Variability of energetic proton flux and pitch angle distributions in the Earth's radiation belt modulated by geomagnetic storms.

This article focuses on the statistical analysis of energetic proton fluxes and their pitch angle distributions (PADs) in Earth's radiation belt in response to 110 geomagnetic storms from 2012 to 2017, using data from the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) onboard the Van Allen Probes. It finds that low-energy protons (55–100 keV) near 90° pitch angles are more readily accelerated during storms, producing pronounced pancake PADs, while higher-energy protons (>400 keV) require more intense storms for acceleration, with enhanced PADs persisting for extended periods. Approximately one-quarter of storms increase proton flux across all energies both inside and outside the plasmapause location (Lpp), with significant day–night asymmetries in PADs linked to spatial location and magnetic local time sectors. The study attributes these variations to competing source and loss processes, including radial diffusion, magnetopause shadowing, and wave–particle interactions, providing quantitative insights into radiation belt proton dynamics.