Mercury's sodium exosphere: Magnetospheric ion recycling

Abstract

A three-dimensional Monte Carlo model of Mercury's neutral sodium exosphere was used to describe correlation between the observed variations in the exospheric density and the variations in the surface concentration of sodium along Mercury's motion around the Sun [Leblanc and Johnson, 2003]. Four processes of ejection were taken into account: thermal and photon stimulated desorptions, solar wind sputtering, and micrometeoroid vaporization. Here a model of ion circulation in Mercury's magnetosphere developed by Delcourt et al. [2002, 2003] is used to examine the contribution from sodium magnetospheric ion recycling. The model is coupled to the exospheric model to track newly ionized sodium formed in Mercury's exosphere. The coupled Na and Na+ model is used to examine the sputtering by and implantation of the reimpacting ions. The magnetospheric ion sputtering does not contribute significantly to the total amount of Na atoms ejected into Mercury's exosphere because of its rather small flux; however, magnetospheric ion implantation in Mercury's surface is sufficiently concentrated inside narrow latitude bands to enhance the local surface concentration of sodium. This enhancement is seen to contribute to the peaks in Mercury's exospheric sodium emission observed at high latitudes when the implanted sodium is exposed to the solar flux near the dayside terminator. Copyright 2003 by the American Geophysical Union.