The Long-Term Flux of the Solar Wind Suprathermal Ions That Precipitate on the Lunar Surface

Solar wind ions weather the optical, physical, and chemical properties of the lunar surface. While the solar wind ion number density is dominated by the thermal population, the study of suprathermal ions is important to understand weathering effects that may develop at depth larger than the penetration depth of the bulk solar wind. Possible manifestations of suprathermal ion weathering include the creation of thick amorphous rims, contamination of isotopic ratios at depth, and alteration of the optical reflectance at infrared wavelengths. Furthermore, while the thermal population forms a beam parallel to the ecliptic plane, suprathermal ions may have a large northward or southward velocity component and can therefore directly access and weather the lunar polar regions, including the interiors of the Permanently Shadowed Regions (PSRs). In this article, we constrain the long-term properties of the solar wind suprathermal ions using 17 years of observations by the SupraThermal Ion Composition Spectrometer onboard the Wind spacecraft (Wind-STICS). We find that the 17-year omnidirectional energy spectra of protons, helium ions, and oxygen ions observed by Wind-STICS are in excellent agreement with the 11-month estimates published by Mewaldt et al. (2001, https://doi.org/10.1063/1.1433995) and Mewaldt et al. (2007, https://doi.org/10.1007/978-0-387-74184-0_32). This agreement validates the conclusions of Nenon and Poppe (2020, https://doi.org/10.3847/PSJ/abbe0c) who proposed that suprathermal ions heavier than helium may increase the rate of amorphous rim formation in lunar samples by 50%. Furthermore, the Wind-STICS observations of out-of-ecliptic ions reveal that the lunar PSRs are weathered by the same flux of >15 keV/nucleon as anywhere else on the lunar surface.