Monte Carlo modeling of north-south asymmetries in Mercury's sodium exosphere

摘要

Ground-based observations and MESSENGER's first flyby show persistent asymmetries in Mercury's neutral sodium tail, with brightest emission often seen in the northern lobe. This feature may be associated with the recent finding that the magnetic dipole moment is offset from the planet's center by 0.2 R _M to the north, while approximately aligned with the spin axis. Such a configuration produces an asymmetry in the magnetosphere cusp whereby more plasma has direct access to the planet's southern hemisphere than the north. Using 3-D numerical modeling, it is demonstrated that ion precipitation, enhanced in the south, can result in the observed Na emission profiles across the tail that are brighter to the north. However, sources located at high-latitude cusp footprints on the dayside are unable to match the observed width of the brightness profiles across the tail. Instead, these simulations provide evidence for lower latitude sources on the dawnside, resulting from the accumulation of sodium during the long Mercury night. Photodesorption, rather than ion sputtering, is determined to be the responsible mechanism for this population's release and escape from the planet's surface.