Plasma Structuring by the Gradient Drift Instability at High Latitudes andComparison with Velocity Shear Driven Processes

摘要

Satellite in-situ measurements made by the Dynamics Explorer 2 satellite wereutilized to describe the nature of plasma structuring at high latitudes caused by the gradient-drift instability process. The non-midnight and dawn-dusk orbits of the DE 2 satellite were used to study the simultaneous density and electric field spectra of convecting large scale (approximately hundreds of kilometers) plasma density enhancements in the polar cap (known as patches) in directions parallel and perpendicular to their antisunward convection. Distinct differences were noted in the behavior of the ac and dc electric field structure and short scale (<125 m) density irregularities in these two mutually orthogonal directions perpendicular to the geomagnetic field. However, since these two orthogonal directions were not sampled simultaneously, the observed differences cannot be unequivocally related to the direction of convection. Structured plasma density enhancements in the auroral oval (known as blobs) were found to have considerable power spectral density at these short scales in the presence of significant Pedersen and Hall conductances in the 10 to 20-mho range. While density irregularity amplitudes (DeltaN/N)sub RMS were found to be as large as 15-20% using 8-s samples of the DE 2 data, the corresponding dc electric field fluctuation DeltaE was found to be less than a few millivolts for both patches and blobs. This (DeltaN/N)sub RMS vis-a-vis DeltaE behavior for the gradient drift process provided a fairly dramatic contrast with velocity shear driven processes where the DeltaE magnitudes were found to be at least an order of magnitude larger for the same levels of density irregularities. Reprints. (jhd)