Formation of the Mg II h and k polarization profiles in a solar plage model and their suitability to infer magnetic fields

Abstract

The Mg II h and k lines are among the strongest in the near-ultraviolet solar spectrum and their line core originates in the upper chromosphere, just below the transition region. Consequently, they have become one of the main targets for investigating the magnetism of the upper solar atmosphere. The recent CLASP2 mission obtained unprecedented spectropolarimetric data of these lines in an active region plage, which have already been used to infer the longitudinal component of the magnetic field by applying the weak field approximation. In this paper, we aim at improving our understanding of the diagnostic capabilities of these lines by studying the emergent Stokes profiles resulting from radiative transfer calculations in a radiative magneto-hydrodynamic (rMHD) time-dependent model representative of a solar plage. To this end, we create a synthetic observation with temporal and spatial resolutions similar to those of CLASP2. We find strong asymmetries in the circular polarization synthetic profiles which considerably complicate the application of the weak field approximation. We demonstrate that the selective application of the weak field approximation to fit different spectral regions in the profile allows to retrieve information about the longitudinal component of the magnetic field at different regions of the model atmosphere, even when the circular polarization profiles are not anti-symmetric and are formed in the presence of strong velocity and magnetic field gradients.

Accepted for publication in ApJ

References in corpus (13)

• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Overview and Performance
• Magnetic Diagnostics of the Solar Chromosphere with the Mg II h-k Lines
• Mapping Solar Magnetic Fields from the Photosphere to the Base of the Corona
• Ion-neutral interactions and non-equilibrium ionization in the solar chromosphere
• The magnetic sensitivity of the Mg ii k line to the joint action of Hanle, Zeeman and magneto-optical effects
• The magnetic sensitivity of the resonance and subordinate lines of Mg II in the solar chromosphere
• Frequency Redistribution Function for the Polarized Two-Term Atom
• The transfer of resonance line polarization with partial frequency redistribution and J-state interference
• Modelling Mg II During Solar Flares. I. Partial Frequency Redistribution, Opacity, and Coronal Irradiation
• The effects of three-dimensional radiative transfer on the resonance polarization of the Ca I 4227 $\mathring{\mathrm{A}}$ line
• Explicit Form of the Radiative and Collisional Branching Ratios in Polarized Radiation Transport with Coherent Scattering
• A Note on the Radiative and Collisional Branching Ratios in Polarized Radiation Transport with Coherent Scattering
• Effects of spectral resolution on simple magnetic field diagnostics of the Mg II h & k lines

Cited by in corpus (4)

• Tomography of a solar plage with the Tenerife Inversion Code
• Estimating the longitudinal magnetic field in the chromosphere of quiet-Sun magnetic concentrations
• The magnetic sensitivity of the (250-278nm) Fe II polarization spectrum
• Derivation of Instrument Requirements for Polarimetry using Mg, Fe, and Mn lines between 250 and 290 nm