Poynting flux of MHD modes in magnetic solar vortex tubes

TL;DR

This study investigates how background rotational flows in solar vortex tubes influence the Poynting flux of MHD waves, revealing significant increases in energy transport with stronger flows and emphasizing the necessity of perturbations for upward energy flux.

Contribution

It provides a detailed modeling and analysis of Poynting flux in magnetic solar vortex tubes, highlighting the impact of background flows on energy transport and clarifying the origin of upward Poynting flux.

Findings

Poynting flux increases with background rotational flow strength.

Non-axisymmetric modes show over 1000% increase in Poynting flux under strong flows.

Untwisted flux tubes cannot produce upward Poynting flux without perturbations.

Abstract

Magnetic flux tubes in the presence of background rotational flows, known as solar vortex tubes, are abundant throughout the solar atmosphere and may act as conduits for MHD waves to transport magnetic energy to the upper solar atmosphere. We aim to investigate the Poynting flux associated with these waves within solar vortex tubes. We model a solar vortex tube as a straight magnetic flux tube with a background azimuthal velocity component. The MHD wave solutions in the equilibrium configuration of a vortex tube are obtained using the SESAME code and we derive an expression for the vertical component of the Poynting flux, $S_z$, associated with MHD modes. In addition, we present 2D visualisations of the spatial structure of $S_z$ for different MHD modes under different background flow strengths. We show that $S_z$ increases in the presence of a background rotational flow when compared to a flux tube with no rotational flow. When the strength of the background flow is greater than $100$ times the strength of the perturbation, the $S_z$ associated with non-axisymmetric ($|m|>0$) modes increases by over $1000\%$ when compared to a magnetic flux tube in the absence of a background rotational flow. Furthermore, we present a fundamental property of solar vortices that they cannot solely produce an upwards Poynting flux in an untwisted tube, meaning that any observed $S_z$ in straight flux tubes must arise from perturbations, such as MHD waves.