Mercury's exosphere as seen by BepiColombo/PHEBUS visible channels during the first two flybys.
Résumé
BepiColombo, the ESA/JAXA joint mission has already performed 2 flybys of Mercury on October 1, 2021 and June 23, 2022. PHEBUS (Probing of Hermean Exosphere By UltraViolet Spectroscopy) was able to observe during these flybys notably with its two visible channels. The channel c404 is centered on 404 nm (K emission line), and the other one, c422, is centered on 422 nm (Ca emission line). During both flybys, the slit was removed and the acquisitions were made every 10s with a 8s exposure time.
The count rate as a function of time clearly depicts the observation geometry of the flybys, the transit in the shadow of Mercury in particular, and the maximum of the emission on the dayside after the closest approach. c422 made Ca measurements during both flybys. We have observed an extended Ca corona (up to 10 000 km) which was not observed to that extent with MESSENGER. As for c404, it is more difficult to state whether K was detected or not during the 1st flyby. However, the 2nd flyby shows two different scale heights between c422 and c404; it is thus a species detection. It is more likely to be Mn (emission line at 403 nm) rather than K. As a reminder, K has not been observed by MESSENGER at this wavelength [1]. Mn has been observed only at the end of the mission, in the pre-dawn nightside region and between TAA of 0° and 70° [1], which does not correspond to the TAA of BepiColombo flybys.
We also notice peaks on top of the general emission profiles that take place at dawn. They may be related to dust particles or the crossing of magnetospheric structures.
We correct the data for different contributions (dark current, zodiacal light and stars in PHEBUS field of view) and convert them to radiance. We then consider the data on the dawn side to derive an exponential fit I = I0 e-z/h, where z is the tangent altitude of PHEBUS line of sight above the surface, I0 the radiance at the surface and h is the e-folding distance. The best fit found indicates two different populations with two scale heights: ~2 200 km and ~10 300 km. Exponential fits to MESSENGER limb scan data at dawn gives a single population [2].
To conclude, the flybys have allowed the detection of Ca and probably Mn. Two different high energy source processes seem to be at the origin of Ca. The origin of the surprising emission bursts at dawn is still under investigation.