Mercury’s plasma environment and BepiColombo spacecraft's third flyby of Mercury

The instruments designed to measure ions in Mercury’s plasma environment are: 

Mass Spectrum Analyzer (MSA), Mass Ion Analyzer (MIA), and Mass Electron Analyzer (MEA).

• Mass Spectrum Analyzer: This instrument has a spherical top-hat analyzer for energy analysis and a Time-Of-Flight (TOF) chamber for mass analysis.
• For ions passing through the top-hat analyzer, the energy-to-charge ratio is:

• This energy allows the MSA to filter ions with specific energies before they enter the Time Of Flight chamber. TOF analyzer measures the time t it takes for ions to travel a fixed distance d after they pass through the top-hat analyzer. From the TOF data, the ion’s velocity can be calculated. Then the mass-to-charge ratio m/q is calculated.

• The MSA uses a reflectron TOF system, which improves the mass resolution by reflecting ions back and forth within the TOF analyzer. If there are small differences in ion velocities, more accurate m/q can be measured.

Differential Directional Energy Flux (DDEF) of the ions and electrons is measured by MSA, MIA and MEA.

• N is the number of particles,
• E is the particle energy,
• A is the detector area,
• T is time, Ω is the solid angle,
• Θ and ϕ represent the direction of travel of the particles in spherical coordinates.

Magnetospheric regions and ion composition:

• The low latitude boundary layer(LLBL) is a region where magnetosheath and magnetospheric plasmas are mixed along the magnetospheric side of the low-latitude. There is presence of an energy dispersion( how particles with different energies spread out as they travel through a magnetic field of the ions). This dispersion extends from ~20 keV e−1 in the outer part of the flank down to 10s of eVs per e in the inner part.

• Kelvin-Helmholtz Instability occurs at the interface between two plasma flows with different velocities, such as between the solar wind (a stream of charged particles from the Sun) and a planetary magnetosphere or at the boundary of different plasma regions.

• In study, H+  trajectories were computed using a modified Luhmann–Friesen model for the magnetic field combined with convection pattern for the electric field. The full equation of motion was integrated  using a fourth-order Runge–Kutta technique.

• The plasmas sheet horns: In this region, there is presence of ~1 keV e−1 ions in the near-tail central plasma sheet extending to the higher latitudes.

• The flyby provided direct evidence of Mercury’s ring current. It is a circulating flow of charged particles around the planet, having energetic hydrogen ions (H⁺) and heavier ions like oxygen (O⁺)
• The ion observations highlight the presence of cold ions (≤50 eV e−1 ) and energetic ions (up to 38 keV e−1 ) in the environment. Energetic electrons up to 10 keV e−1 were also observed in the deep magnetosphere. 

https://www.nature.com/articles/s42005-024-01766-8