| Summary: | The aim of this thesis has been to gain a better understanding of how the structure of, and physical conditions in, the lower solar atmosphere change as a result of intense energy input during solar flares. This has been achieved by employing high time resolution, broadband spectroscopic observations from the SDO/EVE instrument, in conjunction with numerical modelling of solar flares using a 1D non-LTE radiation hydrodynamics code (RADYN). The entire EUV spectrum of a sample of thirteen solar flares was characterised in terms of continuum colour temperatures, line ratios, and line to continuum ratios. It was found that the observed flare spectra were remarkably homogeneous, with little variation in the measured ratio values between different events. The Lyman continuum was found to have colour temperatures in the range 8 - 13 kK, and the continuum flux was strongly correlated with the colour temperature. The low colour temperature of the He I continuum (11 - 17 kK) suggested that it was formed through the photoionisation/recombination process in solar flares. The observed ratios of the higher Lyman lines were found to be invariant throughout each studied flare, and had values that were close to those that were calculated in the optically thin, LTE regime. These observations were compared to a grid of model atmospheres generated using the RADYN code. By examining the physical conditions in the model atmospheres it was determined that the increased electron density in the lower atmosphere of solar flares drives the hydrogen level populations closer to their LTE values. This offered an explanation for the observed temporal behaviour of the Ly a/p line ratio, and the observed ratios of the other Lyman lines. However, the simulations failed to reproduce the observed spectral energy distribution in extreme ultraviolet lines and continua.
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