Electron Heating Near the Lunar Surface and Implications for Wave-Particle Interactions

The Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) mission has been in orbit around the terrestrial moon for over a decade. Within this time the spacecraft has collected a wealth of data on the lunar plasma environment, providing new insights into moon-plasma interactions occurring near the lunar surface. One such moon-plasma interaction is related to electron heating. Incident solar wind plasma can be reflected near lunar crustal magnetic fields or absorbed into the lunar regolith near unmagnetized regions. Additionally, secondary electrons and photoelectrons can also be emitted from the lunar surface. The modification of the incident solar wind ion and electron dynamics as well as the production of secondary and photoelectrons can lead to unstable particle distributions that in turn can excite a variety of plasma waves which can heat the ambient electrons. Previous studies suggest that perpendicular heating can be accomplished through wave-particle interactions with electrostatic waves generated by the electron cyclotron drift instability (ECDI) as well as electromagnetic waves. Meanwhile, parallel heating can be accomplished through wave-particle interactions with electrostatic waves generated by the electron two-stream and modified two-stream instabilities. This study focuses on characterizing the near lunar surface plasma environment using ten years of ARTEMIS data when the spacecraft was less than 200 km above the lunar surface. We find that electrostatic wave emissions consistent with ECDI generation plays an increasingly important role in perpendicular electron heating as the reflected ion density increases, while electromagnetic interactions display the opposite trend. Furthermore, the electrostatic waves associated with parallel heating exhibit plasma characteristics that are consistent with both the electron two-stream and modified two-stream instabilities. These results showcase similarities between wave-particle interactions that lead to electron heating near quasi-perpendicular shocks with moon-plasma interactions occurring near the lunar surface.