High-energy particle observations from the Moon

The Moon is a unique location to study the deep space plasma environment. During most part of its orbit around the Earth it is directly exposed to the solar wind. Due to the absence of a substantial intrinsic magnetic field and of a collisional atmosphere, solar wind and solar energetic particles arrive almost without any deviation or absorption and impact directly on its surface, interacting with the lunar regolith and the tenuous lunar exosphere. Energetic particles arriving at the Moon’s surface can be absorbed, or scattered, or can remove another atom from the lunar regolith by sputtering or desorption. The same phenomenon occurs also with the galactic cosmic rays, which present fluxes and energy spectra typical of interplanetary space. During 5 – 6 days every orbit, however, the Moon crosses the tail of the terrestrial magnetosphere. It then offers the possibility to study in-situ the terrestrial magnetotail plasma environment as well as atmospheric escape from the Earth’s ionosphere, in the form of heavy ions accelerated and streaming downtail. The lunar environment offers thus a unique opportunity to study the interaction of the solar wind, the cosmic rays and the magnetosphere with the surface, the immediate subsurface, and the surface-bounded exosphere of an unmagnetized planetary body.