Characteristics of High‐Energy Protons in the Equatorial Plane of Inner Radiation Belt Observed by Van Allen Probes

Abstract

Long‐term and short‐term variations of high‐energy protons in the inner radiation belt are of major importance in geospace research. The entire inner radiation belt at 1.1 < L < 3.0 have been investigated based on Van Allen Probes Relativistic Electron‐Proton Telescope data during the descending phase of the solar cycle (2013–2018). It is found that two belts with peaks near L = 1.5 and L = 2.0 for E < 50 MeV protons and one belt at L = 1.5 for E > 50 MeV protons, once they are formed, last for more than 6 years. In the outer zone of the inner belt (1.8 < L < 2.4), the variations of high‐energy proton fluxes with E < 50 MeV are well correlated with SYM‐H index, while only weak correlation is found for them in the inner region at 1.3 < L < 1.6. Geomagnetic disturbances do not show clear correlation with variations for E > 50 MeV protons in the whole inner radiation belt. By analyzing the structure evolution of the inner radiation belt during half of the solar cycle, we found that the entire inner radiation belt shrinks at solar minimum and stretches at solar maximum. Our results reveal the relationship between high‐energy protons and SYM‐H index and solar cycle modulation on the whole configuration of inner radiation belt, which may assist in understanding and modeling of the inner radiation belt protons.