Protons in the near-lunar wake observed by the Sub-keV Atom Reflection Analyzer on board Chandrayaan-1

Significant proton fluxes were detected in the near-wake region of the Moon by an ion mass spectrometer on board Chandrayaan-1. The energy of these nightside protons is slightly higher than the energy of the solar wind protons. The protons are detected close to the lunar equatorial plane at a 140° solar zenith angle, that is, ~50° behind the terminator at a height of 100 km. The protons come from just above the local horizon and move along the magnetic field in the solar wind reference frame. We compare the observed proton flux with the predictions from analytical models of an electrostatic plasma expansion into a vacuum. The observed velocity is higher by a factor of 2 to 3 than the velocity predicted by analytical models. The simple analytical models cannot explain the observed ion dynamics along the magnetic field in the vicinity of the Moon