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The sizable linear polarization signals produced by the scattering ofanisotropic radiation in the core of the Ca I 4227 $\mathrm{\r{A}}$ lineconstitute an important observable for probing the inhomogeneous and dynamicplasma of the lower solar chromosphere. Here we show the results of athree-dimensional (3D) radiative transfer complete frequency redistribution(CRD) investigation of the line's scattering polarization in amagneto-hydrodynamical 3D model of the solar atmosphere. We take into accountnot only the Hanle effect produced by the model's magnetic field, but also thesymmetry breaking caused by the horizontal inhomogeneities and macroscopicvelocity gradients. The spatial gradients of the horizontal components of themacroscopic velocities produce very significant forward scattering polarizationsignals without the need of magnetic fields, while the Hanle effect tends todepolarize them at the locations where the model's magnetic field is strongerthan about 5 G. The standard 1.5D approximation is found to be unsuitable forunderstanding the line's scattering polarization, but we introduce a novelimprovement to this approximation that produces results in qualitativeagreement with the full 3D results. The instrumental degradation of thecalculated polarization signals is also investigated, showing what can weexpect to observe with the Visible Spectro-Polarimeter at the upcoming DanielK. Inouye Solar Telescope.

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