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Title: The effects of energetic particles on radiative transfer and emission from hydrogen in solar flares
Author: Druett, Malcolm
ISNI:       0000 0004 7430 0890
Awarding Body: Northumbria University
Current Institution: Northumbria University
Date of Award: 2017
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There are rapid increases of hard and soft X-rays (HXR, SXR) and ultraviolet (UV) emission with large Doppler blue-shifts associated with plasma up-flows observed at flare onsets accompanied by broadened chromospheric emission with large redshifts. Hα shows red-shifts of 1–4 Å in the impulsive phase of solar flares observed with various past (Ichimoto and Kurokawa, 1984;Wuelser and Marti, 1989) and current (the Swedish Solar Telescope, SST) instruments (Druett et al., 2017). HXR footpoints are observed to be co-temporal and co-spatial with increases in white light (WL) and continuous emission during the impulsive phase. These effects point to fast, effective sources of excitation and ionisation of hydrogen atoms in flaring atmospheres associated with HXR emission. Most current radiative hydrodynamic models can account for SXR and UV emission, but fail to explain correctly the strongly red-shifted Hα line emission occurring at the flare onsets or the locations of the white light sources, and offer little explanation of the origin of seismic sources in flaring event. We investigate electron beams as the agents accounting for the observed hydrogen line and continuum emission by considering a 1D hydrodynamic response of the quiet Sun chromosphere to injection of an electron beam and its conversion into a flaring atmosphere with its own kinetic temperatures, densities and macro-velocities (Zharkova and Zharkov, 2007). A radiative response in these atmospheres is simulated using a fully non-local thermodynamic equilibrium (NLTE) approach for a 5 levels plus continuum hydrogen atom model. Simultaneous steady state and integral radiative transfer equations in all optically thick transitions (Lyman and Balmer series) are solved iteratively for all the transitions to define their source functions with the relative accuracy of 10. The solutions of the radiative transfer equations were found using the L2 approximation. Resulting intensities of hydrogen line and continuum emission are calculated for Lyman, Balmer and Paschen series. The hydrodynamic model is shown to account closely for the timing and magnitude of upward motion to the corona observed in 171Å by the Atmospheric Imaging Assembly/Solar Dynamics Observatory for C1.5 flaring event onset, published in Nature communications (Druett et al., 2017), and suggests that both red and blue Doppler-shifts should be observed in the hydrogen Lyα line (in prep). Inelastic collisions with beam electrons are shown to strongly increase excitation and ionisation of hydrogen atoms at all depths from the chromosphere to photosphere. This leads to an increase in Lyman continuum radiation, which governs the hydrogen ionisation and leads to strong enhancement of emission in Balmer and Paschen continua. The contribution functions for Paschen continuum emission indicate a close correlation of the emission induced by electron beams with the observations of heights of WL and HXR emission reported for limb flares, (Druett and Zharkova, 2018) unlike other published simulations. This process also leads to a strong increase of wing emission (Stark's wings) in Balmer and Paschen lines combined with large red-shifted enhancements of Hα line emission resulting from a downward motion by hydrodynamic shocks. In contrast to other existing simulations, our work reproduces very closely the observed Hα line profiles with large red-shifts in a C1.5 flare by the Swedish Solar Telescope (Druett et al., 2017), the large red-shifts previously observed (Ichimoto and Kurokawa, 1984; Zarro et al., 1988; Wuelser and Marti, 1989) and explains dimming of Hα emission at flare onsets if observed with narrow spectral windows of 2-3Å. The hydrodynamic models are able to account for the delivery of momentum below the photosphere in the hydrodynamic shocks that result form the beam injection. The supersonic velocities and heights of these shocks can be used to predict a seismic response (in prep), and a method is proposed for detecting the propagation of hydrodynamic shocks that are capable of triggering a seismic response (Druett and Zharkova, 2018).
Supervisor: Zharkova, Valentina ; Mclaughlin, James Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: F500 Astronomy