In-flight calibration of RADEM, the JUICE mission radiation monitor

Context. The RADiation-hard Electron Monitor (RADEM) aboard the JUpiter ICy moons Explorer (JUICE) mission, launched on April 14, 2023, measures high-energy protons and electrons during the cruise phase and will continue to do so during the nominal mission phase. However, ground calibration results were unable to explain the initial flight observations, which prompted an in-flight calibration campaign. Aims. Our main goal was to calibrate RADEM and develop a procedure to compute particle fluxes from the count rates obtained by the RADEM detector heads. Methods. We used galactic cosmic rays (GCRs) to calibrate RADEM’s sensors by increasing the respective thresholds and therefore modifying their response to high-energy particles. We then compared the count rates obtained in flight for each threshold to theoretical count rates calculated using the Badhwar-O’Neill 2020 (BON2020) GCR model and threshold-dependent response functions. We used these results to develop a flux-reconstruction algorithm based on the bow-tie method. Results. We derived a new set of in-flight calibration coefficients for all sensors. In several cases, the in-flight calibration slopes differ by up to an order of magnitude from pre-flight ground calibration values. Proton fluxes from solar energetic particle (SEP) events, reconstructed using the bow-tie method, show good agreement (within a factor of two) with measurements from the SOlar and Heliospheric Observatory (SOHO). Conclusions. The RADEM provides accurate measurements of proton fluxes in interplanetary space and is well suited for both single-spacecraft analyses and coordinated multi-mission studies of SEPs. While electrons have been clearly identified during the JUICE Lunar-Earth gravity assist (LEGA), reconstructing their fluxes needs a more detailed analysis.