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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A09204, doi:10.1029/2007JA012284, 2007

Solar wind control of plasma number density in the near-Earth plasma sheet

D. Nagata

Department of Geophysics, Kyoto University, Kyoto, Japan


S. Machida

Department of Geophysics, Kyoto University, Kyoto, Japan


S. Ohtani

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA


Y. Saito

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan


T. Mukai

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan


Abstract

The plasma number density in the plasma sheet depends on the solar wind number density and the north-south component of interplanetary magnetic field (IMF B z ) with the time lag of several hours. We examined such dependences as functions of (X, Y) coordinates in the near-Earth plasma sheet by fitting observations of plasma sheet and solar wind to an empirical model equation. We explored shortest and longest response time lags to IMF B z by optimizing the correlation coefficient of fit. Analyses were conducted separately for northward and southward IMF dominant conditions. The dependence of plasma sheet number density on solar wind number density is stronger in the near-tail region (r > 20 R E ) under the southward IMF dominant condition. The dependence has weak dawn-dusk asymmetry under the southward IMF dominant condition, whereas it is stronger in the dusk flank under the northward IMF dominant condition. The dependence on IMF B z is globally positive under the northward IMF dominant condition, whereas the dependence is negative in the near-Earth premidnight region under the southward IMF dominant condition. Both shortest and longest time lags increase from the mid-tail to the near-Earth premidnight region under the southward IMF dominant condition. On the other hand, the shortest (longest) time lag increases antisunward (sunward) along flanks under the northward IMF dominant condition. These features can be explained in terms of reconnection and Kelvin-Helmholtz diffusion mechanisms for entry of magnetosheath plasma into the plasma sheet and electric and magnetic drift transport in the plasma sheet.

Received 19 January 2007; accepted 11 June 2007; published 8 September 2007.

Keywords: mass loading; plasma sheet; solar wind.

Index Terms: 2764 Magnetospheric Physics: Plasma sheet; 2784 Magnetospheric Physics: Solar wind/magnetosphere interactions; 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; 2748 Magnetospheric Physics: Magnetotail boundary layers; 2760 Magnetospheric Physics: Plasma convection (2463).

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Citation: Nagata, D., S. Machida, S. Ohtani, Y. Saito, and T. Mukai (2007), Solar wind control of plasma number density in the near-Earth plasma sheet, J. Geophys. Res., 112, A09204, doi:10.1029/2007JA012284.