Ionospheric signatures from 2 years continuous monitoring of the equatorial ionosphere over Nigeria with HF Doppler sounder

Abstract

A receiver that measures and logs Doppler shifts of high frequency (HF) radio signals reflected from the ionosphere was installed in Lagos (geographic: 6.48°N, 3.27°E; dip latitude −4.66°), Nigeria in March 2011. However, continuous monitoring of the ionosphere was facilitated by the installation of a transmitter in Abuja (geographic: 8.99°N, 7.39°E; dip latitude 1.01°), Nigeria in July 2019, dedicated to transmitting HF signals for this remote sensing system. This provided the opportunity to study ionospheric signatures in HF Doppler data obtained from an equatorial location. This paper presents a summary of key findings from analysis of data from the instrument during the first 28 months of operation from July 2019 to November 2021. In this work, a statistical analysis of two main daytime features in the Doppler data is presented. The first feature is irregularities that appear as spreading of the Doppler trace, while the second feature is wave structures consistent with travelling ionospheric disturbances (TIDs). In order to highlight trends in the data, the events were separated into morning (occurring from sunrise to 1159 LT) and afternoon (occurring from 1200 LT to sunset) events. The results showed that occurrences of either type of features were more frequent during the afternoon compared with morning hours. The occurrences of irregularities that appeared as a spread in the Doppler trace peaked in the month of July in the morning, while, in the afternoon these occurrences peaked in the month of March. The duration of most of these irregularities was ≥60 min. A peak in occurrences of TIDs was observed in the morning and afternoon epochs during the March equinox. It was also observed that over 80 % of occurrences of spreading in the Doppler trace during the equinoxes were associated with the occurrences of TIDs. In the month of July, despite a paucity of TIDs, there was a peak in occurrences of spreading in the Doppler trace. Overall, the lowest number of occurrences of both types of features was recorded in the month of August. Detailed analysis of selected events showed that the morning occurrences of spreading in the Doppler trace were mostly remnants of post-midnight spread F, while the afternoon occurrences were due to either TIDs or severe E-layer irregularities. The results from this study show the potential of the low-cost HF Doppler instrument as a real-time monitor of space weather activity at equatorial latitudes. However, further work will be required to establish what percentage of daytime spreading in the F-layer is caused by the remnants of nocturnal plasma bubbles, TIDs, or irregularities in the E-layer.