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

Energetic protons trapped in the radiation belt, as a vital component of the ring current system, are observationally and theoretically modulated by geomagnetic disturbances. Utilizing Van Allen Probe observations, we statistically analyzed their temporal variations at 55–489 keV as well as their pitch angle distributions (PADs) index n (fitted by sinn∂, where ∂ is the pitch angle) in response to geomagnetic storms. It shows that protons at low energies are more easily accelerated during storms. The threshold of accelerations becomes greater for high-energy protons, while a large value of n can persist for a few days to months. Further investigations suggest that one-quarter of the storms increase the proton flux at all energy channels (55–489 keV) both inside and outside the plasmapause location (Lpp). Specifically, more than half of the storms enhance the flux for protons at Ek > 400 keV inside and close to the Lpp as well as protons at Ek < 100 keV deep inside the Lpp. Comparably, protons at larger pitch angles (near 90°) are more easily lost outside the Lpp, which results in more pronounced pancake PADs with larger n. The index n preferentially decreases at L > 5 during 75% of the storms on the dayside, while it decreases at L = ∼4 during 50% of the storms on the nightside, showing significant day–night asymmetry. Further detailed investigations revealed that source and loss processes, including radial diffusion, magnetopause shadowing, and wave–particle interactions, account for the statistical results. The present study provides quantitative information on the overall characteristics of energetic proton fluxes, which can enhance the comprehension of the radiation belts.