Scattering Polarization of Hydrogen Lines in Weakly Magnetized Stellar Atmospheres I. Formulation and Application to Isothermal Models

Abstract

Although the spectral lines of hydrogen contain valuable information on the physical properties of a variety of astrophysical plasmas, including the upper solar chromosphere, relatively little is known about their scattering polarization signals whose modification via the Hanle effect may be exploited for magnetic field diagnostics. Here we report on a basic theoretical investigation of the linear polarization produced by scattering processes and the Hanle effect in Ly-a, Ly-b and H-a taking into account multilevel radiative transfer effects in an isothermal stellar atmosphere model, the fine-structure of the hydrogen levels, as well as the impact of collisions with electrons and protons. The main aim of this first paper is to elucidate the physical mechanisms that control the linear polarization in the three lines, as well as its sensitivity to the perturbers density and to the strength and structure of micro-structured and deterministic magnetic fields. To this end, we apply an efficient radiative transfer code we have developed for performing numerical simulations of the Hanle effect in multilevel systems with overlapping line transitions. For low density plasmas such as that of the upper solar chromosphere collisional depolarization is caused mainly by collisional transitions between the fine-structure levels of n=3, so that it is virtually insignificant for Ly-a but important for Ly-b and H-a. We show the impact of the Hanle effect on the three lines taking into account the radiative transfer coupling between the different hydrogen line transitions. For example, we demonstrate that the linear polarization profile of the H-a line is sensitive to the presence of magnetic field gradients in the line core formation region and that in solar-like chromospheres selective absorption of polarization components does not play any significant role on the emergent scattering polarization.

24 pages, 16 figures, 2 tables, accepted for publication in ApJ

References in corpus (10)

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• Zero-Field Dichroism in the Solar Chromosphere
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Cited by in corpus (14)

• PORTA: A three-dimensional multilevel radiative transfer code for modeling the intensity and polarization of spectral lines with massively parallel computers
• The Hanle Effect of the Hydrogen Ly-alpha Line for Probing the Magnetism of the Solar Transition Region
• 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
• Scattering Polarization and Hanle Effect in Stellar Atmospheres with Horizontal Inhomogeneities
• The transfer of resonance line polarization with partial frequency redistribution and J-state interference
• The Ly-alpha Lines of H I and He II: A Differential Hanle Effect for Exploring the Magnetism of the Solar Transition Region
• The impact of quantum interferences between different J-levels on scattering polarization in spectral lines
• The Hanle effect of Ly$\alpha$ in an MHD model of the Solar Transition Region
• On the inversion of the scattering polarization and the Hanle effect signals in the hydrogen Lyman-$\alpha$ line
• Diagnosis of Magnetic and Electric Fields of Chromospheric Jets through Spectropolarimetric Observations of HI Paschen Lines
• Constraining the shaping mechanism of the Red Rectangle through spectro-polarimetry of its central star
• Does H$\alpha$ Stokes~$V$ profiles probe the chromospheric magnetic field? An observational perspective
• Simultaneous spectropolarimetric observations in the H$\alpha$ and Ca II 8662 Å lines of an active region
• Spectropolarimetric observations of the solar atmosphere in the H$\alpha$ 6563 Å line