Sodium gyro-resonance with Kelvin-Helmholtz waves at Mercury.
Local hybrid simulations of shear layers in Mercury's magnetotail reproduces spectral signatures of a hot sodium ion population in Kelvin-Helmholtz (K-H) vortices. The gyro-resonance mechanism enhances dawn-dusk asymmetry in the growth of the K-H instability.
Observations of Mercury's local plasma environment by NASA's MESSENGER spacecraft have revealed that the planet hosts a strongly asymmetric magnetosphere. The planet's off-axis internal field and finite Larmor radius physics contribute to differences in the structure and dynamics of the magnetosphere. For example, Kelvin-Helmholtz waves are observed to have a faster growth rate on the dusk flank than the dawn flank. Additionally, Gershman et al. (2015) have revealed an apparent bias towards the growth of K-H vortices at scales associated with the gyration of a hot sodium ion population.
We have reproduced this observed gyro-resonance using a set of a two-dimensional hybrid simulations of the shear layers between the solar wind proton population and the magnetospheric proton and sodium ion populations at both dawn and dusk flanks of the magnetotail. For our simulations, the growth of K-H vortices is suppressed at the dawn flank in the presence of Na+, and the spectrum of K-H waves at the dusk flank features a strong peak at Na+ gyro-scales (Fig 1).
Fig 1 caption: Colour map of solar wind proton density shown sodium-scale vortex growth at the dusk boundary (left) and suppression of vortex growth at the dawn boundary (right).
Our test particle simulations of the interaction of sodium ions with K-H vortices have shown that this gyro-resonance occurs due to acceleration and subsequent trapping of sodium ions by the vortex electric field (Fig 2). At the dawn flank, for which the direction of gyration relative to the vorticity is reversed, sodium ions are instead deflected by the vortex electric field. Despite the fact that ion gyration is in the opposite direction to the vorticity at the dusk flank, the current contribution of population trapped ions is co-rotating, leading to the gyro-resonant effect observed in our simulations and by MESSENGER.
Fig 2 caption: Test particle trajectories for sodium ions at dusk (red) and dawn (blue) boundaries, showing trapping and deflection behaviour when gyration and vorticity are counter-rotating and co-rotating respectively.
Posted by: Peter Gingell