THE MHD STRUCTURE OF THE PLASMASHEET BOUNDARY .1. TANGENTIAL MOMENTUM BALANCE AND CONSISTENCY WITH SLOW MODE SHOCKS

Differences between observations in the near and distant tail and MHD simulations have motivated a statistical study of the MHD structure of the plasmasheet boundary in the near tail, utilizing data from the AMPTE/IRM spacecraft. The relationship between the change in the tangential velocity predicted by the Rankine-Hugoniot relations for a discontinuity with a normal magnetic field and the measured change in the tangential velocity for approximately 80 crossings is presented. The measured change is almost always much less than the predicted value. This suggests that either: (1) There is not usually a normal component of the magnetic field across the boundary; combined with previous work on pressure balance, this implies that the boundary is a tangential discontinuity; or (2) The boundary is not usually well-modeled as a planar, time-stationary MHD discontinuity. This is very different from the case of the deep tail where the plasmasheet boundary is usually a slow mode shock tailward of the neutral line. It agrees with MHD simulations, including the effects of the ionosphere, which do not have slow mode shocks earthward of the neutral line. These results suggest that the magnetic connection to the earth inhibits the formation of slow mode shocks. There is a small subset of the crossings which are consistent with the identification of part of the plasmasheet boundary as a slow mode shock. These may be cases where the substorm neutral line formed in close proximity to the satellite.