Relativistic turning acceleration of radiation belt electrons by whistler mode chorus

Abstract

We perform test particle simulations assuming whistler mode chorus wave packets that are generated at the geomagnetic equator propagate away from the equator in both poleward directions. While electrons in the energy range 10–100 keV are primarily responsible for the generation of chorus waves through pitch angle diffusion into the loss cone, it has been found that a fraction of the higher‐energy electrons from a few hundred keV to a few MeV are effectively accelerated by chorus due to a special nonlinear trapping process called relativistic turning acceleration (RTA). This mechanism has been recently demonstrated for a coherent whistler mode wave packet with a constant amplitude and constant frequency. In the present study we confirm that the RTA process takes place for a wave packet with variable frequency such as that occurring in a rising tone of chorus emissions. We study the efficiency of the RTA process for different particle energies. A Green's function method is used to describe the evolution of the particle energy distribution function. The RTA process due to chorus emissions creates a high‐energy tail in the electron energy distribution function. The shape of the high‐energy tail is determined by the distribution function of the seed electrons in the lower‐energy range. RTA can accelerate electrons in a much shorter timescale than that estimated by quasi‐linear diffusion theory, e.g., it typically takes tens of minutes to hours for ∼100 keV seed electrons to be accelerated to energies of a few MeV by RTA.