A Novel Approach to Resonant Absorption of the Fast MHD Eigenmodes of a Coronal Arcade

TL;DR

This paper introduces a new spectral decomposition method to analyze the resonant absorption of fast MHD eigenmodes in coronal arcades, revealing a heavily damped mode caused by resonant coupling with Alfvén waves.

Contribution

It develops a spectral decomposition approach with real frequencies and complex wavenumbers to derive dispersion relations and damping rates for fast eigenmodes in coronal arcades, highlighting a new heavily damped mode.

Findings

Resonant absorption creates a new heavily damped fast mode.

The mode travels only a few wavelengths before damping.

The method derives dispersion relations and eigenfunctions for these modes.

Abstract

The arched field lines forming coronal arcades are often observed to undulate as magnetohydrodynamic (MHD) waves propagate both across and along the magnetic field. These waves are most likely a combination of resonantly coupled fast magnetoacoustic waves and Alfv\'en waves. The coupling results in resonant absorption of the fast waves, converting fast wave energy into Alfv\'en waves. The fast eigenmodes of the arcade have proven difficult to compute or derive analytically, largely because of the mathematical complexity that the coupling introduces. When a traditional spectral decomposition is employed, the discrete spectrum associated with the fast eigenmodes is often subsumed into the continuous Alfv\'en spectrum. Thus fast eigenmodes, become collective modes or quasi-modes. Here we present a spectral decomposition that treats the eigenmodes as having real frequencies but complex wavenumbers. Using this procedure we derive dispersion relations, spatial damping rates, and eigenfunctions for the resonant, fast eigenmodes of the arcade. We demonstrate that resonant absorption introduces a fast mode that would not exist otherwise. This new mode is heavily damped by resonant absorption, only travelling a few wavelengths before losing most of its energy.