Magnetospheric protons and electrons encountered by the Moon in the plasma sheet

During its passage through the geomagnetic tail, the Moon encounters the plasma sheet. Properties of plasma sheet electrons and protons, first detected at lunar distances by Explorer 35 are described. The electrons have a rapidly fluctuating non-Maxwellian energy distribution with a mean energy of several hundred electron volts and density approximately 0.2cm-3. The protons, of energy approximately 1keV, were usually detected above the instrument background when flowing towards the Earth at approximately 200km s-1. Implications for migration of grains on the lunar surface are also pointed out and it is suggested that strong terrestrial polar winds in the early history of the Earth-Moon system may have caused some erosion of the Earth-facing side of the Moon, and that gravitational shielding of interplanetary rock flux by the Earth may also be an explanation of the relative smoothness of the front side