Transition to turbulence in nonuniform coronal loops driven by torsional Alfvén waves

Abstract

It has been shown in a previous work that torsional Alfvén waves can drive turbulence in nonuniform coronal loops with a purely axial magnetic field. Here we explore the role of the magnetic twist. We modeled a coronal loop as a transversely nonuniform straight flux tube, anchored in the photosphere, and embedded in a uniform coronal environment. We considered that the magnetic field is twisted and control the strength of magnetic twist by a free parameter of the model. We excited the longitudinally fundamental mode of standing torsional Alfvén waves, whose temporal evolution was obtained by means of high-resolution three-dimensional ideal magnetohydrodynamic numerical simulations. We find that phase mixing of torsional Alfvén 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. When the magnetic twist remains weak, it delays the onset of the KHi and slows the development of turbulence down. In contrast, magnetic tension can suppress the nonlinear growth of the KHi when the magnetic twist is strong enough, even when the KHi has locally been excited by the phase-mixing shear. Thus, turbulence is not generated in strongly twisted loops.