Effects of ionospheric conductivity on convective flow of plasma in the magnetosphere

Abstract

Convective flow of plasma in the magnetosphere is apparently driven by the interaction between the solar wind and the magnetosphere, but the flow pattern is regulated by the ionosphere and by pressure gradients in the magnetospheric plasma. The equations for conservation of ionospheric currents are used here to deduce theoretical flow patterns. The currents caused by the pressure of magnetospheric plasma are neglected. When all merging and friction processes are assumed to take place in the tail, and the dayside magnetopause is assumed to be an equipotential, computed trajectories of plasmasphere particles generally exhibit bulges in the dusk-to-midnight sector; if the conductivity model includes a sharp drop in conductivity at sunset, the computed bulge has a sharp onset near local sunset. When convection is assumed to be caused by merging or some other friction process operating near the nose of the magnetosphere, the outer-plasmasphere trajectories have pronounced bulges in the dusk-to-midnight sector, with sharp onset near local sunset, if the conductivity model includes a sharp drop in conductivity at local sunset and a band of substantially enhanced Pedersen and Hall conductivities in the auroral zone. Vasyliunas' observations indicate that the plasma sheet is generally well defined throughout the evening and afternoon sectors of the magnetosphere, to about 1300 LT. Comparing the observed plasma-sheet region with the regions of the computed flow patterns that are accessible to kilovolt electrons from the tail, we find agreement only in cases where the day-night asymmetry is included in the conductivity model, and only when a potential drop ≳35 kv is assumed to exist across the nose region of the magnetosphere. Throughout most of the afternoon and evening sectors, the computed shape of the inner edge of the plasma sheet is insensitive to the assumption about the rate of precipitation. The computed ionospheric current patterns resemble observed currents only if the auroral-zone Pedersen and Hall conductivities are assumed to be enhanced by an order of magnitude or more over the midlatitude nightside conductivities. When peak auroral-zone enhancements of the Pedersen and Hall conductivities are taken to be 3.3 and 10 mhos, respectively, current patterns computed assuming a large voltage drop across the nose of the magnetosphere are roughly consistent with observed DP2 fluctuations; currents computed assuming that all merging activity takes place in the tail then generally resemble DS currents.