Resonant absorption of kink magnetohydrodynamic waves by a magnetic twist in coronal loops

TL;DR

This paper investigates how magnetic twist in coronal loops causes resonant absorption of kink MHD waves, providing a potential explanation for their rapid damping observed in the solar corona.

Contribution

It models a coronal loop with magnetic twist and derives analytical expressions for wave frequencies and damping rates, linking magnetic twist to observed wave damping.

Findings

Resonant absorption by magnetic twist explains rapid damping of kink waves.

Magnetic twist causes deviations in period ratios P1/P2 and P1/P3 consistent with observations.

Analytical dispersion relation derived for kink MHD modes in twisted flux tubes.

Abstract

There is ample evidence of twisted magnetic structures in the solar corona. This motivates us to consider the magnetic twist as the cause of Alfven frequency continuum in the coronal loops, which can support the resonant absorption as a rapid damping mechanism for the observed coronal kink magnetohydrodynamic (MHD) oscillations. We model a coronal loop with a straight cylindrical magnetic flux tube which has constant but different densities in the interior and exterior regions. The magnetic field is assumed to be constant and aligned with the cylinder axis everywhere except a thin layer near the boundary of the flux tube which has an additional small magnetic field twist. Then, we investigate a number of possible instabilities that may arise in our model. In the thin tube thin boundary approximation, we derive the dispersion relation and solve it analytically to obtain the frequencies and damping rates of the fundamental (l=1) and first/second overtone (l=2,3) kink (m=1) MHD modes. We conclude that the resonant absorption by the magnetic twist can justify the rapid damping of kink MHD waves observed in coronal loops. Furthermore, the magnetic twist in the inhomogeneous layer can cause deviations from P1/P2=2 and P1/P3=3 which are comparable with the observations.