MAGNETIC STORM EFFECTS ON THE MIDLATITUDE PLASMASPHERE

Whistler mode group delays observed at Faraday, Antarctica (65-degrees-S, 64-degrees-W) decrease after the onset of magnetic storms, and slowly recover to normal levels in 1 or 2 days. This is interpreted as a decrease (typically of approximately 50%) and recovery of the plasmaspheric electron density at L = 2.5. Within 1 day of the main phase of storms with K(p)(max) between 6 and 8, the number of observed whistler ducts increases by a factor of 2 or 3, recovering in a few days. During the most intense storms (K(p) > 8) the duct number decreases. The frequency of occurrence of observed whistler mode signals increases during storms, due probably to enhanced ionospheric propagation of the signals; the storm time dependence implies that there is no link with the apparent increase in duct numbers. The amplitudes of received whistler mode signals are increased by up to a factor of 10 during storms: this is interpreted in terms of magnetospheric amplification through wave-particle interactions, though the evidence suggests that amplification is not necessarily the mechanism by which increased duct numbers are observed. There appears to be a real increase in the duct formation rate, consistent with Walker's (1978) theory in which ring current penetration of the plasmasphere creates a preferential region for duct formation 1.5 R(E) inside the plasmapause.