Characterization and formation of the Mg I 12.32 μm line in the quiet Sun and sunspot

¹ State Key Laboratory of Solar Activity and Space Weather, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China
² School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing, China
³ School of Astronomy and Space Science, Nanjing University, Nanjing, China
⁴ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing, China
⁵ State Key Laboratory of Solar Activity and Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing, China

^⋆ Corresponding authors: wuyc@bao.ac.cn; wxli@nao.cas.cn

Received: 25 June 2025
Accepted: 18 August 2025

Abstract

Context. The Mg I 12.32 μm line is highly sensitive to magnetic fields due to its long wavelength, making it a promising tool for precise solar-magnetic-field measurements. The formation of this line is significantly influenced by nonlocal thermodynamic equilibrium (NLTE) effects.

Aims. Previous studies have shown that the Mg I 12.32 μm line exhibits different behaviors in various regions of the Sun. This study focuses on the peak intensity of the Mg I 12.32 μm line to analyze its relationship with the physical parameters of the solar atmosphere and its formation mechanism.

Methods. We employed the Rybicki-Hummer (RH) 1.5D radiative transfer code to synthesize the Stokes profiles of the Mg I 12.32 μm line based on a three-dimensional solar atmospheric model of a sunspot and its surrounding quiet Sun. By computing R¯_xiΔx_i, where R¯_xi is the average response function and Δx_i is the difference in physical parameters between the two models being compared, we identified the atmospheric height and physical parameters that most significantly influence the normalized peak intensity in the quiet Sun and the active region, respectively.

Results. In analyzing the synthesized Stokes profiles, we found two key features: (1) in the quiet Sun, the normalized peak intensity is strong at the centers of the granules and weakens in the intergranular lanes; (2) in the sunspot umbra, the normalized peak intensity is generally weak, with only a few areas showing evident emission. Through the analysis of the response functions, we identified the causes of these differences. In the quiet Sun, the differences in normalized peak intensity are primarily attributed to temperature variations at log τ₅₀₀; the logarithm of the continuum optical depth at λ = 500 nm, ranging from −0.21 to 0.91 and from −1.65 to −0.76; as well as to temperature and density variations at log τ₅₀₀ ranging from −3.86 to −2.38. In the sunspot umbra, the differences are mainly due to density variations at log τ₅₀₀ ranging from −0.96 to 1.26. In addition, we discussed the mechanisms through which these physical parameters influence the normalized peak intensity.