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Ann. Geophys., 24, 3011-3026, 2006
www.ann-geophys.net/24/3011/2006/
© European Geosciences Union 2006
Cluster survey of the mid-altitude cusp: 1. size, location, and dynamics
F. Pitout1, C. P. Escoubet2, B. Klecker1, and H. Rème3
1Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85741 Garching, Germany
2European Space Agency, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
3Centre d'étude Spatiale des Rayonnements, 9, avenue du Colonel Roche, BP 4346, 31028 Toulouse Cedex 4, France
Abstract. We present a statistical study of four years of Cluster
crossings of the mid-altitude cusp. In this first part of the study, we
start by introducing the method we have used a) to define the cusp
properties, b) to sort the interplanetary magnetic field (IMF) conditions or
behaviors into classes, c) to determine the proper time delay between the solar
wind monitors and Cluster. Out of the 920 passes that we have analyzed, only 261
fulfill our criteria and are considered as cusp crossings. We look at the
size, location and dynamics of the mid-altitude cusp under various IMF
orientations and solar wind conditions. For southward IMF, Bz rules the
latitudinal dynamics, whereas By governs the zonal dynamics, confirming
previous works. We show that when |By| is larger than
|Bz|, the cusp widens and its location decorrelates from
By. We interpret this feature in terms of component reconnection
occurring under By-dominated IMF. For northward IMF, we demonstrate
that the location of the cusp depends primarily upon the solar wind dynamic
pressure and upon the Y-component of the IMF. Also, the multipoint
capability of Cluster allows us to conclude that the cusp needs typically
more than ~20 min to fully adjust its location and size in
response to changes in external conditions, and its speed is correlated to
variations in the amplitude of IMF-Bz. Indeed, the velocity in °ILAT/min
of the cusp appears to be proportional to the variation in Bz
in nT: Vcusp=0.024 ΔBz. Finally, we observe differences in
the behavior of the cusp in the two hemispheres. Those differences suggest
that the cusp moves and widens more freely in the summer hemisphere.
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