Arase HEP-L electron measurement calibration and comparisons of pitch angle distributions with Van Allen Probes

The Arase satellite observes the dynamics of the Earth’s radiation belts, including the electron fluxes over a wide energy range from a few electronvolts to several MeV. This work focuses on the measurements of the Arase high-energy electron experiment (HEP), specifically its instrument HEP-L that observes electrons from 60 keV to 1.5 MeV. It is a state-of-the-art instrument capable of distinguishing the incoming direction of electrons by position sensitive detectors, thus accurately determining the pitch angle. HEP-L has been previously calibrated based on the modeled response functions of the instrument’s energy channels using Geant4 simulation. The current work utilizes the same simulation, but now considering the calibration in terms of the azimuthal channels, i.e., the direction of measured counts. As shown by the simulation, the distribution of counts in each azimuthal channel is broader than the nominal range in the initial direction angle, causing cross-channel contamination. We propose a new method to calibrate HEP-L data to diminish this contamination, and applying the correction method lowers the electron fluxes especially at field-aligned pitch angles, where uncorrected HEP-L data were overestimated, as seen in comparison with other instruments on board Arase and the Van Allen Probes. Pitch angle distributions (PADs) from Arase and Van Allen Probes were compared during close conjunctions of the spacecraft. Previous comparisons have only considered the omnidirectional fluxes which offer a limited view of the radiation belt dynamics. PADs provide a more detailed comparison and shed light on, e.g., wave-particle interactions. Here we derived PADs from HEP-L and extremely high-energy experiment (XEP) on Arase, and the Magnetic Electron Ion Spectrometer (MagEIS) and the Relativistic Electron-Proton Telescope (REPT) instruments on Van Allen Probes to cover a large energy range. There is a remarkable agreement within a factor of 2 for all energies, for all pitch angles for XEP data and for most pitch angles (20 to 160 degrees) for uncorrected HEP-L data. By applying the new correction, the discrepancy at the field-aligned pitch angles is reduced.