原子模型对散射偏振理论轮廓的影响

The Influence of Atomic Model on Scattering Polarization Profile

太阳磁场的诊断是太阳物理研究中非常重要的一部分.斯托克斯参量I、Q、U和V可以用来完整描述偏振光的信息,对观测得到的斯托克斯光谱进行反演可以诊断太阳磁场的信息.近几十年来,塞曼效应成为磁场诊断的主要手段,利用塞曼效应使谱线产生的分裂可以诊断强度达到几百高斯的强磁场,但是在太阳宁静区中存在大量强度小于100 Gs的弱磁场,对于弱磁场可以利用汉勒效应来诊断,应用汉勒效应诊断弱磁场一直是磁场诊断的主要内容之一,需要对偏振的产生机制有更完整的理解.文章的主要内容是研究在采用不同原子模型的假设下,由散射产生的谱线轮廓的区别,以及在存在磁场时,不同原子模型谱线的汉勒效应的区别.以中性镁为研究对象,选取了7能级、4能级和2能级原子模型,分别研究了这几个原子模型对散射偏振和汉勒效应的影响.发现2能级原子模型和多能级(7能级和4能级)原子模型产生的谱线的偏振度有很大的区别,并且在2能级原子模型假设下b_4线不存在下能级的汉勒效应,但是在多能级的原子模型下b_4线仍存在下能级的汉勒效应.对比7能级原子模型和4能级原子模型,谱线的偏振度只有很小的变化,汉勒效应的表现也基本相同.主要结论如下:在利用中性镁b 3线的线偏振轮廓Q/I反演磁场时,至少需要采用4个能级的原子模型.这主要依赖于谱线形成区域的原子的能级分布,原子能级占有数多的能级在研究中是必须考虑的.

The diagnostic of magnetic field is very important in solar physics. Stokes parameters I, Q, U, and V can give a complete description of the state of polarization of a beam of light. By inversion of Stokes parameters, the magnetic field in the sun can be obtained. In the recent decades, the diagnostic of magnetic field has been almost based on the Zeeman effect in presence of a strong magnetic field. But in the quiet sun region, the turbulent magnetic field strength is usually less than 100 Gs. Because the Hanle effect is sensitive to a weak magnetic field, it can provide a diagnostic tool for a weak magnetic field. However, it is not easy to obtain the weak field with this effect, and a complete understanding of the polarization mechanism is needed. This paper focuses on the influence of different atomic models on the scattering polarization and Hanle effect. We adopt the seven-, four-, and two-level atomic models to investigate the theoretical linear polarization profiles of MgI b triplet so that we can probe the magnetic field more precisely. In this calculation, we use a typical temperature of 5755 K, corresponding to the temperature of MgI b triplet formation region. It does find that the results for the seven- and four-level atomic models are almost the same. The polarization degrees are nearly unaffected by the populations of MgI atom＇s levels above the transition levels. There is a big difference between the results from the two- and multi-level atomic models. For the two-level atomic model, the b4 line is not affected by the Hanle effect in the lower level. But for the multi-level atomic models, it does. And the lower level Hanle effect of b2 line in the multi-level approximation is much more obvious than that in the two-level approximation. This study is thus useful to produce the theoretical profiles for diagnosing a weak magnetic field, and finally b2 line is the most suitable candidate for the purpose.