Letter to the Editor

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

¹ Istituto Ricerche Solari Locarno, via Patocchi, 6605 Locarno-Monti, Switzerland
e-mail: mbianda@irsol.ch
² Institute of Astronomy, ETH Zürich, 8093 Zürich, Switzerland
³ Indian Institute of Astrophysics, Koramangala, Bangalore 560 034, India
⁴ MPI für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
⁵ UNS, CNRS, OCA, Laboratoire Cassiopée, 06304 Nice Cedex, France

Received: 7 April 2011
Accepted: 5 May 2011

Abstract

Chromospheric magnetic fields are notoriously difficult 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 10 830 Å and the Ca i 4227 Å 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 Å 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.