Polarization Of Coronal Forbidden Line

Since the magnetic field is responsible for manifestations of most activities in the solar corona,one of the most challenging problems in solar physics is the diagnostics of solar magnetic fields.This paper is devoted to analyzing the diagnostic content of linear polarization observations in coronal forbidden lines.Although this technique is restricted to off-limb observations,it represents a significant tool to diagnose the magnetic field structure in the solar corona,where the magnetic field is intrinsically weak and still poorly known.We adopt the quantum theory of polarized line formation developed in the framework of the density matrix formalism,and synthesize maps of the emergent linear polarization signal in coronal forbidden lines using potential-field source-surface magnetic field models.In contrast to the classical theory,quantum theory provides us more accurate pictures and can be applied to a more complex atom system.As the Hanle effect in the forbidden lines is saturated in the corona,the linear polarization is either parallel or perpendicular to the direction of magnetic field projected on the plane of sky.Besides,the inclination between the magnetic field and the plane of sky can also be estimated near the van Vleck region.Continuous and long-term observations of forbidden lines are able to provide the information about the evolution of magnetic field structure in the corona,especially in the polar region,since the linear polarization of forbidden lines is sensitive to the extremely weak magnetic field.Polarimetries of suitable lines may give a more comprehensive understanding about how the magnetic field evolves in the polar region and how solar polarity reverses.The influence of electronic collisions,active regions,and Thomson scattering on the linear polarization of coronal forbidden lines is also examined.It is found that electronic collisions can not influence the direction of linear polarization of coronal forbidden line,and it is mainly influenced by active regions and Thomson scattering.These two effects have to be carefully taken into account to increase the accuracy of the field diagnostics.The influence of active region is relevant only near the largest active regions,and the influenceof Thomson scattering is decided by the ratio between ion density and electron density.Polarizations of Fe XIII 10747 ?A and Si IX 39290 ?A lines are weakly influenced by Thomson scattering? Fe XIII 10747 ?A line is suitable for the magnetic field diagnostics in a higher temperature region(1.78MK),and Si IX39290 ?A line is suitable in a lower temperature region(1.12MK)? Si IX 39290?A may be suitable for the magnetic field diagnostics in the polar region.