On solar radiation-driven surface transport of sodium atoms at mercury

The ballistic motion of the exospheric sodium atoms on the surface of Mercury is modeled, taking into account the solar radiation pressure acceleration and partial surface thermal accommodation. The Monte Carlo simulations show that there should be a significant degree of limb brightening as well as brightness enhancement over the poles. To maintain the observed sodium optical emission, a surface production rate on the order of 5-9 × 10²⁴ atoms s^-1 is needed. It is also found that, under the present set of assumptions, a reasonable agreement can be reached between theoretical results and ground-based measurements for the dependence of the disk-averaged abundance of the sodium atoms on the solar radiation pressure acceleration. If the low-latitude portion of the planetary surface is shielded from the magnetospheric convective electric field, the effective loss rate of the sodium atoms via photoionization and magnetospheric pickup may be reduced to about 2 × 10²⁴ atoms s^-1, with the polar regions acting as the main area of ion outflows.