THE HANLE AND ZEEMAN POLARIZATION SIGNALS OF THE SOLAR Ca II 8542 Å LINE

Abstract

We highlight the main results of a three-dimensional (3D) multilevel radiative transfer investigation about the solar disk-center polarization of the Ca ii 8542 Å line. First, through the use of a 3D model of the solar atmosphere, we investigate the linear polarization that occurs due to the atomic level polarization produced by the absorption and scattering of anisotropic radiation, taking into account the symmetry-breaking effects caused by its thermal, dynamic, and magnetic structure. Second, we study the contribution of the Zeeman effect to the linear and circular polarization. Finally, we show examples of the Stokes profiles produced by the joint action of the atomic level polarization and the Hanle and Zeeman effects. We find that the Zeeman effect tends to dominate the linear polarization signals only in the localized patches of opposite magnetic polarity, where the magnetic field is relatively strong and slightly inclined; outside such very localized patches, the linear polarization is often dominated by the contribution of atomic level polarization. We demonstrate that a correct modeling of this last contribution requires taking into account the symmetry-breaking effects caused by the thermal, dynamic, and magnetic structure of the solar atmosphere, and that in the 3D model used the Hanle effect in forward-scattering geometry (disk-center observation) mainly reduces the polarization corresponding to the zero-field case. We emphasize that, in general, a reliable modeling of the linear polarization in the Ca ii 8542 Å line requires taking into account the joint action of atomic level polarization and the Hanle and Zeeman effects.

Comparison of theoretical and observed Ca II 8542 Stokes profiles in quiet regions at the centre of the solar disc

“…The discrepancy in the widths of the observed profiles and those resulting from the model atmosphere we used is well described for Stokes I profiles for several spectral lines (see e.g. De Carlsson et al 2015;Stěpán & Trujillo Bueno 2016).…”

“…It is important to note that the Stokes profiles resulting from the joint action of scattering polarisation and the Hanle and Zeeman effects (solid lines) are not simply the sum of the scattering polarisation and the Hanle effect (dashed lines) and those caused by the Zeeman effect (dash-dotted lines). For a more detailed discussions of the role of the different polarising mechanisms, seě Stěpán & Trujillo Bueno (2016) We also wish to stress that the simulation box contains magnetic fields stronger than 10 G at the formation height of the Ca ii 8542 Å line core (Hanle saturation regime for this line) in more than 82% of the grid points. Despite this fact, the maximum linear polarisation signal caused by the scattering polarisation and the Hanle effect (LP H ) is stronger than the maximum linear polarisation signal caused by the Zeeman effect (LP Z ) in more than 85% of the pixels.…”

Solar polarimetry through the K i lines at 770 nm

“…The thick solid green line designates the region where the plasma β ∼ 1 and the thick dashed purple line designates the height where optical depth is unity at 500 nm. Stěpán & Trujillo Bueno (2016). We believe that it will be interesting to examine how large are the atomic polarization signals at different heliocentric angles and how much the presence of a magnetic field alter them through the Hanle effect.…”

Chromospheric heating during flux emergence in the solar atmosphere

“…This time was chosen to minimize the time difference between the Ca ii K and Ca ii 854.2 nm scans owing to the different cadence of the CRISP and CHROMIS observations. The Ca ii 854.2 nm line is not only sensitive to the Zeeman effect, but also to atomic level polarization due to anisotropy in the radiation field and depolarization by the Hanle effect (Manso Sainz & Trujillo Bueno 2010;Carlin & Asensio Ramos 2015;Štěpán & Trujillo Bueno 2016). Three-dimensional radiative transfer calculations computed with a 3D radiation-MHD simulation as input atmosphere indicate that atomic level polarization and the Hanle effect generate linear polarization signals at the 10 −3 level.…”