Revisiting the relation between nonthermal line widths and transverse MHD wave amplitudes

TL;DR

This study investigates the relationship between nonthermal line widths and transverse MHD wave amplitudes in the solar corona, revealing that previous assumptions overestimate wave energies and questioning their role in coronal heating.

Contribution

The paper derives a new relation between nonthermal line widths and wave amplitudes using mathematical modeling and 3D MHD simulations, correcting previous overestimations.

Findings

RMS wave amplitudes are never greater than nonthermal line widths.

Previous estimates of wave energy may be significantly overestimated.

The study questions the sufficiency of transverse waves in coronal heating.

Abstract

Observations and 3D MHD simulations of the transverse MHD waves in the solar corona have established that true wave energies hide in the nonthermal line widths of the optically thin emission lines. This displays the need for a relation between the nonthermal line widths and transverse wave amplitudes for estimating the true wave energies. In the past decade, several studies have assumed that the root mean square (rms) wave amplitudes are larger than nonthermal line widths by a factor of $\sqrt{2}$. However, a few studies have ignored this factor while estimating rms wave amplitudes. Thus there appears to exist a discrepancy in this relation. In this study, we investigate the dependence of nonthermal line widths on wave amplitudes by constructing a simple mathematical model followed by 3D MHD simulations. We derive this relation for the linearly polarised, circularly polarised oscillations, and oscillations excited by multiple velocity drivers. We note a fairly good match between mathematical models and numerical simulations. We conclude that the rms wave amplitudes are never greater than the nonthermal line widths which raises questions about earlier studies claiming transverse waves carry enough energy to heat the solar corona.