Abstract
The solar terminator is a moving boundary between day-side and night-side regions on the Earth, which is a substantial source of perturbations in the ionosphere. In the vicinity of the solar terminator, essential parameters like S4 index measurements are widely analyzed in order to monitor and predict perturbations in the ionosphere. The utilization of the scintillation index S4 is a well-accepted approach to describe the amplitude/intensity fluctuation of a received signal, predominantly caused by small-scale irregularities of the ionospheric plasma. We report on the longitudinal daily and seasonal occurrence of GNSS signal scintillations, using the data derived from the GNSS stations in Bahir Dar, Ethiopia, Lomé, Togo and Dakar, Senegal. The observed seasonal climatology of GNSS signal scintillations in equatorial Africa is adequately explained by the alignment of the solar terminator and local geomagnetic declination line. It should be pointed out that the strongest scintillations are most frequently observed during the time when the solar terminator is best aligned with the geomagnetic declination line. At all three stations, the comparison of computational and observational results indicated that the scintillation activity culminated around equinoxes in the years 2014, 2015 and 2016. Comparatively, the western equatorial Africa sector has the most intense, longest-lasting, and highest scintillation occurrence rate in equinoctial seasons in all three years. For the first time, we show that the seasonal variation of the scintillation peaks changes systematically from west to east at equatorial GNSS stations over Africa. A detailed analysis of the solar day–night terminator azimuth at ionospheric heights including the time equation shows that the scintillation intensity has a maximum if the azimuth of the terminator coincides with the declination line of the geomagnetic field. Due to the remarkable change of the declination by about 10° at the considered GNSS stations, the distance between scintillation peaks increases by 46 days when moving westward from the Bahir Dar to the Dakar GNSS station. The observations agree quite well with the computational results, thus confirming Tsunoda’s theory.
Highlights
It is believed that the linear growth rate variations of generalized Rayleigh–Taylor instability [3] and the presence of seed perturbation [4] primarily account for the occurrence phenomenology of GNSS signal scintillations between different regions in equatorial Africa
The seasonal climatology of GNSS signal scintillations was in a good agreement with Tsunoda’s solar terminator and local geomagnetic field declination hypothesis, that is, the scintillations were most frequently observed during the time when the alignment of the solar terminator with the geomagnetic declination was best
Our key results demonstrate a noticeable change of the seasonal behavior in the occurrence of ionospheric irregularities over Africa as a function of longitude, which was closely related to the alignment of the solar day–night terminator with the geomagnetic declination line
Summary
The degree of perturbations in the ionosphere due to small-scale irregularities and plasma bubbles can be largely responsible for the fluctuations in the amplitude and phase of GNSS signals, referred to as scintillations. It is believed that the linear growth rate variations of generalized Rayleigh–Taylor instability [3] and the presence of seed perturbation [4] primarily account for the occurrence phenomenology of GNSS signal scintillations between different regions in equatorial Africa. Many reasonable studies have been conducted to understand the possible driving mechanisms for the occurrence of GNSS signal scintillations in Africa (e.g., [8,9,10,11]). We focus on carefully studying the relationship between the solar day–night terminator and the geomagnetic declination regarding the occurrence of small-scale irregularities in the equatorial ionosphere over the west-to-east extension of Africa. Because the declination of the geomagnetic field varies by more than 10◦ from East to West Africa, here we evaluate the related impact on the occurrence of irregularities as expected by Tsunoda’s theory
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
