Mercury's plasma environment after BepiColombo's third flyby

Mercury’s proximity to the Sun and its weak intrinsic magnetic field lead to a highly dynamic space plasma environment tightly coupling the surface, exosphere, and magnetosphere. Ion populations derive from both the solar wind (notably H+ and He2+) and planetary sources via ionization of exospheric neutrals. Early observations by Mariner 10 identified heavy atoms in Mercury’s exosphere, and ground-based telescopes detected species such as Na+, K+, and Ca+. MESSENGER’s Fast Imaging Particle Spectrometer (FIPS) later established that Mercury’s magnetosphere contains solar-wind–derived light ions and heavier planetary-origin ions (He+, O+-group, Na+-group), but FIPS’ limited mass resolution and restricted field-of-view and energy range hampered species differentiation (e.g., O+ vs. water group, Na+ vs. Mg+) and full characterization of ion energies. BepiColombo, an ESA/JAXA mission, performed three Mercury flybys during cruise (2021–2023) prior to orbit insertion in 2025. This study reports ion and electron measurements from the MPPE suite (MIA, MSA, MEA 2) on the Mio spacecraft during the third flyby (June 19, 2023), which reached ~235 km altitude. We provide a synoptic view of Mercury’s magnetospheric structure, composition, and key regions (LLBL, plasma sheet horns, and ring current), leveraging MSA’s high mass resolution and broader energy coverage than prior missions. Due to the cruise-phase stacked configuration, particle sensors’ fields-of-view were severely restricted by MOSIF, and magnetic field data were unavailable (under calibration) at the time of analysis.

Prior missions and observations established key aspects of Mercury’s plasma environment. Mariner 10 first revealed Mercury’s exosphere and magnetic environment, with later ground-based observations identifying Na, K, and Ca. MESSENGER’s FIPS instrument demonstrated a mix of solar-wind ions (H+, He2+) and planetary-origin heavy ions (He+, O+-group m/q = 16–20, Na+-group m/q = 21–30), but was limited by mass resolution, field-of-view, and energy range (~50 eV/e to ~13 keV/e), preventing clear separation of specific ion species (e.g., O+ vs. water group, Na+ vs. Mg+) and limiting energy characterization. Prior studies proposed and debated the existence and nature (full vs partial) of a ring current at Mercury, with simulations and statistics suggesting quasi-trapped protons at ~1.3–1.5 RM but also highlighting compression events that can disrupt trapping. Plasma sheet horns (PSH) and plasma mantle have been reported, as well as LLBL characteristics on the dayside. These gaps motivated higher mass-resolution and wider energy-range measurements, now provided by BepiColombo/MPPE.

Observations were acquired during BepiColombo’s third Mercury flyby (MFB3) on June 19, 2023, along a near-equatorial trajectory from dusk–nightside through post-midnight and out to dawn–dayside. Instruments: MIA (Mercury Ion Analyzer) and MSA (Mass Spectrum Analyzer) measured ion fluxes, and MEA 2 (Mercury Electron Analyzer) measured electron fluxes. MIA and MEA 2 are top-hat energy analyzers for total flux; MSA combines a spherical top-hat analyzer with a linearly polarized Time-Of-Flight (TOF) chamber using a reflectron design to achieve high mass resolution (m/Δm > 40). During cruise, MOSIF obstructed sensor fields-of-view, reducing coverage to ~0.13–0.1 π sr, with only two ion sensor entrance windows along +Z recording flux. Ion thrusters were off during flyby, avoiding contamination. MSA TOF spectra of straight-through particles in low mode (TSTL) were integrated over 1024 s, limiting temporal resolution for species identification. Energy–time spectrograms (DDEF) from MSA, MIA, and MEA 2 were used to identify magnetospheric regions and boundary crossings. Bow shock crossings occurred at 18:44:22 UT (inbound) and 19:52:00 UT (outbound); magnetopause crossings at 19:14:00 UT (inbound) and 19:45:00 UT (outbound). Mass-per-charge distributions were derived from MSA TOF to identify ion species across regions (solar wind, magnetosheath, LLBL, PSH, ring current). To interpret ion origins and dynamics, backward test-particle simulations were performed using static electromagnetic field models: a modified Luhmann–Friesen magnetospheric magnetic field and a two-cell convection electric field. Trajectories for H+ were traced backward from observed locations to infer source regions (LLBL, PSH, ring current), with a subsolar stand-off distance of 1.31 RM in the model. Magnetic field in situ data were not used because calibration was ongoing at the manuscript time. Data sources include spacecraft SPICE kernels for trajectory and MPPE instrument data archived on Zenodo and AMDA/CDPP.

