Non-LTE formation of the Fe I 6173 Å line in the solar atmosphere

Abstract

The current analysis is dedicated to a detailed investigation of the non-local thermodynamic equilibrium (NLTE) effects influencing the formation of the Fe i 6173 Å line, which is widely used by many instruments, including the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and the Polarimetric and Helioseismic Imager on board the Solar Orbiter. We synthesize the Stokes profiles in a snapshot of a three-dimensional magnetohydrodynamic simulation of the solar photosphere under both LTE and NLTE conditions. The simulation cube contains a sunspot and a plage region around it. The LTE and NLTE Stokes profiles formed in different features are compared and analysed. NLTE effects are evident in both intensity and polarization profiles. For the 6173 Å line, UV overionization is the dominant NLTE mechanism, and scattering effects are much less important. In addition to Fe, an NLTE treatment of Si, Mg, and Al is necessary to set the right photon density in the UV. This is found to further enhance the LTE departures compared to the case where Fe alone is treated in NLTE. These effects in the Stokes profiles 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. The deviations from the LTE profiles are stronger in the Fe i 6173 Å compared to the 6301 Å-6302 Å lines because in the latter case, line scattering compensates the effect of UV overionization. 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.

Fleeting Small-scale Surface Magnetic Fields Build the Quiet-Sun Corona

“…However, this line has been shown to be affected by non-LTE conditions in the solar atmosphere. Ignoring these effects might result in an underestimation of magnetic fields (Smitha et al 2023). Such non-LTE effects, however, are not considered in this study.…”

Deconstructing Photospheric Spectral Lines in Solar and Stellar Flares

“…As the Fe I lines of interest in this study are not included in RADYN, and need to be treated in NLTE (Holzreuter & Solanki 2012Smitha et al 2020Smitha et al , 2021Smitha et al , 2023, it becomes necessary to utilize an additional radiative transfer code. The RH code (Uitenbroek 2001) can take snapshots from an RADYN model as input to perform the radiative transfer and spectral synthesis of the Fe lines.…”

Spectropolarimetric investigation of magnetohydrodynamic wave modes in the photosphere: First results from PHI on board Solar Orbiter

“…The solar scene is therefore sampled at different spectral points due to the cavity errors of the etalon, which are anchored to the detector plane. The effect of this shift in the sampling wavelength during the accumulations of each frame was tested using synthetic Stokes profiles of the Fe i line with nonlocal thermodynamic equilibrium (NLTE) effects (described in Smitha et al 2023). The effects are negligible and do not produce any artefacts in the data.…”