COUPLING OF MAGNETOPAUSE-BOUNDARY LAYER TO THE POLAR IONOSPHERE

The ultraviolet auroral images obtained with the Viking satellite often show spatially periodic bright spots which resemble ''beads'' or ''pearls'' aligned on the postnoon auroral oval, which may be due to the coupling of the plasma vortices formed in the boundary layer to the polar ionosphere. Geomagnetic pulsations in the ULF range observed in the cusp region may also be associated with the boundary layer vortices. To explain the observed auroral bright spots and the cusp geomagnetic pulsations, we have developed a model for the plasma dynamics in the low-latitude boundary layer and its interaction with the polar ionosphere via the field-aligned current (FAC). In the presence of a driven plasma flow along the magnetopause, the Kelvin-Helmholtz instability can develop, leading to the formation and growth of plasma vortices in the boundary layer. On the other hand, the finite ionospheric conductivity leads to the decay of these vortices. The competing effect of the formation and decay of vortices leads to the formation of strong vortices only in a limited region. Several enhanced field-aligned power density regions associated with the boundary layer vortices and the upward FAC filaments can be found along the postnoon auroral oval. These enhanced field-aligned power density regions may account for the observed auroral bright spots. In addition, we found that the FACs are stronger in the prenoon sector than in the postnoon sector due to the presence of the field-aligned potential drop in the postnoon sector. The larger prenoon FACs produce stronger cusp pulsations in the prenoon sector, consistent with observations. The frequency spectrum and polarization of the generated pulsations are also discussed. On the other hand, the vortices in the postnoon boundary layer are found to be stronger than those in the prenoon boundary layer.