SMALL-SCALE ELECTRODYNAMICS OF THE CUSP WITH NORTHWARD INTERPLANETARY MAGNETIC-FIELD

We report on possible, low-altitude field signatures of merging occurring at high-latitudes during a period of strong, northward directed interplanetary magnetic field. Our study of the small-scale electrodynamics in a vicinity of the poleward cusp boundary is based on multisensor measurements from a single DE 2 satellite pass over northern hemisphere cusp/cleft regions. Observed sunward convection over the polar cap and presence of the reverse ion dispersion feature are in agreement with expectations for merging occurring at the magnetopause, on the field lines mapping to the poleward boundary of the cusp. Large electric and magnetic field spikes detected at the poleward edge of the magnetosheathlike particle precipitation are interpreted as field signatures of the low-altitude footprint of such merging line locations. A train of phase-shifted, almost linearly polarized electric and magnetic field fluctuations was detected just equatorward of the large electromagnetic spike. These may be due to either ion cyclotron waves excited by penetrating magnetosheath ions or transient oscillations in the frame of convecting plasma, brought about by the sudden change in the flow at the magnetospheric end of the field line.