Geotail observations of magnetosheath flow near the magnetopause, using wind as a solar wind monitor

Abstract

We have examined several Geotail passes through the magnetosheath region downstream of the Earth, when the solar wind conditions as observed by the Wind spacecraft were steady. We have found that near the magnetopause the bulk speed of the magnetosheath plasma is strongly dependent upon the local direction of the magnetic field with respect to the local velocity vector. In one set of observations, the observed magnetosheath speed is slowest when the magnetosheath magnetic field and velocity vectors are aligned (either parallel or antiparallel), and the speed is larger when the magnetic field is oriented perpendicular to the magnetosheath velocity. However, the speed of the magnetosheath plasma is not observed to exceed that of the solar wind plasma. The enhanced speed for perpendicular flows is due to acceleration of the plasma by means of magnetic field line tension, as the magnetosheath plasma is dragged around the magnetosphere. The maximum enhancement in magnetosheath plasma speed can be quantitatively estimated by a consideration of the intermediate mode of the magnetohydrodynamic (MHD) relations. A simple model for the velocity enhancement is derived, and we find extremely good agreement between the Geotail magnetosheath observations and this model. The second set of observations are similar to the first, except that when the magnetosheath velocity and magnetic field vectors are parallel to one another, the magnetosheath flow is higher than when the vectors are antiparallel or perpendicular to each other, and is even larger than the solar wind speed. This behavior cannot be explained by nonresistive ideal MHD theory, and other explanations are investigated.