Response of low-latitude lower ionosphere during solar flare events over the complete solar cycle 24

The lower ionosphere of Earth is greatly impacted by solar flares. During a solar flare event, a sudden enhancement in X-ray flux leads to additional ionization, which increases electron density in the lower ionosphere. In this paper, we first investigated time the behaviour of lower ionosphere during solar flare events (SFE) over the complete solar cycle 24 during the years 2011 to 2018 by using amplitude measurement of very low frequency (VLF) waves and GOES 0.1–0.8 nm X-ray flux. The fixed frequency (19.8 kHz) VLF wave transmitted from the NWC-transmitter, Australia is observed at our low-latitude station Varanasi (geom. lat. 14^o 55^/ N, geom. long. 154^o E), India. The amplitude enhancements associated with solar flares are characterised by the two traditional Wait parameters, virtual reflection height (H^/ in km) and the sharpness factor (β in km^− 1) i.e. electron density gradient. These empirically determined values of H^/ and β were employed in Long Wave Propagation Capability (LWPC) to predict VLF amplitude perturbations induced by the solar flare throughout a wider frequency range than was observable. It is found that the sharpness factor increases with the increasing strength of solar flares, but the virtual reflection height decreases. These observations show a decrease in H^/ from 78 km to 62 km and an increase in β from 0.34 km^− 1 up to a ‘saturation’ level of 0.51 km^− 1. A comparative study of these parameters during different phases of the solar cycle shows that during the rising phase of the solar cycle, β is found to be lower. In contrast, during the declining phase, its value is higher. Also, H^/ decreases more during the decreasing phase of the cycle than during the rising phase. During the peak of the solar cycle, H^/ and β values are found to lie between rising and decreasing phase values, although more dispersed. This indicates that the lower ionosphere behaves differently during different phases of the solar cycle.