Transition to turbulence in nonuniform coronal loops driven by torsional Alfv\'en waves. II. Extended analysis and effect of magnetic twist

TL;DR

This study investigates how magnetic twist influences turbulence development in coronal loops driven by torsional Alfvén waves, revealing that strong twist can suppress turbulence by inhibiting Kelvin-Helmholtz instability growth.

Contribution

It extends previous work by analyzing the impact of magnetic twist on turbulence onset, showing that strong twist can delay or suppress turbulence in coronal loops.

Findings

Weak twist delays Kelvin-Helmholtz instability onset.

Strong twist suppresses turbulence development.

Turbulence occurs only in weakly twisted loops.

Abstract

It has been shown in a previous work that torsional Alfv\'en waves can drive turbulence in nonuniform coronal loops with a purely axial magnetic field. Here we explore the role of the magnetic twist. We model a coronal loop as a transversely nonuniform straight flux tube, anchored in the photosphere, and embedded in a uniform coronal environment. We consider that the magnetic field is twisted and control the strength of magnetic twist by a free parameter of the model. We excite the longitudinally fundamental mode of standing torsional Alfv\'en waves, whose temporal evolution is obtained by means of high-resolution three-dimensional ideal magnetohydrodynamic numerical simulations. We find that phase mixing of torsional Alfv\'en waves creates velocity shear in the direction perpendicular to the magnetic field lines. The velocity shear eventually triggers the Kelvin-Helmholtz instability (KHi). In weakly twisted magnetic tubes, the KHi is able to grow nonlinearly and, subsequently, turbulence is driven in the coronal loop in a similar manner as in the untwisted case. Provided that magnetic twist remains weak, the effect of magnetic twist is to delay the onset of the KHi and to slow down the development of turbulence. In contrast, magnetic tension can suppress the nonlinear growth of the KHi when magnetic twist is strong enough, even if the KHi has locally been excited by the phase-mixing shear. Thus, turbulence is not generated in strongly twisted loops