Effects of unresolved magnetic field on Fe I 617.3 and 630.2 nm line shapes

TL;DR

This study investigates how unresolved magnetic fields influence the shapes of Fe I spectral lines at 617.3 nm and 630.2 nm, revealing that different line parameters respond distinctly to magnetic and temperature variations, aiding in disentangling these effects.

Contribution

It introduces a method to differentiate magnetic and thermodynamic effects on spectral line shapes using observational and synthetic data of Fe I lines.

Findings

IC is sensitive to temperature and magnetic flux

FWHM is mainly affected by magnetic field changes

EQW primarily responds to temperature variations

Abstract

The contribution of the quiet Sun to solar irradiance variability, due either to changes of the solar effective temperature or to the presence of unresolved magnetic field, is still poorly understood. In this study we investigate spectral line diagnostics that are sensitive to both temperature variations and the presence of small scale unresolved magnetic features in these areas of the solar atmosphere. Specifically we study the dependence on the magnetic flux density of three parameters describing the shape of two magnetically sensitive FeI lines, at 630.2 nm and 617.3 nm, namely the line core intensity (IC), full width at half maximum (FWHM), and the equivalent width (EQW). To this aim we analyze observations of active region NOAA 11172, acquired with IBIS at the Dunn Solar Telescope, as well as results from numerical synthesis. Our results show that IC is sensitive to both temperature and magnetic flux density variations, FWHM is mostly affected by magnetic field changes, and EQW is mostly sensitive to temperature. Variations of a few percent of the measured line parameters are found in observational data that was spatially degraded to represent quiet-Sun, disk-centre, medium resolution observations. It is therefore possible to disentangle magnetic from pure thermodynamic effects by comparison of temporal variations of the EQW and the FWHM of either the two FeI lines.