Response of D-region ionosphere due to solar flares during solar cycle 24 using VLF measurement

The effects of solar flares on the propagation of subionospheric VLF signals from NWC transmitter station monitored at a low-latitude station, Varanasi (lat. 14 degrees 55/N, long. 154 degrees E), India, during solar cycle 24 have been analyzed. The amplitude and phase enhancements associated with solar flares were observed in the signals which are due to an increase in the electron density of the D-region as a result of extra ionization caused by the solar flares. The solar flare-induced perturbations in the VLF signals were used to determine D-region ionospheric parameters: H' (the ionospheric refection height) and beta (rate of increase in electron density with height) using Long Wave Propagation Capability (LWPC) modeling. H' decreases in proportion to the logarithm of the X-ray flux intensity. It is found that there is a typical reduction in H' which occur from a midday unperturbed value of about 71 km to about 58 km for an X6 class solar fare where as beta increases from 0.39 km(-1) to a saturation level of about 0.52 km(-1).