Spatial Distribution of Radiation Belt Protons Deduced from Solar Cell Degradation of the Arase Satellite

Abstract

Analysis of solar-cell array panel (SAP) data from the Arase satellite orbiting in the inner magnetosphere showed a clear degradation of solar cells that could be attributed to trapped protons with energies greater than 6 MeV. Proton fluence was determined based on variations in the open-circuit voltage (Voc) of the solar cells, which we compared with that expected based on various distribution models (AP8MAX, AP9 mean and CRRESPRO quiet) of trapped protons. We found a general agreement, confirming the major contribution of trapped protons to the degradation, as well as a slight difference in the fluence expected based on the model calculations. To minimize this difference, we slightly modified the models, and found that concentrating the energetic protons on the magnetic equator provided a better agreement. Our results indicate that >6 MeV protons also has the equatorial concentration as reported for >18 MeV protons from the Van Allen Probes observation, and are interpreted as two components of the trapped protons, i.e. , those of solar energetic particle (SEP) origin have an anisotropic pitch-angle distribution and are confined near the magnetic equator.