Radial diffusion simulations of the 20 September 2007 radiation belt dropout

Abstract. This is a study of a dropout of radiation belt electrons, associated with an isolated solar wind density pulse on 20 September 2007, as seen by the solid-state telescopes (SST) detectors on THEMIS (Time History of Events and Macroscale Interactions during Substorms). Omnidirectional fluxes were converted to phase space density at constant invariants M = 700 MeV G⁻¹ and K = 0.014 R_E G^1/2, with the assumption of local pitch angle α ≈ 80° and using the T04 magnetic field model. The last closed drift shell, which was calculated throughout the time interval, never came within the simulation outer boundary of L^* = 6. It is found, using several different models for diffusion rates, that radial diffusion alone only allows the data-driven, time-dependent boundary values at L_max = 6 and L_min = 3.7 to propagate a few tenths of an R_E during the simulation; far too slow to account for the dropout observed over the broad range of L^* = 4–5.5. Pitch angle diffusion via resonant interactions with several types of waves (chorus, electromagnetic ion cyclotron waves, and plasmaspheric and plume hiss) also seems problematic, for several reasons which are discussed.