The polarization angle in the wings of Ca i 4227: A new observable for diagnosing unresolved photospheric magnetic fields

TL;DR

This study demonstrates that the polarization angle in the wings of the Ca I 4227 line is sensitive to unresolved photospheric magnetic fields through magneto-optical effects, offering a new diagnostic tool for solar magnetic field analysis.

Contribution

The paper introduces the polarization angle in the wings of Ca I 4227 as a novel observable sensitive to unresolved magnetic fields via magneto-optical effects, expanding diagnostic capabilities.

Findings

The polarization angle in Ca I 4227 wings responds to magnetic flux and filling factor.

Magneto-optical effects influence linear polarization in resonance line wings.

The observable can constrain the magnetic field's unresolved properties.

Abstract

When observed in quiet regions close to the solar limb, many strong resonance lines show conspicuous linear polarization signals, produced by scattering processes, with extended wing lobes. Recent studies indicate that, contrary to what was previously believed, the wing lobes are sensitive to the presence of relatively weak longitudinal magnetic fields through magneto-optical (MO) effects. We theoretically investigate the sensitivity of the scattering polarization wings of the Ca I 4227 {\AA} line to the MO effects, and we explore its diagnostic potential for inferring information on the longitudinal component of the photospheric magnetic field. We calculate the intensity and polarization profiles of the Ca I 4227 {\AA} line by numerically solving the problem of the generation and transfer of polarized radiation under non-local thermodynamic equilibrium conditions in one-dimensional semi-empirical models of the solar atmosphere, taking into account the joint action of the Hanle, Zeeman, and MO effects. We consider volume-filling magnetic fields as well as magnetic fields occupying a fraction of the resolution element. In contrast to the circular polarization signals produced by the Zeeman effect, we find that the linear polarization angle in the scattering polarization wings of Ca I 4227 presents a clear sensitivity, through MO effects, not only to the flux of the photospheric magnetic field, but also to the fraction of the resolution element that the magnetic field occupies. We identify the linear polarization angle in the wings of strong resonance lines as a valuable observable for diagnosing unresolved magnetic fields. Used in combination with observables that encode information on the magnetic flux and other properties of the observed atmospheric region, it can provide constraints on the filling factor of the magnetic field.