Van Allen Probe observations of the pitch angle evolutions of high-energy protons near the second peak of inner radiation belt

Abstract

Utilizing the full pitch angle ($\alpha$) measurements by the Relativistic Electron-Proton Telescope (REPT) onboard Van Allen Probe A, we analyzed the pitch angle distribution and storm-time variation mechanism of high-energy protons (< 30MeV) near the second peak (L* = 1.84 – 1.98) of Earth’s inner radiation belt (L*≤ 2.5). We found that the fluxes of the ${40}^{\circ }$ to ${140}^{\circ }$ protons decrease during the main phase of storms and then gradually increase during the recovery phase, whereas those fluxes at smaller or larger pitch angles ($\alpha <{40}^{\circ }$ or > ${140}^{\circ }$) remain relatively stable during the whole storms. Theoretic calculations indicate that the bounce latitude ($\lambda$ > 26°) of the quasi-parallel or anti-parallel moving protons ($\alpha <{40}^{\circ }$ or > ${140}^{\circ }$) exceeds the influence range ($\lambda$≤ 20°) of storm-time ring currents in the inner belt region (L*≤ 2.5), but those of the oblique and quasi-perpendicular protons (${40}^{\circ }<\alpha <{140}^{\circ }$) overlap with the low-latitude ring currents. During the main/recovery phase of storms, the oblique and quasi-perpendicular protons are transported outwards/inwards and thus experience Fermi and Betatron decelerations/accelerations. The adiabatic decelerations/accelerations modify the pancake-like pitch angle distributions of the tens of MeV protons in the outer half part (L* = 1.6 – 2.5) of the inner radiation belt, whereas field line curvature scatterings become effective outside the proton radiation belt (L* > 2.5).