| Issue |
A&A
Volume 669, January 2023
|
|
|---|---|---|
| Article Number | A144 | |
| Number of page(s) | 11 | |
| Section | The Sun and the Heliosphere | |
| DOI | https://doi.org/10.1051/0004-6361/202245130 | |
| Published online | 24 January 2023 | |
Non-LTE formation of the Fe I 6173 Å line in the solar atmosphere
1
Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: smitha@mps.mpg.de, narayanamurthy@mps.mpg.de
2
School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-701, Republic of Korea
Received:
4
October
2022
Accepted:
18
November
2022
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.
Key words: radiative transfer / line: formation / line: profiles / Sun: magnetic fields / Sun: photosphere / polarization
© The Authors 2023
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
This article is published in open access under the Subscribe-to-Open model.
Open access funding provided by Max Planck Society.
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.