Nonstorm-time acceleration and transport of radiation belt electrons: Physical mechanisms and favored conditions

Abstract:

The radiation belt electron dynamics during geomagnetic storms have been widely investigated for many years, and the strength of a geomagnetic storm has been suggested to be a poor indicator of the net change in the radiation belt relativistic electron fluxes. Here we report several events of the new electron radiation belt formation observed by the Van Allen Probes during nonstorm times. Before these events, the outer radiation belt relativistic electron fluxes peaked in the spatial region L=3.5-4.5. During these events, a new electron radiation belt gradually emerged in the spatial region L=5.0-6.0, and the corresponding relativistic electron fluxes increased by several orders of magnitude on a timescale of hours to days. By analyzing the high-resolution data and performing the detailed simulation, we identify the physical mechanisms and the favored conditions for these nonstorm-time radiation belt events. Depending on the specific interplanetary and magnetospheric conditions, the dominant mechanism could be the local acceleration driven by the whistler-mode chorus waves, the radial diffusion driven by the ultra-low-frequency waves, or the adiabatic transport associated with the magnetospheric shrinkage. The intermittent southward interplanetary magnetic field favored the occurrence of substorms. The prolonged substorms injected plenty of hot electrons into the inner magnetosphere, allowing the excitation of whistler-mode chorus waves and then the local acceleration of the accumulated seed electrons. In the absence of significant substorm activities, the solar wind dynamic pressure fluctuations enhanced the magnetospheric ultra-low-frequency waves and then caused the inward radial diffusion and acceleration of the radiation belt electrons. The magnetospheric shrinkage by the enhanced solar wind compression produced the adiabatic enhancement of the radiation belt electron fluxes. These results emphasize the complexity of the radiation belt dynamics and the importance of monitoring the radiation belt environment in nonstorm times.