Two‐dimensional HaIl‐MHD simulation of current sheet dynamics during substorm growth phase

Abstract

In this paper we study the dynamical evolution of the current sheet in the near‐Earth geomagnetic tail region during the substorm growth phase by means of HaIl‐MHD simulation. Special attention is paid to the development of cross‐tail electric current components attributed to the electrons and ions. It is found that the induced component of the electric field generated during the current sheet thinning process leads to E×B drift of both the electrons and ions toward the dawnward direction, which augments the electron diamagnetic drift but opposes the ion diamagnetic drift. This effect causes the electrons to carry a higher and higher portion of the cross‐tail current as time progresses, even if the bulk of the initial current may be carried by the ions. However, the,increase of electron current strongly depends on the boundary condition and on the location along the distant‐tail/near‐Earth axis. It is found that close to the near‐Earth simulation boundary, a sizable normal magnetic field component Bz persists even in the late phase of the simulation. This limits the north‐south electric field Ez to a relatively low value, and as a result, the cross‐tail current is largely carried by the ions there. However, toward the distant tail, Bz is much weaker, and thus the electron current is seen to increase quite rapidly.