Lunar Photoemission Yields Inferred From ARTEMIS Measurements

Abstract

Photoemission yield (the number of emitted electrons per incoming photon) is one of the fundamental properties of solid materials but is not yet well constrained for the lunar surface for photon energies >∼20 eV. In this study, we constrain this yield for incident photons with energies of ∼10–500 eV with data from the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission along with solar irradiance spectra from Version 2 of the Flare Irradiance Spectral Model. We also report the first oxygen Auger electron observations at the Moon by the ARTEMIS spacecraft, which provides a unique feature to identify photoelectrons emitted from the lunar surface. With lunar photoelectron observations identified in both Earth's magnetotail lobes and the solar wind for four selected days, we infer a lower bound of 10−3 in yield for photon energies >∼20 eV. However, our investigation also reveals an uncertainty over ∼4 orders of magnitude in derived yields with a sensitivity study, owing to a poorly constrained photoelectron energy probability function. This uncertainty motivates future experiments on lunar samples to better characterize the lunar surface charging environment.

Characteristics of Lunar Surface Electrons Inferred From ARTEMIS Observations: 1. Backscattered Electrons

“…(1972), and also two photoemission yields shown in Figure 7 in Xu et al. (2021) (referred as “high” and “low” yields because of the uncertainty in surface potentials). The modeled photoelectron energy spectra are shown as the green dashed lines in Figure 1, lower and higher fluxes corresponding to low and high photoemission yields, respectively.…”

“…To remove the possible contamination of PHE in our selected BSE cases, we employ the methodology described by Xu et al. (2021) to model the PHE energy spectrum. More specifically, we use the solar irradiance spectra from Version 2 of the Flare Irradiance Spectral Model (FISM2) (Chamberlin et al., 2007, 2008, 2020), the probability function adopted from Feuerbacher et al.…”

“…Among electron populations that are emitted from the lunar surface, secondary electrons have been well characterized by previous studies (e.g., Halekas et al, 2009a) but the BSE and photoelectrons that make up the high-energy tail have been less well characterized. Recently, Xu et al (2021) reported oxygen Auger electron observations at the Moon by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft, reproduced in the modeling effort by Kato et al (2023), which provides a unique feature to identify lunar photoelectrons (PHE). Thus, this spectral feature can be used to separate photoelectrons from the BSE.…”

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

“…We first present a case study of ARTEMIS observations in the magnetotail lobe region (Figure 5). This event is one of the reported Auger electron events by Xu et al (2021) and was analyzed by Harada et al (2017), who derived the lunar surface potential U M of +15-25 V based on the observed energy-dependent loss cone of electrostatically reflected lobe ions (ion reflectometry). For a non-zero potential difference, we shift the 1-eV resolution model energy spectrum such that the distribution function is conserved, and the shifted spectrum is convolved with the instrument response to derive the model-predicted measurements.…”

“…In this section, we briefly introduce one of the Auger electron observations reported by Xu et al. (2021). Figure 1 shows a case of Auger electron observations in the solar wind.…”