Differences in ground‐observed chorus in geomagnetic storms with and without enhanced relativistic electron fluxes

Abstract

It has been suggested that whistler mode chorus waves play a role in the acceleration and loss of radiation belt electrons during geomagnetic storms. In a previous statistical study of chorus received at Halley station, Antarctica (76°S 27°W, L = 4.3), during storms of the solar cycle 1992–2002, we found that on average, chorus intensities were significantly enhanced in the storm recovery phase. In this paper we extend that study to provide stronger evidence of the link between chorus and electron acceleration. We selected a set of 244 storms in 1992–2002 having a minimum Dst less than −50 nT, for which average 1.8–3.5 MeV electron fluxes 2–3 days after the storm were available from the LANL satellites. This set was classified into two subsets according to whether the flux was less than or greater than an arbitrary threshold of 0.5 electrons cm−2 s−1 sr−1 keV−1, near to the median value. A superposed epoch analysis of the whole set, using the times of minimum Dst as the set of epochs, reproduced the results of the earlier study, but when the two subsets were analyzed separately, it was found that for the lower‐frequency channels, 0.5–1.0 kHz, characteristic of the main chorus band, the average intensities of chorus observed were larger for storms when the poststorm flux was high. The long‐enduring (several days) poststorm depression of wave power at frequencies above the chorus band, found in the previous study and attributed to increased precipitation from the radiation belts, was again noted. The depression was greater for the set of storms characterized by high poststorm fluxes, which strengthens this interpretation. It was accompanied by longer‐lasting geomagnetic and substorm activity after the storm, as indicated by the Kp and AE indices, consistent with recent suggestions that relativistic electron acceleration is more likely to occur in storms with substantial substorm activity in the recovery phase.