AMPTE/IRM observations of the MHD structure of the plasmasheet boundary: Evidence for a normal component of the magnetic field

We present preliminary results of a study of the MHD structure of the plasmasheet boundary in the near tail, utilizing plasma and magnetic field data from the AMPTE/IRM spacecraft. To understand the MHD structure of the boundary, it is first necessary to determine whether there is a normal component of the magnetic field. Three methods were utilized herein: (1) the tangential momentum balance relationship for a discontinuity with a nonzero B_n (2) examination of the 3d electron distributions; and (3) direct determination from the measured magnetic field for various calculated normals. The first two do not depend on the calculated normal to the boundary, while the third does. Statistical results on tangential momentum relation for ~80 crossings are presented. The statistical study shows that the measured change in the tangential velocity is almost always less than that predicted by the Rankine-Hugonoit relations for a boundary with non-zero B_n. 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. For selected events, Bn has been directly calculated and the 3d electron distribution functions have been examined for evidence of a normal magnetic field component as discussed by Feldman et al. [1984]. The expected signature of tailward field aligned electron beams was found both during events for which the measured change in the tangential velocity was close to that predicted and during cases for which the measured change was much smaller than the predicted value. The explanation may be that there are other mechanisms which produce tailward, field-aligned electron beams in the plasmasheet boundary. 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.