Statistical characteristics of the storm time plasma sheet

Abstract

The present study addresses the convection and configuration of the storm time plasma sheet by examining the occurrence ratios of fundamental plasma quantities. Measurements made by the Geotail spacecraft in the midnight sector are used. The results are summarized as follows: (1) the occurrence ratio of the plasma flow velocity is not noticeably different between storm and nonstorm. time or between the main and recovery phases; (2) the convection electric field is more intense during the main phase; (3) the equatorial magnetic field is stronger during the main phase; (4) the ion pressure is higher, therefore the tail current is more intense, during the main phase. It is suggested that the stronger equatorial magnetic field (result 3) counteracts the enhancement of the convection electric field (result 2) so that the occurrence ratio of the flow velocity does not depends on geomagnetic activity (result 1). The storm time increase in the equatorial magnetic field can be attributed to the X-dependent intensification of the dawn-to-dusk electric current (the ring current and tail current); if the current is more intensified on the earthward side than on the tailward side, the net change in BZ is positive. It is also found that the reconnection takes place, if infrequently, at X > - 15 RE during storm periods. However, result 1 suggests that despite the intense tail current (result 4), the X distribution of the near-Earth reconnection site does not statistically depend on storm activity. This can also be attributed to the stronger equatorial magnetic field as well as to the storm time enhancement of the near-Earth plasma number density. It is therefore suggested that the ring current affects. the plasma sheet dynamics through its contribution to the tail magnetic configuration. Copyright 2008 by the American Geophysical Union.