Observations of the forward scattering Hanle effect in the Ca i 4227 {\AA} line

TL;DR

This paper reports spectropolarimetric observations of the Ca i 4227 Å line near the solar disk center, detecting forward-scattering Hanle effect signatures that help diagnose chromospheric magnetic fields.

Contribution

It provides the first observational evidence of the forward-scattering Hanle effect in the Ca i 4227 Å line and models these signatures using polarized radiative transfer.

Findings

Detection of forward-scattering Hanle effect signatures in Ca i 4227 Å line

Successful modeling of observations with polarized radiative transfer

Demonstration of the potential of this effect for chromospheric magnetic diagnostics

Abstract

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.