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 (similar to 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. Mercury's exosphere and magnetosphere contain various heavy planetary elements. These elements are released from the planet's surface due to different space weathering processes. One notable process is the direct bombardment of the solar wind ions on the planetary surface through open magnetic field lines (i.e., the polar cusp), which releases neutral atoms with low energies around similar to eV. Our simulation results indicate that these low energy similar to eV atoms may travel a significant distance before their photoionization or return to the surface. Consequently, Na atoms originating from the polar cusps can supply ions to different regions in the vicinity of Mercury. This mechanism offers an explanation for the concentration of sodium group ions in the dayside ring current, as observed by MESSENGER. Approximately 2.7% of the solar wind-sputtered sodium atoms directly contribute to the magnetospheric ions Cusp-originated sodium atoms can supply Na+ ions to each magnetospheric region due to their sufficient initial velocity The pre-ionization transport of sodium atoms could potentially account for the observed dayside ring current Na+-group ions