Flight data analysis of the BERM radiation monitor aboard the BepiColombo mission to Mercury

Abstract

BepiColombo is the first European mission to the Hermean system. It was launched in 2018 and is predicted to enter Mercury’s orbit in late 2025. It is composed of two spacecraft, ESA's Mercury Planetary Orbiter (MPO) and JAXA’s Mercury Magnetospheric Orbiter (nicknamed Mio).

Among the instrument suit of the MPO is the BepiColombo Radiation Monitor (BERM), that can detect high energy protons (~1 to 200 MeV), electrons (~1 to 10 MeV) and heavy ions. BERM is part of the spacecraft housekeeping, with the objective of monitoring radiation hazards to prevent possible damage on the spacecraft and instruments. Despite not being part of the scientific payload, its capability of measuring such high energies and its operational state during all phases of the mission makes it an asset for scientific objectives as well. BERM consists of a single stack with 11 Silicon detectors interleaved by aluminum and tantalum absorbers. BERM identifies particles type and energies through the signals resultant from its interaction with the stack, assigning each particle to one of 18 channels: five dedicated to electrons, eight to protons, and five to heavy ions. The monitor provides daily files with the number of registered counts in each channel integrated over 30 seconds sampling intervals. Obtaining particle fluxes from the BERM channel counts is not straightforward. In this work, the bow-tie method, introduced by Van Allen in 1979, was applied to convert flight count rates to proton and electron fluxes. The results were used to analyze solar events detected by BERM.

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