Storm Responses of Radiation Belts During Solar Cycle 23: HEO Satellite Observations

We examined the energetic electron responses to storms with D-st <= -75 nT that occurred in the 1998-2008 period deep in the inner magnetosphere 2 <= L <= 4 using HEO3 data. These observations cover the peak of solar cycle 23. We show that the observed poststorm >1.5 MeV electron flux decay rates for L similar to 3 had multiple time scales. Early in the recovery phase, the distribution of e-folding decay time scales had two peaks near 6 and 14 days, while late recovery decay rates had a peak near 20 days. Examination of the times from Dst minimum to the peak of enhanced electron fluxes at L similar to 3 showed that seed population electrons (>230 keV) had a mean e-folding rise time of similar to 0.9 days, while for relativistic electrons (>1.5 MeV), it was similar to 3.3 days. Examination of the poststorm electron flux responses in the 2.5 < 4 showed that (1) there were essentially no poststorm electron flux decreases observed in the slot region and that occurrence of flux increases rose with decreasing Dst, (2) poststorm electron fluxes at L = 3 showed decreases only for events with minimum Dst > -175, and (3) at L = 4 statistics of events that caused flux increases versus no change or flux decreases approached those of earlier results.