The effect of twisted magnetic field on the resonant absorption of MHD waves in coronal loops

TL;DR

This paper investigates how magnetic twist in coronal loops affects MHD wave frequencies and damping, revealing that twist increases these parameters and alters period ratios, aligning with observational data.

Contribution

It provides a numerical analysis of the impact of magnetic twist on MHD wave properties in coronal loops, including dispersion relations and damping rates.

Findings

Magnetic twist increases wave frequencies and damping rates.

Twist reduces the period ratio P_1/P_2 below 2, matching observations.

Specific twist values can replicate observed deviations.

Abstract

The standing quasi modes in a cylindrical incompressible flux tube with magnetic twist that undergoes a radial density structuring is considered in ideal magnetohydrodynamics (MHD). The radial structuring is assumed to be a linearly varying density profile. Using the relevant connection formulae, the dispersion relation for the MHD waves is derived and solved numerically to obtain both the frequencies and damping rates of the fundamental and first-overtone modes of both the kink (m=1) and fluting (m=2,3) waves. It was found that a magnetic twist will increase the frequencies, damping rates and the ratio of the oscillation frequency to the damping rate of these modes. The period ratio P_1/P_2 of the fundamental and its first-overtone surface waves for kink (m=1) and fluting (m=2,3) modes is lower than 2 (the value for an untwisted loop) in the presence of twisted magnetic field. For the kink modes, particularly, the magnetic twists B_{\phi}/B_z=0.0065 and 0.0255 can achieve deviations from 2 of the same order of magnitude as in the observations. Furthermore, for the fundamental kink body waves, the frequency bandwidth increases with increasing the magnetic twist.