Nighttime and seasonal variation of lower ionospheric parameters using tweek analysis during minimum solar activity period over low latitude

Abstract

VLF signals are continuously recorded, monitored, and observed by an Automatic Whistler Detector (AWD) installed at our low latitude Indian station, Banaras Hindu University, Varanasi (geom. lat. 14° 55^’ N, geom. long. 154° E and geog. lat. 25°15′ 20″ N, long. 82° 59’ 21” E). In the present study, the nighttime and seasonal variation of lower ionospheric parameters mainly D-region during a low solar minimum period from 2018 to 2019 has been analyzed at a low latitude station in Varanasi. Since large numbers of tweeks have been observed at Varanasi, we have randomly chosen three days in a month during the summer (June), winter (November), and equinox (September/October) seasons in the years, 2018 and 2019, where good-quality, continuous nighttime data were available. A total of 35,000 tweeks recorded up to n = 1–10 harmonics in different seasons: summer, winter, and equinox showed percentage occurrences of approximately 37%, 32%, and 31% in 2018 and 38%, 31%, and 31% in 2019 respectively during the solar minimum period of solar cycle 24. The analysis of tweeks with harmonics up to tenth modes in different seasons showed higher occurrence in summer, reflecting at less diverse altitudes than winter and equinox seasons, with propagation paths ranging from 500 to 8500 km in 2018 and 800–8000 km in 2019. Nighttime lower ionospheric daily average electron density (n_e) values varied from 23 to 26 cm⁻³ at altitudes of 80–94 km during 2018 and 2019. The D-region is less uniform at higher altitudes during summer. The effect of reflection height on frequency, attenuation factor, time delay, and the usefulness of tweek method for estimating electron density in the nighttime D region ionosphere for different harmonics are also discussed in this study. The electron density (n_e) values obtained by the tweek method during various seasons are compared to IRI-2016, radar data, and rocket data, all of which fall within a similar range of variations specifically between 80 and 90 km showing a better estimation by the tweek method. Further, we have also evaluated the attenuation coefficient of the nighttime tweek propagation. We have also simulated the frequency-time spectrum of different modes of tweeks at various heights and propagation paths.