Abstract
Sharp rises of hard X-ray emission and chromospheric line profiles with strong Doppler red-shifts, observed at the onset of solar flares are not fully explained by existing models. Moreover, continuum emission reveals strong co-temporal enhancements, co-spatial with HXR emission. This suggests a fast effective source of excitation and ionisation of hydrogen atoms associated with HXR emission. We investigate electron beams as agents accounting for these observations.
A 1D hydrodynamic response to the electron beam simulates the flaring atmosphere’s kinetic temperatures, densities and macrovelocities. A radiative response in these atmospheres is produced using a fully non-local thermodynamic equilibrium approach for a 5-level plus continuum hydrogen atom model. Excitation and ionisation are considered by spontaneous, external and internal diffusive radiation and by inelastic collisions with thermal and beam electrons. Simultaneous steady state and integral radiative transfer equations (in optically thick transitions) are solved iteratively for all the transitions.
Inelastic collisions with beam electrons strongly increase excitation and ionisation of hydrogen atoms from the chromosphere to photosphere. This leads to increased Lyman continuum radiation, which governs hydrogen ionisation and results in great enhancement of Balmer and Paschen continuous emission. Height distributions of contribution functions for Paschen continuum emission agree with observations of heights of WL and HXR emission reported for limb flares. This process also increases wing emission in Balmer and Paschen lines, which is superimposed on large redshifted enhancements of line emission resulting from downward motion by hydrodynamic shocks. Simulated line profiles closely fit the observations for different flaring events.