On the nature of kink MHD waves in magnetic flux tubes

TL;DR

This paper investigates the true nature of MHD kink waves in magnetic flux tubes, revealing they are primarily Alfvénic with mixed properties, challenging traditional classifications and emphasizing their robustness across different plasma conditions.

Contribution

The study provides a comprehensive analysis of MHD kink waves, showing they are mainly incompressible and Alfvénic, with implications for their identification and understanding in solar atmospheric phenomena.

Findings

Kink waves are predominantly incompressible with horizontal motions.

The main restoring force for kink waves is magnetic tension.

Kink waves exhibit mixed properties and are not strictly fast or Alfvénic.

Abstract

Magnetohydrodynamic (MHD) waves are often reported in the solar atmosphere and usually classified as slow, fast, or Alfv\'en. The possibility that these waves have mixed properties is often ignored. The goal of this work is to study and determine the nature of MHD kink waves. 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 with non-zero plasma pressure which allows for MHD radiation. 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 $(k_z R)^2$. 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 Alfv\'{e}n frequency, i.e. in the resonant layer. 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 Alfv\'{e}nic. However, it is better to realize that kink waves have mixed properties and cannot be put in one single box.