SOLARNET Conference: The Many Scales of the Magnetic Sun

The Fe I 6173 Å line is widely used to observe the solar photosphere by many instruments. This includes the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and the Polarimetric and Helioseismic Imager on board the Solar Orbiter. During analysis, this line is often assumed to have formed in Local Thermodynamic Equilibrium (LTE) conditions. However, the UV overionization effects acting on the iron species has a strong influence on the Fe I abundance, which affects the strength of this line, resulting in departures from LTE. By synthesizing the Stokes profiles from snapshot of a three-dimensional magnetohydrodynamic simulation of the solar photosphere, we have carried out a detailed investigation on the nature and strength of the non-LTE (NLTE) effects. We find that both intensity and polarization profiles can be strongly affected by the NLTE effects, and that these effects survive even when the profiles are averaged spatially or sampled on a coarse wavelength grid such as that used by the SDO/HMI and other magnetographs. Based on the nature of departures from LTE, treating the 6173 Å line in LTE will likely result in an overestimation of temperature and an underestimation of the magnetic field strength.