Why do Some Storms Decrease the Flux of Relativisitic Electrons in the Radiation Belts ?

An analysis by Reeves et al.(2003) of 276 geomagnetic storms during the period 1989 to 2000 indicates that while about one half of the storms resulted in an increase in the relativistic electron flux in the radiation belts, about one quarter actually decreased the fluxes. In this paper, we demonstrate quantitatively that whether or not storms produce an increase or decrease of relativistic electrons in the inner magnetosphere depends on the competition between the physical processes producing the energization and loss of electrons. We construct a model for the electron energy distribution function incorporating electron energization by cyclotron resonant interaction with whistler-mode chorus, and electron losses due to pitch-angle scattering into the loss cone by combined plasma waves (in particular EMIC waves and plasmaspheric hiss). We then carry out numerical experiments, treating the chorus wave amplitude and electron loss rate as model input variables, and computing the solution for the electron energy distribution function (at a specified time after the storm) as the model output. We find that the extent of the electron flux increase or decrease over the course of the storm is controlled by the magnitudes of the wave amplitude and the particle loss rate. For instance, the results show that if the timescale for particle loss is several days, then a wave amplitude of 10 pT is sufficient to generate a relativistic electron flux increase, while even wave amplitudes as high as 100 pT, if accompanied by a particle loss rate near to that corresponding to strong diffusion, would result in a flux decrease. This study re-enforces the assertion that in order to model the relativistic electron flux variations in the radiation belts over the course of any given geomagnetic storm, a proper accounting must be made of particle losses.