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Several strong resonance lines, such as H I Ly-$\alpha$, Mg II k, Ca II K, CaI 4227 \AA\, which are characterized by deep and broad absorption profiles inthe solar intensity spectrum, show conspicuous linear scattering polarizationsignals when observed in quiet regions close to the solar limb. Such signalsshow a characteristic triplet-peak structure, with a sharp peak in the linecore and extended wing lobes. The line core peak is sensitive to the presenceof magnetic fields through the Hanle effect, which however is known not tooperate in the line wings. Recent theoretical studies indicate that, contraryto what was previously believed, the wing linear polarization signals are alsosensitive to the magnetic field through magneto-optical effects (MO). We searchfor observational indications of this recently discovered physical mechanism inthe scattering polarization wings of the Ca I 4227 \AA\ line. We performed aseries of spectropolarimetric observations of this line using the ZurichIMaging POLarimeter (ZIMPOL) camera at the Gregory-Coud\'e telescope of IRSOL(Switzerland) and at the GREGOR telescope in Tenerife (Spain). Spatialvariations of the total linear polarization degree and of the linearpolarization angle are clearly appreciable in the wings of the observed line.We provide a detailed discussion of our observational results, showing that thedetected variations always take place in regions where longitudinal magneticfields are present, thus supporting the theoretical prediction that they areproduced by MO effects.

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