The Relationship Between Chorus and Enhanced Relativistic Electron Fluxes in Geomagnetic Storms

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, Antarctica (76^oS 27^oW, 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 D st 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^-2s^-1sr^-1keV^-1. A superposed epoch analysis of the whole set, using the times of minimum D st as the set of epochs, reproduced the results of the earlier study, but when the two subsets were analysed 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 post-storm flux was high, consistent with theories that chorus is at least partly responsible for accelerating the electrons. Near the top of the chorus band (2.0 kHz) in the first 12 hours of the recovery phase, the reverse was the case; we explain this in terms of the opposing effects of a chorus source function with a frequency-dependent time constant, and a propagation function representing the wave attenuation between the source region in the magnetosphere and the receiver on the ground. A long (several-day) post-storm depression of wave power at frequencies above the chorus band is attributed to increased precipitation from the radiation belts, affecting absorption of radio atmospherics from tropical thunderstorms propagating to the receiver. The depression was greater for the set of storms characterised by high post-storm fluxes, which strengthens this interpretation. It was accompanied by longer lasting geomagnetic and substorm activity after the storm, as indicated by the K p 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.