The impact of a hot sodium ion population on the growth of the Kelvin-Helmholtz instability in Mercury's magnetotail

Abstract:

Observations of Mercury's local plasma environment by MESSENGER have revealed that the planet hosts a strongly asymmetric magnetosphere as a result of an off-axis dipolar or quadrupolar internal field, and significant finite Larmor radius effects at the boundary layer between magnetospheric and solar wind plasma environments. One important asymmetry appears in the growth and evolution of Kelvin-Helmholtz (K-H) waves at the dawn and dusk flanks of the magnetopause. Linear analysis and global hybrid simulations support a dusk-dawn asymmetry in the growth rate caused by finite Larmor radius effects, and indeed K-H waves have been exclusively observed at the dusk magnetopause during northward IMF. Observations of these K-H waves at sodium gyro-scales invites investigation into the impact of the hot planetary sodium ion population, itself distributed preferentially on the dusk flank, on the growth of the K-H instability and associated plasma transport. We present local two- and three-dimensional hybrid simulations of the dusk and dawn boundary layers, with varying magnetospheric sodium ion number density, and examine the associated changes in the growth rates of the K-H instability, K-H wave spectra, and cross-boundary particle transport. We show that gyroresonance between growing K-H vortices and sodium ion gyration introduces a strong spectral peak at sodium gyro-scales at the dusk magnetopause, that an increase in sodium ion number density increases dawn-dusk asymmetry of K-H growth rates, and that cross-boundary particle transport increases with sodium number density.