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Ann. Geophys., 24, 3391-3401, 2006
www.ann-geophys.net/24/3391/2006/
© European Geosciences Union 2006
On the structure of field-aligned currents in the mid-altitude cusp
A. Marchaudon1,2, J.-C. Cerisier3, J.-M. Bosqued4, C. J. Owen1, A. N. Fazakerley1, and A. D. Lahiff1
1Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, RH5 6NT, Surrey, UK
2Now at: Laboratoire de Physique et Chimie de l'Environnement, 3A avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France
3Université Pierre et Marie Curie-Paris6; Centre d'Etude des Environnements Terrestre et Planétaires, 4 avenue de Neptune, 94107 Saint-Maur-des-Fossés Cedex, France
4Centre d'Etude Spatiale des Rayonnements, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
Abstract. We analyse two crossings of the polar cusp at mid-altitudes (≈4 RE)
by Cluster in order to study the structure of field-aligned
currents associated with the injection of magnetosheath plasma. The current
density is deduced independently from magnetic field and from particle flux
measurements. In both cases the data are carefully tested. Magnetic
fluctuations are analysed by discriminating between those compatible with
the plane current sheet hypothesis under which the current density can be
calculated safely, and those resulting from filamentary current structures.
At medium transverse scales (80 km), the structure of the currents is more
often tube-like than sheet-like, and current sheets are not systematically
elongated in the east-west direction. The total particle current is
calculated from the electron and ion measurements. For electrons, the full
energy range is taken into account, from above the photoelectron threshold
up to 32 keV. Magnetosheath plasma injections are well correlated with pairs
of field-aligned currents. In both cases, the parallel current is mainly
carried by electrons while ions contribute for about 20%. In the plane
current sheets, the ratio between magnetic and particle currents shows large
variations between 0.4 and 1.1. These fluctuations can be explained by the
convective motion of the current sheets.
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