Book excerpt: The study presents the results of a theoretical analysis of the propagation of electromagnetic waves in the very-low-frequency (VLF) and extremely-low-frequency (ELF) ranges in the earth-ionosphere cavity during solar flares. The calculations, which are based upon nominal models of ambient conditions and solar-flare x-ray fluxes and spectra, result in mode-attenuation coefficients which are in good general agreement with observations. The propagation is enhanced relative to ambient conditions at ELF and upper VLF frequencies but is degraded in the lower VLF range. This general behavior is quite insensitive to the details of the assumed x-ray spectrum, the main requirement being a sufficient concentration of x-ray energy in the 1 - 8 A wavelength band. This class of spectrum tends to cause a moderate lowering of the D-region accompanied by an increase in the height gradient of conductivity in the reflection region; this in turn causes the observed phenomena at ELF and VLF during sudden ionospheric disturbances. (Author).