Simulation for the Calibration of Radiation Housekeeping Monitor Onboard BepiColombo/MMO and Application to the Inner Heliosphere Exploration

Abstract

Although primarily a housekeeping instrument for measuring ambient radiation, the Solar Particle Monitor (SPM) onboard BepiColombo can measure high-energy particles, making it useful for observing phenomena such as galactic cosmic rays and Solar Energetic Particles (SEPs). However, it only records time-series data of particle energy loss and counts, which requires characterization by radiation simulation for scientific analysis. In this study, a physical model of the SPM was constructed using the “Geant4” radiation simulation toolkit to investigate its response to charged particles. The probability density functions were derived from the response functions to indicate the proportion of particles in each energy range among the SPM counts. Finally, we inverse-calculated the flux from the counts in the corresponding energy ranges. We applied this method to data from the terrestrial radiation belt and SEPs in March 2022. The results agreed with the empirical radiation belt model and another instrument onboard BepiColombo, demonstrating the validity of the method. This study highlights the potential for scientific applications of housekeeping instruments and suggests the broader use of similar methods on other missions for expanding inner heliosphere multi-point exploration.