On the (mis)interpretation of the scattering polarization signatures in the Ca II 8542 A line through spectral line inversions

Abstract

Scattering polarization tends to dominate the linear polarization signals of the Ca II 8542 A line in weakly magnetized areas ($B\lesssim 100$ G), especially when the observing geometry is close to the limb. In this paper we evaluate the degree of applicability of existing non-LTE spectral line inversion codes (which assume that the spectral line polarization is due to the Zeeman effect only) at inferring the magnetic field vector and, particularly, its transverse component. To this end, we use the inversion code STiC to extract the strength and orientation of the magnetic field from synthetic spectropolarimetric data generated with the Hanle-RT code. The latter accounts for the generation of polarization through scattering processes as well as the joint actions of the Hanle and the Zeeman effects. We find that, when the transverse component of the field is stronger than $\sim$80 G, the inversion code is able to retrieve accurate estimates of the transverse field strength as well as its azimuth in the plane of the sky. Below this threshold, the scattering polarization signatures become the major contributors to the linear polarization signals and often mislead the inversion code into severely over- or under-estimating the field strength. Since the line-of-sight component of the field is derived from the circular polarization signal, which is not affected by atomic alignment, the corresponding inferences are always good.

arXiv commentAccepted for publication in ApJ

References in corpus (27)

• Spectroscopy Made Easy: Evolution
• Advanced Forward Modeling and Inversion of Stokes Profiles Resulting from the Joint Action of the Hanle and Zeeman Effects
• STiC -- A multi-atom non-LTE PRD inversion code for full-Stokes solar observations
• Inversion of the radiative transfer equation for polarized light
• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Optimization of the Spectral Line Inversion Code
• Non-LTE inversions of the Mg II h&k and UV triplet lines
• DELO-Bezier formal solutions of the polarized radiative transfer equation
• Non-local thermodynamic equilibrium inversions from a 3D MHD chromospheric model
• Mapping the magnetic field of flare coronal loops
• Spectropolarimetric capabilities of Ca II 8542 A line
• Spectropolarimetric NLTE inversion code SNAPI
• Magnetic Diagnostics of the Solar Chromosphere with the Mg II h-k Lines
• Fast approximation of angle-dependent partial redistribution in moving atmospheres
• Measurements of Photospheric and Chromospheric Magnetic Fields
• Inference of the chromospheric magnetic field orientation in the Ca II 8542 A line fibrils
• Non-equilibrium calcium ionisation in the solar atmosphere
• On the weak field approximation for CaII 8542 A
• Scattering Polarization of the Ca II IR Triplet for Probing the Quiet Solar Chromosphere
• Zero-Field Dichroism in the Solar Chromosphere
• The Hanle and Zeeman polarization signals of the solar Ca II 8542 Å line
• Temperature constraints from inversions of synthetic solar optical, UV and radio spectra
• Chromospheric polarimetry through multi-line observations of the 850 nm spectral region
• Scattering polarization in the CaII Infrared Triplet with Velocity Gradients
• Temporal Evolution of the Scattering Polarization of the CaII IR Triplet in Hydrodynamical Models of the Solar Chromosphere
• The effects of three-dimensional radiative transfer on the resonance polarization of the Ca I 4227 $\mathring{\mathrm{A}}$ line
• Non-LTE inversions of a confined X2.2 flare: I. Vector magnetic field in the photosphere and chromosphere
• Depolarizing collisions with hydrogen: neutral and singly ionized alkaline earths

Cited by in corpus (1)

• Recovering thermodynamics from spectral profiles observed by IRIS (II): improved calculation of the uncertainties based on Monte Carlo experiments