The plasmapause and its relation to the ion composition in the topside ionosphere

Abstract

The high O +-H + transition level at latitudes above 50° and the plasmapause with its characteristic density plateau suggest that both features are strongly correlated; upward fluxes of protons can consistently account for both observations. It is shown that plasma escape can produce sufficiently large fluxes at latitudes where the field lines are open, thus significantly decreasing the hydrogen-ion density. As a result, plasma diffusion across field lines is induced—coupled with upward fluxes of ionization along field lines, which leads to the formation of a plateau that characterizes the transition region from the open to the closed magnetosphere. Collision-type perpendicular diffusion would produce an enormously rapid transition over a distance of a few meters, which is not observed. This suggests that a turbulent state is built up in the transition region; thus, Bohm's coefficient for turbulent diffusion is employed, and this leads to a quantitatively satisfying description of the plasmapause.