Statistical study of the equatorial F2 layer critical frequency at Ouagadougou during solar cycles 20, 21 and 22, using Legrand and Simon's classification of geomagnetic activity

This paper presents the statistical analysis of the diurnal variations of the F layer at the equatorial station of Ouagadougou (Lat: 12.4 N; Long: 358.5 E; dip: 5.9 ) from 1966 to 1998 (=> ~11 680 days). We consider three main factors of variability: (1) the season (spring, summer, autumn and winter), (2) the phase of the sunspot cycle (ascending, maximum, descending and minimum) and (3) the geomagnetic activity classified by Legrand and Simon in four groups: slow solar wind, high solar wind streams, fluctuating solar wind and shock activity. We easily identify the influence of the solar wind speed and shock activity on the diurnal pattern of the F layer. Shock and recurrent activities tend to enhance or diminish the morning or afternoon maximum of the F2 layer critical frequency. The difference of the diurnal foF2 variation during the increasing and decreasing phases of the sunspot solar cycle is explained by different solar wind regimes. The slow solar wind dominates during the increasing phase of the sunspot cycle and the fluctuating solar wind dominates during the decreasing phase of the sunspot cycle. This paper demonstrates that it is possible using a large database, to bring up significant morphologies of the diurnal variation of the foF2 critical frequency as a function of (1) different solar events such as quiet solar wind, fluctuating wind, recurrent high stream wind and Coronal Mass Ejections (CMEs); (2) solar cycle phases and (3) seasons. It is an approach directly connecting the critical frequency of the F2 layer to the solar parameters.