Energetic Particle Observations with SIXS During BepiColombo’s Sixth Mercury Flyby

Mercury hosts a small global magnetic field, approximately 1% of Earth’s magnetic field strength, capable of standing off the solar wind, resulting in a magnetosphere that is qualitatively similar in structure to Earth’s. However, due to its closer proximity to the Sun, Mercury’s magnetosphere experiences much stronger solar wind pressure than that at Earth, resulting in dynamic magnetospheric processes that occur on much shorter timescales. These extreme conditions can result in processes such as particle energization, transport and precipitation onto the planetary surface, which are strongly influenced by the external solar wind conditions. Here, we present observations of energetic protons (> ~ 1 MeV) and electrons (> ~ 70 keV) from the Solar Intensity X-Ray and Particle Spectrometer (SIXS) onboard the BepiColombo spacecraft during its sixth and final Mercury flyby on 8 January 2025. Similar to the spacecraft’s fourth Mercury flyby in 2024, a solar energetic particle event occurred a few days before closest approach, resulting in elevated fluxes of energetic particles both outside and within the Hermean magnetosphere. Furthermore, the KTH22 Mercury magnetic field model was used to help interpret these energetic particle observations and to evaluate whether features in the data were consistent with the aforementioned magnetospheric processes.