Ultraviolet Spectropolarimetry: A New Diagnostic Window In Solar Physics

Spectropolarimetry of the solar ultraviolet spectrum has opened a new window to study the magnetism of the upper layers of the solar chromosphere. Theoretical investigations and the results of the CLASP (2015) and CLASP2 (2019) suborbital space missions demonstrated the diagnostic potential of the polarization produced by scattering processes and the Hanle and Zeeman effects in some ultraviolet spectral lines, such as Ly α, Mg II h & k, or the Mn I resonance lines to uncover the physics of the upper chromosphere and the transition region.

Motivated by these recent results, we have performed radiative transfer calculations of the Mg II h and k lines in a radiative magneto-hydrodynamic (rMHD) model representative of a solar plage region. We built a synthetic observation that mimics the resolution of CLASP2 and used it to prove the suitability of the Weak-field Approximation (WFA) to infer the longitudinal component of the magnetic field.

After validating the result provided by the CLASP2 mission, we explored the spectropolarimetric capabilities of other spectral regions in the near-UV. In particular, the spectral region between 250 and 280 nm includes a significant number of Fe II lines. Up to now, we lack theoretical and observational investigations about their polarization signals. In this talk, we present the first detailed theoretical study about their intensity and polarization, including the effects of radiative transfer and the Hanle and Zeeman effects. We present a selection of Fe II spectral lines with significant linear and circular polarization signals and evaluate their diagnostic capabilities by studying their scattering polarization signals, formation heights, and magnetic sensitivity. In addition, we include a detailed study of a weak Fe II emission line that is located in the far wings between the Mg II h & k lines, whose intensity and polarization has been recently observed by the CLASP2 mission.