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Ion kinetic properties in Mercury's pre‐midnight plasma sheet

With data from the Fast Imaging Plasma Spectrometer sensor on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, we demonstrate that the average distributions for both solar wind and planetary ions in Mercury's pre‐midnight plasma sheet are well‐described by hot Maxwell‐Boltzmann distributions. Temperatures and densities of the H+‐dominated plasma sheet, in the ranges ~1–10 cm−3 and ~5–30 MK, respectively, maintain thermal pressures of ~1 nPa. The dominant planetary ion, Na+, has number densities about 10% that of H+. Solar wind ions retain near‐solar‐wind abundances with respect to H+ and exhibit mass‐proportional ion temperatures, indicative of a reconnection‐dominated heating in the magnetosphere. Conversely, planetary ion species are accelerated to similar average energies greater by a factor of ~1.5 than that of H+. This energization is suggestive of acceleration in an electric potential, consistent with the presence of a strong centrifugal acceleration process in Mercury's magnetosphere.