Three-dimensional radiative transfer simulations of the scattering polarization of the hydrogen Ly$\alpha$ line in a MHD model of the chromosphere-corona transition region

TL;DR

This study uses advanced 3D radiative transfer simulations to analyze the scattering polarization of hydrogen Lyα in the solar chromosphere, revealing significant polarization signals sensitive to magnetic fields and the effects of spatial resolution.

Contribution

It applies a novel 3D radiative transfer code to model Lyα polarization in a realistic MHD solar atmosphere, highlighting the impact of magnetic fields and spatial resolution on polarization signals.

Findings

Line-center polarization exceeds 1% at full resolution.

Hanle effect sensitivity to magnetic fields as low as 15 G.

Polarization signals decrease below 1% when spatial resolution is degraded.

Abstract

Probing the magnetism of the upper solar chromosphere requires measuring and modeling the scattering polarization produced by anisotropic radiation pumping in UV spectral lines. Here we apply PORTA (a novel radiative transfer code) to investigate the hydrogen Ly$\alpha$ line in a 3D model of the solar atmosphere resulting from a state of the art MHD simulation. At full spatial resolution the linear polarization signals are very significant all over the solar disk, with a large fraction of the field of view showing line-center amplitudes well above the 1% level. Via the Hanle effect the line-center polarization signals are sensitive to the magnetic field of the model's transition region, even when its mean field strength is only 15 G. The breaking of the axial symmetry of the radiation field produces significant forward-scattering polarization in Ly$\alpha$, without the need of an inclined magnetic field. Interestingly, the Hanle effect tends to decrease such forward-scattering polarization signals in most of the points of the field of view. When the spatial resolution is degraded, the line-center polarization of Ly$\alpha$ drops below the 1% level, reaching values similar to those previously found in 1D semi-empirical models (i.e., up to about 0.5%). The center to limb variation of the spatially-averaged polarization signals is qualitatively similar to that found in 1D models, with the largest line-center amplitudes at $\mu=\cos\theta\approx 0.4$ ($\theta$ being the heliocentric angle). These results are important, both for designing the needed space-based instrumentation and for a reliable interpretation of future observations of the Ly$\alpha$ polarization.