Strong Zeeman effects in the Fe I 5434.5 Å line observed in a sunspot on August 17, 2024

Abstract

The Fe I 5434.5 Å line is widely used as a non-magnetic reference due to its small effective Landé factor of $-0.014$. Nevertheless, this line can show significant non-zero Stokes $V$ polarization in active regions on the Sun. In this work, we study the distribution of magnetic fields in the sunspot observed on August 17, 2024 using the high-fidelity Stokes $I+V$ and $I-V$ spectra of three photospheric spectral lines of different magnetic sensitivity: Fe I 5434.5 Å, Ni I 5435.9 Å and Mn I 5432.5 Å. The observed splitting of the Stokes $I\pm V$ bisectors in the Fe I 5434.5 Å line follows the variation of the corresponding splitting in the two other spectral lines, indicating the presence of a spatial component with an extraordinarily strong magnetic field reaching 22 kG.