Comparing observations of the closely located JUICE and STEREO-A spacecraft during the widespread solar energetic particle event of 2024 May 13
JUICE was launched in April 2023, and it is now in its cruise phase to Jupiter, where it is scheduled to arrive in July 2031. JUICE carries a radiation monitor, namely the RADiation hard Electron Monitor (RADEM) to measure protons, electrons, and ions, detecting particles coming mainly from the anti-Sun direction. On 2024 May 13, a large solar energetic particle (SEP) event took place in association with an eruption close to the western limb of the Sun as seen from Earth. Providentially, at that time JUICE was located very close to STEREO-A, being separated by 0.13 au in radial distance, 0.3 deg in latitude, and 1.6 deg in longitude. Methods. We analysed the IP context using in-situ measurements and studied the proton anisotropies measured by near-Earth spacecraft and STEREO-A. We focused on an isotropic period during the decay phase of the SEP event to compute the proton energy spectrum. We fit the STEREO-A spectrum and compared it to that measured by SOHO and JUICE. Conclusions. The RADEM instrument aboard JUICE is a valuable tool for measuring SEP events in the heliosphere, providing an excellent opportunity to study and characterise the energetic particle environment in the solar wind between 0.65 and 5.2 au. The intercalibration factors between the fluxes measured by STEREO-A and JUICE at the effective energies of 6.9 MeV, 13.3 MeV, 21.6 MeV, and 31.2 MeV are 1.02, 1.23, 1.12, and 0.95 respectively. These intercalibration factors are valid only until 2024 July 10, when the configuration of the RADEM instrument was changed.