Dynamics of Sputtered Neutral Sodium Atoms in the Near-Mercury Space

The solar wind sputtering in the magnetospheric polar cusp is an important source of heavy atoms in Mercury’s exosphere and magnetosphere. However, the majority of ejected atoms are neutral, undergoing an extended period before photoionization occurs. In this study, we employ an ab initio simulation to investigate the behavior of sodium (Na) atoms prior to their photoionization. Our results reveal that overall only approximately 2.7% of the sputtered atoms contribute to magnetospheric ions, while the vast majority of these ions (∼82.9%) escape into interplanetary space. The remaining fraction (14.4%) eventually returns to the planetary surface. For Na atoms ionized inside the magnetosphere, a larger proportion of Na⁺ (53.5%) is supplied to the magnetotail compared to the polar cusp (39.4%), which is due to the tailward acceleration caused by solar radiation. Additionally, the remaining Na⁺ (7.1%) contributes to the dayside ring current region, as demonstrated by the observation of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Our research introduces a perspective on Na⁺ transport in the magnetosphere that complements and coexists with traditional mechanisms.