The global statistical response of the outer radiation belt and inner magnetosphere during geomagnetic storms

Despite recent advances in understanding the nature of competing storm-time electron loss and acceleration processes the dynamics of the outer radiation belt during storms remains poorly understood. Previous studies have shown the outer radiation belt can exhibit either no change, an enhancement, or depletion in radiation belt electrons during storms. Analyzing the total radiation belt electron content, calculated from the Van Allen probes phase space density (PSD), we investigate the time-dependent and global response of the outer radiation belt during 80+ storms. We demonstrate that by removing adiabatic effects there is a clear and repeatable sequence of events in storm-time radiation belt electron dynamics. Namely, the relativistic (μ=1000 MeV/G) and ultra-relativistic (μ=4000 MeV/G) electron populations can be separated into two phases; an initial phase dominated by loss followed by a second phase dominated by acceleration. At lower energies, the radiation belt seed population of electrons (μ=150 MeV/G) shows no evidence of loss but rather a net enhancement during storms. Furthermore, we investigate the dynamics of the physical processes which can drive these sequential periods of net electron loss and acceleration at relativistic and ultra-relativistic electron energies. Key indicators include EMIC, ULF, and VLF wave amplitudes, the location of the plasmapause and magnetopause, and substorm activity. This investigation demonstrates two key aspects of storm-time radiation belt electron dynamics. First, the radiation belt responds in a repeatable manner to both solar wind and magnetospheric driving during geomagnetic storms and follows a clear sequence of events. Second, the response of the radiation belt is energy dependent, relativistic electrons behaving differently than lower energy seed electrons. These results have important implications for radiation belt research; the repeatability in electron dynamics coupled with observations of processes leading to electron loss (EMIC waves) and acceleration (VLF or ULF waves) can be used to quantify the relative importance of physical processes in radiation belt dynamics during storms.