Plasma entry across the distant tail magnetopause 2. Comparison between MHD theory and observation

Abstract

A total of 1263 lobe magnetopause crossings were identified utilizing the plasma and magnetic field data obtained by the GEOTAIL spacecraft at large downtail distances (−100 RE > X > −210 RE). On the basis of these crossing events, we compare the observed variations in the plasma and magnetic field parameters with the ones predicted from the MHD framework and study the structure of the tail boundary layer as well as the nature of the solar wind plasma entry. The above crossing events have been classified into three (open, closed, and ambiguous) types based on the jump condition for the MHD discontinuities [Hasegawa et al., 2002]. For the open‐type boundaries, the changes in the plasma flow velocity agree well with the Walén relation, i.e., the tangential stress balance relation expected for a rotational discontinuity (RD), at the external interface of the magnetopause transition region. In the lobe/mantle region inside the interface, however, the velocity changes deviate from the Walén relation and the plasma and field parameters exhibit a smooth variation from the values characteristic to the magnetosheath to those characteristic to the core lobe. This behavior is in good agreement with the prediction that a slow‐mode expansion fan is formed in the tail lobe inside the RD magnetopause. Our comparison suggests that the magnetosheath plasma is decelerated and rarefied mainly across the expansion fan rather than across the RD, as it is transferred to the inner part of the tail lobe.