7.3 Changes in the cold plasmasphere

The plasmasphere encircling the Earth is formed by cold ionospheric plasma flowing outward along magnetic field lines. At low latitudes, close to the Earth, magnetic flux tubes follow drift paths that co-rotate with the Earth and therefore are filled with escaping ionospheric plasma in time scales of several days. The location of the outer boundary of the plasmasphere, the plasmapause, is controlled by the relative intensities of the solar wind-imposed electric field and the co-rotation electric field. Although roughly circular in shape, the plasmapause often shows an elongation in the duskside, following the general electric field pattern (Lyons and Williams, 1984). Statistical studies have shown that the position of the plasmapause is correlated with geomagnetic activity, being at smaller radial distances during higher levels of activity (Moldwin et al., 2003). This can be understood by the enhanced convection electric field moving the boundary between the convection-dominated outer region and the co-rotation-dominated inner region closer to the Earth. During magnetic quiescence, the plasmasphere is expanded and the plasmapause can be located outside geostationary orbit. Particularly during low magnetic activity conditions, there is significant variability in the plasmapause location.

During storms, the plasmapause moves inward due to the enhanced solar wind driving, while a drainage plume develops in the dusk sector (Elphic et al., 1996). Within this plume, the cold plasma flows outward toward the magnetopause thus escaping from the plasmasphere. In the nightside, the inward motion of the plasmapause is of the order of 0.5 RE/h, with the changes following the interplanetary magnetic field variations with 20 – 30 min delay (Spasojević et al., 2003). After the field lines reconnect at the magnetopause, they convect over the polar cap toward the magnetotail, thus providing an additional source of plasma to the tail plasma sheet during storms (Elphic et al., 1997). As the storm driving subsides, the plasmasphere slowly recovers its quiet-time size. The recovery time scale is associated both with the recovery of the quiet-time electric field structure and with the outflow time scale of the cold plasma from the ionosphere. As discussed below, the location of the plasmapause during storms has been shown to be a significant factor in determining the fate of the relativistic electron population in the outer van Allen belt.

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