The Influence of Atomic Model on the Theoretical Profile of Scattering Polarization

The diagnosis of solar magnetic field is a very important part in the studies of solar physics. The Stokes parameters I, Q, U, and V can give a complete description of the state of polarized light. By an inversion of the observed Stokes parameters, the information of solar magnetic field can be diagnosed. In the recent decades, the diagnosis of magnetic field is mainly based on the Zeeman effect, by which the produced line splitting is used for the diagnosis of a strong magnetic field up to several hundred Gauss. But in the solar quiet regions, there exist a large amount of weak magnetic fields with a strength less than 100 Gauss, which can be diagnosed by the Hanle effect, thus the diagnosis of weak magnetic field by the Hanle effect is one of the main topics in the diagnostics of magnetic field, and a more completely understanding for the polarization mechanism is needed. This paper focuses on the influences of atomic model on the scattering-caused line profile, and the Hanle effect when there is magnetic field existed. We adopt the neutral magnesium as a target in the study, the seven-, four-, and two-level atomic models are selected to investigate respectively the influences of these atomic models on the scattering polarization and the Hanle effect. It is found that there is a great difference between the polarization degrees produced by the two-level atomic model and multi-level ones (the seven- and four-levels). For the two-level atomic model, the b4 line is not affected by the Hanle effect of low levels, but for the multi-level atomic models, it does. Comparing the seven- and four-level atomic models, the polarization degree of spectral lines only has a small variation, and the behavior of Hanle effect is basically the same. The main conclusion of this paper is that when the linear polarization Q/I profiles of the 3 neutral magnesium b lines are used for the inversion of magnetic field, at least the four-level atomic model should be adopted. This mainly depends on the atomic level distribution in the forming region of the spectral line, the atomic levels with a greater occupation number must be considered in the study.