Investigation of solar wind dependence of the plasma sheet based on long-term Geotail/LEP data evaluation

Abstract:

It is observationally known that the plasma density and temperature in plasma sheet are significantly changed by solar wind conditions [e.g., Terasawa et al., 1997]. Thus it is considered that the plasma sheet plasma is originated from the solar wind, and several entry mechanisms have been suggested. When the interplanetary magnetic field (IMF) is southward, the solar wind plasma enters the plasma sheet mainly through magnetic reconnection at the dayside magnetopause. In contrast, for the northward IMF, the double-lobe reconnection [Song et al., 1999], abnormal diffusion [Johnson and Cheng., 1997], and plasma mixing through the Kelvin-Helmholtz instability caused by viscous interaction [Hasegawa et al., 2004] have been proposed. Relative contribution of each process is, however, far from understood.

In the present study, we use magnetotail observations by the Geotail spacecraft at radial distances of 10-32 Re during 12-year period from 1995 to 2006 to investigate properties of the plasma sheet. We conducted a statistical analysis with calibrated LEP-EA [Mukai et al., 1994] ion and electron data. We selected central plasma sheet observations and derived electron and ion temperature and density using the same method and criteria as Terasawa et al. [1997]. In addition, OMNI solar-wind data are used. The results show that the plasma sheet density (both ion and electron temperatures) has a good correlation with the solar wind density (kinetic energy) over the whole solar cycle. We find clear dawn-dusk asymmetry in the temperature ratio Ti/Te, i.e., the average Ti/Te is higher on the duskside than the dawn. The density also shows the dawn-dusk asymmetry and higher on the duskside than on the dawnside. A previous study by Wang et al. [2012] showed that Ti/Te is high (typically 5-10) in the magnetosheath. The statistical results, therefore, suggest that the shocked solar wind plasma can easily enter the duskside plasma sheet rather than the dawnside. We will discuss the possible mechanisms of the entry of the cold plasma into the duskside plasma sheet.