On the Diurnal Variation of Doppler Frequency Shift of Trans-Ionospheric Radio Waves Within Equatorial Ionospheric Anomaly Region in Africa

Abstract This paper examined the diurnal variation of the doppler frequency shift (DFS) of a trans-ionospheric high frequency (HF) radio wave signal within the equatorial ionospheric anomaly region. Within the framework of the International Space Weather Initiative, a multi-institutional cooperation set up a HF transmitter and receiver systems at the Space Environment Research Laboratory, Abuja, Nigeria (Geographic: 7.39°E, 8.99°N; dip latitude – 1.37°) and the Department of Physics, University of Lagos, Nigeria (Geographic: 3.27°E, 6.48°N; dip latitude – 1.72°) respectively. Medium-term digital recording of waveforms of trans-ionospheric HF radio signals were obtained from the experimental observation to investigate the diurnal variation of DFS along Abuja – Lagos radio path between 2019 and 2022. The DFS exhibited diurnal variation with maximum at sunrise (5:00–7:00 UT) and sunset (17:00–18:00 UT) periods, as well as day-to-day variability which was attributed to day-to-day variability in the dynamics of flow of plasma in the Equatorial Ionization Anomaly region, meridional neutral winds and local atmospheric conditions in the thermosphere. A large spread of amplitudinal variations of DFS was observed in post Sunset and post-midnight period, while the observed small variations of DFS in midday and afternoon hours is attributed to the displacement of reflection point downward.