, P. A. Puhl-Quinn1,2, H. Rème4, and R. B. Torbert2
1Max-Planck-Institut für extraterrestrische Physik, 85748 Garching, Germany
2University of New Hampshire, Durham, NH 03824, USA
3University of Iowa, Iowa City, IA 52242, USA
4CESR-CNRS, 31028 Toulouse, France
Abstract. In this paper we report measurements of the convection obtained with the
Electron Drift Instrument (EDI) on Cluster. We use 20 passes that cross the
between its dayside and nightside boundaries (or vice versa) at
geocentric distances ranging from about 5 to about 13RE, and at
interspacecraft separations (transverse to the ambient magnetic field)
between a few km and almost 10000km. We first illustrate the nature of
the data by presenting four passes in detail. They demonstrate that the sense
of convection (anti-sunward vs. sunward) essentially agrees with the
expectations based on magnetic reconnection occurring on the dayside or
poleward of the cusp. The most striking feature in the EDI data is the
occurrence of large-amplitude fluctuations that are superimposed on the
average velocities. One type of fluctuation appears to grow when approaching
the dayside boundary. The examples also show that there is a variable
degree of inter-spacecraft correlation, ranging from excellent to poor. We
then present statistical results on all 20 passes. Plotting 10-min averages
of the convection velocities vs. IMF Bz one recovers the expected
dependence, albeit with large scatter. Looking at the variances computed over
the same 10-min intervals, one confirms that there is indeed one type of
contribution that grows towards the dayside boundary, but that variances can
be high anywhere. Finally, computing the inter-spacecraft correlations as a
function of their separation distance transverse to the magnetic field shows
that the average correlation drops with increasing distance, but that even at
distances as large as 5000km the correlation can be very good. To put those
scales into context, the separation distances have also been scaled to
ionospheric altitudes where they range between a few hundred meters and
600km.
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