Seasonal quiet-day morphology of VLF signal strength at Ibadan

Abstract

Very Low Frequency (VLF) signal amplitude is enhanced during solar flare events. Hence, it can be used as remote sensing for tracking solar flares and studying the lower ionosphere. However, the enhancement is not directly proportional to the magnitude of a solar flare hindering the effectiveness of the VLF signal as a remote sensor for tracking solar flare. Preliminary analysis shows that the level of enhancement recorded for a particular solar flare occurrence depends on the background fluctuations. Thus, this study aims to explore the background fluctuations of the VLF signal using quiet-day data and investigate the impact of background fluctuations on solar flare detectability using the VLF signal. The VLF data used for this study were collected using a VLF receiver installed at KolaDaisi University, Ibadan (KDU-I). The VLF data were from three stations; GBZ (Freq.: 19.8 kHz, Lat.: 21.82°, Long.: 114.17°), NWC (Freq.: 19.58 kHz, Lat.: 54.91°, Long.: 003.28°) and VTX4 (Freq.: 19.2, kHz Lat.: 08.38°, Long.: 077.75°) recorded between 2019 and 2022 at Ibadan (Lat.: 7.6248°, Long.: 3.9131°). The results revealed that the time series has a cyclic pattern that corresponds to each global season classification in the northern hemisphere (Summer, autumn, winter and spring) with an upward trend from wet to dry season. This observation shows that similar geophysical parameters that drive meteorology also have an impact on the background fluctuations of VLF amplitude. It was also observed that the sensitivity of VLF amplitude in sensing solar flare depends on the background fluctuation. It is recommended that the effect of meteorology and background fluctuation be considered when modelling VLF amplitude as a remote sensor for tracking solar flares.