Apparent cross-field superslow propagation of magnetohydrodynamic waves in solar plasmas

TL;DR

This paper demonstrates that the observed cross-field propagation of certain magnetohydrodynamic waves in solar plasmas is an illusion caused by phase mixing of continuum waves, not actual energy transfer across magnetic surfaces.

Contribution

It reveals that apparent superslow wave propagation in solar structures results from phase mixing of continuum Alfvén and slow waves, clarifying a common observational misinterpretation.

Findings

Apparent cross-field propagation is due to phase mixing, not real energy transfer.

The phase velocity is slower than typical Alfvén or sound speeds.

Understanding this helps correctly interpret solar plasma observations.

Abstract

In this paper we show that the phase mixing of continuum Alfv\'{e}n waves and/or continuum slow waves in magnetic structures of the solar atmosphere as, e.g., coronal arcades, can create the illusion of wave propagation across the magnetic field. This phenomenon could be erroneously interpreted as fast magnetosonic waves. The cross-field propagation due to phase mixing of continuum waves is apparent because there is no real propagation of energy across the magnetic surfaces. We investigate the continuous Alfv\'{e}n and slow spectra in 2D Cartesian equilibrium models with a purely poloidal magnetic field. We show that apparent superslow propagation across the magnetic surfaces in solar coronal structures is a consequence of the existence of continuum Alfv\'{e}n waves and continuum slow waves that naturally live on those structures and phase mix as time evolves. The apparent cross-field phase velocity is related to the spatial variation of the local Alfv\'{e}n/slow frequency across the magnetic surfaces and is slower than the Alfv\'{e}n/sound velocities for typical coronal conditions. Understanding the nature of the apparent cross-field propagation is important for the correct analysis of numerical simulations and the correct interpretation of observations.