Blend lines in the polarized spectrum of the Sun

TL;DR

This paper develops a theoretical framework and numerical methods to include non-LTE blend lines in the polarized spectrum modeling of the Sun, highlighting their subtle but important influence on spectral line interpretation.

Contribution

It introduces a new approach to incorporate non-LTE blend lines into radiative transfer models for solar polarization analysis.

Findings

Blend lines affect polarization in the solar spectrum.

Non-LTE effects are significant for accurate modeling.

Interaction of blend lines with main lines is complex but important.

Abstract

Blend lines form an integral part of the theoretical analysis and modeling of the polarized spectrum of the Sun. Their interaction with other spectral lines needs to be explored and understood before we can properly use the main spectral lines to diagnose the Sun. They are known to cause a decrease in the polarization in the wings of the main line on which they superpose, or in the polarization of the continuum, when they are assumed to be formed either under the local thermodynamic equilibrium (LTE) conditions, or when their intrinsic polarizability factor is zero. In this paper, we describe the theoretical framework to include the blend lines formed under non-LTE conditions, in the radiative transfer equation, and the numerical techniques to solve it. The properties of a blend line having an intrinsic polarization of its own and its interaction with the main line are discussed. The results of our analysis show that the influence of the blend lines on the main spectral lines, though small in the present context, is important and needs to be considered when interpreting the polarized spectral lines in the second solar spectrum.