Na exospheric emission in relation to Sun events during the BepiColombo Second Mercury’s Fly-by

During BepiColombo second Mercury’s flyby on 23 June 2022, a ground-based campaign with the THEMIS solar telescope provided simultaneous observations of the sodium (Na) exosphere while Bepicolombo measured interplanetary and magnetospheric conditions. Between 8:53 and 9:34 UT, the exospheric Na emission abruptly increased in intensity by about 70% across the entire day side, followed by a slow recovery to the preceding level over approximately four hours. BepiColombo did not register neither relevant perturbations of the solar wind nor solar energetic particles during the Na increase. The configuration of Mercury, Earth and Solar Orbiter enabled observations of the solar regions facing Mercury from both sides. They revealed that an active region facing Mercury, but hidden from Earth, produced a flare and an extreme-ultraviolet (EUV) brightening when the Na emission increased, whereas subsequent emissions from post-flare loops produced no further increase. While photon-stimulated desorption is one of the main mechanisms releasing Na from the surface, short-therm Na variability has so far been attributed mainly to plasma precipitation. Our observations suggest that solar flares can trigger rapid Na variability at Mercury, and point to a multi-phase release from a finite reservoir of surface sodium, reminiscent of the steeper than linear dependence on solar EUV reported for the Moon on longer timescales. This result marks a significant step forward in understanding the mechanisms governing the interaction of Mercury and the Moon with the Sun and it was possible thanks to multi-messenger observations highlighting the potential of investigations during BepiColombo nominal mission.