Magneto-Acoustic Waves in Magnetic Twisted Flux Tube

TL;DR

This paper derives a comprehensive dispersion relationship for magneto-acoustic waves in twisted magnetic flux tubes, including the previously omitted m=-1 mode, and examines the impact of magnetic twist on wave patterns in sunspots.

Contribution

It presents a complete dispersion relationship that incorporates magnetic twist and the m=-1 wave mode, filling gaps in previous models.

Findings

The m=-1 mode exhibits mode change from body to surface at finite kr0.

The phase speeds of m=-1 are generally lower than m=+1.

Magnetic twist has minimal effect on sunspot spiral wave morphology.

Abstract

At present, many works about magneto-hydrodynamics wave diagnostics in magnetic flux tubes are based on some pioneer works not considering the contributions of magnetic twist. Other works considered the effect on magneto-hydrodynamics waves, but the dispersion relationship they presented only gave the wave modes of m=0,1,2. The kink mode of m=-1 was absent. Therefore, in this work we would like to present a complete dispersion relationship that includes both magnetic twist and the wave mode of m=-1. Analogous to the m=+1 wave mode, the mode of m=-1 also exhibits the mode change at finite kr0, from body to surface mode. The phase speeds of this mode are usually less than those of m=+1 mode. The harmonic curves of m=+1/-1 modes in dispersion relationship diagrams are approximately symmetric in respect to a characteristic velocity, e.g. the tube velocity in flux tubes. Based on the present dispersion relationship, we revisit the issue of spiral wave patterns in sunspot and find that the magnetic twist has no great influence on their morphology in the frame of linear perturbation analysis.