Recurrent Events’ Impacts on foF2 Diurnal Variations at Dakar Station during Solar Cycles 21-22

Abstract

This paper deals with recurrent events’ (REs’) impacts on foF2’s diurnal variations at Dakar station (lat: 14.8° N, long: 342.6° E, Senegal) during solar cycles 21 and 22. Data were analyzed by taking in account solar phases and seasons influence. REs generate positive storms in all seasons and on all solar phases and also weak negative storms in all seasons and at minimum and maximum solar phases. The results suggest on the one hand that vertical drift $E\times B$ due to strong electrojet currents cannot be considered an important mechanism of positive storms and on the other hand that prereversal enhancement (PRE) phenomenon (with a delay due to Dakar station position) and equatorward thermospheric neutral winds circulation (TNWC) contribute mainly to their generation. The intense positive storms, which appear mostly at night, are observed in decreasing phase and in spring which correspond, respectively, to the solar phase and the season of maximum REs occurrence. Then, positive storms’ strength presents seasonal and solar cycle dependence and it is related to REs days occurrence. REs’ negative storms strength are linked directly with REs days’ occurrence on solar phase scale, while they are not on seasonal scale.