An investigation into the ionospheric effects of solar flares.
thesisposted on 19.11.2015, 09:18 by Ian Christopher. Wand
Impulsive bursts of solar radiation in the X-ray and extreme ultra violet (EUV) wavelength bands give rise to enhanced ionization rates within the ionosphere. Such effects are known as sudden ionospheric disturbances (SID) and have important effects on radiowave propagation in both the very low frequency (VLF) and high frequency (HF) bands causing phase, frequency and amplitude changes in such signals. These disturbances are highly correlated with optical solar flare events. Using satellite observations of X-ray and EUV solar flux during disturbed conditions models have been constructed of typical flare enhancements in the 0-10 A, 10-100 A and EUV wavelength ranges. The solar flux in the 0-10 A range affects the lowest ionosphere and thus directly influences the propagation characteristics of VLF radiowaves. The response of the ionosphere to such an impulsive burst has been studied by a numerical solution of the appropriate continuity equation, a full-wave method has then been used to determine the changes in phase height, reflection and conversion coefficients as the flare progresses. The analysis indicates that the flare radiation produces changes in these parameters and the preflare electron density profile is found to be an important influence on these effects, however the decay rate is not in good agreement with experimental observations. The solar flux in the 10-100 A and EUV wavelength bands affects the E and F regions of the ionosphere, the excess ionization- formed by a burst of these wavelengths has been calculated by numerical methods and the effect on vertical incidence HF propagated radiowaves determined by a ray tracing method. It is found that the two wavelength bands can have differing results on such radio signals. The standard frequency transmissions have been monitored for SID effects in both VLF and HF frequency bands and these results are compared with the theoretical models which have been developed. In some of the events observed deductions are possible regarding the wavelength of the solar spectrum responsible for the flare effects.