3-D non-LTE radiative transfer effects in Fe I lines: III. Line formation in magneto-hydrodynamic atmospheres

Abstract

Non-local thermodynamic equilibrium (NLTE) effects in diagnostically important solar Fe I lines are important due to the strong sensitivity of Fe I to ionizing UV radiation, which may lead to a considerable under-population of the Fe I levels in the solar atmosphere and, therefore, to a sizeable weakening of Fe I lines. Such NLTE effects may be intensified or weakened by horizontal radiative transfer (RT) in a three-dimensionally (3-D) structured atmosphere. We analyze the influence of horizontal RT on commonly used Fe I lines in a snapshot of a 3-D radiation magneto-hydrodynamic (MHD) simulation of a plage region. NLTE- and horizontal RT effects occur with considerable strength (up to 50% in line depth or equivalent width) in the analyzed snapshot. As they may have either sign and both signs occur with approximately the same frequency and strength, the net effects are small when considering spatially averaged quantities. The situation in the plage atmosphere turns out to be rather complex. Horizontal transfer leads to line-weakening relative to 1-D NLTE transfer near the boundaries of kG magnetic elements. Around the centers of these elements, however, we find an often significant line-strengthening. This behavior is in contrast to that expected from previous 3-D RT computations in idealized flux-tube models, which display only a line weakening. The origin of this unexpected behavior lies in the fact that magnetic elements are surrounded by dense and relatively cool down-flowing gas, which forms the walls of the magnetic elements. The continuum in these dense walls is often formed in colder gas than in the central part of the magnetic elements. Consequently, the central parts of the magnetic element experience a sub-average UV-irradiation leading to the observed 3-D NLTE line strengthening.

13 pages, 11 figures, accepted for publication in A&A

References in corpus (4)

• The formation of IRIS diagnostics II. The formation of the Mg II h&k lines in the solar atmosphere
• Properties of solar plage from a spatially coupled inversion of Hinode SP data
• Comparison of solar photospheric bright points between SUNRISE observations and MHD simulations
• 3-D non-LTE radiative transfer effects in Fe I lines: II. Line formation in 3-D radiation hydrodynamic simulations

Cited by in corpus (18)

• Non-LTE line formation of Fe in late-type stars - III. 3D non-LTE analysis of metal-poor stars
• Non-LTE line formation of Fe in late-type stars IV: Modelling of the solar centre-to-limb variation in 3D
• High-frequency Oscillations in Small Magnetic Elements Observed with Sunrise/SuFI
• An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres I. Formation of the G-band in metal-poor dwarf stars
• The influence of NLTE effects in Fe I lines on an inverted atmosphere I. 6301 A and 6302 A lines formed in 1D NLTE
• Spatially resolved spectroscopy across stellar surfaces. I. Using exoplanet transits to analyze 3-D stellar atmospheres
• Non-LTE inversions of a confined X2.2 flare: I. Vector magnetic field in the photosphere and chromosphere
• Diagnostic capabilities of spectropolarimetric observations for understanding solar phenomena I. Zeeman-sensitive photospheric lines
• The non-LTE formation of the Fe I 6173 A line in the solar atmosphere
• Spatially resolved spectroscopy across stellar surfaces. V. Observational prospects: Toward Earth-like exoplanet detection
• Influence of NLTE effects in Fe I lines on inverted atmosphere II. 6301 A and 6302 A lines formed in 3DNLTE
• Numerical non-LTE 3D radiative transfer using a multigrid method
• Spatially resolved spectroscopy across stellar surfaces. IV. F, G, & K-stars: Synthetic 3D spectra at hyper-high resolution
• Solar Photospheric Spectrum Microvariability I. Theoretical searches for proxies of radial-velocity jittering
• Solar Hα excess during Solar Cycle 24 from full-disk filtergrams of the Chromospheric Telescope
• Three-dimensional non-LTE radiative transfer effects in Fe I lines IV. Line formation at high spatial resolution
• Formation of chromospheric Fe I lines in the near ultraviolet in 1D atmospheres
• Solar photospheric spectrum microvariability III. Radial velocities and line profiles in magnetic active-region granulation