Satellite in situ measurements made by the Dynamics Explorer 2 (DE 2) satellite were utilized to describe the nature of plasma structuring at high latitudes caused by the gradient drift instability process. Specifically, by using noon-midnight and dawn-dusk orbits of the DE 2 satellite it was found possible 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 (ΔN/N)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 ΔE was found to be less than a few millivolts per meter for both patches and blobs. This (ΔN/N)RMS vis-a-vis ΔE behavior for the gradient drift process provided a fairly dramatic contrast with velocity shear driven processes where the ΔE magnitudes were found to be at least an order of magnitude larger for the same levels of density irregularities. The electric field spectra for the moderate shear category discussed by Basu et al. (1988a) were also found to have a significantly different spectral index as compared to such spectra associated with the gradient drift process. The results of this paper together with those of Basu et al. (1988a) provide fairly conclusive evidence for the existence of at least two generic classes of instabilities operating in the high-latitude ionosphere: one driven by large-scale density gradients in a homogeneous convection field with respect to the neutrals and the other driven by the structured convection field itself in an ambient ionosphere where density fluctuations are ubiquitous. ¿ American Geophysical Union 1990