First remote measurements of lunar surface charging from ARTEMIS: Evidence for nonmonotonic sheath potentials above the dayside surface

[1] During an early lunar encounter, ARTEMIS‐P2 passed earthward from the Moon in the terrestrial magnetotail. Fortuitously, though more than 8000 km away, magnetic field lines connected the spacecraft to the dayside lunar surface during several time periods in both the lobe and plasma sheet. During these intervals, ARTEMIS made the first accurate and quantitative remote measurements of lunar surface charging from an observation point almost 100 times more distant than previous remote measurements of surface potentials. ARTEMIS also measured incident plasma, including hot tenuous electrons from a source deeper in the tail, portions of that population mirrored near the Earth, and cold ions from the terrestrial ionosphere. The spatial and temporal variation of these sources, combined with shadowing by the lunar obstacle and motion and curvature of magnetotail field lines, leads to highly variable charging currents to the surface. ARTEMIS measurements provide evidence for negative dayside surface potentials, likely indicative of nonmonotonic sheath potentials above the sunlit surface, in the plasma sheet and, for the first time, in the tail lobe. These nonmonotonic potentials, and the resulting accelerated outward going beams of lunar photoelectrons, may help maintain quasi‐neutrality along magnetic field lines connected to the Moon.