Jorrit Leenaarts, Mats Carlsson, Luc Rouppe van der Voort
We use state-of-the-art radiation-MHD simulations and 3D non-LTE radiative
transfer computations to investigate \Halpha\ line formation in the solar
chromosphere and apply the results of this investigation to develop the
potential of \Halpha\ as diagnostic of the chromosphere.
We show that one can accurately model \Halpha\ line formation assuming
statistical equilibrium and complete frequency redistribution provided the
computation of the model atmosphere included non-equilibrium ionization of
hydrogen, and the Lyman-$\alpha$ and Lyman-$\beta$ line profiles are described
by Doppler profiles.
We find that 3D radiative transfer is essential in modeling hydrogen lines
due to the low photon destruction probability in \Halpha. The \Halpha\ opacity
in the upper chromosphere is mainly sensitive to the mass density and only
weakly sensitive to temperature.
We find that the \Halpha\ line-core intensity is correlated with the average
formation height: the larger the average formation height, the lower the
intensity. The line-core width is a measure of the gas temperature in the
line-forming region. The fibril-like dark structures seen in \Halpha\ line-core
images computed from our model atmosphere are tracing magnetic field lines.
These structures are caused by field-aligned ridges of enhanced chromospheric
mass density that raise their average formation height, and therefore makes
them appear dark against their deeper-formed surroundings. We compare with
observations, and find that the simulated line-core widths are very similar to
the observed ones, without the need for additional microturbulence.
View original:
http://arxiv.org/abs/1202.1926
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