Metals in Earth’s Magnetosphere: From Ionosphere or Moon?

Recent observations by the Geotail and Cluster missions in the magnetosphere have revealed the presence of singly charged metallic ions, such as Mg⁺ and Fe⁺. However, the origins and transport mechanisms of these metallic ions are unknown. Metallic ions are prevalent in the Earth's lower atmosphere, primarily produced from the ablation of meteoroids and the formation of metal layers. Similarly, metallic ions are also common in the vicinity of the Moon's surface, where they are created from the ionization of the lunar exosphere, a thin neutral atmosphere that surrounds the Moon. To deepen our understanding of the sources of the metallic ions in the magnetosphere, this study develops different strategies to evaluate the contributions of metallic ion outflow from the ionosphere and the Moon across different solar and geomagnetic conditions. 

 

To estimate the metallic ion outflow from the ionosphere, we utilize a physics-based model, PWOM, which solves the transport of ionospheric outflow for all the relevant ion species, such as H⁺, He⁺, N⁺, O⁺, and molecular ions. As metallic ions are mainly produced via charge exchange between molecular ions and metal atoms in the ionosphere, the abundance of metallic ion outflow is represented by the densities of molecular ions with adjusted ratios. In addition, the metallic ion upflow will be further accelerated to outflow in the high-altitude ionosphere via the energization of wave-particle interaction. This vertical transport of metallic ions is further tracked by PWOM and shows a strong connection with the wave energy input.

 

On the other hand, metallic ion outflow from the Moon is inferred from LADEE and ARTEMIS observational data. These pickup ions are generated through charge exchange or electron impact from the metallic neutrals, which are primarily produced through either charged particle sputtering or micrometeoroid impact vaporization of the lunar surface. The abundances of metallic neutrals are derived from information on sputter yields and observed ion fluxes. Once the densities of neutral metal species are obtained, the metallic pickup ion fluxes are further calculated based on the relevant ionization cross sections with incoming electron and ion fluxes to the lunar exosphere. We use in-situ ARTEMIS observations of ion and electron fluxes to properly calculate these ionization rates. 

 

This study will be the first to compare the contributions of ionospheric outflow and lunar pickup ions to the magnetosphere. Moreover, by utilizing observational data from ARTEMIS, the variations of sources of metallic ions will be analyzed as the Moon passes through the upstream solar wind, magnetosheath, and magnetotail. The differences between the outflow of metallic ions from the ionosphere and the Moon can be used as a tracer to better understand the transport and energization processes of heavy ion plasma in the magnetosphere.