Structure and composition of the distant lunar exosphere: Constraints from ARTEMIS observations of ion acceleration in time‐varying fields

Abstract

By analyzing the trajectories of ionized constituents of the lunar exosphere in time‐varying electromagnetic fields, we can place constraints on the composition, structure, and dynamics of the lunar exosphere. Heavy ions travel slower than light ions in the same fields, so by observing the lag between field rotations and the response of ions from the lunar exosphere, we can place constraints on the composition of the ions. Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) provides an ideal platform to utilize such an analysis, since its two‐probe vantage allows precise timing of the propagation of field discontinuities in the solar wind, and its sensitive plasma instruments can detect the ion response. We demonstrate the utility of this technique by using fully time‐dependent charged particle tracing to analyze several minutes of ion observations taken by the two ARTEMIS probes ~3000–5000 km above the dusk terminator on 25 January 2014. The observations from this time period allow us to reach several interesting conclusions. The ion production at altitudes of a few hundred kilometers above the sunlit surface of the Moon has an unexpectedly significant contribution from species with masses of 40 amu or greater. The inferred distribution of the neutral source population has a large scale height, suggesting that micrometeorite impact vaporization and/or sputtering play an important role in the production of neutrals from the surface. Our observations also suggest an asymmetry in ion production, consistent with either a compositional variation in neutral vapor production or a local reduction in solar wind sputtering in magnetic regions of the surface.

ARTEMIS observations of the solar wind proton scattering function from lunar crustal magnetic anomalies

“…For the sputtered lunar neutral exosphere, Poppe et al [] made theoretical predictions for the degree to which exospheric neutral densities may be affected by the presence of crustal magnetic anomalies, finding that densities can be diminished by over an order of magnitude in crustal magnetic anomaly regions. Preliminary evidence for such an effect has been found in analysis of exospheric pickup ion distributions observed by the ARTEMIS mission [ Halekas et al , ]. Thus, the solar wind proton reflection percentage map constructed here is critical in correlating reduced solar wind sputtering fluxes with variations in both the fluxes of energetic neutral atoms and exospheric pickup ions.…”

“…Crustal magnetic anomalies are also believed to play a role in locally suppressing the formation of the lunar neutral exosphere by decelerating and/or reflecting solar wind protons such that charged‐particle sputtering of the lunar surface is locally diminished [ Poppe et al , ]. Preliminary evidence for this phenomenon has been identified in ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) observations of exospheric pickup ion distributions at the Moon [ Halekas et al , ]. Finally, the reflection of solar wind protons from crustal anomalies has significant impact on the near‐lunar plasma environment.…”

A Comprehensive Model for Pickup Ion Formation at the Moon

“…Compared to a Maxwellian distribution, the S‐T distribution has an extended tail to higher energies leading to a more extended neutral distribution (see also Wurz et al., 2007). Furthermore, sputtered species are only considered to be emitted from the dayside of the Moon despite the potential presence of off‐axis particle fluxes in the terrestrial foreshock (e.g., Nénon & Poppe, 2021; Nishino et al., 2017) We also do not consider second‐order effects in the sputtered distribution at the Moon from either self‐sputtering (Poppe, Halekas, Sarantos, & Delory, 2013) or local shielding by crustal magnetic anomalies (e.g., Vorburger et al., 2012; Poppe et al., 2014; Halekas et al., 2016). Sputtered species include carbon (C), oxygen (O), sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), potassium (K), calcium (Ca), titanium (Ti), and iron (Fe).…”

“…The Wind spacecraft, during a series of lunar flybys en route to L1, detected several instances of pickup ions originating from the Moon primarily at mass 16 (O + or $${\mathrm{C}\mathrm{H}}_{4}^{+}$$), mass 27 (Al + ), and mass 28 (Si + or CO + ) (Mall et al., 1998). More recent measurements of lunar pickup ions have come from Kaguya (Tanaka et al., 2009; Yokota et al., 2009, 2014a, 2020), Chang’E−1 (Wang et al., 2011), the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission (Halekas et al., 2012, 2013, 2016, 2018; Harada et al., 2015; Liuzzo et al., 2021; Poppe et al., 2012, Poppe, Halekas, Samad, et al., 2013; Zhou et al., 2013), and the Lunar Atmospheric and Dust Environment Explorer (LADEE) mission (Halekas et al., 2016; Poppe et al., 2016). Together, these observations have confirmed the presence of multiple additional exospheric species including $${\mathrm{H}}_{2}^{+}$$, He + , C + , O + , Ne + , Na + , Al + , Si + /CO + , K + , and 40 Ar + .…”

New suprathermal proton population around the Moon: Observation by SARA on Chandrayaan‐1

“…Solar wind plasma interacts continuously with the lunar surface thereby controlling the lunar plasma environment [Halekas et al, 2011;Bhardwaj et al, 2015]. Observations from different missions have shown the presence of several populations of energetic ions around the Moon, such as solar wind protons scattered from the dayside lunar surface [Saito et al, 2008;Holmström et al, 2010] and magnetic anomalies [Lue et al, 2011;Saito et al, 2010], and lunar exospheric ions picked up by the solar wind [Hilchenbach et al, 1992;Mall et al, 1998;Wang et al, 2011;Halekas et al, 2013Halekas et al, , 2015Halekas et al, , 2016Poppe et al, 2016].…”