Seasonal, Diurnal, and Solar-Cycle Variations of Electron Density at Two West Africa Equatorial Ionization Anomaly Stations

We analyse the variability of foF2 at two West Africa equatorial ionization anomaly stations (Ouagadougou and Dakar) during three solar cycles (from cycle 20 to cycle 22), that is, from 1966 to 1998 for Ouagadougou and from 1971 to 1997 for Dakar. We examine the effect of the changing levels of solar extreme ultraviolet radiation with sunspot number. The study shows high correlation between foF2 and sunspot number (Rz). The correlation coefficient decreases from cycle 20 to cycle 21 at both stations. From cycle 21 to cycle 22 it decreases at Ouagadougou station and increases at Dakar station. The best correlation coefficient, 0.990, is obtained for Dakar station during solar cycle 22. The seasonal variation displays equinoctial peaks that are asymmetric between March and September. The percentage deviations of monthly average data from one solar cycle to another display variability with respect to solar cycle phase and show solar ultraviolet radiation variability with solar cycle phase. The diurnal variation shows a noon bite out with a predominant late-afternoon peak except during the maximum phase of the solar cycle. The diurnal Ouagadougou station foF2 data do not show a significant difference between the increasing and decreasing cycle phases, while Dakar station data do show it, particularly for cycle 21. The percentage deviations of diurnal variations from solar-minimum conditions show more ionosphere during solar cycle 21 at both stations for all three of the other phases of the solar cycle. There is no significant variability of ionosphere during increasing and decreasing solar cycle phases at Ouagadougou station, but at Dakar station there is a significant variability of ionosphere during these two solar-cycle phases. 1. Introduction Many ionosphere studies concern ionosphere parameter variability [1, 2] and do not include the African sector [3]. Moreover, some papers deal with the comparison between ionospheric data and the International Reference Ionosphere (IRI) [4–9]. On the other hand, many studies have investigated the solar-cycle variation and/or geomagnetic activity variation of the critical frequency of the F2 layer ([10–16]. It is important to know that few studies integrate African sector data, as noted by Bilitza et al. [3]), and take into account long series of data. In fact we have in the African sector the works which treat the variability of equatorial F2 density [17–23] in the equatorial ionization anomaly (EIA) trough, and in the Asian sector we have the works of Le Huy et al. [24] and Pham Thi Thu et al. [25], which concern