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Ann. Geophys., 24, 1015-1027, 2006
www.ann-geophys.net/24/1015/2006/
© European Geosciences Union 2006
Cluster observations of a structured magnetospheric cusp
N. Balan1, H. Alleyne1, S. Walker1, H. Réme2, P. M. E. Décréau3, A. Balogh4, M. André5, A. N. Fazakerley6, N. Cornilleau-Wehrlin7, D. Gurnett8, and M. Fraenz9
1Automatic Control and Systems Engineering, University of Sheffield, Sheffield, UK
2CESR, Toulouse, France
3LPCE/CNRS and Universite d’Orleans, France
4Blackett Laboratory, Imperial College, London, UK
5SISP, Uppsala, Sweden
6Mullard Space Science Lab, Uni. College London, UK
7CETP-IPSL/CNRS, Vélizy, France
8University of Iowa, Iowa, USA
9MPI für Sonnen Systemforschung, Lindau, Germany
Abstract. On 18 April 2002 the Cluster spacecraft crossed through the northern outer
magnetospheric cusp region during 16:25-17:55 UT when the solar wind dynamic
pressure was rather low (<2 nPa) and IMF Bz was more negative than
IMF By. The Cluster data from the FGM, CIS, PEACE, EFW, WHISPER and
STAFF instruments reveal that the cusp is structured with three anti-sunward
ion flow events of durations ≈1.5, 17.5 and 19.0 min, with bulk
plasma flow roughly parallel to the magnetopause toward north. The ion and
electron densities within the events are much greater than those outside. The
zonal electric field in the ion flow events turns eastward as expected from
V×B effect. The sharp inward boundaries of the ion flow
events cross the four spacecraft in one time sequence, and the outward
boundaries of the events cross the spacecraft in the reverse time sequence.
The observations studied using magnetosphere and magnetopause models suggest
that the structured cusp is a temporal feature that arises due to three
inward and outward movements of the magnetopause by about 1.5RE so that
Cluster, while crossing through the cusp, happened to be in the magnetosheath
(ion flow event) and cusp alternately. The magnetopause moved due to the
changes in the solar wind dynamic pressure by up to 100%.
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