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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 26, issue 12
Ann. Geophys., 26, 3653-3667, 2008
https://doi.org/10.5194/angeo-26-3653-2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: STAMMS: Spatio-Temporal Analysis and Multipoint Measurements...

Ann. Geophys., 26, 3653-3667, 2008
https://doi.org/10.5194/angeo-26-3653-2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.

  20 Nov 2008

20 Nov 2008

The statistics of foreshock cavities: results of a Cluster survey

L. Billingham1, S. J. Schwartz1, and D. G. Sibeck2 L. Billingham et al.
  • 1Space and Atmospheric Physics, Blackett Laboratory, Imperial College London, London, UK
  • 2Goddard Space Flight Center, Greenbelt, MD, USA

Abstract. We use Cluster magnetic field, thermal ion, and energetic particle observations upstream of the Earth's bow shock to investigate the occurrence patterns of foreshock cavities. Such cavities are thought to form when bundles of magnetic field connect to the quasi-parallel bow shock. Shock-processed suprathermal ions can then stream along the field, back against the flow of the solar wind. These suprathermals enhance the pressure on shock-connected field lines causing them to expand into the surrounding ambient solar wind plasma. Foreshock cavities exhibit depressions in magnetic field magnitude and thermal ion density, associated with enhanced fluxes of energetic ions. We find typical cavity duration to be few minutes with interior densities and magnetic field magnitudes dropping to ~60% of those in the surrounding solar wind. Cavities are found to occur preferentially in fast, moderate magnetic field strength solar wind streams. Cavities are observed in all parts of the Cluster orbit upstream of the bow shock. When localised in a coordinate system organised by the underlying physical processes in the foreshock, there is a systematic change in foreshock cavity location with IMF cone angle. At low (high) cone angles foreshock cavities are observed outside (inside) the expected upstream boundary of the intermediate ion foreshock.

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