The Exosphere of Mercury during BepiColombo's Flybys 2, 3, and 4

Mercury’s exosphere has been extensively investigated through ground-based observations. Since 2007, the THEMIS solar telescope was used to conduct a long-term monitoring campaign of the sodium (Na) exosphere, enabling detailed studies of its morphology and variability at different timescales driven by interactions between Mercury’s surface, intrinsic magnetic field, neutral and ionized environment, and the solar wind and radiation. They allowed to investigate the contribution of the different source processes as well as the statistical frequency of the different morphological patterns, and the rapid change from one to another at the sudden change of interplanetary magnetic field conditions.

In 2022, 2023, and 2024, we carried out targeted observations of Mercury’s sodium exosphere in the days surrounding three of the six flybys performed by the ESA–JAXA BepiColombo spacecraft around the planet. The objective was to provide a global view of the exospheric morphology and dynamics during the spacecraft’s close-approach phase, when also several in-situ instruments were simultaneously operating. These measurements offered crucial information on the planetary environment, including magnetic fields, as well as ion, electron, and neutral populations across a wide range of energies, and radiation.

We present and compare the three exospheric configurations observed in the days around these three flybys, highlighting their morphological and dynamical similarities and differences.