The effects of three-dimensional radiative transfer on the resonance polarization of the Ca I 4227 $\mathring{\mathrm{A}}$ line

Abstract

The sizable linear polarization signals produced by the scattering of anisotropic radiation in the core of the Ca I 4227 $\mathring{\mathrm{A}}$ line constitute an important observable for probing the inhomogeneous and dynamic plasma of the lower solar chromosphere. Here we show the results of a three-dimensional (3D) radiative transfer complete frequency redistribution (CRD) investigation of the line's scattering polarization in a magneto-hydrodynamical 3D model of the solar atmosphere. We take into account not only the Hanle effect produced by the model's magnetic field, but also the symmetry breaking caused by the horizontal inhomogeneities and macroscopic velocity gradients. The spatial gradients of the horizontal components of the macroscopic velocities produce very significant forward scattering polarization signals without the need of magnetic fields, while the Hanle effect tends to depolarize them at the locations where the model's magnetic field is stronger than about 5 G. The standard 1.5D approximation is found to be unsuitable for understanding the line's scattering polarization, but we introduce a novel improvement to this approximation that produces results in qualitative agreement with the full 3D results. The instrumental degradation of the calculated polarization signals is also investigated, showing what can we expect to observe with the Visible Spectro-Polarimeter at the upcoming Daniel K. Inouye Solar Telescope.

18 pages, 12 figures; accepted in ApJ

References in corpus (10)

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Cited by in corpus (7)

• Modeling the scattering polarization of the solar Ca i 4227 Å line with angle-dependent partial frequency redistribution
• Novel framework for the three-dimensional NLTE inverse problem
• Evidence for the Operation of the Hanle and Magneto-Optical Effects in the Scattering Polarization Signals Observed by CLASP2 Across the Mg II h and k Lines
• Spectropolarimetric observations of the solar atmosphere in the H$\alpha$ 6563 Å line
• Evaluating the reliability of a simple method to map the magnetic field azimuth in the solar chromosphere
• The potential of the wavelength-integrated scattering polarization of the hydrogen Ly-alpha line for probing the solar chromosphere
• On the (mis)interpretation of the scattering polarization signatures in the Ca II 8542 A line through spectral line inversions