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

Abstract:

Fifteen month of pitch angle resolved Van Allen Probes Relativistic Electron-Proton Telescope (REPT) measurements of differential electron flux are analyzed to investigate the characteristics of the pitch angle distribution of radiation belt ultrarelativistic(> 2 MeV) electrons during storm conditions and during the long-storm decay. By modeling the ultrarelativistic electron pitch angle distribution as ,where is the equatorial pitch angle， we examine the spatiotemporal variations of n value. The results show that， in general， n values increases with the level of geomagnetic activity. In principle the ultrarelativistic electrons respond to geomagnetic storms by becoming peaked at 90° pitch angle with n-values of 2 - 3 as a supportive signature of chorus acceleration outside the plasmasphere. High n-values also exists inside the plasmasphere, being localized adjacent to the plasmapause and energy dependent, which suggests a significant contribution from electronmagnetic ion cyclotron (EMIC) waves 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 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 ultrarelativistic electrons decay fastest, possibly resulting from efficient EMIC wave scattering. As L shell continues to increase, generally becomes larger again, indicating an overall slower loss process by waves at high L shells. Our investigation based upon the 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.