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

Abstract

Polarization that is produced by coherent scattering can be modified by magnetic fields via the Hanle effect. According to standard theory the Hanle effect should only be operating in the Doppler core of spectral lines but not in the wings. In contrast, our observations of the scattering polarization in the Ca {\sc i} 4227 Åline reveals the existence of spatial variations of the scattering polarization throughout the far line wings. This raises the question whether the observed spatial variations in wing polarization have a magnetic or non-magnetic origin. A magnetic origin may be possible if elastic collisions are able to cause sufficient frequency redistribution to make the Hanle effect effective in the wings without causing excessive collisional depolarization, as suggested by recent theories for partial frequency redistribution with coherent scattering in magnetic fields. To model the wing polarization we apply an extended 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 around the Doppler core, but it fails to reproduce the observed spatial variations of the wing polarization in terms of magnetic field effects with frequency redistribution. This null result points in the direction of a non-magnetic origin in terms of local inhomogeneities (varying collisional depolarization, radiation-field anisotropies, and deviations from a plane-parallel atmospheric stratification).

Accepted in May 2009 for publication in The Astrophysical Journal

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Cited by in corpus (10)

• FTS atlas of the Sun's spectrally resolved center-to-limb variation
• Center to limb observations and modeling of the Ca I 4227 A line
• Magneto-optical effects in the scattering polarization wings of the Ca I resonance line at 4227 angstroms
• Observations of the forward scattering Hanle effect in the Ca i 4227 Å line
• Polarized line formation with J-state interference in the presence of magnetic fields: A heuristic treatment of collisional frequency redistribution
• Spatio-temporal evolution of Hanle and Zeeman synthetic polarization in a chromospheric spectral line
• Observational indications of magneto-optical effects in the scattering polarization wings of the Ca I 4227 Å line
• Multi-dimensional radiative transfer to analyze Hanle effect in Ca {\sc ii} K line at 3933 Å\,
• Assessment of the CRD approximation for the observer's frame RIII redistribution matrix
• The polarization angle in the wings of Ca i 4227: A new observable for diagnosing unresolved photospheric magnetic fields