On the nature of spectral line broadening in solar coronal dimmings

TL;DR

This study investigates the causes of spectral line broadening in solar coronal dimmings, revealing that inhomogeneous flows and unresolved velocity structures, rather than Alfvén waves, are primary contributors.

Contribution

It introduces empirical coefficients to quantify line profile distortions and demonstrates that inhomogeneities and multiple velocity components explain observed broadening.

Findings

Line broadening is mainly due to flow inhomogeneities and unresolved velocity components.

Double-Gaussian fits reveal a nearly stationary component and a shifted component with larger Doppler velocities.

Alfvén wave amplitude increase alone cannot account for the observed spectral features.

Abstract

We analyze the profiles of iron emission lines observed in solar coronal dimmings associated with coronal mass ejections, using the EUV Imaging Spectrometer on board Hinode. We quantify line profile distortions with empirical coefficients (asymmetry and peakedness) that compare the fitted Gaussian to the data. We find that the apparent line broadenings reported in previous studies are likely to be caused by inhomogeneities of flow velocities along the line of sight, or at scales smaller than the resolution scale, or by velocity fluctuations during the exposure time. The increase in the amplitude of Alfv\'en waves cannot, alone, explain the observed features. A double-Gaussian fit of the line profiles shows that, both for dimmings and active region loops, one component is nearly at rest while the second component presents a larger Doppler shift than that derived from a single-Gaussian fit.