2 research outputs found
3-D non-LTE radiative transfer effects in Fe I lines: III. Line formation in magneto-hydrodynamic atmospheres
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.Comment: 13 pages, 11 figures, accepted for publication in A&