OBSERVATIONS OF A TRANSVERSE MAGNETIC-FIELD PERTURBATION AT 2 ALTITUDES ON THE EQUATORWARD EDGE OF THE MAGNETOSPHERIC CUSP

On January 28, 1990, the Dynamics Explorer 1 and Akebono satellites cr ossed a magnetic field structure at the equatorward edge of the polar cusp at altitudes of 22,000 and 5000 km, respectively, within 6 min of each other. Locally measured plasma particles and fields and magnetom eter data from a ground station near the foot of the magnetic field li ne are more consistent with an interpretation of the structure as that of a standing Alfven wave than that of a quasi-steady field-aligned c urrent sheet. We discuss the observations supporting this conclusion a nd other related observations of field-aligned currents, Alfven waves, and ion energization near the equatorward edge of the cusp. These obs ervations suggest that Alfven waves are commonly present near the equa torward edge of the cusp.