The magnetic sensitivity of the Ca II resonance and subordinate lines in the solar atmosphere

¹ Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
³ Consejo Superior de Investigaciones Científicas, Madrid, Spain

^★ Corresponding author: ijuanikorenab@gmail.com

Received: 10 July 2025
Accepted: 22 October 2025

Abstract

Aims. The polarization of the Ca II resonant doublet (H and K lines) and subordinate infrared triplet lines are valuable observables for diagnosing the magnetism of the solar chromosphere. It is thus necessary to understand in detail the physical mechanisms that play a role in producing their Stokes profiles in the presence of magnetic fields.

Methods. We use the spectral synthesis module of the HanleRT-TIC code to study the impact of anisotropic radiation pumping with partial frequency redistribution (PRD) and J-state interference (JSI), considering a plane-parallel semi-empirical static solar atmospheric model. We also study the sensitivity of the lines to magnetic fields of various strengths and orientations accounting for the joint action of the Hanle and Zeeman effects.

Results. Taking PRD into account is required to suitably model the polarization in the core regions of the resonant lines, whereas JSI plays a crucial role in their far wings. We confirm that the metastable lower levels of the subordinate lines also contribute to the scattering polarization of the K line. In the presence of horizontal magnetic fields, we find that the resonant lines are sensitive to a wide range of field strengths (sub-gauss to tens of gauss), whereas the scattering polarization of the infrared triplet lines are mainly sensitive to milligauss field strengths. At a near-limb line of sight (LOS) with μ = 0.1, the Hanle effect modifies the scattering polarization via a depolarization and a rotation in the plane of linear polarization. At disk center, horizontal fields in the 1D semi-empirical model give rise to linear polarization signals; in the K line, this is governed by the Hanle effect in the sub-gauss to few tens of gauss range and by the Zeeman effect in stronger fields. For vertical magnetic fields, the Hanle effect does not operate, but the linear polarization wings of the resonance lines are sensitive to magneto-optical effects. Finally, we find that the atomic level polarization exerts an influence on the outer circular polarization lobes of the the resonant lines and that the weak field approximation tends to overestimate the LOS magnetic field component if this frequency range is considered.