Neutral and ion-exospheres in the solar wind with applications to mercury

A model of planetary neutral and ion-exospheres in the solar wind is formulated for weak or lunar like solar-wind interaction with a planet. The neutral exosphere model allows for density and temperature variations and for rotation at the exobase. The ion-exosphere is produced by ionization of the neutral exosphere in the solar wind and its density distribution is obtained by solving the continuity equation in the drift approximation. Applying to Mercury a surface temperature distribution inferred from infra-red data and a vanishing bound neutral flux at the base, He and He⁺ density distributions are found. When the He atmosphere of Mercury is due entirely to the surface bombardment by solar wind He⁺⁺, the resulting He⁺ density is found to vary from 1.5 × 10^?1 to 10^?3 cm^?3 over the range 1.5–5 planetocentric radii on the dayside. These densities are found to be detectable by typical solar-wind plasma instruments. The possible effects of cyclotron-resonant scattering by interplanetary magnetic field fluctuations are examined and shown to be negligible. An electromagnetic plasma instability, triggered by the birth of ions in the exosphere, is shown to be important for the thermalization of the energy mode transverse to the interplanetary magnetic field, allowing more ions to be detected by solar-wind ion probes.