Developing an emission model of photoelectrons and Auger electrons from the sunlit lunar surface and its application to evaluation of the lunar electrostatic environment

Since the Moon does not possess its intrinsic magnetic field and thick atmosphere, the ambient plasma near the Moon interacts with the lunar surface directly. Above the sunlit lunar surface, photoelectrons and Auger electrons are emitted by solar irradiation. We developed a numerical model of emissions of these electrons, which is derived from the data of solar photon flux, photoionization cross sections of elements, and the chemical composition of the lunar surface. Comparing our model with observational data from Electrostatic Analyzer (ESA) onboard ARTEMIS spacecraft, the model successfully reproduces the observed energy spectrum within one order of magnitude. Since Auger electrons have intrinsic energies characteristic of the emitting elements and molecules, it was proposed that the observations of Auger electrons can apply to chemical composition mapping of the dayside lunar surface. Our model may contribute to explorations of the surface composition. It is also thought that electrostatic potential difference between the spacecraft and the lunar surface can be detected by comparing observed energy spectra with the predicted energy characteristics of photoelectrons and Auger electrons from our model.