Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...285..655f&link_type=abstract
Astronomy and Astrophysics 285, 655-662 (1994)
Astronomy and Astrophysics
Astrophysics
24
Polarization, Scattering, Lines: Formation, Sun: Chromosphere, Sun: Magnetic Fields
Scientific paper
The center-to-limb variations of the linear polarization observed by Stenflo et al. (1980) outside active regions, in the resonance lines of CaI at 4227 A and of SrII at 4078 A, are analyzed. Both lines are formed in the low chromosphere by multiple scattering. Their linear polarization is a consequence of the coherent scattering of the anisotropic radiation field by the atoms. It is very sensitive to partial frequency redistribution, depolarizing collisions, turbulent velocity fields and to the Hanle effect due to weak magnetic fields. The rates of depolarizing collisions may be determined by fitting the polarization profiles observed in the line wings, which are not affected by the Hanle effect. The quadratic mean of the turbulent velocity field is adjusted in order to recover the observed line core intensity. It is then possible to estimate the resonance polarization in the line cores. These estimates are in good agreement with the observations performed close to the solar limb (at heliocentric angles larger than about 70deg). Closer to disk center the observed polarization rates are significantly larger than what is expected in the absence of a magnetic field. We investigate the Hanle effect of an almost horizontal magnetic field of random azimuthal orientation lying in the chromosphere, which may constitute the base of a magnetic canopy. If the base of the canopy lies slightly below the depth where the line core is formed, i.e. between 700km and 1000km above τ_5000_=1, then the line core polarization close to disk center is actually enhanced as compared to its non-magnetic value. This surprising effect is a consequence of multiple scattering of line photons in the chromosphere. The enhancement is however somewhat smaller than what is required to fit the observations. In contrast, the polarization is reduced when the canopy base lies in the region where the line core is formed, i.e. between 1000km and 1300km above τ_5000_=1; it becomes insensitive to any horizontal magnetic field lying higher in the chromosphere.
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