Milne-Eddington inversion of the Fe I line pair at 630~nm

TL;DR

This paper investigates the validity of using the same thermodynamic parameters for two Fe I lines at 630 nm in Milne-Eddington inversions of the solar photosphere, demonstrating improved accuracy with dual-line inversion.

Contribution

It assesses the assumption of constant opacity ratio between the lines and confirms the effectiveness of simultaneous inversion for better magnetic and velocity diagnostics.

Findings

Dual-line inversion yields more accurate results than single-line methods.

The lines can be coupled and inverted with shared thermodynamic parameters.

The constant opacity ratio assumption is validated for these lines.

Abstract

The iron lines at 630.15 and 630.25 nm are often used to determine the physical conditions of the solar photosphere. A common approach is to invert them simultaneously under the Milne-Eddington approximation. The same thermodynamic parameters are employed for the two lines, except for their opacities, which are assumed to have a constant ratio. We aim at investigating the validity of this assumption, since the two lines are not exactly the same. We use magnetohydrodynamic simulations of the quiet Sun to examine the behavior of the ME thermodynamic parameters and their influence on the retrieval of vector magnetic fields and flow velocities. Our analysis shows that the two lines can be coupled and inverted simultaneously using the same thermodynamic parameters and a constant opacity ratio. The inversion of two lines is significantly more accurate than single-line inversions because of the larger number of observables.