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

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 \r{A}. Methods. The Mg I b$_2$ line was analyzed alongside the Fe I 6173 \r{A} 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.