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.
