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Polarization that is produced by coherent scattering can be modified bymagnetic fields via the Hanle effect. According to standard theory the Hanleeffect should only be operating in the Doppler core of spectral lines but notin the wings. In contrast, our observations of the scattering polarization inthe Ca {\sc i} 4227 \AA line reveals the existence of spatial variations of thescattering polarization throughout the far line wings. This raises the questionwhether the observed spatial variations in wing polarization have a magnetic ornon-magnetic origin. A magnetic origin may be possible if elastic collisionsare able to cause sufficient frequency redistribution to make the Hanle effecteffective in the wings without causing excessive collisional depolarization, assuggested by recent theories for partial frequency redistribution with coherentscattering in magnetic fields. To model the wing polarization we apply anextended version of the technique based on the "last scattering approximation".This model is highly successful in reproducing the observed Stokes $Q/I$polarization (linear polarization parallel to the nearest solar limb),including the location of the wing polarization maxima and the minima aroundthe Doppler core, but it fails to reproduce the observed spatial variations ofthe wing polarization in terms of magnetic field effects with frequencyredistribution. This null result points in the direction of a non-magneticorigin in terms of local inhomogeneities (varying collisional depolarization,radiation-field anisotropies, and deviations from a plane-parallel atmosphericstratification).

Origin of spatial variations of scattering polarization in the wings of the Ca {\sc i} 4227 \AA line