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- Title
The Long‐Term Flux of the Solar Wind Suprathermal Ions That Precipitate on the Lunar Surface.
- Authors
Nénon, Q.; Raines, J. M.; Poppe, A. R.
- Abstract
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%5f32). This agreement validates the conclusions of Nénon 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. Plain Language Summary: The lunar surface is directly exposed to space and constantly bombarded by positively charged particles known as "ions." These particles in turn modify the optical, physical, and chemical properties of the planetary surface. In this article, we use 17 years of ion observations by the Wind spacecraft to constrain the long‐term‐averaged characteristics of the ions flowing in the solar wind with a velocity much greater than the solar wind group speed. Owing to their greater velocity, these fast ions can modify the surface at larger depth than typical solar wind particles. We find that the long‐term 17‐year flux of fast ions observed by Wind is consistent with an 11‐month estimate previously published in 2001. We also find that the fluxes of ions flowing in and out of the ecliptic plane, where most planets orbit, become similar above a minimum ion velocity. This finding reveals that the interiors of the lunar polar craters, which are not exposed to the typical solar wind, are in fact altered by at least these out‐of‐ecliptic ions. The presented characterization of precipitating ions is a first step toward understanding the alteration of polar craters by space plasma ions, including the sources and sinks of water ice. Key Points: Wind‐STICS constrains the long‐term flux of the suprathermal solar wind ions that weather the lunar surfaceThe 17‐year‐averaged energy spectra of protons, helium ions, and oxygen ions are in agreement with an 11‐month estimate published in 2001Lunar Permanently Shadowed Regions are weathered by the same flux of >15 keV/nucleon ions as anywhere else on the lunar surface
- Subjects
SOLAR wind; LUNAR surface; HELIUM ions; LUNAR craters; FAST ions; ION migration &; velocity
- Publication
Journal of Geophysical Research. Planets, 2023, Vol 128, Issue 11, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2023JE007958