Forward modeling of the Mg I 12.32 μm line from a 3D magnetohydrodynamic model of an enhanced network

¹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
e-mail: xybai@bao.ac.cn
² School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, PR China
³ Solar Research Laboratory, National Research Institute of Astronomy and Geophysics Helwan, Cairo 11421, Egypt
⁴ Key Laboratory of Solar Activity and Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, PR China

Received: 4 January 2024
Accepted: 17 April 2024

Abstract

Context. The Mg I 12 μm lines, 12.22 and 12.32 μm, represent a pair of emission lines, and their line cores originate around the temperature minimum region. These lines exhibit the highest ratio of Zeeman to Doppler broadening in the infrared solar spectrum, making them crucial for accurately investigating the solar magnetic field.

Aims. We synthesized the Mg I 12.32 μm Stokes profiles from a 3D magnetohydrodynamic (MHD) model and studied the validity of different methods for extracting the magnetic field. The observational profiles at different spatial resolution were simulated, which are helpful for the design of future solar telescopes with large apertures.

Methods. We used a 3D MHD simulation model for an enhanced network computed using the Bifrost code. We performed nonlocal thermal equilibrium calculations for Stokes profiles of the Mg I 12.32 μm line using the Rybicki–Hummer code.

Results. From the simulation we determined the average formation height of the Mg I 12.32 μm line to be around 450 km. The various solar features have different formation heights, and the variance of formation height in magnetic concentration regions is about 160 km. The wavelength-integrated method is proven effective in calibrating the integrated Stokes profiles to obtain the longitudinal (B_l) and horizontal (B_H) field components for weak magnetic fields; the B_l is below 300 G. Furthermore, the weak field approximation was found to be valid only for estimating magnetic fields with B_l below 150 G. The Stokes I profiles clearly show Zeeman triple splitting around the magnetic flux concentration with a grid resolution of 48 km. We determined that a resolution of 0.97″, equivalent to the diffraction limit of a telescope with a diameter of 3.2 m, was necessary to detect the Zeeman splitting for the simulated snapshot. Our results from this 3D MHD model are valuable for interpreting data from the Accurate Infrared Magnetic Field Measurements of the Sun (AIMS) telescope and designing future solar infrared telescopes.