Combined radial diffusion and adiabatic transport of radiation belt electrons with arbitrary pitch angles

Storm-time radial diffusion of radiation belt electrons with arbitrary pitch angles in a time-varying geomagnetic field is simulated based on our recently developed STEERB code. In particular, the fully adiabatic response of energetic electrons to the variation of geomagnetic field is self-consistently incorporated. Simulation results show that the outward adiabatic transport (instead of outward radial diffusion) is primarily responsible for the main phase depletion of energetic electron fluxes at large pitch angles beyond 5R(e) (R-e is the Earth's radius). However, combined radial diffusion and adiabatic transport contributes insignificantly to the main phase depletion of energetic electron fluxes within 5R(e), or the recovery phase enhancement of energetic electron fluxes in the outer radiation belt. Moreover, the simulation with both radial diffusion and adiabatic transport shows that the pitch angle distribution of energetic outer zone electrons can evolve from a rounded 90 degrees-peaked distribution to a butterfly-shaped distribution during the main phase, and back to a rounded 90 degrees-peaked distribution during the recovery phase. Such essential changes of pitch angle distribution may further affect the efficiency of other local loss and energization mechanisms.