Temporal and seasonal variability of low-latitude D-region ionospheric response to solar flares using VLF observations

The present work investigates the impact of solar flares on the D-region ionosphere using Very Low Frequency (VLF) signals transmitted from NWC (21.81°S, 114.16°E; 19.8 kHz) and recorded at a low-latitude receiving station in Dehradun, India (30.31°N, 78.03°E). The analysis covers the period from July 2022 to June 2023 and includes a total of 234 solar flare events, comprising 153 C-class and 81 M-class flares. Variations in VLF signal amplitude were examined in conjunction with solar X-ray flux data obtained from the GOES satellite. The amplitude difference (ΔA) between flare and non-flare days, as well as the time delay (Δt) between the GOES X-ray flux peak and the corresponding VLF response, were analyzed as functions of local time and season. The results reveal a pronounced local time dependence of both amplitude enhancements and response delays, with maximum perturbations observed during the morning hours for both flare classes. Seasonal analysis indicates stronger ionospheric responses during winter compared to summer and equinox periods, likely due to reduced background electron densities in the D-region. Several anomalous amplitude variations that could not be explained solely by local time or seasonal effects were attributed to factors such as the location of the flare on the solar disk and prevailing geomagnetic conditions. These findings suggest complex interdependencies among solar, ionospheric, and geomagnetic parameters. This study extends previous investigations by providing a comprehensive temporal and seasonal characterization of the low-latitude D-region ionospheric response to solar flare activity, emphasizing the roles of solar zenith angle, flare intensity, and ionospheric relaxation processes.