Surface Alfven waves in solar flux tubes

TL;DR

This paper investigates surface Alfven waves in solar flux tubes, showing how non-uniform density affects wave properties and vorticity distribution, and identifying fundamental modes as surface Alfven waves in thin magnetic cylinders.

Contribution

It demonstrates that non-axisymmetric fundamental radial modes in non-uniform flux tubes behave as surface Alfven waves, extending the understanding of wave modes in solar atmospheric structures.

Findings

Fundamental radial modes have surface Alfven wave properties at density discontinuities.

Vorticity is confined at the boundary in uniform density, but spreads in continuous density profiles.

Radial overtones relate to fast magneto-sonic modes, unlike fundamental modes.

Abstract

Magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere. Alfven waves and magneto-sonic waves are particular classes of MHD waves. These wave modes are clearly different and have pure properties in uniform plasmas of infinite extent only. Due to plasma non-uniformity MHD waves have mixed properties and cannot be classified as pure Alfven or magneto-sonic waves. However, vorticity is a quantity unequivocally related to Alfven waves as compression is for magneto-sonic waves. Here, we investigate MHD waves superimposed on a one-dimensional non-uniform straight cylinder with constant magnetic field. For a piecewise constant density profile we find that the fundamental radial modes of the non-axisymmetric waves have the same properties as surface Alfven waves at a true discontinuity in density. Contrary to the classic Alfveen waves in a uniform plasma of infinite extent, vorticity is zero everywhere except at the cylinder boundary. If the discontinuity in density is replaced with a continuous variation of density, vorticity is spread out over the whole interval with non-uniform density. The fundamental radial modes of the non-axisymmetric waves do not need compression to exist unlike the radial overtones. In thin magnetic cylinders the fundamental radial modes of the non-axisymmetric waves with phase velocities between the internal and the external Alfven velocities can be considered as surface Alfven waves. On the contrary, the radial overtones can be related to fast-like magneto-sonic modes.