Chinese Academy of Sciences The Sun is not as quiet as it seems, and large or small solar activities caused by solar magnetic field occur every moment. Up to now, all measurements related to the solar magnetic field have been polarization measurements. Based on the principle of the Zeeman effect, the magnetic field strength can be simply obtained by measuring the Zeeman splitting of a magnetic sensitive spectral line. However, this is almost inapplicable for visible light and near-infrared spectral lines. The Mg I 12.32 and 12.22 μm lines in the mid-infrared are a pair of emission lines that present a great advantage for accurate solar magnetic field measurement due to their long wavelength, making it possible to directly use the Zeeman splitting formula to measure the magnetic field.