Planetary ion species and sporadic plasma sheet ion injections observed during BepiColombo’s sixth Mercury flyby

On 8 January 2025, BepiColombo performed its sixth and final Mercury gravity assist, following a unique trajectory during cruise from the night-side southern hemisphere to the day-side northern hemisphere, crossing the cusp region at altitudes as low as ~295 km. This high-latitude, low-altitude path provided an unprecedented opportunity to investigate Mercury’s inner magnetosphere dynamics. Despite limitations due to the spacecraft’s cruise-mode configuration, the magnetometer (MPO-MAG; Heyner et al., 2021) and the Mercury Plasma Particle Experiment (MPPE; Saito et al., 2021), particularly the Mass Spectrum Analyzer (MSA; Delcourt et al., 2016) and Mercury Ion Analyzer (MIA), acquired valuable ion measurements.

We present new observations of cold planetary ions (Na⁺/Mg⁺ and K⁺/Ca⁺, down to ~10 eV/e) and a potential detection of additional species such as Si⁺ in the central plasma sheet. Energetic H⁺ ions (~10 keV/e) were also detected and are consistent with a ring current configuration, as supported by backward-traced test particle simulations.

Most notably, we report the first evidence of sporadic ion injections into Mercury’s polar regions. Dispersed H⁺ signatures (>1 keV/e) suggest injections of solar wind ions from the night-side magnetotail, while heavy ion dispersions indicate localized planetary ion injections and bounce motion near the planet. These results highlight dynamic processes, possibly substorm-like activity, shaping Mercury’s plasma environment.