Three-dimensional Radiative Transfer Modeling of the Polarization of the Sun's Continuous Spectrum

TL;DR

This paper develops a 3D radiative transfer model for the Sun's polarized continuous spectrum, demonstrating improved agreement with observations over 1D models and providing insights into solar magnetic energy and atmospheric structure.

Contribution

It introduces a comprehensive 3D radiative transfer approach that accounts for horizontal inhomogeneities and anisotropy, enhancing the modeling of solar polarization.

Findings

Model shows remarkable agreement with empirical polarization data.

3D model better than 1D models in matching observations.

Hints at slight discrepancies in temperature gradients in the solar atmosphere.

Abstract

Here we formulate and solve the 3D radiative transfer problem of the polarization of the solar continuous radiation. Our approach takes into account not only the anisotropy of the continuum radiation, but also the symmetry-breaking effects caused by the horizontal atmospheric inhomogeneities produced by the solar surface convection. Interestingly, our radiative transfer modeling in a well-known 3D hydrodynamical model of the solar photosphere shows remarkable agreement with the empirical data, significantly better than that obtained via the use of 1D atmospheric models. Although this result confirms that the above-mentioned 3D model was indeed a suitable choice for our Hanle-effect estimation of the substantial amount of "hidden" magnetic energy that is stored in the quiet solar photosphere, we have found however some small discrepancies whose origin may be due to uncertainties in the empirical data and/or in the thermal and density structure of the 3D model. For this reason, we have paid some attention also to other (more familiar) observables, like the center-limb variation of the continuum intensity, which we have calculated taking into account the scattering contribution to the continuum source function. The overall agreement with the observed center-limb variation turns out to be impressive, but we find a hint that the model's temperature gradients in the continuum forming layers could be slightly too steep, perhaps because all current simulations of solar surface convection and magnetoconvection compute the radiative flux divergence ignoring the fact that the effective polarizability is not completely negligible, especially in the downward-moving intergranular lane plasma.