Investigating coronal wave energy estimates using synthetic non-thermal line widths

Abstract

Aims. Estimates of coronal wave energy remain uncertain as a large fraction of the energy is likely hidden in the non-thermal line widths of emission lines. In order to estimate these wave energies, many previous studies have considered the root mean squared wave amplitudes to be a factor of $ \sqrt{2} $ greater than the non-thermal line widths. However, other studies have used different factors. To investigate this problem, we consider the relation between wave amplitudes and the non-thermal line widths within a variety of 3D magnetohydrodynamic (MHD) simulations. Methods. We consider the following 3D numerical models: Alfvén waves in a uniform magnetic field, transverse waves in a complex braided magnetic field, and two simulations of coronal heating in an arcade. We applied the forward modelling code FoMo to generate the synthetic emission data required to analyse the non-thermal line widths. Results. Determining a single value for the ratio between the non-thermal line widths and the root mean squared wave amplitudes is not possible across multiple simulations. It was found to depend on a variety of factors, including line-of-sight angles, velocity magnitudes, wave interference, and exposure time. Indeed, some of our models achieved the values claimed in recent articles while other more complex models deviated from these ratios. Conclusions. To estimate wave energies, an appropriate relation between the non-thermal line widths and root mean squared wave amplitudes is required. However, evaluating this ratio to be a singular value, or even providing a lower or upper bound on it, is not realistically possible given its sensitivity to various MHD models and factors. As the ratio between wave amplitudes and non-thermal line widths is not constant across our models, we suggest that this widely used method for estimating wave energy is not robust.

Coronagraphic Observations of Si x 1430 nm Acquired by DKIST/Cryo-NIRSP with Methods for Telluric Absorption Correction

“…The excess width of coronal emission lines beyond that induced by thermal broadening is well known (see, e.g., Boland et al 1973;Doschek et al 1976;Singh et al 2006;Tomczyk et al 2007, among others) and may be used to investigate the role of wave motions and/or turbulence in the coronal energy balance (see, e.g., McIntosh & De Pontieu 2012;Fyfe et al 2021) The nonthermal velocities derived in Section 4.7 and shown in Figure 8 range from 8 to 23 km s −1 , which is largely consistent with the previous measurements of nonthermal line widths and larger than the CryoNIRSP instrumental line width (≈c/45000 ; 6.5 km s −1 ). While the measurement errors are expected to depend on the signal strength, we can estimate the error in the derived nonthermal velocity for the spectrum that is repeatedly fit in the Appendix (see Figure 9).…”