Mercury’s Environment Observed by BepiColombo during the Second Mercury’s Swing-by

Thanks to MESSENGER observations, we know that Mercury’s magnetosphere is highly dynamic, and it can be fully reconfigured in a few minutes, with strong influences of external conditions.

BepiColombo mission includes a comprehensive payload for the investigation of the environment. During the swing-bys the magnetic field and particles in Mercury’s magnetosphere were successfully measured by the MPO and Mio payloads. In this presentation, we will focus on Mercury’s swing-by 2 (MSB2) on 23 June 2022 showing a good example of highly dynamic magnetosphere.

During this swing by, BepiColombo passed from dusk in the far tail toward dawn in the dayside. The trajectory was in the southern hemisphere in a nearly equatorial path. Simultaneous Na ground-based observations have been obtained by the THEMIS solar telescope during the whole day. Solar Orbiter was at about 90° est of BepiColombo observing the Sun remotely.

These two simultaneous observations allowed to observe the magnetosphere in situ while the Na global exosphere was imaged.

According to the magnetic field data (MPO-MAG and MGF) before and after the flyby, the IMF z component was around 0 nT varying between northward to southward. The solar wind observed by SERENA-PICAM before and after the swing-by shows a high variability in the energy. Ion and electron populations in the plasma sheet and close to the planet at dawn have been observed.

When the spacecraft was entering in the far tail at about 9 UT, an abrupt increase in dayside exospheric intensity has been registered by THEMIS. This intensity slowly recovered to the previous values in about 3 hours. When the spacecraft exited from the planetary shadow, at 9:50 UT, SIXS-P observed an electron population at energy between 70-280 keV.

The magnetopause boundary was clearly identifiable together with a weak low latitude boundary layer. While the bow shock crossing was not clearly distinguishable, showing energy-dispersion signatures and a flapping boundary. Upstream the bow shock, foreshock ions have been observed by SERENA-PICAM and MPPE-MSA in agreement with a quasi-parallel IMF configuration.

On the same day, Solar Orbiter/FSI observed a M2 and long-lasting flare from 9:00 UT to 12:UT in the southern solar hemisphere toward Mercury quadrant.

While the Na exospheric variability is clearly linked to solar conditions, it is still difficult to describe the exact mechanism responsible of the Na release without two vantage-point measurements providing information of external conditions and magnetospheric dynamic and exosphere.