Ionospheric signatures of solar flares.
VLF waves propagate in the Earth-ionosphere waveguide (EIW). The EIW is bounded below by the surface of the Earth and above by the ionospheric D-region (50–90 km altitude). The conditions for wave propagation in the EIW are studied and derived specifically for VLF propagation. The D-region is maintained by shortwave solar radiation that ionises the neutral atmosphere. The Wait parameters, H′ (reflection height) and (sharpness), describe the lower boundary of the D-region. Any enhancement in solar X-rays modifies these parameters, leading to a change in the propagation conditions for VLF signals. The effect of the terminator is presented where, it is found to narrow the depression of the monthly averaged diurnal amplitude profile from summer to winter. A series of solar flares were identified of which two case studies are presented. H′ and are calculated from the VLF signals by the Long Wave Propagation Code (LWPC). It is found that H′ decreased and increased at the time of flare. Once H′ and are obtained, the electron density profile can be constructed which is of crucial importance for VLF waves propagating in the EIW. The gradient of the electron density profile is found to increase as increases. It’s found that all the modal interference minima are moved towards the transmitter at the time of the flare. For flares of great magnitude, extrapolation is required to classify the flare in a magnitude class using VLF data. The change in the phase of the VLF signal is found to be linearly proportional to the change in the X-ray flux.