Diagnosing the solar atmosphere through the Mg I b$_2$ 5173 Å line. I. Nonlocal thermodynamic equilibrium inversions versus traditional inferences

A. L. Siu-Tapia; L. R. Bellot Rubio; D. Orozco Suárez

doi:10.1051/0004-6361/202453230

paper

Diagnosing the solar atmosphere through the Mg I b $_{2}$ 5173 Å line. I. Nonlocal thermodynamic equilibrium inversions versus traditional inferences

arXiv:2503.12498 · doi:10.1051/0004-6361/202453230

Abstract

Aims. We examined the capabilities of methods based on the weak-field approximation and line bisectors to extract fast and reliable information about the height stratification of the magnetic field and line-of-sight velocities, respectively, from high spatial resolution observations of the Mg I b $_{2}$ line at 5173 Å. Methods. The Mg I b $_{2}$ line was analyzed alongside the Fe I 6173 Å line to help constrain the physical conditions of the photosphere. Additionally, we present the first high-resolution inversions of the Mg I b $_{2}$ line under nonlocal thermodynamic equilibrium (NLTE) conditions conducted over a large field of view using a full-Stokes multiline approach. To determine the optimal inversion strategy, we performed several tests on the Mg I b $_{2}$ line using the Fourier Transform Spectrometer atlas profile before applying it to our observations. Results. The good correlations between the traditional methods and the NLTE inversions indicate that the weak-field approximation is generally a reliable diagnostic tool at moderate field strengths for the rapid inference of the longitudinal magnetic field from the Mg I b $_{2}$ line. In contrast, line bisectors exhibit poorer correlations with the NLTE inferred plasma velocities, suggesting that they might not be suitable for deriving velocity gradients from the Mg I b $_{2}$ line. Furthermore, to accurately derive the thermodynamic properties of the solar atmosphere from this line, the more complex, and time-consuming, NLTE Stokes inversions are necessary. This work also provides observational evidence of the existence of low-lying canopies expanding above bright magnetic structures and pores near the low chromosphere.

Paper accepted for publication in Section 9. The Sun and the Heliosphere of Astronomy and Astrophysics. 18 pages, 11 figures, 3 tables

Diagnosing the solar atmosphere through the Mg I b2​ 5173 Å line. I. Nonlocal thermodynamic equilibrium inversions versus traditional inferences

Abstract

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Diagnosing the solar atmosphere through the Mg I b $_{2}$ 5173 Å line. I. Nonlocal thermodynamic equilibrium inversions versus traditional inferences