Investigation of Mercury exosphere from ground-based observations: the cases of BepiColombo’s flybys 2 and 3

The ground-based observations over years have shown to be a crucial mean to study the exosphere of Mercury and to investigate the complex environment of the planet and its interactions with the solar wind and the embedded interplanetary magnetic field.

In fact, the Na exospheric pattern have shown to map the magnetic cusp and to be very sensitive to the IMF-planetary magnetic field coupling.

Now, with the era of space mission at Mercury (MESSENGER in the last decade, and now BepiColombo), the ground-based observations are playing the new role of complementary observation to the in-situ, high resolution measurements of exospheric components, but also ion population and magnetic field.

In this frame, we present the observations performed from the ground-based solar telescope THEMIS during the second and third flyby of BepiColombo at Mercury in June 2022 and 2023. Exospheric data were acquired in a period of 4-5 days around the flybys dates during all daytime hours with a high resolution spectrograph able to adequately image both the D2 and D1 emission lines of sodium.

We will highlight analogies and differences in the morphology and how they relate to the IMF experienced by the spacecraft during its approach. Ion populations encountered will also be investigated.

Space- and Earth-based coordinated observations will have to be performed also during BepiColombo nominal mission because they are an outstanding tool for making contemporary detailed and global investigations of the exosphere.