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Ann. Geophys., 22, 2577-2585, 2004
www.ann-geophys.net/22/2577/2004/
© European Geosciences Union 2004
Density structures inside the plasmasphere: Cluster observations
F. Darrouzet1, P. M. E. Décréau2, J. De Keyser1, A. Masson3, D. L. Gallagher4, O. Santolik5,*, B. R. Sandel6, J. G. Trotignon2, J. L. Rauch2, E. Le Guirriec2, P. Canu7, F. Sedgemore8, M. André9, and J. F. Lemaire1
1Belgian Institute for Space Aeronomy (IASB-BIRA), 1180 Brussels, Belgium
2Laboratoire de Physique et Chimie de l’Environnement (LPCE/CNRS), Université d’Orléans, 45071 Orléans, France
3Research and Scientific Support Department (RSSD), ESTEC-ESA, 2201-AG Noordwijk, The Netherlands
4Marshall Space Flight Center (MSFC), NASA, Huntsville, AL 35805, USA
5Faculty of Mathematics and Physics (MFF), Charles University, 121 16 Prague, Czech Republic
6Lunar and Planetary Laboratory (LPL), University of Arizona, Tucson, AZ 85721, USA
7Centre d’étude des Environnements Terrestre et Planétaires (CETP/CNRS), Université de Versailles Saint-Quentin-en-Yvelines, 78140 Vélizy, France
8Formerly at Danish Space Research Institute (DSRI), 2100 Copenhagen, Denmark
9Swedish Institute of Space Physics (IRFU), Uppsala division, 751 21 Uppsala, Sweden
*also at Institute of Atmospheric Physics (IAP/CAS), 141 31 Prague, Czech Republic
Abstract. The electron density profiles derived from the EFW and
WHISPER instruments on board the four Cluster spacecraft reveal density
structures inside the plasmasphere and at its outer boundary, the
plasmapause. We have conducted a statistical study to characterize these
density structures. We focus on the plasmasphere crossing on 11 April
2002, during which Cluster observed several density irregularities inside
the plasmasphere, as well as a plasmaspheric plume. We derive the density
gradient vectors from simultaneous density measurements by the four
spacecraft. We also determine the normal velocity of the boundaries of the
plume and of the irregularities from the time delays between those
boundaries in the four individual density profiles, assuming they are
planar. These new observations yield novel insights about the occurrence of
density irregularities, their geometry and their dynamics. These in-situ
measurements are compared with global images of the plasmasphere from the
EUV imager on board the IMAGE satellite.
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