The influence of NLTE effects in Fe I lines on an inverted atmosphere

I. 6301 Å and 6302 Å lines formed in 1D NLTE

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: smitha@mps.mpg.de
² Institute of Astronomy, ETH Zentrum, 8092 Zürich, Switzerland
³ School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-701, Republic of Korea

Received: 1 November 2019
Accepted: 15 December 2019

Abstract

Context. Ultraviolet overionisation of iron atoms in the solar atmosphere leads to deviations in their level populations based on Saha-Boltzmann statistics. This causes their line profiles to form in non-local thermodynamic equilibrium (NLTE) conditions. When inverting such profiles to determine atmospheric parameters, the NLTE effects are often neglected and other quantities are tweaked to compensate for deviations from the LTE.

Aims. We investigate how the routinely employed LTE inversion of iron lines formed in NLTE underestimates or overestimates atmospheric quantities, such as temperature (T), line-of-sight velocity (v_LOS), magnetic field strength (B), and inclination (γ) while the earlier papers have focused mainly on T. Our findings has wide-ranging consequences since many results derived in solar physics are based on inversions of Fe I lines carried out in LTE.

Methods. We synthesized the Stokes profiles of Fe I 6301.5 Å and 6302.5 Å lines in both LTE and NLTE using a snapshot of a 3D magnetohydrodynamic simulation. The profiles were then inverted in LTE. We considered the atmosphere inferred from the inversion of LTE profiles as the fiducial model and compared it to the atmosphere resulting from the inversion of NLTE profiles. The observed differences have been attributed to NLTE effects.

Results. Neglecting the NLTE effects introduces errors in the inverted atmosphere. While the errors in T can go up to 13%, in v_LOS and B, the errors can go as high as 50% or above. We find these errors to be present at all three inversion nodes. Importantly, they survive degradation from the spatial averaging of the profiles.

Conclusions. We provide an overview of how neglecting NLTE effects influences the values of T, v_LOS, B, and γ that are determined by inverting the Fe I 6300 Å line pair, as observed, for example, by Hinode/SOT/SP. Errors are found at the sites of granules, intergranular lanes, magnetic elements, and basically in every region susceptible to NLTE effects. For an accurate determination of the atmospheric quantities and their stratification, it is, therefore, important to take the NLTE effects into account.