R. Holzreuter

Scattering polarization in strong chromospheric lines. I. Explanation of the triplet peak structure

Although the triplet polarization structure of the Na  D2 and Ca  4227 A 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  D2 has basically the same origin as for the Ca  4227 A 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 W2, which is independently obtained with the density-matrix formalism.

Ca II K polarization as a diagnostic of temperature bifurcation

Aims. We compute the linearly polarized spectrum of Ca ii K caused by coherent scattering and exploit the line for chromospheric diagnosis, with particular attention to temperature bifurcation, by comparing the theory with solar observations. Methods. We numerically solve the statistical equilibrium equations and the radiative transfer equation taking into account polarized coherent scattering with partial frequency redistribution. All calculations are performed in 1D within a plane-parallel atmosphere. Results. We find strong evidence of a chromospheric temperature bifurcation. This suggests that the linearly polarized spectrum of Ca ii K might become a valuable tool to study cool components and the dynamics of the chromosphere independently of observations of molecular CO lines and millimeter and sub-millimeter continua.

Spatial mapping of the Hanle and Zeeman effects on the Sun

Spatial mapping of the Hanle and Zeeman eects on the Sun has been done for the rst time, through Stokes vector imaging with a narrow-band (0.2 A) universal lter. It is shown how the polarization signatures of the Hanle and Zeeman eects can be cleanly distinguished from each other by comparing the Stokes images recorded at dierent, specially selected wavelengths within the Na i D2{D1 line system. Examples of the polarization signatures of sunspots, faculae, the supergranulation network, and large-scale canopy elds are shown. The most striking result of our observations is that the scattering polarization has an extremely intermittent structure rather than being a simple function of limb distance. These intermittent scattering polarization signals are cospatial with the facular and supergranulation network seen both in intensity and circular polarization. The observed pattern can be explained in terms of magnetic enhancement of the scattering polarization in the network and/or Hanle depolarization of the scattering polarization outside the network. Since however no magnetic elds are seen in circular polarization outside the network, the relative absence of linear scattering polarization there may be explained by Hanle depolarization only if the volume lling, depolarizing magnetic eld has mixed polarities on a subarcsec scale that is not resolved.

Scattering polarization in strong chromospheric lines - II. Influence of the temperature curve on the Ca II K line

In a previous paper we analyzed the polarization profile of the Ca ii K line near the limb at µ = 0.1. We now extend this work to model the center-to-limb variation of the linear polarization of the Ca ii K line and compare it with calculated spectra based on standard 1D model atmospheres. Our previous two-component approach with a hot and a cool atmospheric component is re-examined. We confirm our previous result that no single model is able to explain the observations. While self-consistent single atmosphere solutions may fit the polarization spectra, they fail to simultaneously fit the corresponding intensity spectra or the polarization spectra at other heliocentric angles. The mixing of a cool and a hot component is however a good approach for all disk positions, although the optimum mixing ratio varies strongly with heliocentric angle. As we approach the limb the hot component gains in importance, which is consistent with the scenario of diverging magnetic canopies overlying a cool atmosphere.