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

TL;DR

This study investigates how the polarization of Ca II lines in the solar atmosphere is affected by magnetic fields, radiation processes, and atomic effects, providing insights for solar magnetism diagnostics.

Contribution

It models the polarization of Ca II lines considering PRD, JSI, and magnetic effects, revealing their roles in different solar atmospheric conditions.

Findings

PRD is essential for core polarization modeling.

JSI significantly influences far-wing polarization.

Hanle and Zeeman effects dominate different magnetic field regimes.

Abstract

Aims: The polarization of the Ca II resonant doublet (H and K lines) and the subordinate infrared triplet lines are key observables for diagnosing solar chromospheric magnetism. It is thus necessary to understand the physical mechanisms that shape their Stokes profiles in magnetic environments. Methods: Using the spectral synthesis module of the HanleRT-TIC code, we study the effects of anisotropic radiation pumping with partial frequency redistribution (PRD) and J-state interference (JSI) in a plane-parallel semi-empirical static solar atmospheric model. We also analyze the sensitivity of these lines to magnetic fields of varying strengths and orientations, accounting for the combined action of the Hanle and Zeeman effects. Results: Including PRD is crucial to model the polarization in the core regions of the resonant lines, while JSI strongly affects their far wings. The metastable lower levels of the subordinate lines also influence the scattering polarization of the K line. With horizontal magnetic fields, the resonant lines respond to field strengths from sub-gauss to tens of gauss, whereas the infrared triplet scattering polarization is mainly sensitive to milligauss fields. At a near-limb line of sight (LOS) with $\mu = 0.1$, the Hanle effect modifies the scattering polarization via a depolarization and a rotation in the plane of linear polarization. At disk center, horizontal fields generate linear polarization in the 1D model: for the K line, the Hanle effect dominates from sub-gauss to a few tens of gauss, and the Zeeman effect dominates in stronger fields. For vertical fields, the Hanle effect vanishes, but magneto-optical effects affect the linear polarization wings. Finally, atomic level polarization impacts the outer circular polarization lobes of the resonant lines, and the weak-field approximation overestimates the LOS magnetic component in this frequency range.