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The modeling of the peculiar scattering polarization signals observed in somediagnostically important solar resonance lines requires the consideration ofthe detailed spectral structure of the incident radiation field as well as thepossibility of ground level polarization, along with the atom's hyperfinestructure and quantum interference between hyperfine F-levels pertaining eitherto the same fine structure J-level, or to different J-levels of the same term.Here we present a theoretical and numerical approach suitable for solving thiscomplex non-LTE radiative transfer problem. This approach is based on thedensity-matrix metalevel theory (where each level is viewed as a continuousdistribution of sublevels) and on accurate formal solvers of the transferequations and efficient iterative methods. We show an application to theD-lines of NaI, with emphasis on the enigmatic D1 line, pointing out theobservable signatures of the various physical mechanisms considered. Wedemonstrate that the linear polarization observed in the core of the D1 linemay be explained by the effect that one gets when the detailed spectralstructure of the anisotropic radiation responsible for the optical pumping istaken into account. This physical ingredient is capable of introducingsignificant scattering polarization in the core of the NaI D1 line without theneed for ground-level polarization.

Radiative transfer modeling of the enigmatic scattering polarization in the solar NaI D1 line