Observations of the forward scattering Hanle effect in the Ca i 4227 Å line

M. Bianda, R. Ramelli, L. S. Anusha, J. O. Stenflo, K. N. Nagendra, R. Holzreuter, M. Sampoorna, H. Frisch, H. N. Smitha

Published 2011-05-11Version 1

Chromospheric magnetic fields are notoriously diffcult to measure. The chromospheric lines are broad, while the fields are producing a minuscule Zeeman-effect polarization. A promising diagnostic alternative is provided by the forward-scattering Hanle effect, which can be recorded in chromospheric lines such as the He i 10830 {\AA} and the Ca i 4227 {\AA} lines. We present a set of spectropolarimetric observations of the full Stokes vector obtained near the center of the solar disk in the Ca i 4227 {\AA} line with the ZIMPOL polarimeter at the IRSOL observatory.We detect a number of interesting forward-scattering Hanle effect signatures, which we model successfully using polarized radiative transfer. Here we focus on the observational aspects, while a separate companion paper deals with the theoretical modeling.