Vlasov simulations of Kinetic Alfv\'en Waves at proton kinetic scales

TL;DR

This study uses hybrid Vlasov-Maxwell simulations to explore kinetic Alfvén waves at proton scales in solar wind conditions, revealing how wave-particle interactions deform proton velocity distributions and contribute to plasma heating.

Contribution

It provides detailed insights into the linear and nonlinear behaviors of kinetic Alfvén waves at proton scales, highlighting the role of wave-particle interactions in plasma dynamics.

Findings

Wave-particle interactions cause significant deformations in proton velocity distributions.

Kinetic Alfvén waves exhibit small phase speeds near proton thermal velocities.

Nonlinear effects lead to non-Maxwellian velocity profiles and phase space vortices.

Abstract

Kinetic Alfv\'en waves represent an important subject in space plasma physics, since they are thought to play a crucial role in the development of the turbulent energy cascade in the solar wind plasma at short wavelengths (of the order of the proton inertial length $d_p$ and beyond). A full understanding of the physical mechanisms which govern the kinetic plasma dynamics at these scales can provide important clues on the problem of the turbulent dissipation and heating in collisionless systems. In this paper, hybrid Vlasov-Maxwell simulations are employed to analyze in detail the features of the kinetic Alfv\'en waves at proton kinetic scales, in typical conditions of the solar wind environment. In particular, linear and nonlinear regimes of propagation of these fluctuations have been investigated in a single-wave situation, focusing on the physical processes of collisionless Landau damping and wave-particle resonant interaction. Interestingly, since for wavelengths close to $d_p$ and proton plasma beta $\beta$ of order unity the kinetic Alfv\'en waves have small phase speed compared to the proton thermal velocity, wave-particle interaction processes produce significant deformations in the core of the particle velocity distribution, appearing as phase space vortices and resulting in flat-top velocity profiles. Moreover, as the Eulerian hybrid Vlasov-Maxwell algorithm allows for a clean almost noise-free description of the velocity space, three-dimensional plots of the proton velocity distribution help to emphasize how the plasma departs from the Maxwellian configuration of thermodynamic equilibrium due to nonlinear kinetic effects.