Three-dimensional non-LTE radiative transfer effects in Fe I lines

Abstract

Context. In the first three papers of this series, we investigated the formation of photospheric neutral iron lines in different atmospheres ranging from idealized flux tube models to complex three-dimensional magneto-hydrodynamic (3D MHD) simulations. The overarching goal was to understand the role of non-local thermodynamic equilibrium (NLTE) and horizontal radiative transfer (RT) effects in the formation of these lines.Aims. In the present paper, we extend this investigation using a high-resolution MHD simulation, with a grid spacing much smaller than the scales currently resolvable by telescopes. We aim to understand whether the horizontal RT effects imposes an intrinsic limit on the small-scale structures that can be observed by telescopes, by spatially smearing out these structures in the solar atmosphere.Methods. We synthesized the Stokes profiles of two iron line pairs, one at 525 nm and other at 630 nm in 3D NLTE. We compared our results with the ones in previous papers and checked the impact of horizontal transfer on the quality of the images.Results. Our results with the high-resolution simulations align with the ones inferred from lower-resolution simulations in the previous papers of this series. The spatial smearing due to horizontal RT, although present, is quite small. The degradation caused by the point spread function of a telescope is much stronger.Conclusions. In the photospheric layers, we do not see an image degradation caused by horizontal RT that is large enough to smear out the small-scale structures in the simulation box. The current generation of telescopes with spatial resolutions smaller than the horizontal photon mean free path should in principle be able to observe the small-scale structures, at least in the photosphere.