Perpendicular ion heating effects on the refilling of the outer plasmasphere

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Abstract
The trapping and the thermalization of the outflowing ionospheric plasma in the outer plasmaspheric flux tubes during refilling after geomagnetic storms involves microscopic processes, which in the initial stage of the refilling are essentially collisionless. Although there are several observations on upflowing ions with anisotropic pitch angle distributions at superthermal energies in the region of the refilling, the role of such heated ions in the refilling has not been examined. It is shown here that an extended perpendicular ion heating by a low‐level background plasma noise near the ion cyclotron frequency with spectral power density ψ ∼ 10−11 V² m−2 Hz−1 can be effective in trapping the bulk of the ions in the flux tube. Such electric field noise levels can be generated by fluctuations associated with the equilibrium near marginal stability of a low‐density plasma mainly consisting of very hot ions originating from the ring current. The heated bulk ions have energies (∼ 1 eV) characteristic of the filled plasmasphere. When the turbulence level exceeds the above level, the heated ions show the features of the ion conies in the superthermal energy range observed along the field lines of refilling. In contrast to the weak extended heating, when the heating is intense and localized in the equatorial region, the trapped ions are shown to set up parallel electric fields pointing away from the equator. The potential drop associated with such fields are found to be large enough to stop interhemispheric flow by reflecting the ionospheric plasma streams.