MAGNETOSPHERIC BOUNDARY DYNAMICS - DE-1 AND DE-2 OBSERVATIONS NEAR THE MAGNETOPAUSE AND CUSP

A broad spectrum of particle and field measurements was taken near local noon by the Dynamics Explorer satellites during the magnetic storm of September 6, 1982. While at apogee, DE 1 sampled the magnetospheric boundary layer at mid southern latitudes and, due to the passage of an intense solar wind burst, briefly penetrated into the magnetosheath. In the boundary layer and the adjacent magnetosheath the plasma flow was directed toward dawn. Variance and de Hoffmann-Teller analyses of electric and magnetic field data during the magnetopause crossing showed the magnetopause structure to be that of a rotational discontinuity or an intermediate shock with a substantial normal magnetic field component. This is consistent with an open magnetosphere model in which significant magnetic merging occurs at the local time of the spacecraft. The orbit of DE 2 carried it through the morning sector of the low-altitude, southern cusp. The measurements show a well-defined, cusp current system occurring on open magnetic field lines. At both cusp and subcusp latitudes the electric field was equatorward indicating a strongly eastward plasma flow. The boundary between these two regions was marked by the onset of magnetosheath precipitation and an electric field structure containing both poleward and equatorward spikes. The poleward spike has associated field-aligned currents which are closed by Pedersen currents and, from force balance considerations, is interpreted as the signature of a magnetic merging event at the magnetopause. The equatorward spike has the characteristics of a down-coming and reflected Alfven wave packet of finite dimensions. The high-altitude measurements suggest that the dayside boundary layer is made up of closed magnetic flux tubes, a large fraction of which drift to the magnetopause where merging with the IMF occurs. The merging line maps to the ionosphere as a "gap" across which the polar cap potential is applied to the magnetosphere. The potential is applied from a magnetosheath generator to the polar ionosphere by means of the cusp, field-aligned current system. The electric fields provide an ionospheric indicator of the mapping of the merging line location.