Resonant absorption: transformation of compressive motions into vortical motions

TL;DR

This study explores how resonant damping in magnetohydrodynamic waves causes a significant transformation of compressive motions into highly amplified parallel vortical motions, especially in non-uniform plasma regions.

Contribution

It provides a detailed analysis of the spatial evolution of MHD waves during resonant damping, highlighting the dramatic increase in parallel vorticity and wave behavior resembling Alfvén waves with pressure variations.

Findings

Parallel vorticity explodes in non-uniform plasma regions.

Resonant damping transforms compressive motions into vortical motions.

MHD waves resemble Alfvén waves with pressure variations in non-uniform areas.

Abstract

This paper investigates the changes in spatial properties when magnetohydrodynamic (MHD) waves undergo resonant damping in the Alfv\'en continuum. The analysis is carried out for a 1D cylindrical pressure-less plasma with a straight magnetic field. The effect of the damping on the spatial wave variables is determined by using complex frequencies that arise as a result of the resonant damping. Compression and vorticity are used to characterise the spatial evolution of the MHD wave. The most striking result is the huge spatial variation in the vorticity component parallel to the magnetic field. Parallel vorticity vanishes in the uniform part of the equilibrium. However, when the MHD wave moves into the non-uniform part, parallel vorticity explodes to values that are orders of magnitude higher than those attained by the transverse components in planes normal to the straight magnetic field. In the non-uniform part of the equilibrium plasma, the MHD wave is controlled by parallel vorticity and resembles an Alfv\'en wave, with the unfamiliar property that it has pressure variations even in the linear regime.