Variability of the pitch angle distribution of radiation belt ultrarelativistic electrons during and following intense geomagnetic storms: Van Allen Probes observations

Fifteen months of pitch angle resolved Van Allen Probes Relativistic Electron-Proton Telescope (REPT) measurements of differential electron flux are analyzed to investigate the characteristic variability of the pitch angle distribution of radiation belt ultrarelativistic (>2MeV) electrons during storm conditions and during the long-term poststorm decay. By modeling the ultrarelativistic electron pitch angle distribution as sin(n)alpha, where alpha is the equatorial pitch angle, we examine the spatiotemporal variations of the n value. The results show that, in general, n values increase with the level of geomagnetic activity. In principle, ultrarelativistic electrons respond to geomagnetic storms by becoming more peaked at 90 degrees pitch angle with n values of 2-3 as a supportive signature of chorus acceleration outside the plasmasphere. High n values also exist inside the plasmasphere, being localized adjacent to the plasmapause and exhibiting energy dependence, which suggests a significant contribution from electromagnetic ion cyclotron (EMIC) wave scattering. During quiet periods, n values generally evolve to become small, i.e., 0-1. The slow and long-term decays of the ultrarelativistic electrons after geomagnetic storms, while prominent, produce energy and L-shell-dependent decay time scales in association with the solar and geomagnetic activity and wave-particle interaction processes. At lower L shells inside the plasmasphere, the decay time scales tau(d) for electrons at REPT energies are generally larger, varying from tens of days to hundreds of days, which can be mainly attributed to the combined effect of hiss-induced pitch angle scattering and inward radial diffusion. As L shell increases to L similar to 3.5, a narrow region exists (with a width of similar to 0.5L), where the observed ultrarelativistic electrons decay fastest, possibly resulting from efficient EMIC wave scattering. As L shell continues to increase, tau(d) generally becomes larger again, indicating an overall slower loss process by waves at high L shells. Our investigation based upon the sin(n)alpha function fitting and the estimate of decay time scale offers a convenient and useful means to evaluate the underlying physical processes that play a role in driving the acceleration and loss of ultrarelativistic electrons and to assess their relative contributions.