Center to limb observations and modeling of the Ca I 4227 A line

Abstract

The observed center-to-limb variation (CLV) of the scattering polarization in different lines of the Second Solar Spectrum can be used to constrain the height variation of various atmospheric parameters, in particular the magnetic fields via the Hanle effect. Here we attempt to model non-magnetic CLV observations of the $Q/I$ profiles of the Ca I 4227 A line recorded with the ZIMPOL-3 at IRSOL. For modeling, we use the polarized radiative transfer with partial frequency redistribution with a number of realistic 1-D model atmospheres. We find that all the standard FAL model atmospheres, used by us, fail to simultaneously fit the observed ($I$, $Q/I$) at all the limb distances ($\mu$). However, an attempt is made to find a single model which can provide a fit at least to the CLV of the observed $Q/I$ instead of a simultaneous fit to the ($I$, $Q/I$) at all $\mu$. To this end we construct a new 1-D model by combining two of the standard models after modifying their temperature structures in the appropriate height ranges. This new combined model closely reproduces the observed $Q/I$ at all the $\mu$, but fails to reproduce the observed rest intensity at different $\mu$. Hence we find that no single 1-D model atmosphere succeeds in providing a good representation of the real Sun. This failure of 1-D models does not however cause an impediment to the magnetic field diagnostic potential of the Ca I 4227 A line. To demonstrate this we deduce the field strength at various $\mu$ positions without invoking the use of radiative transfer.

20 pages, 10 figures, Accepted for publication in ApJ

Cited by in corpus (8)

• FTS atlas of the Sun's spectrally resolved center-to-limb variation
• The effects of three-dimensional radiative transfer on the resonance polarization of the Ca I 4227 $\mathring{\mathrm{A}}$ line
• Modeling the scattering polarization of the solar Ca i 4227 Å line with angle-dependent partial 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
• The quantum interference effects in the Sc II 4247 A line of the Second Solar Spectrum
• 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