On the nature of kink MHD waves in magnetic flux tubes

¹ Centre Plasma Astrophysics and Leuven Mathematical Modeling and Computational Science Center, Katholieke Universiteit Leuven, 3001 Leuven, Belgium e-mail: Marcel.Goossens@wis.kuleuven.be
² Centre for Stellar and Planetary Astrophysics, Monash University, 3800 Victoria, Australia e-mail: Jesse.Andries@wis.kuleuven.be
³ Departament de Física, Universitat de les Illes Balears, Spain e-mail: Inigo.Arregui@uib.es

Received: 28 April 2009
Accepted: 4 June 2009

Abstract

Context. Magnetohydrodynamic (MHD) waves are often reported in the solar atmosphere and usually classified as slow, fast, or Alfvén. The possibility that these waves have mixed properties is often ignored.

Aims. The goal of this work is to study and determine the nature of MHD kink waves.

Methods. This is done by calculating the frequency, the damping rate and the eigenfunctions of MHD kink waves for three widely different MHD waves cases: a compressible pressure-less plasma, an incompressible plasma and a compressible plasma which allows for MHD radiation.

Results. In all three cases the frequency and the damping rate are for practical purposes the same as they differ at most by terms proportional to . In the magnetic flux tube the kink waves are in all three cases, to a high degree of accuracy incompressible waves with negligible pressure perturbations and with mainly horizontal motions. The main restoring force of kink waves in the magnetised flux tube is the magnetic tension force. The total pressure gradient force cannot be neglected except when the frequency of the kink wave is equal or slightly differs from the local Alfvén frequency, i.e. in the resonant layer.

Conclusions. Kink waves are very robust and do not care about the details of the MHD wave environment. The adjective fast is not the correct adjective to characterise kink waves. If an adjective is to be used it should be Alfvénic. However, it is better to realize that kink waves have mixed properties and cannot be put in one single box.