Study of the thermal plasma on closed field lines outside the plasmasphere

Abstract

Using OGO 4 ion composition measurements near the midlatitude light ion trough below 1000 km, a model for the plasma state along the magnetic field lines is developed which produces, in the equatorial plane, the sharp density gradient characteristic of the plasmapause. In this model the field aligned plasmasphere boundary is a boundary across which the characteristics of the proton flux along the field lines change. Within the plasmasphere the proton fluxes along the field lines are of small or vanishing magnitude and may be directed either away from or towards the Earth. By contrast, the small equatorial ion densities (∼1/cc) known to exist on closed field lines outside the plasmasphere are consistent with the measured low altitude densities only if large proton fluxes, of the order of the critical flux, are directed along the field lines. These latter fluxes are directed away from the Earth and correspond to the attainment of supersonic proton flow velocities on closed field lines outside the plasmasphere. This is consistent with the model of a supersonic polar wind convecting onto closed field lines.