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

Fifteen months of pitch angle resolved Van Allen Probes REPT measurements of differential electron flux are analyzed to investigate the characteristic variability of the pitch angle distribution (PAD) of radiation belt ultra-relativistic (>2 MeV) electrons during storm conditions and during the long-term post-storm decay. By modeling the ultra-relativistic electron pitch angle distribution as sinn α, where α is the equatorial pitch angle, we examine the spatio-temporal variations of the n-value. The results show that in general n-values increase with the level of geomagnetic activity. In principle, ultra-relativistic electrons respond to geomagnetic storms by becoming more peaked at 90\textdegree pitch angle with n-values of 2\textendash3 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 EMIC waves scattering. During quiet periods, n-values generally evolve to become small, i.e., 0\textendash1. The slow and long-term decays of the ultra-relativistic electrons after geomagnetic storms, while prominent, produce energy and L-shell dependent decay timescales in association with the solar and geomagnetic activity and wave-particle interaction processes. At lower L shells inside the plasmasphere, the decay timescales τd 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 ~ 3.5, a narrow region exists (with a width of ~0.5 L) where the observed ultra-relativistic electrons decay fastest, possibly resulting from efficient EMIC wave scattering. As L shell continues to increase, τd generally becomes larger again, indicating an overall slower loss process by waves at high L shells. Our investigation based upon the sinn α function fitting and the estimate of decay timescale offers a convenient and useful means to evaluate the underlying physical processes that play a role in driving the acceleration and loss of ultra-relativistic electrons and to assess their relative contributions.