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

TL;DR

This study demonstrates that nonlinear evolution of torsional Alfven waves in coronal loops rapidly induces turbulence via Kelvin-Helmholtz instability, significantly enhancing energy dissipation and potentially explaining coronal heating.

Contribution

It reveals that nonlinear phase mixing and Kelvin-Helmholtz instability accelerate turbulence onset in coronal loops, advancing understanding of coronal heating mechanisms.

Findings

Turbulence develops within a few wave periods due to KHi.

Turbulence is highly anisotropic and transverse to magnetic field.

Reynolds number decreases faster than linear phase mixing predicts.

Abstract

Both observations and numerical simulations suggest that Alfvenic waves may carry sufficient energy to sustain the hot temperatures of the solar atmospheric plasma. However, the thermalization of wave energy is inefficient unless very short spatial scales are considered. Phase mixing is a mechanism that can take energy down to dissipation lengths, but it operates over too long a timescale. Here, we study how turbulence, driven by the nonlinear evolution of phase-mixed torsional Alfven waves in coronal loops, is able to take wave energy down to the dissipative scales much faster than the theory of linear phase mixing predicts. We consider a simple model of a transversely nonuniform cylindrical flux tube with a constant axial magnetic field. The flux tube is perturbed by the fundamental mode of standing torsional Alfven waves. We solved the three-dimensional (3D) ideal magnetohydrodynamics equations numerically to study the temporal evolution. Initially, torsional Alfven waves undergo the process of phase mixing because of the transverse variation of density. After only few periods of torsional waves, azimuthal shear flows generated by phase mixing eventually trigger the Kelvin-Helmholtz instability (KHi), and the flux tube is subsequently driven to a turbulent state. Turbulence is very anisotropic and develops transversely only to the background magnetic field. After the onset of turbulence, the effective Reynolds number decreases in the flux tube much faster than in the initial linear stage governed by phase mixing alone. We conclude that the nonlinear evolution of torsional Alfven waves, and the associated KHi, is a viable mechanism for the onset of turbulence in coronal loops.