Observation of High-Energy Particles in the Inner Radiation Belt by the HEP Instrument of the Arase Satellite

Abstract

The high-energy electron experiments (HEP) instrument on board the Arase satellite employs two sensors, HEP-L and HEP-H, and was designed to measure electrons with energies from 70 keV to 2 MeV. The recent Van Allen Probes observations indicate that MeV electron flux is very small in the inner radiation belt, while the HEP has detected significant counts at MeV energy channels in the inner radiation belt. Counts in the inner radiation belt are registered similarly at different energy channels of HEP-H and higher energy channels of HEP-L, and show no clear energy dependence. Their properties suggest contamination of high-energy protons that populate densely the inner radiation belt. In order to identify the energy of the penetrating protons we compare the spatial distribution of the HEP counts with NASA's AP9 mean model. We find that the primary peak of the count distribution measured with HEP in MeV energy range is located at L = 1.5 at the magnetic equator, which in in agreement of > 60 MeV inner belt protons of AP9 mean model. The secondary distribution is also found at higher L values, which can be attributed to MeV protons. We have been conducting Geant4 simulation for penetrating protons into the HEP. Our result of the simulation is consistent with suggestions of analysis on the spatial distribution.