Global MHD simulation of the geomagnetic sudden commencement on 21 October 1999

Abstract

Recently, Shinbori et al. (2004) examined the electric field variations associated with geomagnetic sudden commencements (SC) by using data from the Akebono satellite in the inner magnetosphere (L < 5) and reported the following characteristics of the SC‐associated electric field variations. (1) The electric field shows a bipolar change. (2) The initial excursion of the electric field tends to be directed westward. (3) The electric field amplitude does not show a dependence on magnetic local time. By using a global three‐dimensional MHD simulation model, we examine how and where such SC‐associated electric field variations are established. In our study, we used the SC event that occurred on 21 October 1999, 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 the Wind satellite near GSM (x, y, z) ∼ (22, −62, 20) RE are used as the simulation input parameters. The numerical simulation shows that inward flow is first excited near local noon and then flow vortex is generated near the flankside as the solar wind discontinuity is passing over the magnetosphere. Thus, the convection electric field variations change with local time. 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 a bipolar structure. We suggest that the flow vortex in our simulation is associated with the main impulse of SC and that the SC‐associated electric fields observed at Akebono are due to the convection electric field.