Electron Scale Structure of Magnetic Holes in the Plasmasheet

Details Electron Scale Structure of Magnetic Holes in the Plasmasheet Electron Scale Structure of Magnetic Holes in the Plasmasheet

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm41a1860m&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SM41A-1860

Physics

[2740] Magnetospheric Physics / Magnetospheric Configuration And Dynamics, [2752] Magnetospheric Physics / Mhd Waves And Instabilities, [2764] Magnetospheric Physics / Plasma Sheet, [2788] Magnetospheric Physics / Magnetic Storms And Substorms

Scientific paper

Magnetic holes have been observed in the Earth's magnetosheath, in the upstream solar wind, and on occasion in the magnetosphere. These holes are generally attributed to the magnetic mirror instability growing from free energy available in high-Beta, anisotropic plasmas with Tperp>Tpar. Since ions are much hotter or more energetic than electrons, it is generally assumed that ion anisotropy drives these instabilities. Ge et al. (2010) has recently submitted a report on THEMIS observations of magnetic holes in the plasmasheet following substorm dipolarizations. These observations are curious in that the surrounding plasma was mirror stable, the structures were of thermal ion gyro-radius scale, and changes in electron anisotropy characterized the holes. Ions were found to be isotropic both inside and outside the structures, while electrons had Tpar>Tperp outside the holes and Tpar=Tperp inside the holes. In this paper we report on additional observations of these small-scale plasmasheet magnetic holes. In particular we show direct measurements (Vi-Ve) of a thin Hall current that forms at the edges of these holes. This current, which results from an electron pressure gradient, strongly suggests that these structures grow from free energy in the electron distribution even though Te is much less than Ti. These measurements allow a accurate calculation of the velocities of the holes, which can be translated directly into magnetic hole scale size.

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