Magnetohydrodynamic kink waves in nonuniform solar flux tubes: phase mixing and energy cascade to small scales

TL;DR

This paper investigates how phase mixing of MHD kink waves in nonuniform solar flux tubes leads to small-scale structures, facilitating energy dissipation in the solar atmosphere through a modal analysis approach.

Contribution

It introduces a modal expansion method to analyze MHD kink wave damping and small-scale formation, linking resonant absorption and phase mixing as a unified energy cascade mechanism.

Findings

Modal analysis reveals detailed damping and small-scale generation.

Phase mixing and resonant absorption are interconnected processes.

Energy cascade facilitates efficient wave energy dissipation.

Abstract

Magnetohydrodynamic (MHD) kink waves are ubiquitously observed in the solar atmosphere. The propagation and damping of these waves may play relevant roles for the transport and dissipation of energy in the solar atmospheric medium. However, in the atmospheric plasma dissipation of transverse MHD wave energy by viscosity or resistivity needs very small spatial scales to be efficient. Here, we theoretically investigate the generation of small scales in nonuniform solar magnetic flux tubes due to phase mixing of MHD kink waves. We go beyond the usual approach based on the existence of a global quasi-mode that is damped in time due to resonant absorption. Instead, we use a modal expansion to express the MHD kink wave as a superposition of Alfv\'en continuum modes that are phase mixed as time evolves. The comparison of the two techniques evidences that the modal analysis is more physically transparent and describes both the damping of global kink motions and the building up of small scales due to phase mixing. In addition, we discuss that the processes of resonant absorption and phase mixing are intimately linked. They represent two aspects of the same underlying physical mechanism: the energy cascade from large scales to small scales due to naturally occurring plasma and/or magnetic field inhomogeneities. This process may provide the necessary scenario for efficient dissipation of transverse MHD wave energy in the solar atmospheric plasma.