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 cruise phase, not all the sensors can operate for science measurements, however, 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 as shown in figure 1.

Figure 1. Second Mercury swing-by trajectory in x-y MSO coordinates

According to the magnetic field data (MPO-MAG and MGF) before and after the flyby, the IMF z component turned from northward to southward, as it was suggested by comparing the observed magnetic field data during the crossing to the simulated ones (Exner et al., JGR-SP, 129,  2024). The solar wind observed by SERENA-PICAM before and after the swing-by shows a high variability in the energy. In the inner tail, signatures of plasma sheet populations streaming toward the planet have been observed. The ion energies (SERENA-MIPA and PICAM) reached up to 10 keV and electron (MPPE-MEA) up to 1 keV close to the dawn side of the planet. This population could be the signature of plasma convected and energized from the tail toward the so-called horn, or it could be a signature of partial ring current. Mass analysis (MPPE-MSA) shows that the origin of these ions is probably the solar wind. In the dayside, 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. Mass analysis show that the foreshock ions have a component of He⁺⁺ and heavy ions. A population at lower energy (about 10s eV) observed by MPPE-MSA is the signature of spacecraft outgassing.

Simultaneous Na ground-based observations have been obtained by the THEMIS solar telescope during the whole day. These observations show a highly variable exospheric intensities, that could be related to the variability of the solar wind and of the interplanetary magnetic field.