The formation of the Halpha line in the solar chromosphere

Abstract

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.