Global MHD simulation of the geomagnetic sudden commencementon 21 October 1999

摘要

Recently, Shinbori et al. (2004) examined the electric field variations associated withgeomagnetic sudden commencements (SC) by using data from the Akebono satellite in theinner magnetosphere (L < 5) and reported the following characteristics of the SC-associatedelectric field variations. (1) The electric field shows a bipolar change. (2) The initialexcursion of the electric field tends to be directed westward. (3) The electric field amplitudedoes not show a dependence on magnetic local time. By using a global three-dimensionalMHD simulation model, we examine how and where such SC-associated electric fieldvariations are established. In our study, we used the SC event that occurred on 21 October1999, which was caused by a sudden increase in the solar wind dynamic pressure from ,3 to 13 nPa. The solar wind and interplanetary magnetic field conditions observed from theWind satellite near GSM (x, y, z) (22, —62, 20) RE are used as the simulation inputparameters. The numerical simulation shows that inward flow is first excited near local noonand then flow vortex is generated near the flankside as the solar wind discontinuity is passingover the magnetosphere. Thus, the convection electric field variations change with localtime. The vortical structure has a duration of 3-4 min and propagates with a flow speed of,75% of the solar wind speed. The electric fields associated with flow vortices show abipolar structure. We suggest that the flow vortex in our simulation is associated with themain impulse of SC and that the SC-associated electric fields observed at Akebono are due tothe convection electric field.