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

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.

15 pages, 15 figures, accepted for publication in The Astrophysical Journal

References in corpus (4)

• Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere
• The Ly-alpha profile and center-to-limb variation of the quiet Sun
• Scattering Polarization of Hydrogen Lines in Weakly Magnetized Stellar Atmospheres I. Formulation and Application to Isothermal Models
• On the inversion of the scattering polarization and the Hanle effect signals in the hydrogen Lyman-$\alpha$ line

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• Evidence for the Operation of the Hanle and Magneto-Optical Effects in the Scattering Polarization Signals Observed by CLASP2 Across the Mg II h and k Lines
• Solar Hα excess during Solar Cycle 24 from full-disk filtergrams of the Chromospheric Telescope
• Diagnostic potential of wavelength-integrated scattering polarisation signals of the solar He II Ly-alpha line
• Spectral Characteristics of the He I D3 Line in a Quiescent Prominence Observed by THEMIS
• The potential of the wavelength-integrated scattering polarization of the hydrogen Ly-alpha line for probing the solar chromosphere