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.

MESSENGER observations of the dayside low‐latitude boundary layer in Mercury's magnetosphere

“…Moreover, the study showed that the large majority of the KH waves occurred for northward IMF. Different theories explain the asymmetry observed, where two are connected either to an asymmetric mass loading in the velocity shear layer where the KH instability forms [e.g., Anderson et al , ; Sundberg and Slavin , ] or to the finite Larmor radius (FLR) effects and the broadening of the shear layer on the dawnside magnetopause [e.g., Glassmeier and Espley , ; Nakamura et al , ; Gershman et al , ; Gingell et al , ]. However, the asymmetry is still viewed as an open issue, and both theories need to be confirmed by further observations.…”

Pickup ion processes associated with spacecraft thrusters: Implications for solar probe plus

“…A consequence of this is that where usually the planetary atmosphere is an important source of ions, at Mercury the magnetosphere couples directly to the planet's surface which causes the surface to act as both a sink for ions and also a source as ions are sputtered away [Koehn et al, 2002]. These pickup ions, especially the sodium ions, tend to have gyro-radii within an order of the size of the Hermean magnetosphere and therefore kinetic effects become important [Gingell et al, 2015, Trávnícek et al, 2010.…”

Interplay between Kelvin–Helmholtz and lower-hybrid drift instabilities

“…2006; Faganello et al. 2008 a ; Matsumoto & Seki 2010; Leroy & Keppens 2017) or by adopting a hybrid kinetic model but neglecting electron inertia (Gingell, Sundberg & Burgess 2015). In this case, however, the LHDI cannot grow since the FGM scales as (Gary & Sgro 1990), where is the wave number associated to the FGM and the mass ratio between ions and electrons, and stabilizes more and more when the density gradient scale length becomes larger than a few ion inertial lengths (Gary 1993).…”