Non-LTE Modeling of the Ba II D2 Line Resonance Polarization

The Ba II resonance line at 455.4 nm is formed in the low solar chromosphere. It shows significant linear polarization outside active regions close to the solar limb. This so-called resonance polarization is sensitive to the Hanle effect of weak magnetic fields. We report on numerical simulations of the intensity and resonance polarization center-to-limb variations in the line and in the adjacent continuum, in a quiet solar atmosphere and we compare them to observations performed at THEMIS in August 2007. In the simulations we take into account non-LTE multilevel coupling, multiple scattering and partial frequency redistribution, and we neglect the hyperfine structure of the odd isotopes of Barium. As resonance polarization and partial frequency redistribution effects are very sensitive to elastic collisions with hydrogen atoms we use accurate depolarizing collisional rates recently computed for this line by a semi-classical method. Our radiative transfer calculations allow us to model the central part of the line core and the wings quite well. We show that the line polarization depends very much indeed on partial frequency redistribution effects. Then we investigate its diagnostic potential for weak unresolved magnetic fields in the low chromosphere. We find that the observed polarization rates are in good agreement with the simulations if we take into account the Hanle effect due to an isotropic turbulent magnetic field of the order of 30 Gauss at the altitude where the line core is formed, i.e. between 900 km and 1300 km above the basis of the photosphere.