Responses of outer radiation belt electron phase space densities to geomagnetic storms: A statistical analysis based on Van Allen Probes observations

Using Van Allen Probes observations spanning September 2012 to June 2019, we statistically investigate responses of electron phase space densities (PSDs) to 131 isolate storms in the Earth’s outer radiation belt. Electron PSDs for μ = 50-5000 MeV/G and K = 0.11 G^1/2R_E are calculated to evaluate three distinct responses (i.e., enhancement, depletion and no change), showing strong dependences on μ, L* and storm magnitude. Seed population is dominant by enhancement- and no change-type events, while relativistic and ultrarelativistic populations exhibit dynamical evolutions regardless of storm level. As μ increases, enhancement-type events decrease and tend to higher L*, while depletion-type events are increased for relativistic and ultrarelativistic populations. Comparing with small storms, large storms tangibly increase enhancement-type events at broader L* resulting in peak occurrences of relativistic population in the heart of the Earth’s other radiation belt. In contrast, large storms are likely decreasing ~20% depletion-type events for relativistic population and ~10% for ultrarelativistic population, as well as occurring at lower L*. No change-type events are primarily concentrated on inner part of the Earth’s other radiation belt, the L* coverages of which are sensitive to storm magnitude, especially for relativistic and ultrarelativistic populations. We also suggested that large storms are potentially accompanied by more intense solar and geomagnetic activities than small storms. While solar wind speed performs similarly for both storm levels and exhibits μ-dependent variations. Our results improve the current studies of storm-time electron PSD responses, thus providing a more comprehensive investigation to in-depth understanding dynamics of the radiation belt electrons during different storm levels.