Center-to-limb Observations and Modeling of the Ca I 4227 Å 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 the nonmagnetic CLV observations of the Q/I profiles of the Ca I 4227 Å line recorded with the Zurich Imaging Polarimeter-3 at IRSOL. For modeling, we use the polarized radiative transfer with partial frequency redistribution with a number of realistic one-dimensional (1D) model atmospheres. We find that all the standard Fontenla-Avrett-Loeser (FAL) model atmospheres, which we used, fail to simultaneously fit the observed (I, Q/I) at all the limb distances (μ). However, an attempt is made to find a single model which can provide a fit to at least the CLV of the observed Q/I instead of a simultaneous fit to the (I, Q/I) at all μ. To this end we construct a new 1D 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 μ but fails to reproduce the observed rest intensity at different μ. Hence we find that no single 1D model atmosphere succeeds in providing a good representation of the real Sun. This failure of 1D models does not, however, cause an impediment to the magnetic field diagnostic potential of the Ca I 4227 Å line. To demonstrate this we deduce the field strength at various μ positions without invoking the use of radiative transfer.