Diagnosing the solar atmosphere through the Mg I b₂ 5173 Å line

II. Morphological classification of the intensity and circular polarization profiles

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Apdo. 3004, 18080 Granada, Spain
² Spanish Space Solar Physics Consortium, Granada, Spain
³ Geophysical and Astronomical Observatory, Faculty of Science and Technology, University of Coimbra, 3040-004 Coimbra, Portugal
⁴ Instituto de Astrofísica e Ciências do Espaço, Department of Physics, University of Coimbra, 3040-004 Coimbra, Portugal

^⋆ Corresponding author; azaymisiu@gmail.com

Received: 29 November 2024
Accepted: 5 March 2025

Abstract

Context. The Mg I b₂ line at 5173 Å is primarily magnetically sensitive to heights between the mid photosphere and the low chromosphere, a region that has not been sufficiently explored in the solar atmosphere but is crucial for understanding the magnetic coupling between the two layers. New generation solar observatories, both space-borne and ground-based, are now performing polarimetric observations of this spectral line, enabling simultaneous measurements with multiple spectral lines. This allows for detailed studies of the magnetism around the temperature minimum region of the solar atmosphere at high spatial, temporal, and spectral resolutions.

Aims. We present a first classification of the Stokes I and V profiles of the Mg I b₂ line using high spatial resolution observations from the Swedish 1-m Solar Telescope.

Methods. We used the Euclidean distance to perform a morphological classification of the intensity and circular polarization profiles of the Mg I b₂ line. The physical properties of the resulting classes were analyzed using classical inference methods. Additionally, we present a two-line full-Stokes inversion of the representative profiles in which the Mg I b₂ line is treated fully under non-local thermodynamic equilibrium (NLTE) conditions, while the Fe I 6173 Å line is simultaneously inverted under LTE assumptions to provide photospheric constraints. This approach offers insights into the temperature stratification and other physical gradients involved in the formation of the different profile morphologies.

Results. The intensity and polarization profiles of the Mg I b₂ line show clear spectral signatures depending on the magnetic environment and the variety of physical processes occurring in the different solar magnetic regimes within the line formation range. We found nine classes of Stokes V profiles and 16 classes of Stokes I profiles in our Mg I b₂ dataset. These classes can be further grouped into families based on shared characteristics, physical properties, and location.

Conclusions. Our classification provides important information on the different environments and processes occurring in the solar atmosphere around the temperature minimum region. It is also relevant for improving the performance of NLTE inversions.