The third BepiColombo flyby to Mercury: ground based observation of the exosphere and in-situ measurements

On 19 June 2023 the ESA-JAXA double spacecraft BepiColombo performed its third flyby around Mercury with a closest approach on the nightside at an altitude of 200 km above the surface and an almost equatorial trajectory that crossed the tail magnetosphere (Figure 1, left).

During the days around the flybys many instruments onboard the spacecraft were operative to get new in-situ measurements of the planetary environment, providing data of magnetic field, of the solar wind and plasma, ions, electrons and neutral particles, radiation, dust particles.

Also from the Earth, an observing campaign of Mercury was performed from the solar telescope THEMIS in the Canary Island of Tenerife, to detect the exospheric morphology and variability for the 5 days around the flyby. Thanks to the favourable orbital configuration (Figure 1, right), the good performances of the high resolution spectrograph MTR and of the specific characteristics of the solar telescope THEMIS and its adaptive optics, it is possible to observe Mercury during all the daytime, i.e. many hours/day.

The high resolution spectrograph can image separately the two D emission lines at 5890-96 A of sodium, and a scanning system provide a full image of the planet in about 30 minutes. Sodium is a well-known exospheric component of Mercury, and it is well detectable also from the ground, thanks to the fact that its emission lies in a region free from telluric lines, and it is then able to provide a time series of sodium exospheric images to study its morphology and variability.

The exosphere, that is neutral gaseous environment of the planet, is the result of many different interactions occurring between the outer space (solar wind particles and radiation, dust…) with the planetary surface, causing release of particles. The interplanetary magnetic field, interacting with the intrinsic planetary magnetic field, seems to be a primary driver of the two peculiar peaks of emission occurring at high latitude, often observed during ground based observations (Figure 2).

To study to exosphere thus means to enhance our understanding of Mercury’s overall environment and the physical processes that generate and sustain it.

In our work we provide the results coming from the analysis of the days around the third flyby at Mercury by completing ground-based data with in-situ measurements of the magnetic field and the ion and electron populations.

Figure 1: (left) BepiColombo trajectory on the XY plane during the third flyby (in red) compared to the previous ones; (right) Inner Solar System configuration during the contemporary observations from Earth-based solar telescope THEMIS (Sun in the center, Earth in blue, Mercury in green).

 

Figure 2: Example of sodium intensity emission in D2 line observed on June 19, 2023, showing the typical 2-peaks pattern in the solar direction, with the northern one more intense. Sun in on the left, the disk of Mercury is shown (black circle) and also the dusk terminator (white line).