• Boundary crossings: Inbound bow shock at 18:44:22 UT; inbound magnetopause at 19:14:00 UT; outbound magnetopause at 19:45:00 UT; outbound bow shock at 19:52:00 UT.
• Low-Latitude Boundary Layer (LLBL): Upon entering the dusk magnetosphere after 19:14 UT, ions exhibited strong energy dispersion from ~20 keV/e at the outer flank down to tens of eV/e interiorward, consistent with mantle-like convection effects observed at near-equatorial latitudes. Bursty ion flux enhancements inside the LLBL imply impulsive injection processes at the duskside flank, potentially linked to Kelvin–Helmholtz instabilities or magnetic reconnection. Heavy ions (m/q ~16–23) with energies ≳10 keV/e (consistent with O+ and Na+) and cold heavy ions (m/q = 16 and 39; O+, Ca+/K+) coexisted with lighter ions (H+, He2+).
• Cold ion plasma detection: Near 19:24:25 UT, intense cold ions between ~30–100 eV/e were detected while MEA 2 observed electron flux depletion as the spacecraft entered Mercury’s umbra. The event is interpreted as spacecraft negative charging accelerating ambient low-energy ions (likely planetary in origin) toward the spacecraft, making them detectable.
• Plasma Sheet Horns (PSH): Starting ~19:28:41 UT, a thermalized hot ion population and, for the first time, electron populations of a few keV/e were observed in the near-tail plasma sheet. ~1 keV/e ions extending to higher latitudes are characteristic of PSH; ions injected from the distant tail are gradually energized by convection toward the planet.
• Ring current evidence: From ~19:32:00 to 19:44:04 UT (magnetic latitude −3° to 12°), intense ion fluxes in the 5–40 keV/e range and energetic electrons up to ~10 keV were measured at low altitudes and near-equatorial latitudes. The broader energy range of MSA/MIA (vs. MESSENGER/FIPS) enabled the first full energy distribution measurement of this population, including trapped energetic H+ around ~20 keV/e, providing strong evidence for a (partial or full) ring current at Mercury. Test-particle simulations indicate a 10 keV/e H+ can complete a planetary drift in ~4 minutes for a magnetopause at 1.31 RM.
• Composition across regions: In magnetosheath and solar wind, plasma was dominated by H+ and He2+. In the magnetosphere, heavy ions were detected with two prominent bands: ~2 keV/e (O+, m/q ~16) and ~10 keV/e (K+/Ca+, m/q ~39), with O+ dominant among heavy ions in the central plasma sheet. Cold ions near ~15 eV/e with m/q ~16–23 were also present, though their precise origin could not be resolved with available TOF time resolution.
• Outgassing signature: Narrowband ions at ~10 and ~20 eV/e with m/q = 1 and 16 were attributed to spacecraft outgassing of water group molecules, observed before/after bow shock crossings and outbound.

The MPPE observations during MFB3 address key open questions about Mercury’s plasma environment by revealing the structure, composition, and energization processes across major magnetospheric regions. The detection of trapped energetic H+ peaking near 20 keV/e and the complete energy distributions supports the existence of a Hermean ring current and clarifies its energy content and spatial occurrence during a compressed magnetospheric state. The LLBL observations, including near-equatorial energy dispersion and bursty ion enhancements, demonstrate direct coupling between mantle-like transport and low-latitude mixing layers, pointing to dynamic processes such as Kelvin–Helmholtz instability and magnetic reconnection at the magnetopause that drive injections and energization. The identification of PSH with thermalized hot ions and keV electrons, and the presence of heavy planetary ions (notably O+, with contributions from Na+ and K+/Ca+), highlight the role of planetary-source ions in pressure balance, convection-driven acceleration, and ion recycling. The cold ion detections in umbra emphasize spacecraft charging effects but also reveal the presence of very low-energy planetary ions that are otherwise difficult to observe. Together, these findings provide a dawn–dusk synoptic view of Mercury’s magnetosphere with higher mass resolution and extended energy coverage than previous missions, improving constraints on ion sources, transport, trapping, and energization, and setting the stage for comprehensive 3D measurements during the orbital phase.

BepiColombo/MPPE observations during Mercury flyby 3 provide a synoptic dawn–dusk view of the magnetosphere, revealing cold ions (≤50 eV/e), energetic ions up to ~38–40 keV/e, and energetic electrons up to ~10 keV. Key regions were characterized, including a Low-Latitude Boundary Layer exhibiting impulsive injections, plasma sheet horns with keV ions and electrons, and compelling evidence for a Mercury-encircling ring current with characteristic H+ energies around ~20 keV/e. Heavy planetary-origin ions were resolved by mass-to-charge, with O+ dominant and additional contributions from Na+, K+, and Ca+, underscoring the importance of heavy ions to magnetospheric pressure and dynamics. These cruise-phase results foreshadow richer discoveries once in orbit, when full 3D coverage will allow definitive assessment of ring current spatial extent (full vs partial), detailed ion composition anisotropies (e.g., Na+), and the interplay of reconnection and instabilities in forming and sustaining LLBL and PSH. Two-spacecraft measurements by MPO and Mio will enable new insights into temporal–spatial variability, plasma transport, and coupling throughout Mercury’s magnetosphere.

• Magnetic field data were unavailable (under calibration) at the time of analysis, precluding direct field-aligned context and current determination.
• Cruise-phase stacked configuration and MOSIF severely limited particle sensor fields-of-view (~0.1–0.13 π sr), restricting directional coverage and anisotropy assessment; this limits definitive discrimination between full versus partial ring current and affects Na+ detectability due to anisotropic distributions.
• MSA TOF spectra (TSTL) were integrated over 1024 s, limiting temporal resolution for species identification and hindering precise spatial mapping of composition gradients or rapid injections.
• Spacecraft charging in umbra complicates interpretation of cold ion observations, which may include accelerated ambient low-energy ions; distinguishing planetary-origin cold ions from outgassing products is challenging at the available resolution.
• Test-particle simulations used static, idealized field models and did not include IMF configuration or solar wind variability, limiting quantitative interpretation of drift paths and injection sources.
• Outgassing signatures (e.g., water group) can contaminate low-energy mass channels in the solar wind/magnetosheath, requiring careful separation from ambient populations.