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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 24, issue 3
Ann. Geophys., 24, 1015–1027, 2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 24, 1015–1027, 2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.

  19 May 2006

19 May 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 N. Balan et al.
  • 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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