Scattering polarization in strong chromospheric lines

Abstract

Aims. We explore the spatial variations of the chromospheric Call K scattering polarization on the quiet Sun, in particular the fluctuations of the linear polarizations Q/I and U/f, their correlations, varying profile shapes, and center-to-limb variations. Methods. A set of high precision polarimetric recordings with ZIMPOL (Zurich Imaging Polarimeter) at distinct heliographic positions is used. Results. Two main effects contribute to the observed fluctuations: (i) Chromospheric magnetic fields (located in the K3 layer), which induce polarization variations via the Hanle effect. (ii) Hot spots due to small-scale temperature structuring of the chromosphere, which induce variations of the local scattering geometry. The observed signatures of both effects are influenced by the turbulent broadening, which steeply increases with height, by optical depth variations in the K3 layer, and by the chromospheric temperature stratification. Almost all the fluctuations that are seen with our spatial resolution (approximately 5 arcsec) originate in the high chromospheric K3 layer, while the lower chromospheric layers are much more homogeneous at the resolved scales. The relatively small Q/I fluctuations observed between the K1 and K2 wavelengths constrain the size and lifetime of cold and hot areas in the low and mid chromosphere. Conclusions. At present it is not possible to disentangle the Hanle effect and the hot-spot effect from each other. This would require 2D spatial mapping of the linear polarization at the K3, K2, and Kl wavelengths with higher spatial resolution, which is hard to do with existing telescopes, since the observations are photon starved and require larger telescope apertures.