ARTEMIS Observations of Proton Scattering off the Lunar Surface

Solar wind protons that have been scattered off the lunar surface constitute an important plasma population in the lunar space environment [1]. To better understand the scattering process as well as the effects of the scattered protons, we here aim to constrain the scattering characteristics.We study the characteristics of scattered protons from the Moon using data from the ARTEMIS spacecraft, and put our results in the context of previous findings from Kaguya [2] and Chandrayaan-1 [3]. We study individual cases in detail, and proceed to characterize the scattering comprehensively using data collected over several years.Our observations are generally consistent with expectations from previous studies: we confirm a scattering rate of 0.1%-1% [c.f. 2] and an energy spectrum with a peak at 60%-70% of the incident solar wind energy [c.f. 3]. The observed directional scattering function appears consistent with that of scattered neutral hydrogen atoms [c.f. 4]. However, we observe a weaker dependence on the solar wind speed than reported by [3].From these observations, we make updated empirical models for solar wind scattering off the lunar surface. We also discuss possible explanations for the weaker solar wind speed dependence observed here. Finally, we discuss implications of these results for the scattering mechanism.[1] Nishino et al., GRL, 2010[2] Saito et al., GRL, 2008[3] Lue et al., JGR, 2014[4] Schaufelberger et al., GRL, 2011