Multi-instrument study of the response of intense solar flares during the descending period of the 24th solar cycle

This study delves into the ionosphere’s response in low and equatorial regions of the two most powerful solar flares in the descending phase of the 24th solar cycle, which was attained on July 23, 2016, and April 02, 2017, using multi-instrument measurements using very low frequency (VLF) waves, global positioning system (GPS) based total electron contents (TEC) over four different IGS stations, electron density profiles derived from COSMIC and SWARM satellite. Due to the sudden increase in ionization of the D layer , VLF signals exhibited an elevation in amplitude during a solar flare at a low-latitude station, Varanasi. A strong correlation exceeding 70% was identified between X-ray flux and VLF amplitude variation. Additionally, a sudden rise in ionospheric TEC from 4 different IGS stations indicated an augmentation up to ∼4.40 TECU. Notably, the M7.16 flare, which occurred at 02:12 UT, induced a greater enhancement in electron density and TEC compared to the X1.09 flare, largely due to its timing. Ground-based measurements further corroborated electron density profiles derived from COSMIC and SWARM satellite data, showcasing significant variations near the F-region, where electron density peaked dramatically. The study highlights how solar flares impact different ionospheric layers based on region, with increased extreme ultraviolet (EUV) flux driving photo-ionization and TEC enhancements in the upper ionosphere (F region), while elevated flux triggers ionization and disturbances in the lower ionosphere (D region).