Sharp rises of hard X-ray (HXR) emission accompanied by Hα line profiles with strong red shifts up to 4 Å from the central wavelength, often observed at the onset of flares with the Specola Solare Ticinese Telescope (STT) and the Swedish Solar Telescope (SST), are not fully explained by existing radiative models. Moreover, the observations of white light (WL) and Balmer contin- uum emission with the Interface Region Imaging Spectrograph (IRIS) reveal strong co-temporal enhancements and often nearly co-spatial with HXR emission. These effects indicate a fast ef- fective source of excitation and ionisation of hydrogen atoms in flaring atmospheres associated with HXR emission. In this paper we investigate electron beams as the agents accounting for the observed hydrogen line and continuum emission.
Flaring atmospheres are considered to be produced by a 1D hydrodynamic response to the injection of an electron beam defining their kinetic temperatures, densities and macrovelocities. A radiative response in these atmospheres is simulated using a fully non-local ther- modynamic equilibrium (NLTE) approach for a 5 levels plus continuum hydrogen atom model, considering its excitation and ionisation by spontaneous, external and internal diffusive radiation and by inelastic collisions with thermal and beam electrons. Resulting intensities of hydrogen line and continuum emission were also calculated for Balmer and Paschen series.
We find that inelastic collisions with beam electrons strongly increase excitation and ionisation of hydrogen atoms from the chromosphere to photosphere. This leads to an increase in Lyman continuum radiation, which has high optical thickness, and after the beam is off it governs hydrogen ionisation and leads to the long lasting order of magnitude enhancement of emission in Balmer and Paschen continua. The ratio of Balmer-to-other-continuum head intensities are found correlated with the beams's initial flux. The height distribution of contribution functions for Paschen continuum emission indicate a close correlation with the observations of heights of WL and HXR emission reported for limb flares. This process also leads to a strong increase of wing emission (Stark's wings) in Balmer and Paschen lines, that is superimposed on large red-shifted enhancements of Hα-Hγ line emission resulting from a downward motion by hydrodynamic shocks. The simulated line profiles are shown to closely fit the observations for different flaring events.
Authors: Druett M. and Zharkova V.V.
Publication Status: Accepted
Last Modified: 2017-10-11 06:21