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Ann. Geophys., 25, 1417-1432, 2007
www.ann-geophys.net/25/1417/2007/
© European Geosciences Union 2007
Temperature anisotropies of electrons and two-component protons in the dusk plasma sheet
M. N. Nishino1, M. Fujimoto1, T. Terasawa2, G. Ueno3, K. Maezawa1, T. Mukai4, and Y. Saito1
1ISAS/JAXA, Kanagawa 229-8510, Japan
2Tokyo Institute of Technology, Tokyo 152-8551, Japan
3Institute of Statistical Mathematics, Tokyo 106-8569, Japan
4JAXA, Tokyo 100-8260, Japan
Abstract. To investigate the cold plasma sheet formation under northward IMF, we study
the temperature anisotropies of electrons and two-component protons observed
by the Geotail spacecraft. The two-component protons, which are occasionally
observed in the dusk plasma sheet near the low-latitude boundary, are the
result of spatial mixing of the hot protons of the magnetosphere proper and
the cold protons from the solar wind. Recent research focusing on the
two-component protons reported that the cold proton component at times has a
strong anisotropy, and that the sense of the anisotropy depends on the
observed locations. Since electrons have been known to possess a strong
parallel anisotropy around the low-latitude boundary layer, we compare
anisotropies of electrons and protons to find that the strengths of parallel
anisotropies of electrons and the cold proton component are in good
correlation in the tail flank. The parallel anisotropy of electrons is
stronger than that of the cold proton component, which is attributed to
selective heating of electrons. We further find that the strengths of the
parallel anisotropies in the tail flank depend on the latitudinal angle of
the IMF; strong parallel anisotropies occur under strongly northward IMF. We
discuss that the Kelvin-Helmholtz vortices, which developed under strongly northward
IMF, and the resultant magnetic reconnection therein may lead to the strong
parallel anisotropies observed in the tail flank.
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