Solar Activity Dependence of the Dayside Lunar Surface Potential in the Terrestrial Magnetotail

Abstract On the dayside of the Moon, the surface potential is primarily determined by (a) photoelectron emission by solar radiation and (b) the ambient plasma environment. Both of these drivers have large variability, but the resulting variability of the lunar surface potential has not been fully characterized yet. To investigate the solar activity dependence of the dayside lunar surface potential in the terrestrial magnetotail, we compared long‐term observations of lunar surface photoelectrons with numerical calculations of photo‐emitted electron energy spectra. The comparison demonstrates that the observed solar activity dependence of lunar photoelectrons is well reproduced by the model. The current balance surface potential based on this model predicts more positive potentials for higher solar activities, possibly exceeding in the magnetotail lobes during solar flares. These results highlight the importance of evaluating the lunar electrostatic environment under various conditions, including extreme solar events.