Three-dimensional radiative transfer simulations of the scattering polarization of the hydrogen Lyα line in a magnetohydrodynamic model of the chromosphere-corona transition region

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 α line in a three-dimensional model of the solar atmosphere resulting from a state of the art magnetohydrodynamic ( MHD ) simulation. At full spatial resolution the linear polarization signals are very signi fi cant all over the solar disk, with a large fraction of the fi eld of view ( FOV ) showing line-center amplitudes well above the 1% level. Via the Hanle effect the line-center polarization signals are sensitive to the magnetic fi eld of the model ʼ s transition region, even when its mean fi eld strength is only 15 G. The breaking of the axial symmetry of the radiation fi eld produces signi fi cant forward-scattering polarization in Ly α , without the need of an inclined magnetic fi eld. Interestingly, the Hanle effect tends to decrease such forward-scattering polarization signals in most of the points of the FOV. When the spatial resolution is degraded, the line-center polarization of Ly α drops below the 1% level, reaching values similar to those previously found in one-dimensional ( 1D ) semi-empirical models ( i.e., up to about 0.5 % ) . The center to limb variation ( CLV ) of the spatially averaged polarization signals is qualitatively similar to that found in 1D models, with the largest line-center amplitudes at μ = cos θ ≈ 0. 4 ( θ 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 α polarization. Reproduced with permission from the Astrophysical Journal. © IOP Publishing