Metallic Ions Near the Moon: Impact of Solar Activity and Lunar Position

Abstract

Metallic ions are commonly found in the cis-lunar environment, primarily produced through the neutral lunar exosphere. They become prevalent species of lunar pickup ions as the Moon moves through the solar wind upstream, magnetosheath, and magnetotail. Extensive studies on the composition of lunar pickup ions from the Lunar Atmosphere and Dust Environment Explorer and THEMIS-ARTEMIS missions have revealed the significant presence of ions with around 28 and 40 amu near the Moon, which are later identified as metallic species such as Al⁺, Si⁺ and K⁺ ions. However, while these studies have provided valuable insights, the abundance of metallic ions and their variations with the Moon's location and solar activity has yet to be understood. This study calculates the production and ionization rates of metallic ions based on in-situ THEMIS-ARTEMIS observations. Our analysis indicates that the magnetosphere effectively reduces the production of metallic neutrals and ions due to the reduction of ionization and sputtering rates. The statistical analysis of the 12-year data set further shows that the lunar pickup ion fluxes are not heavily reliant on solar activity, and the median values remain relatively consistent over time. Therefore, the source rates of metallic pickup ions are associated with the location of the Moon rather than being dependent on solar activity. The outflow rates of heavy ion species from the Moon are comparable with the molecular and metallic ion rates from Earth's ionosphere, suggesting their essential roles in the dynamics of heavy ions in Earth's terrestrial environment.