On the production of a magnetic-field-aligned electric field by the interaction between the hot magnetospheric plasma and the cold ionosphere

The interpretation of recent observations of field-aligned ions with energies in the keV range in terms of an electric field parallel to the geomagnetic field lines between the hot plasma from the plasma sheet, in its horns going to the atmosphere, and the cold ionospheric plasma is discussed. Some quantitative results, based on the assumption that some kind of diffusion dominates the propagation through the horns of the plasma sheet, show that magnetic-field-aligned electric fields may sometimes exist and that the total potential difference between the ionosphere and the plasma sheet may even exceed 10 kV. The effects on the voltage difference of the detailed shapes of electron density and equivalent temperature distributions between the ionosphere and the plasma sheet are discussed, as well as the dependence of the electric field on various other parameters.