Effects on the plasmasphere of a time-varying convection electric field

Abstract

The time evolution of the plasmasphere has been investigated theoretically, using simple computational models. The magnetic field is assumed to be dipolar and time-independent, but the convection electric field is allowed to vary in time. For purposes of comparison, various spatial distributions of the magnetospheric electric field are considered. Plasmasphere flux tubes are assumed to be filled by diffusion of plasma upwards from the dayside ionosphere. Following a reduction in the convection field, the bulge of the original plasmasphere develops into a long tail that gradually wraps itself around the main plasmasphere. Periodic gusts in a spatially uniform convection field produce extremely complicated fine structure that depends strongly on both local time and universal time. Each large gust produces a distinct tail of cold plasma that stretches from the main body of the plasmasphere to the magnetopause, and causes a peak in density, outside the main plasmapause; similar features have been observed by OGO satellites. The calculations indicate that a periodic gusty field has a major effect on the size of the plasmasphere if the field has large Fourier components close to the drift period of cold plasma near the plasmapause. Gusts occurring randomly, at an average rate of several a day, can also cause substantial reduction in the size of the plasmasphere. The assumption that the convection field is spatially uniform, but gusty, leads to better agreement with the observed average shape of the plasmasphere than the assumption of a constant, uniform electric field. The theory indicates that the thickness of the plasmasphere boundary should be inversely correlated with magnetic activity, in general agreement with OGO 5 observations.