Resonant absorption: Transformation of compressive motions into vortical motions

¹ Centre for mathematical Plasma Astrophysics, KU Leuven, Celestijnenlaan 200B bus 2400, 3001 Leuven, Belgium
e-mail: marcel.goossens@kuleuven.be
² Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁴ Departament de Física, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain
⁵ Institut d’Aplicacions Computacionals de Codi Comunitari (IAC 3), Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain

Received: 11 May 2020
Accepted: 30 June 2020

Abstract

This paper investigates the changes in spatial properties when magnetohydrodynamic (MHD) waves undergo resonant damping in the Alfvén 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 Alfvén wave, with the unfamiliar property that it has pressure variations even in the linear regime.