Diagnostic potential of wavelength-integrated scattering polarisation in the solar He II Ly-α line

Abstract

Aims. The main goal of this work is to study the potential of the He II Ly-α wavelength-integrated scattering polarisation for probing the magnetism of the solar upper chromosphere. Meanwhile, the suitability of different modelling approximations is investigated. Methods. Radiative transfer calculations are performed in semi-empirical 1D solar atmospheres, out of local thermodynamic equilibrium, 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 by 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. The synthetic Stokes profiles resulting from CRD, PRD–AA, and PRD–AD calculations show a very good agreement in the line core, while some differences are observed in Q/I outside this spectral region. Moreover, 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-α 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 Ū/Ī signals, showing observable sensitivity to horizontal magnetic fields in the 0–1000 G range, are also found, particularly near the limb. 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. Conclusions. These results, to be confirmed by 3D calculations including the impact of horizontal inhomogeneities and bulk velocity gradients, show that filter polarimetry in the He II Ly-α line has a promising potential for chromospheric magnetic-field diagnostics. In near-limb plage regions, this could already be assessed through sounding-rocket experiments.