Alfv\`en wave phase-mixing and damping in the ion cyclotron range of frequencies

TL;DR

This study investigates how the Hall term influences Alfvén wave damping and phase mixing in plasmas, revealing that Hall effects modify damping rates in non-uniform conditions but not in uniform plasmas, with implications for plasma wave dynamics.

Contribution

It provides the first analytical and numerical analysis of Hall effects on Alfvén wave damping and phase mixing in the ion cyclotron frequency range.

Findings

Hall term does not affect algebraic decay in uniform plasma.

Damping rate is reduced in Hall MHD for non-uniform plasma.

Wave dispersion effects diminish damping rate differences in phase mixing.

Abstract

Aims. To determine the effect of the Hall term in the generalised Ohm's law on the damping and phase mixing of Alfven waves in the ion cyclotron range of frequencies in uniform and non-uniform equilibrium plasmas. Methods. Wave damping in a uniform plasma is treated analytically, whilst a Lagrangian remap code (Lare2d) is used to study Hall effects on damping and phase mixing in the presence of an equilibrium density gradient. Results. The magnetic energy associated with an initially Gaussian field perturbation in a uniform resistive plasma is shown to decay algebraically at a rate that is unaffected by the Hall term to leading order in k^2di^2 where k is wavenumber and di is ion skin depth. A similar algebraic decay law applies to whistler perturbations in the limit k^2di^2>>1. In a non-uniform plasma it is found that the spatially-integrated damping rate due to phase mixing is lower in Hall MHD than it is in MHD, but the reduction in the damping rate, which can be attributed to the effects of wave dispersion, tends to zero in both the weak and strong phase mixing limits.