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

Abstract

The modeling of the peculiar scattering polarization signals observed in some diagnostically important solar resonance lines requires the consideration of the detailed spectral structure of the incident radiation field as well as the possibility of ground level polarization, along with the atom's hyperfine structure and quantum interference between hyperfine F-levels pertaining either to 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 this complex non-LTE radiative transfer problem. This approach is based on the density-matrix metalevel theory (where each level is viewed as a continuous distribution of sublevels) and on accurate formal solvers of the transfer equations and efficient iterative methods. We show an application to the D-lines of NaI, with emphasis on the enigmatic D1 line, pointing out the observable signatures of the various physical mechanisms considered. We demonstrate that the linear polarization observed in the core of the D1 line may be explained by the effect that one gets when the detailed spectral structure of the anisotropic radiation responsible for the optical pumping is taken into account. This physical ingredient is capable of introducing significant scattering polarization in the core of the NaI D1 line without the need for ground-level polarization.

Accepted for publication in The Astrophysical Journal

References in corpus (4)

• The transfer of resonance line polarization with partial frequency redistribution and J-state interference
• A key physical mechanism for understanding the enigmatic linear polarization of the solar BaII and NaI D1 lines
• Non coherent continuum scattering as a line polarization mechanism
• Physics of polarized scattering at multi-level atomic systems

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