1.5D investigation of the Hanle effect in the Ca I 4227 line using partial frequency redistribution: Comparison of synthetic Stokes profiles in Bifrost and MURaM-ChE rMHD models

Abstract

Context. Modeling scattering polarization in the Ca I 4227 Å line is important for diagnosing chromospheric magnetic fields. This requires solving the polarized radiative transfer equation, including the Hanle effect and partial frequency redistribution (PFR). Planeparallel, semi-empirical models are known to be inadequate for accurately reproducing the observed scattering polarization profiles of this line Aims. The primary aim of this work is to investigate the relative influence of heights of formation and magnetic fields (via the Hanle effect) in realistic solar model atmospheres on the emergent spatially averaged synthetic Stokes profiles of the Ca I 4227 Å line. Methods. For this purpose, we used 1D vertical columns derived from snapshots of recent 3D radiative magnetohydrodynamic (rMHD) simulations of the solar chromosphere performed with the Bifrost and MURaM-ChE codes as model atmospheres. We then carried out 1.5D nonlocal thermodynamic equilibrium (NLTE) polarized radiative transfer calculations, including the PFR and the Hanle effect. Results. The intensity profiles from both simulations coincide at the line core, whereas the line wings in MURaM-ChE display a higher intensity as these signals originate from deeper, hotter regions. The polarization signals are formed at greater heights in MURaM-ChE than in Bifrost. In the nonmagnetic case, the emergent, spatially averaged profiles from Bifrost exhibit a larger polarization amplitude in the line core, whereas MURaM-ChE ones show enhanced polarization in the wings. When the impact of magnetic fields from the 3D rMHD simulations is included, the spatially averaged profiles indicate stronger Hanle depolarization of ⟨Q⟩/⟨I⟩ in the Bifrost snapshot. Conclusions. Because scattering polarization is highly sensitive to the heights at which the polarization signals are formed, the differing thermal and magnetic structures at the corresponding line formation heights in the two simulations lead to distinct Stokes profiles. In particular, the Hanle depolarization signatures indicate that the line-core formation region in the MURaM-ChE snapshot is less strongly magnetized compared to the Bifrost snapshot.