Retrieving 3D Coronal Magnetic Field from Spectropolarimetry of Coronal Emission Lines.

Detailed knowledge of the magnetic and thermal environment of the solar atmosphere, including the photosphere, chromsosphere, and the corona, is crucial for the understanding of the physics of solar eruptions. Recent progresses in spectropolarimetric measurements of magnetically sensitive emission lines provides a viable path towards quantitative characterization of the three-dimensional (3D) structure of coronal magnetic fields - the main driver of coronal phenomena. However, the inference of the 3D coronal vector magnetic fields from observations is not straightforward due to the complex nature of the emission process (resonance scattering through Hanle and Zeeman effects) and line-of-sight (LOS) integration through the optically thin corona. The latter can be resolved by employing tomographic inversion technique using observations from multiple sight lines.

In a recent work (Kramar et al., 2016), we have applied vector tomography method to the coronal emission line (CEL) linear polarization data (I, Q, and U maps) obtained by Coronal Multichannel Polarimeter (CoMP) to derive 3D coronal magnetic fields revealing coronal structures (streamers, pseudostreamers, coronal holes) to demonstrate the feasibility of the methods. Although linear polarization tomography can be used to probe certain coronal field configuration (Kramar et al. 2013), linear polarization data alone does not allow us to uniquely reconstruct all possible field configurations in general.

Vector tomography of coronal magnetic field is currently limited to linear polarization tomography due to the difficulty of circular polarization observations. However, with the pending completion of the Daniel K. Inouye Solar Telescope and the 2020 launch of India's Aditya mission aimed to provide full-Stokes CEL polarization measurements on a routine basis, the full-Stokes vector tomography of coronal magnetic fields will be possible in the near future. We will discuss how inclusion of circular polarization data will improve the accuracy of magnetic field reconstruction, and present new research on the 3D reconstructions of the coronal vector magnetic fields using the full polarization spectra (I, Q, U, and V) of coronal emission lines.