Effect of magnetic twist on nonlinear transverse kink oscillations

TL;DR

This paper investigates how magnetic twist influences nonlinear transverse kink oscillations in solar magnetic tubes, showing that twist delays shear instabilities and affects their development depending on the inhomogeneous layer thickness.

Contribution

It provides a comprehensive analysis of magnetic twist effects on nonlinear kink oscillations, including energy transfer and shear instability development in different layer configurations.

Findings

Twist delays shear instability onset.

Effect of twist is stronger with thicker inhomogeneous layers.

Moderate twist has limited impact on thin layers.

Abstract

Magnetic twist is thought to play an important role in many structures of the solar atmosphere. One of the effects of twist is to modify the properties of the eigenmodes of magnetic tubes. In the linear regime standing kink solutions are characterized by a change in polarization of the transverse displacement along the twisted tube. In the nonlinear regime, magnetic twist affects the development of shear instabilities that appear at the tube boundary when it is oscillating laterally. These Kelvin-Helmholtz instabilities (KHI) are produced either by the jump in the azimuthal component of the velocity at the edge of the sharp boundary between the internal and external part of the tube, or either by the continuous small length-scales produced by phase-mixing when there is a smooth inhomogeneous layer. In this work the effect of twist is consistently investigated by solving the time-dependent problem including the process of energy transfer to the inhomogeneous layer. It is found that twist always delays the appearance of the shear instability but for tubes with thin inhomogeneous layers the effect is relatively small for moderate values of twist. On the contrary, for tubes with thick layers, the effect of twist is much stronger. This can have some important implications regarding observations of transverse kink modes and the KHI itself.