Whistler observations of the depletion of the plasmasphere during a magnetospheric substorm

Whistlers recorded in Antarctica and North America were used to monitor changes in electron concentrations in the dayside plasmasphere during isolated substorm activity. The ‘viewing area’ of the whistler receiver network extended over L ∼ 2 to 6 and ∼6 hours in geomagnetic time. Before the substorm, the plasmasphere was relatively full and smooth, and there was no evidence of large longitudinal variations in plasma distribution. Immediately after the substorm onset, parts of the plasmasphere were depleted rapidly and produced large-scale structures with sharp longitudinal gradients in plasma density. The depletion of the plasmaspheric tubes is interpreted in terms of downward flow of plasma into the underlying ionosphere, and fluxes of the order of 109 el/cm² sec across the 1000 km level are inferred. Simultaneous ionosonde records show enhanced critical frequencies in the underlying F2 layer. These results indicate that significant amounts of plasma in the plasmasphere are dumped into the ionosphere during geomagnetic disturbances and that this is an important process in storm phenomena in both regions.