The FORMATION OF THE Hα LINE IN THE SOLAR CHROMOSPHERE

Abstract

We use state-of-the-art radiation-MHD simulations and 3D non-LTE radiative transfer computations to investigate Hα line formation in the solar chromosphere and apply the results of this investigation to develop the potential of Hα as diagnostic of the chromosphere.We show that one can accurately model Hα line formation assuming statistical equilibrium and complete frequency redistribution provided the computation of the model atmosphere included nonequilibrium ionization of hydrogen, and the Lyman-α and Lyman-β 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 Hα. The Hα opacity in the upper chromosphere is mainly sensitive to the mass density and only weakly sensitive to temperature.We find that the Hα 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 Hα 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.

The magnetic field in the solar atmosphere

“…This, however, is in direct contrast with the results of recent numerical simulations which suggested that Hα fibrils are visible manifestations of high-density ridges aligned with the magnetic field (Leenaarts et al 2012), thus serving as an indirect tracer of the vertical-to-horizontal transition of the magnetic field orientation around magnetic flux concentrations. This was addressed by de la Cruz Rodríguez and Socas-Navarro (2011) who compared the observed orientation of fibrils in Ca ii 8542 Å images to the chromospheric magnetic field vector, inferred from observed polarization signals originating from the same spectral line.…”

Moreton and EUV Waves Associated with an X1.0 Flare and CME Ejection

“…A similar kinematic analysis performed on the He ii distance-time maps shows similar values of acceleration and speed with respect to Hα. This suggests that the upper chromosphere and the transition region respond similarly to the increase of pressure or density produced by the arrival of the coronal shock which leads to emission enhancements (Vršnak and Cliver, 2008;Leenaarts, Carlsson, and Rouppe van der Voort, 2012;Krause et al, 2015).…”

Hαfeatures with hot onsets

“…The 3D case remains prohibitive . Leenaarts et al (2012Leenaarts et al ( , 2015 therefore restricted their numerical 3D Hα formation studies to a single simulation snapshot.…”