Kelvin-Helmholtz instability at proton scales with an exact kinetic equilibrium

TL;DR

This study uses Hybrid Vlasov-Maxwell simulations to explore how Kelvin-Helmholtz instability at proton scales influences particle dynamics and turbulence development in magnetized collisionless plasmas, with implications for space physics observations.

Contribution

It demonstrates the importance of initial equilibrium conditions for turbulence development and reveals how particle velocity distributions deviate from equilibrium due to turbulent fields.

Findings

Turbulence develops efficiently when initial equilibrium is used.

Proton velocity distributions significantly deviate from thermodynamic equilibrium.

Energy cascades to proton scales, affecting particle dynamics.

Abstract

The Kelvin-Helmholtz instability is a ubiquitous physical process in ordinary fluids and plasmas, frequently observed also in space environments. In this paper, kinetic effects at proton scales in the nonlinear and turbulent stage of the Kelvin-Helmholtz instability have been studied in magnetized collisionless plasmas by means of Hybrid Vlasov-Maxwell simulations. The main goal of this work is to point out the back reaction on particles triggered by the evolution of such instability, as energy reaches kinetic scales along the turbulent cascade. Interestingly, turbulence is inhibited when Kelvin-Helmholtz instability develops over an initial state which is not an exact equilibrium state. On the other hand, when an initial equilibrium condition is considered, energy can be efficiently transferred towards short scales, reaches the typical proton wavelengths and drives the dynamics of particles. As a consequence of the interaction of particles with the turbulent fluctuating fields, the proton velocity distribution deviates significantly from the local thermodynamic equilibrium, the degree of deviation increasing with the level of turbulence in the system and being located near regions of strong magnetic stresses. These numerical results support recent space observations from the Magnetospheric MultiScale mission of ion kinetic effects driven by the turbulent dynamics at the Earth's magnetosheath (Perri et al., 2020, JPlPh, 86, 905860108) and by the Kelvin-Helmholtz instability in the Earth's magnetosphere (Sorriso-Valvo et al., 2019, PhRvL, 122, 035102).