Ionospheric conditions affecting the evolution of equatorial plasma depletions

Abstract

Model studies have been carried out to specify the ionospheric conditions and electrodynamical processes required to describe the geometry of westward tilting and bifurcating equatorial plasma depletions inferred from airglow imaging measurements taken on Ascension Island in early 1981. The results indicate that if the westward tilts are to be associated with eastward plasma drifts that decrease with altitude above the F peak, this shear must be set up by a zonal wind pattern of decreasing speed at increasing distances from the equator. Below the peak, calculations of flux tube integrated Pedersen conductivity, including both E and F region contributions, are in agreement with the simulations of Zalesak et al. (1982) that show bifurcated depletions to be associated with a mild gradient in the bottomside conductivity profile.