Simulation for the calibration of radiation data acquired by Solar Particle Monitor/MMO

    The exploration of the inner heliosphere has been limited by large gravitational potential differences. Consequently, the propagation processes of solar ejecta such as Interplanetary Coronal Mass Ejections (ICMEs) and Solar Energetic Particles (SEPs) are somewhat problematic. Recent orbital engineering developments and other advancements have enabled the deployment of multiple probes, such as BepiColombo and Solar Orbiter. This has provided a unique opportunity for multi-point observations of solar eruptions, allowing for the tracking of their radial and longitudinal evolution. 

    This study focuses on radiation data acquired by “Solar Particle Monitor (SPM)” onboard BepiColombo for solar physics. SPM is the housekeeping instrument, particularly more suited for highly energetic particles such as Solar Energetic Particles (SEP) and Galactic Cosmic Ray (GCR) than other instruments. However, it provides only time-series data of count rates and deposited energies. To extract valuable information about the incident charged particles, including their type, number, energy, and direction, a radiation simulation toolkit Geant4 (Allison et al., 2016) is employed.

    The mass SPM model was defined in the model space, incorporating an aluminum shield to estimate radiation shielding effects from surrounding equipment and walls of spacecraft. The thickness of the shield in each direction is determined by comparing SPM measurements with simulation results, identifying the combination of thicknesses that aligns most closely with observed trends. The study also reproduces the electron flux during BepiColombo's Earth swing-by in 2020 using the AE9 radiation model (Ginet et al., 2013), comparing the simulated results with actual measurements to determine effective shield thicknesses. Additionally, the method proposed by Park et al. (2021), utilizing transformation matrices to derive incident particle energy spectra from observed spectra, is applied.

    Our research extends the analysis to the recovery time constants of Forbush Decreases, resulting from ICME shielding GCR which the spacecraft encountered in 2022. The study aims to analyze the structural changes induced by the interaction between ICMEs and solar wind during their propagation. Future applications of the analysis are planned for Solar Energetic Particle (SEP) studies, contributing to the understanding of SEP propagation processes through multi-point observations.

    The implications of the results are significant, especially considering the predicted peak of the 11-year solar activity cycle in 2025. BepiColombo, having already captured numerous phenomena, necessitates further analysis using the established calibration method. The study underscores the potential applicability of housekeeping devices commonly equipped on spacecraft for scientific observations. If applied to other probes, similar methods not only expand the scope of scientific observations but also contribute to the growth of the solar observation network within the heliosphere.