Apparent magnetic field decay in a solar flare from Zeeman splitting of the Mg 12.32 microns observations
The magnetic field is the most fundamental observable in solar physics, while solar flares are the most violent, rapid magnetic energy-release events in the solar atmosphere. Decades of observations and theory have sketched the framework of flare evolution and magnetic-field-related physical process, yet the actual magnetic field variations during flares remained poorly understood. The main obstacle is that magnetic-diagnostic spectral lines are distorted by transient flare activity, making traditional spectro-polarimetry inversions complicated. Using a mid-infrared solar telescope, we investigate spectroscopic evolution of the Mg 12.32 microns line during an X1.5-class flare. Owing to the exceptionally large Zeeman sensitivity of this line, magnetic field strength can be directly determined from the measured line separation. Our analysis reveals apparent magnetic field weakening beneath flare ribbons in the impulsive phase. Radiative-hydrodynamic simulations and atmospheric inversions confirm that the formation height of the Mg 12.32 microns line increases during the flare. We suggest this apparent field change probably originates either from elevated line-formation altitude or magnetic-field redistribution. Additional observations and modeling are needed to clarify these two contributions. This study demonstrates the capability of mid-infrared wavelengths to trace magnetic-field evolution during flares.