Statistical study of non-adiabatic energization and transport in Kelvin-Helmholtz vortices at mercury

Abstract

Non-adiabatic energization and transport of planetary ions in the vicinity of Kelvin-Helmholtz (KH) vortices at Mercury are statistically investigated using a magnetohydrodynamic model combined with a test particle tracing technique. The measurements from the MErcury Surface, Space ENvironment, Geochemistry, and Ranging spacecraft show signatures of KH waves in the magnetospheric flanks, especially on the duskside. We examine here the energization and transport of heavy ions of planetary origin such as Na+ associated with the growth of the KH instability on both the dawn and dusk sides of the magnetosphere, and for both northward and southward interplanetary magnetic field configurations. We find that in all cases those planetary ions in the magnetosphere can be accelerated non-adiabatically with a level of acceleration larger on the dawnside. The transport of those ions is controlled by the convection electric field in the magnetosheath. Those planetary ions when picked up in the duskside magnetosphere where the KH waves have been mainly observed at Mercury stagnate inside the magnetosphere and, hence, are not expected to escape. The present study indicates that these ions may likely stay inside the magnetosphere and sputter the planetary surface on the duskside preferentially, being the source of secondary neutrals from the planetary surface.