Modeling Photoelectron and Auger Electron Emission From the Sunlit Lunar Surface: A Comparison With ARTEMIS Observations

Abstract

Due to the lack of a dense atmosphere, the Moon directly interacts with ambient plasmas and solar radiation, leading to lunar surface charging. Solar X-rays drive the emission of photoelectrons and Auger electrons from the lunar surface to space. The Auger electrons have characteristic energies intrinsic to the photo-emitting atoms and were recently identified at the Moon by Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) observations. In this study, we developed a numerical model of the energy spectrum of lunar photoelectrons and Auger electrons, thereby comparing the predicted and observed energy spectra. By adjusting a scaling factor, the model well reproduces the ARTEMIS observations obtained in the solar wind, where the energy spectra are minimally affected by surface charging. Meanwhile, the energy spectra obtained in the geomagnetic tail can be significantly altered by lunar surface potentials. We show that it is difficult to determine a unique combination of the scaling factor and the lunar surface potential with the ARTEMIS energy resolution because of a strong parameter degeneracy. Nevertheless, for a fixed scaling factor, a strong correlation is identified between the lunar surface potentials inferred from the shifts of the energy spectra and those from the upward photoelectron beam energies, providing a proof of concept for the use of the photo-emitted electrons as a new remote sensing tool of the lunar surface potential. We advocate for future observations of lunar electrons with a high energy resolution.