Inferring lunar dayside surface potential from modeled and observed photoelectrons

The lunar dayside surface is constantly bombarded by solar photons and ambient charged particles, inducing a variety of surface processes (e.g., photoemission, secondary electron emission, sputtering, and absorption of charged particles). The resulting charge transfer between the lunar surface and space causes surface charging such that electric currents into and out of the surface are balanced. Lunar surface charging is both scientifically and practically important (human exploration). The nightside surface charging has been well characterized by previous studies, but not so much for the dayside, mainly because of a poor understanding of lunar photoelectrons and a lack of a robust methodology. Recently, oxygen Auger photoelectrons emitted from the lunar surface have been observed by the ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) spacecraft, which provides a unique feature to identify lunar photoelectrons (PHE) and infer the dayside surface electrostatic potential. With a combination of observations from the ARTEMIS mission and modeling efforts, we can determine the lunar surface potential. In this study, we provide the first statistical analysis of the lunar dayside surface potential when the Moon is located in the Earth’s magnetotail lobes and how it varies with solar zenith angles, local plasma density, and electron temperatures.