Can we rely on EUV emission to identify coronal waveguides?

TL;DR

This study investigates whether EUV emission reliably indicates coronal waveguides, finding that the apparent loops in EUV do not straightforwardly correspond to actual waveguiding regions, supporting the coronal veil model.

Contribution

The paper extends the coronal veil model to coronal oscillations, demonstrating that EUV emission features do not directly trace true waveguides in the corona.

Findings

EUV emission features only partially overlap with true waveguides.

Projected waveguide widths are larger than observed EUV loops.

Coronal veil structure is independent of specific models.

Abstract

Traditional models of coronal oscillations rely on modelling the coronal structures that support them as compact cylindrical waveguides. Recently, an alternative model of the structure of the corona has been proposed, where the thin strand-like coronal loops observable in the EUV emission are a result of line-of-sight integration of warps in more complex coronal structures, referred to as the coronal veil model. We extend the implications of the coronal veil model of the solar corona to models of coronal oscillations. Using the convection-zone-to-corona simulations with the radiation-magnetohydrodynamics code Bifrost, we analysed the structure of the self-consistently formed simulated corona. We focus on the spatial variability of the volumetric emissivity of the Fe IX 171.073 {\AA} EUV line, and on the variability of the Alfv\'en speed, which captures the density and magnetic structuring of the simulated corona. We traced features associated with large magnitudes of the Alfv\'en speed gradient, which are the most likely to trap MHD waves and act as coronal waveguides, and looked for the correspondence with emitting regions which appear as strand-like loops in line-of-sight-integrated EUV emission. The waveguide filling factors corresponding to the fraction of the waveguides filled with plasma emitting in the given EUV wavelength range from 0.09 to 0.44. This suggests that we can observe only a small fraction of the waveguide. Similarly, the projected waveguide widths in the plane of the sky are several times larger than the widths of the apparent loops observable in EUV. We conclude that the 'coronal veil' structure is model-independent. As a result, we find a lack of straightforward correspondence between a peak in the integrated emission profile which constitutes an apparent coronal loop and regions of plasma bound by a large Alfv\'en speed gradient acting as waveguides.