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

TL;DR

This work tackles how Ca II H&K and the infrared Ca II triplet encode chromospheric magnetism by integrating PRD, J-state interference, and Hanle/Zeeman physics in a 1D non-LTE radiative-transfer framework. Using HanleRT-TIC in a FAL-C atmosphere, the authors compare redistribution schemes and atomic models, showing PRD is vital for H&K cores while CRD suffices for the IR triplet, with J-state interference requiring a multi-term treatment in the inter-line region. They demonstrate that metastable lower levels strongly influence the K-line polarization and that magnetic fields across sub-Gauss to tens of Gauss (for H&K) and milligauss to Gauss (for the IR triplet) produce distinct Hanle and MO signatures, including MO-induced wing rotations and Żeeman-dominated wings at higher strengths. Circular polarization profiles (V/I) scale with B and reveal the importance of including atomic polarization to reproduce outer-lobe signals, while WFA tends to overestimate the LOS field in the wings. The results provide a practical guide for interpreting current and upcoming high-precision spectropolarimetric data from DKIST and Sunrise missions, and set the stage for more realistic 3D and angle-dependent PRD analyses.

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 $μ= 0.1$, the Hanle effect modifies the scattering polarization via a depolarization and a rotation in the plane of linear polarization. At disk center, horizontal fields 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.

Figures (17)

• Figure 1: Stokes $I$, normalized to the continuum intensity $I_\mathrm{cont}$ (first and third rows), and fractional linear polarization $Q/I$ profiles (second and fourth rows), shown as a function of wavelength distance from the center of each line under consideration. The intensity profiles of the UV doublet and of the IR triplet lines are normalized to the continuum value at $\lambda = 3954.8 \: \AA$ and at $\lambda = 8500.5 \: \AA$, respectively. The profiles were computed in the semi-empirical model C of FAL_C assuming a 5L atomic model. The emergent profiles are shown at a line of sight with $\mu = 0.1$. The various curves represent the results of different redistribution treatments: CRD (solid black), IRCRD (dashed red), and PRD (dotted blue). The reference direction for positive Stokes $Q$ is the parallel to the nearest limb.
• Figure 2: Fractional linear polarization $Q/I$ profiles for the region around the H and K lines (left panel), and the core and near-wing region of the K line (central panel) and the H line (right panel). The considered redistribution treatment is IRCRD and the emergent profiles are shown for a LOS with $\mu=0.1$. The curves represent the emergent profiles for a 5L atomic model (which cannot account for J-state interference) in solid black, and a 3T model (which accounts for J-state interference) in dashed red. The reference direction for positive Stokes $Q$ is the parallel to the nearest limb.
• Figure 3: Stokes $I$, normalized to the continuum intensity $I_\mathrm{cont}$ (upper panels), and fractional linear polarization $Q/I$ profiles (lower panels) for the IR triplet as a function of wavelength distance from the center of each line; from left to right, the $8498 \: \AA$, $8542 \: \AA$, and $8662 \: \AA$ lines. The emergent profiles are shown for a near the limb line of sight, with $\mu=0.1$. The curves represent emergent profiles for a 5L atomic model (which cannot account for J-state interference) in solid black, and a 3T model (which accounts for J-state interference) in dashed red. The reference direction for positive Stokes $Q$ is the parallel to the nearest limb.
• Figure 4: Stokes $I$, normalized to the continuum intensity $I_\mathrm{cont}$ (upper panel), and fractional linear polarization $Q/I$ profiles (lower panel) for the core and near-wing region of the K line as a function of wavelength distance to the center of the line. The considered redistribution treatment is PRD and the emergent profiles are shown for a near the limb line of sight, with $\mu=0.1$. The solid black and dashed red curves correspond to calculations for 3L (which ignores the metastable levels) and 5L (including the metastable levels) atomic models, respectively.
• Figure 5: Fractional linear polarization $Q/I$ (upper panels) and $U/I$ (lower panels) profiles for the region around the H and K lines (left panels) and the core of the H line (right panels). The 3T multi-term atomic models are considered, so J-state interference is accounted for. The emergent profiles are shown for a LOS with $\mu=0.1$. The curves represent emergent profiles for syntheses considering a horizontal magnetic field ($\theta_{B} = 90^\circ$) with different strengths, indicated in the legend. The reference direction for positive Stokes $Q$ is the parallel to the nearest limb.