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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 27, issue 8
Ann. Geophys., 27, 3043–3053, 2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 27, 3043–3053, 2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.

  05 Aug 2009

05 Aug 2009

Foreshock-like density cavity in the outflow region of magnetotail reconnection

C. L. Cai1,2,3, I. Dandouras1,2, H. Rème1,2, J. B. Cao3,4, G. C. Zhou3, G. K. Parks5, and D. Fontaine6 C. L. Cai et al.
  • 1University of Toulouse, UPS, CESR, 9 avenue colonel Roche, 31028 Toulouse cedex 4, France
  • 2CNRS, UMR5187, 31028 Toulouse, France
  • 3State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Beijing 100080, China
  • 4School of Astronautics, Beijing University of Aeronautics and Astronautics, Beijing 100191, China
  • 5Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
  • 6Centre d'Etudes des Environnements Terrestre et Planétaires, Centre National de la Recherche Scientifique, 78140 Vélizy, France

Abstract. During Cluster spacecraft crossing of the magnetotail, a novel density depleted cavity in association with magnetic compressions in the outflow region of reconnection was observed. It contains intense reflected field-aligned particles, which are produced by a generation mechanism similar to that of the terrestrial foreshock, and hence manifests a foreshock-like morphology. In this cavity, reflected field-aligned proton beams were observed and simultaneously the feature of magnetic-mirror loss-cone proton distributions were found. Magnetic field fluctuations, especially quasi-monochromatic oscillations, were recorded. Both the leading egde and the ULF wave boundary of the ion foreshock are identified from the time sequence of proton and magnetic field observations. Just upstream of the leading egde of the ion foreshock, reflected field-aligned electrons were detected, whose distribution has a narrow bump-on-tail pattern. However, close to the shock front, reflected electrons with a broad bump-on-tail pattern was measured. These two different manifestations of reflected electrons reveal the differences in their microscopic physics of the reflecting process. Moreover, a part of incident ions was further accelerated in the cavity due to trans-time magnetic pumping which provides another possible mechanism in the multi-step acceleration processes in reconnection.

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