Geotail observations of magnetosheath flow properties, with simultaneous observations of the solar wind by the wind spacecraft

Observations from the GEOTAIL spacecraft are used to examine the plasma flow and magnetic field within the magnetosheath at intermediate downtail distances. Simultaneous measurements of the solar wind condition as measured by the WIND spacecraft are also used for this study. Close to the downtail magnetopause, we find that under appropriate solar wind conditions, the plasma flow velocity of the magnetosheath is slowest when the local magnetic field and velocity vectors are aligned, but are largest when the two vectors are perpendicular to one another, though the magnetosheath flow speed remains slower than or equal to the solar wind velocity. A simple model utilizing the magnetic field line tension and Alfven phase speed is used to explain the observations. Occasionally, close to the magnetopause we find magnetosheath flows which are much greater than the solar wind velocity, but only for certain orientations of the local magnetic field. The source of these fast plasma flows is the low latitude boundary layer (LLBL), and is not the shocked solar wind plasma. Possible reasons for the observation of very fast flows near the magnetotail magnetopause are explored. (C) 1997 COSPAR. Published by Elsevier Science Ltd.