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

Abstract

Non-local thermodynamic equilibrium (NLTE) effects in diagnostically important solar Fe i lines are important because of the strong sensitivity of Fe i to ionizing UV radiation, which may lead to a considerable underpopulation of the Fe i levels in the solar atmosphere and, therefore, to a sizeable weakening of Fe i lines. These NLTE effects may be intensified or weakened by horizontal radiative transfer (RT) in a three-dimensionally (3D) structured atmosphere. We analyze the influence of horizontal RT on commonly used Fe i lines in a snapshot of a 3D 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. Because 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 1D NLTE transfer near the boundaries of kG magnetic elements. Around the centers of these elements, however, we find line strengthening, which is often significant. This behavior contrasts with what is expected from previous 3D RT computations in idealized flux-tube models, which only display line weakening. The origin of this unexpected behavior lies in the fact that magnetic elements are surrounded by dense and relatively cool downflowing 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 subaverage UV-irradiation leading to the observed 3D NLTE line strengthening.

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

“…It is negative only in the intergranular lane next to a magnetic element (pos. 4), as discussed also in Holzreuter & Solanki (2015). The weakening of the NLTE line relative to LTE (δE > 0) is due to the opacity deficit resulting from the UV overionization of iron atoms.…”

“…4) and in the middle of a strong magnetic element (pos. 5), the last two of which, following the convention in Holzreuter & Solanki (2015), may be referred to as a flux sheet and a flux tube, respectively. Although we discuss the full maps of the different atmospheric quantities obtained from inversions, at these five spatial locations we also discuss the Stokes profiles in detail.…”

“…In the right panels of the same figure, we also compare the LTE and NLTE intensity profiles at the five points. We compute the relative differences in intensity (or residual intensity) and equivalent widths similar to Equations 1 and 2 of Holzreuter & Solanki (2015). The expressions are repeated here for convenience,…”

“…At log(τ) = 0.0, the temperature in the reference model is slightly higher than in the test model ( Figure 5, bottom row). In principle, the inversion code should return the same temperature in both cases since the continuum intensity is computed strictly in LTE, as in Holzreuter & Solanki (2015). However, in between and away from the two spectral lines, the difference between the NLTE and LTE intensity profiles is non-zero and positive.…”

Numerical non-LTE 3D radiative transfer using a multigrid method

“…The next solar generation telescopes, such as the 4-m DKIST, will obtain a diffraction limit of 0.03 at 500 nm, corresponding to 32 km on the Sun. To be able to compare these high-resolution observations with MHD simulations, 3D radiative transfer is needed even for much weaker photospheric lines (Holzreuter & Solanki 2015;Judge et al 2015). Some important chromospheric diagnostic lines are influenced by partial redistribution effects, such as Lyman-α, Mg ii h&k, and Ca ii H&K. These effects have been included in a 3D RT code (Sukhorukov & Leenaarts 2017), but at the cost of an increase in computational time up to a factor 10.…”