THE MORNINGSIDE LOW-LATITUDE BOUNDARY-LAYER AS DETERMINED FROM ELECTRIC AND MAGNETIC-FIELD MEASUREMENTS ON GEOTAIL

On October 17, 1992, the Geotail satellite crossed the dawnside magnetopause approximately ill times. At the majority of these crossings, the. magnetic field and the normal component of the electric field were larger and the plasma. density was smeller in a low-latitude boundary layer than they were in either the nearby magnetosheath or in the magnetosphere. These results are interpreted in terms of the Kelvin-Helmholtz instability associated with velocity shear at the magnetopause. Consistent with this interpretation, it is shown that the low-latitude boundary layer was the region where the flow decreased from its magnetosheath to magnetospheric value. Evidence is presented that the magnetopause was locally oriented within less than 20 degrees of its nominal geometry on these crossings and that it moved with an amplitude of 1500-6000 kilometers. The thickness of the low-latitude boundary layer averaged 800 kilometers and the electric potential across it averaged 4 kilovolts with a spread of a factor of 2. Thus, the viscous interaction and similar processes are not significant contributors to magnetospheric convection.