Evidence for negative charging of the lunar surface in shadow

Observations of electron distributions above the shadowed surface of the Moon show energy-dependent loss cones which indicate reflection by both magnetic and electric fields. At the same time, low energy (≲100 eV) field aligned upward-going electron beams are observed. Together, these observations imply average night-side potential differences between the surface and the Lunar Prospector (LP) spacecraft of ∼-35 V. The lunar surface may be at an even higher negative potential relative to the ambient plasma, since LP will likely also charge negative. The potential difference is consistent with simple current balance models which include secondary emission. No clear dependence is found on surface terrane type and age, or on ambient electron density and temperature. Instead, the potential difference is found to depend strongly on the angle from the subsolar point and the angle between the magnetic field and the normal to the lunar surface.