Observational indications of magneto-optical effects in the scattering polarization wings of the Ca I 4227 Å line

Abstract

Context. Several strong resonance lines, such as H I Ly-α, Mg II k, Ca II K, and Ca I 4227 Å, are characterized by deep and broad absorption profiles in the solar intensity spectrum. These resonance lines show conspicuous linear scattering polarization signals when observed in quiet regions close to the solar limb. Such signals show a characteristic triplet-peak structure with a sharp peak in the line core and extended wing lobes. The line core peak is sensitive to the presence of magnetic fields through the Hanle effect, which however is known not to operate in the line wings. Recent theoretical studies indicate that, contrary to what was previously believed, the wing linear polarization signals are also sensitive to the magnetic field through magneto-optical (MO) effects. Aims. We search for observational indications of this recently discovered physical mechanism in the scattering polarization wings of the Ca I 4227 Å line. Methods. We performed a series of spectropolarimetric observations of this line using the Zurich IMaging POLarimeter camera at the Gregory-Coudé telescope at Istituto Ricerche Solari Locarno in Switzerland and at the GREGOR telescope in Tenerife (Spain). Results. Spatial variations of the total linear polarization degree and linear polarization angle are clearly appreciable in the wings of the observed line. We provide a detailed discussion of our observational results, showing that the detected variations always take place in regions in which longitudinal magnetic fields are present, thus supporting the theoretical prediction that they are produced by MO effects.