Photoemission and electrostatic potentials on the dayside lunar surface in the terrestrial magnetotail lobes

Abstract

Despite the need to accurately predict and assess the lunar electrostatic environment in all ambient conditions that the Moon encounters, photoemission and electrostatic potentials on the dayside lunar surface in the terrestrial magnetotail lobes remain poorly characterized. We study characteristics and variabilities of lunar photoelectron energy spectra by utilizing Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) and Apollo measurements in combination with the Flare Irradiance Spectral Model (FISM). We confirm that the photoelectron spectral shapes are consistent between ARTEMIS and Apollo and that the photoelectron flux is linearly correlated with the FISM solar photon flux. We develop an observation‐based model of lunar photoelectron energy distributions, thereby deriving the current balance surface potential. The model predicts that dayside lunar surface potentials in the tail lobes (typically tens of volts) could increase by a factor of 2–3 during strong solar flares.

Lunar Photoemission Yields Inferred From ARTEMIS Measurements

“…However, as pointed out by Harada et al. (2017), ion reflectometry is not always available such that the dayside surface potential is hard to determine while in the magnetotail lobes. Meanwhile, the correction to this relatively large surface potential in the magnetotail lobe cannot be neglected.…”

“…Photoelectron fluxes (blue) in Figure 3a are significantly lower than that in Figure 3b because the lunar dayside surface potential is higher in the lobe, which not only traps a greater fraction of the emitted photoelectrons near the surface but also decelerates escaping electrons. The lunar surface potential (U m ) in the magnetotail lobe case is determined to be 15-25 V by Harada et al (2017) with ion reflectometry (an energy-dependent loss cone in the ion distribution) while U m is usually between 0 and 10 V in the solar wind (e.g., Freeman & Ibrahim, 1975;Manka & Michel, 1973;Poppe & Horányi, 2010). However, as pointed out by Harada et al (2017), ion reflectometry is not always available such that the dayside surface potential is hard to determine while in the magnetotail lobes.…”

“…Harada et al. (2017) reported photoelectron observations by ARTEMIS Probe 1 when the Moon was in Earth's magnetotail lobe. It is worthwhile to re‐examine this case study.…”

“…(2017)). Inside the potential sheath, currents are dominated by low‐energy photoelectrons (the cold core population) such that the dayside surface potential generally ranges from 0 to ∼10 V (e.g., Halekas, Saito, et al., 2011; Halekas et al., 2008), and (2) in the magnetotail lobes, the ambient plasma density is so low that the high‐energy (>10 eV) tail population of photoelectrons become important (Harada et al., 2013; Pedersen, 1995), equivalent to a higher effective electron temperature that could lead to large (a few tens of Volts) positive surface potentials (Harada et al., 2017).…”

Electrostatic Waves and Electron Heating Observed Over Lunar Crustal Magnetic Anomalies

“…Moreover, solar wind sputtering from the lunar surface and ionization of the tenuous neutral exosphere can produce heavier lunar pickup ions, which can then be accelerated downstream from the Moon by the motional electric field (Cao, Halekas, Poppe, et al., 2020; Cao, Halekas, Chu, et al., 2020; Halekas, Poppe, Delory, Sarantos, et al., 2012; Yokota et al., 2009). Some other examples of lunar interaction include backscattering of solar wind ions and photoelectron emission from the lunar surface (Bhardwaj et al., 2015; Goldstein, 1974; Harada et al., 2017; Lue et al., 2014; Reasoner & Burke, 1972).…”