||Linear Polarization of the Solar Ca I 4227 Å Line: Modeling with Radiative Transfer and Last Scattering Approximation
||437, Solar Polarization Workshop 6
||Anusha, L. S.; Stenflo, J. O.; Frisch, H.; Bianda, M.; Holzreuter, R.; Nagendra, K. N.; Sampoorna, M.; Ramelli, R.
||To model the Ca I 4227 Å line polarization, radiative
transfer effects with partial frequency redistribution (PRD) must be
taken into account. The numerical solution of the relevant polarized
radiative transfer (RT) equations is computationally very demanding.
The “last scattering approximation” (LSA) is a concept allowing
faster methods to be devised. It is based on the remark that a
single scattering of the radiation field is sufficient for creating
most of the polarization. Its key ingredient is the anisotropy of the radiation
field. If the anisotropy is extracted from the observed center to limb
variation of the intensity profile, only the wings of the Q/I
spectrum can be modeled (Sampoorna et al. 2009). We show
here that the core region may be modeled as well if one takes into
account the depth variation of the anisotropy which is obtained from
an unpolarized multilevel RT (Anusha et al. 2010). After a validation of the LSA approach by comparison with
a polarized RT calculation, we apply both approaches to model
recent observations of the Ca I 4227 Å line polarization
taken on the quiet Sun. Apart from a global scaling factor,
both approaches give a very good fit to the Q/I spectrum for all the
wavelengths. As the LSA is 8 times faster than the RT approach, we
can recommend it as an efficient method to analyze other strong
resonance lines in the second solar spectrum.