Enhanced Phase Mixing of Torsional Alfv\'en Waves in Stratified and Divergent Solar Coronal Structures, Paper II: Nonlinear Simulations

TL;DR

This study uses nonlinear MHD simulations to explore how torsional Alfvén waves in the solar corona experience enhanced damping and heating due to nonlinear effects like wave coupling and shock formation, beyond linear predictions.

Contribution

It introduces nonlinear governing equations for torsional Alfvén waves and compares simulation results to reveal nonlinear effects in wave damping and heating in the solar corona.

Findings

Nonlinear effects excite magnetosonic waves from Alfvén waves.

Self-interaction of Alfvén waves enhances damping.

Shock waves form due to steepening of compressive perturbations.

Abstract

We use MHD simulations to detect the nonlinear effects of torsional Alfv\'en wave propagation in a potential magnetic field with exponentially divergent field lines, embedded in a stratified solar corona. In Paper I we considered solutions to the linearised governing equations torsional Alfv\'en wave propagation and showed, using a finite difference solver we developed named WiggleWave, that in certain scenarios wave damping is stronger than what would be predicted by our analytic solutions. In this paper we consider whether damping would be further enhanced by the presence of nonlinear effects. We begin by deriving the nonlinear governing equations for torsional Alfv\'en wave propagation and identifying the terms that cause coupling to magnetosonic perturbations. We then compare simulation outputs from an MHD solver called Lare3d, which solves the full set of nonlinear MHD equations, to the outputs from WiggleWave to detect nonlinear effects such as: the excitation of magnetosonic waves by the Alfv\'en wave, self-interaction of the Alfv\'en wave through coupling to the induced magnetosonic waves, and the formation of shock waves higher in the atmosphere caused by the steepening of these compressive perturbations. We suggest that the presence of these nonlinear effects in the solar corona would lead to Alfv\'en wave heating that exceeds the expectation from the phase mixing alone.