Abstract
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.
Highlights
Since the first sounding of the ionosphere in 1926 by Breit & Tuve (1926), many studies have been performed on the variability of the F layer
FoF2 increases to reach its maximum value around 10:00 LT for recurrent activity and 11:00 LT for shock activity and it slowly decreases until 20:00 LT and increases after that time
The diurnal variation of the F2 layer critical frequency exhibits a double shape: it decreases from 00:00 LT to 06:00 LT, increases and reaches its first maximum around 10:00 LT
Summary
Since the first sounding of the ionosphere in 1926 by Breit & Tuve (1926), many studies have been performed on the variability of the F layer. One can distinguish two main physical processes generating large-scale electric field which strongly influence low latitudes: (1) the prompt penetration (Vasyliunas 1970) and (2) the disturbance dynamo effects (Blanc & Richmond 1980; Fejer et al 2007; Huang et al 2007; Fejer 2011) All these coupling processes are related to the three main chosen factors of variability: the solar cycle, the season and the geomagnetic activity (shock and solar wind effects). We take into account the influences of the Sun-Earth relative position, sunspot activity, solar wind speed and shock activity to classify the data, in order to understand and predict the shape of the mean diurnal variation of the F2 layer Such a study is only possible when there is a large existing continuous recording database.
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