Scattering polarization in strong chromospheric lines

Abstract

Abstract. Although the triplet polarization structure of the Na  D 2 and Ca  4227 Å lines in the second solar spectrum has been known for more than two decades, a clear and consistent explanation has been lacking. Here we show that the qualitative profile shape may be explained in terms of the anisotropy of the radiation field and partial frequency redistribution (PRD) effects. The complicated frequency and depth dependence of the anisotropy can be understood in terms of simple arguments that involve the source function gradient and boundary effects. We show in particular that the triplet peak structure of the polarization profile of Na  D 2 has basically the same origin as for the Ca  4227 Å line. Hyperfine structure and lower-level atomic polarization only modify the core polarization without altering the overall qualitative features. For our calculations we adopt a numerical method that combines the advantages of both the classical formalism with integral source function and the density-matrix formalism. In a first step, a multi-level, PRD-capable MALI code, which solves the statistical equilibrium and the radiative transfer equation self-consistently, computes intensity, opacities and collision rates. Keeping these quantities fixed, we obtain the scattering polarization in a second step by solving the radiative transfer equation for the transitions of interest with the classical formalism, which assumes a two-level atomic model with unpolarized lower level. Quantum interferences and lower-level atomic polarization are included in terms of a wavelength dependent polarizability W 2 , which is independently obtained with the density-matrix formalism.

NLTE modeling of Stokes vector center-to-limb variations in the CN violet system

“…The anisotropy mainly results from two competing contributions, namely the source function gradient and a surface effect (Trujillo Bueno 2001;Holzreuter et al 2005). The source function gradient is, under LTE conditions, due to the temperature gradient within the atmosphere.…”

Solar turbulent magnetic fields: Non-LTE modeling of the Hanle effect in the C2molecule

“…We found however that this model cannot produce a sufficiently good fit to our current data sets for the R-and P-triplets with very different total angular momentum numbers. To investigate the possibility to fit both triplets simultaneously, we now apply a more consistent approach using radiative transfer, similar to the one employed by Holzreuter et al (2005) and Shapiro et al (2011a) who applied it to the first and second solar spectrum in strong chromospheric lines and the CN violet system, respectively. While a detailed description of the code can be found in those papers, we will emphasize the differences and required changes for our modeling.…”

“…The actual calculation was done in two steps, analogous to the approach by Holzreuter et al (2005). First, Stokes I and opacities were calculated.…”

The Hanle effect in a random medium

“…Here Ω and x are the direction and frequency of the incident beam, Ω and x the direction and frequency of the scattered beam and H the magnetic field vector (see e.g. Domke & Hubeny 1998;Stenflo 1994, and references therein;Faurobert et al 2001;Bommier et al 2005;Holzreuter et al 2005). For a two-level atom model, with unpolarized ground level and a redistribution function of the above type, each Stokes parameter can be written in the form…”