Resonant diffusion of radiation belt energetic electrons by field-aligned propagation whistler-mode chorus waves

Abstract

Adopting the dipole geomagnetic field， Gaussian spectral density for the waves， and semi-empirical latitudinal electron density models obtained from available in situ data， this paper has calculated the local and bounce-averaged quasi-linear resonant electron diffusion coefficients due to chorus and then determined the timescales for electron precipitation loss and stochastic acceleration， in the range of 4≤L≤7 outside the plasmapause. The results indicate that the spatial extent where gyroresonance occurs depends on electron energy， equatorial pitch angle， wave spectrum， and the local electron number density and magnetic field. Besides these five parameters， the actual values of resonant frequency rely on magnetic latitude where resonance occurs. The acceleration of radiation belt energetic electrons occurs predominantly due to equatorial chorus， and the mid-latitude chorus preferably contributes to the precipitation loss of relativistic electrons. The timescales for both electron loss and acceleration due to chorus-driven diffusion have been evaluated to be of hours for lower-energy electrons (about 200keV) and of days for higher-energy electrons (about 1MeV). It is also found that variation of latitudinal density distributions contributes importantly to chorus-driven electron resonant diffusion. In general， an increasing latitudinal electron density increases the loss timescales for untrapped electrons with small equatorial pitch angles， but has negligible effect on the acceleration of trapped electrons with large equatorial pitch angles. The variations of chorus wave amplitude and wave spectrum with magnetic latitude and L-shell also make important contributions to the lifetime and acceleration of radiation belt electrons， which are generally greater than the effects of varying latitudinal distribution of cold plasma density.