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 MErcury Surface, Space ENvironment, GEochemistry, and Ranging 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 almost exclusively observed at the dusk magnetopause during northward interplanetary magnetic field. Observations of these K‐H waves at sodium gyroscales invite 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‐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 gyroscales 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 decreases with sodium number density at the dawn flank.

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ORCID for P.W. Gingell: orcid.org/0000-0003-2218-1909

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