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Solar flares are explosions on the surface of the sun that release a large amount of electromagnetic energy in the form of radio waves at the long wavelength end, through optical emission to X-rays at the short wavelength end. It has long been known that the solar flares, particularly associated with X-rays having wavelengths typically of tenths of nm, penetrate the lower region of the ionosphere (D-region) and increase the electron density via extra ionization (Mitra 1974). The normal unperturbed daytime D-region from which Very Low Frequency (VLF) signals are reflected is maintained mainly by Lyman-α radiation (121.6 nm) from the sun that partially ionizes the minor neutral constituent nitric oxide (at height around 70 km). Under normal conditions, the solar X-ray flux is too small to be a significant source for ionizing the D-region. However, when solar flare occurs, the X-ray flux increases significantly which with wavelengths below 1 nm penetrates down to the D-region and markedly increases the ionization of the neutral constituents particularly nitrogen and oxygen hence increases the electron density. The lower ionosphere can be characterized as the “Wait ionosphere” defined by a reference height H′in km and the exponential sharpness factor in km-1 (Wait and Spies 1964). Researchers have reported changes in the ionospheric parameters, H' and β as a function of solar X-ray flux (Thomson et al. 2004, 2005; Grubor et al. 2005; Zigman et al. 2007). The increase in the D-region electron density can produce significant perturbations in the phase and the amplitude of VLF signals propagating in the Earthionosphere waveguide.

Initial results on solar flare effect on 24.8 kHz subionospheric propagation over long path to Suva