From the Moon to Earth’s Magnetosphere: The Journey of Metallic Ions

The presence of heavy ions in Earth's magnetosphere is crucial for understanding the response of the Earth's terrestrial environment to solar wind driving. The discovery of magnetospheric heavy ions in the 1970s has led to a widespread belief that low-charge state heavy ions in Earth's magnetosphere must originate from the ionosphere. However, this may not be true for all heavy ion species. Observations by the Geotail and Cluster missions have revealed the presence of singly charged metallic ions, such as Mg+ and Fe+ and their abundances were found to vary depending on the Moon's location, as did their density ratios to O+. Therefore, a new source for metallic ions was proposed: the lunar regolith.

Metallic ions are commonly found in the cis-lunar environment, primarily produced through the neutral lunar exosphere. They become a 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 amu near the Moon, which are later identified as metallic species such as Al+ and Si+. The production and ionization rates of metallic ions are derived with the 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. Furthermore, the source rates of metallic pickup ions are associated with the location of the Moon rather than being dependent on solar activity.

By comparing these source rates, our findings suggest that the Moon may contribute more metallic ions to the magnetosphere than the Earth’s ionosphere. Finally, we employed a particle tracing model with time-dependent electromagnetic fields from a global magnetosphere model to trace the metallic ions from the Moon into the magnetosphere. The results of our simulations indicate that metallic ions from the Moon could reach the inner magnetosphere, indicating that the metallic ions observed by the Cluster and Geotail missions may originate from the Moon. Additionally, the Moon’s location plays a crucial role in transporting metallic ions toward Earth, highlighting the presence of rapid Earthward flow in Earth's magnetotail.