Scattering Polarization Property Of Solar Spectrum

Diagnosing the magnetic feld is very important in solar physics. The wayto diagnose the magnetic feld vector is usually through Zeeman efect for thestrong magnetic feld, but in the weak magnetic feld region where Zeeman is lessefcient and Hanle efect can provide a useful tool. Using Hanle efect to diag-nose the weak magnetic feld is popular in the literature these years which needa complete understanding of the polarization mechanisms. A series of Interna-tional Solar Polarization Workshops which focus on exploring the magnetic feldthrough the polarized radiation have been taken place every three years sincethe frst one in St.Petersberg, Russia, in1995. As the high polarimetric precisiontechnology were applied through the ZIMPOL (Zurich Imaging Polarimeter), theso-called Second Solar Spectrum shows a great amount of rich and complicatedstructure of the linear polarization profle. That the linear polarization profleprovide a playground for studying of Hanle efect. Through the efort of a lotof scientists, the theories to compute the Hanle efect were developed like thestatistical equilibrium equations and radiative transfer equation in the densitymatrix form. These theories have make great success and solve many problems inthe polarization profle formation and still face to some difcult. Here we reviewthe basic theory in scattering polarization to deal with Second Solar Spectrumand Hanle. And we also focus on investigating the scattering induced fraction-al linear polarization Q/I and U/I of Mg I b triplet in presence of a magneticfeld. It is found that the triplet is suitable to study the lower level Hanle efectand very sensitive to the sub-gauss magnetic feld. We use the quantum theoryunder the fat-spectrum approximation developed by Landi Degl’Innocenti, E tothe investigate the Hanle efect in the triplet, and adopt a seven-levels atomicmodel of magnesium, and the radiative transfer does not be considered and thepartial frequency efect (PRD) is not taken into account. The fractional linearpolarization profle Q/I and U/I at the triplet are presented. The triplet is sensi-tive to the magnetic feld strength between0.001and0.1gauss corresponding to lower level Hanle efect and between1and10gauss corresponding to upper levelHanle efect. In the lower level Hanle efect region, as the magnetic strengthis increased, the Q/I amplitude of b4line is increased due to the coupling ofatomic alignment in the statistical equations. And in the upper level Hanle ef-fect region, the magnetic sensitivity is proportional to the the parameter ω(2)JuJ introduced by Landi Degl’Innocenti. It is found that the most sensitive line is b4and then followed by b2line. First, the anisotropy factors obtained through thelimb darkening of continuum is not suitable to treat the strong lines. Second,the dichroism contribution needs to be considered to ft the observational profleof triplet. Finally, when the weak magnetic is present, the atomic alignment canbe increased due to the coupling in the statistical equilibrium equations. On theother hand, due to the dichroism contribution, the Hanle efect can infuence theline wings but can not produce any observable signal because of the contributionof the continuum.