Diagnostic potential of wavelength-integrated scattering polarisation signals of the solar He II Ly-alpha line

Abstract

Aims: Main goal of this work is to study the potential of He II Ly-alpha wavelength-integrated scattering polarisation for probing the magnetism of the solar upper chromosphere. Methods: Radiative transfer calculations are performed in semi-empirical 1D solar atmospheres considering a two-term atomic model and accounting for the Hanle, Zeeman, and magneto-optical effects. The problem is suitably linearised and discretised, and the resulting numerical system is solved with a matrix-free iterative method. The results obtained modelling scattering processes with three different descriptions, namely in the limit of complete frequency redistribution (CRD), and accounting for partial frequency redistribution (PRD) effects under the angle-averaged (AA) approximation and in the general angle-dependent (AD) formulation, are compared. Results: In the line-core, the synthetic Stokes profiles resulting from CRD, PRD-AA, and PRD-AD calculations show a very good agreement. On the other hand, relevant differences are observed in Q/I outside the line-core region. Besides, the precise structure of the atmospheric model does not noticeably affect the line-core profiles, but it strongly impacts the Q/I signals outside the line-core. As most of the He II Ly-alpha photons originate in the core region, it turns out that wavelength-integrated linear polarisation signals are almost insensitive to both the scattering description and the atmospheric model. Appreciable wavelength-integrated U/I signals, showing observable sensitivity to horizontal magnetic fields in the range 0-1000 G, are also found, particularly near the limb. It turns out that, while the integration time required to detect magnetic fields in the quiet chromosphere with this line is too long for sounding rocket missions, magnetic fields corresponding to typical plage areas would produce detectable signals, especially near the limb.

References in corpus (15)

• Magnetic Diagnostics of the Solar Chromosphere with the Mg II h-k Lines
• The magnetic sensitivity of the Mg ii k line to the joint action of Hanle, Zeeman and magneto-optical effects
• 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
• The Hanle and Zeeman polarization signals of the solar Ca II 8542 Å line
• The transfer of resonance line polarization with partial frequency redistribution in the general Hanle-Zeeman regime
• The transfer of resonance line polarization with partial frequency redistribution and J-state interference
• Formal Solutions for Polarized Radiative Transfer. I. The DELO Family
• Master equation theory applied to the redistribution of polarized radiation in the weak radiation field limit. V. The two-term atom
• The effects of three-dimensional radiative transfer on the resonance polarization of the Ca I 4227 $\mathring{\mathrm{A}}$ line
• Modeling the scattering polarization of the solar Ca i 4227 Å line with angle-dependent partial frequency redistribution
• Numerical solutions to linear transfer problems of polarized radiation II. Krylov methods and matrix-free implementation
• Solving the paradox of the solar sodium D1 line polarization
• The transfer of polarized radiation in resonance lines with partial frequency redistribution, J-state interference, and arbitrary magnetic fields. A radiative transfer code and useful approximations
• Assessment of the CRD approximation for the observer's frame RIII redistribution matrix
• The potential of the wavelength-integrated scattering polarization of the hydrogen Ly-alpha line for probing the solar chromosphere

Cited by in corpus (1)

• A numerical approach for modelling the polarisation signals of strong resonance lines with partial frequency redistribution. Numerical applications to two-term atoms and plane-parallel atmospheres