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

• Published in: The Astrophysical journal 2015-04, Vol.803 (2)
• Main Authors: Stepan, J, Trujillo Bueno, J, Leenaarts, Jorrit, Carlsson, Mats
• Format: Article
• Language: English
• Subjects: ANISOTROPY; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; AXIAL SYMMETRY; CHROMOSPHERE; COMPUTERIZED SIMULATION; HYDROGEN; MAGNETIC FIELDS; MAGNETISM; MAGNETOHYDRODYNAMICS; MEAN-FIELD THEORY; POLARIZATION; RADIANT HEAT TRANSFER; radiative transfer; SCATTERING; SPACE; SPATIAL RESOLUTION; SUN; Sun: chromosphere; Sun: magnetic fields; Sun: transition region; THREE-DIMENSIONAL CALCULATIONS
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.1088/0004-637X/803/2/65

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