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.