A case study of plasma processes in the dayside cleft

Abstract

On November 28, 1983, between 1638 UT and 1642 UT, the HILAT satellite moving approximately along the noon meridian passed over northeastern Canada and encountered the dayside high latitude cleft. During the passage an upward current sheet bounded to the north and south by weaker downward current sheets was observed. The current sheets, particularly the upward current sheet, were immersed in a region of fairly intense low energy electron precipitation of the type commonly observed in the polar cleft. Simultaneous radar measurements were made in a region slightly to the east of the satellite track and in which beacon measurements between HILAT and a ground station at Sondre Stromfjord, Greenland, were being conducted. The radar indicated an extended, magnetic east‐west‐aligned region of small‐scale irregularities at the same magnetic latitude as the cleft‐related current sheets and precipitation. The HILAT beacon experiment showed the strongest scintillations within the same region. On the basis of the available data, it appears as if the convective flows from the dawn and dusk sectors were converging and rotating poleward within the observational volume. The irregularities producing the scintillations and radar backscatter may have been produced by the combined influence of the gradient drift and current convective plasma instabilities, the former process being excited by the poleward convection of plasma parallel to the precipitation‐induced density gradient and the latter process by the upward field‐aligned current. The apparent absence of irregularities further into the polar cap may be accounted for by the stabilizing effect of plasma drift antiparallel to the density gradient on the poleward edge of the cleft precipitation zone.