The Impenetrable Barrier: Suppression of Chorus Wave Growth by VLF Transmitters

Rapid radiation belt recovery following storm time depletion involves local acceleration of multi-MeV electrons in nonlinear interactions with VLF chorus waves. Previous studies of an apparent impenetrable barrier at L similar to 2.8 focused on diffusion and precipitation loss mechanisms for an explanation of the sharp reduction of multi-MeV electron fluxes earthward of L similar to 3. Van Allen Probes observations for cases when the plasmasphere is contracted earthward of L similar to 3 indicate that strong coherent signals from VLF transmitters can play significant roles in the suppression of nonlinear chorus wave growth earthward of L similar to 3. As a result, local nonlinear acceleration of hundreds of keV electrons to MeV energies does not occur in this region. During the recovery of the outer radiation belt when the plasmasphere is significantly contracted, the suppression of chorus wave growth and local acceleration by the action of the transmitter waves at the outer edge of the VLF bubble contributes to the sharp inner edge of the new MeV electron population and the formation of the impenetrable barrier at L similar to 2.8.