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.
Many ionosphere studies concern ionosphere parameter variability [
The present study relies on the use of long series of data (three solar-cycles of foF2) which are obtained from the African sector and particularly from the Sub-Saharan African sector. It is well-known that in Africa, and especially in Sub-Saharan Africa, there is a lack of data. In the past only a few ionosonde stations operated (see Figure
Ionosondes in operation in 1960.
The objective of this paper is to determine (1) foF2 variability of two West African EIA stations with solar-cycle, season, and time of day and (2) to point out foF2 longitudinal variations. Comparison between data and models will be done in another study. It will be important before testing models to know well the variability of station data.
The structure of the paper is as follows. After the treatment of data and methodology in Sections
For this study, F2 layer critical frequency of (foF2) data obtained from two African EIA ionosonde stations: (1) Ouagadougou (lat: 12.4°N; long: 358.5°E; dip: +1.45) and (2) Dakar (lat: 14.8°N; long: 342.6°E; dip: +5.53). These data covered three solar-cycles (cycles 20, 21 and 22) and are provided by the Ecole Nationale de Télécommunication de Bretagne (ENST-Bretagne).
Sunspot number (
Our database contains hourly foF2 values which are going from June 1966 to February 1998 for Ouagadougou and from January 1971 to February 1997 for Dakar. For this work are considered years with 75% monthly available data (i.e., 9/12 ratio of months per year). With this criterion, the available data go from 1971 to 1996 for Dakar and from 1965 to 1997 for Ouagadougou. For Ouagadougou, year 1986 must be excluded, for its available data are 5/12; but for the analysis of solar-cycle 22 data, weights have been used (1 for years with available number of months more than 75%, 0.75 for years with available number of months between 75% and 50%, and 0.5 when available number of months is less than 50%) in order to integrate year 1986.
It is important to note that, for the retained years, all hourly data are available during a day, and for most of the retained years (more than 98%) the number of months exceeds 75%. Therefore, daily values are an arithmetic mean over all hours, monthly values are an arithmetic mean over all days, and annual values are an arithmetic mean over all months.
As foF2 is greatly influenced by solar ultraviolet radiation, foF2 variability with solar-cycle-phase must show solar ultraviolet radiation variation with respect to solar-cycle phase. Solar-cycle phases are determined by considering the following conditions (see [
foF2 variations are analysed by using (1) annual averaged data for solar-cycle variations, (2) monthly averaged data for seasonal variations, and (3) hourly averaged data for diurnal variations. These analyses are made by taking into account solar-cycle phases.
As the solar-cycle 20 maximum (
Yearly variation of foF2 and
Monthly variations of foF2 at the stations of Ouagadougou and Dakar for cycles 20, 21, and 22 (a) during solar minimum, (b) for increasing solar-activity, (c) during solar maximum; and (d) for decreasing solar-activity.
Diurnal variation of foF2 at Ouagadougou station (left panels) and at Dakar station (right panels). Panel (a) concerns solar-cycle 31 and panel (b) solar-cycle 22.
For analyzing foF2 variability, we will use qualitative analysis based on examination of data plots (error bars will help us for this analysis) and quantitative analysis based on percentage deviation, expressed as
Figure
Table
Correlation coefficients between foF2 and sunspot number (
Cycle | 20 | 21 | 22 | Location |
---|---|---|---|---|
foF2 correlation coefficient | 0.977 | 0.973 | 0.948 | Ouagadougou |
0.950 | 0.883 | 0.990 | Dakar |
Figure
The red lines represent monthly mean variation of cycle 20 data, green lines those of cycle 21 and blue lines cycle 22 data.
The left panel (a) shows a lack of data during the minimum phase of solar-cycle 20 (absence of red line) for Ouagadougou, which operated since 1966 (with data available since 1967). The lack of data is also observed in the right panels (a), (b), and (c) (absence of red lines in these panels), because available data at Dakar station begins in 1971.
Figure
During June solstice, at Dakar for all solar-cycles, the density of ionization is the same. Whatever the station, the maximum of ionization appears always in October. The other maximums appear sometimes in March and sometimes in April. During solar-cycle maximum phase, foF2 profiles are regular and the density of ionization grows from cycle 20 to cycle 21.
Table
Predominance of March or September equinoctial peak for different solar-cycle phases.
Solar cycle phases | Solar cycles | Nature of peak predominance | |||||
Ouagadougou station | Dakar station | ||||||
20 | 21 | 22 | 20 | 21 | 22 | ||
Minimum | March/April | ||||||
September/October | |||||||
Increasing | March/April | ||||||
September/October | |||||||
Maximum | March/April | ||||||
September/October | |||||||
Decreasing | March/April | ||||||
September/October |
Years of the different solar-cycle phases and their
Solar-cycles | Solar-cycle phases | ||||
Minimum | Increasing | Maximum | Decreasing | Years and | |
20 | 1964-1965 | 1966-1967 | 1968–1970 | 1971–1974 | Years |
12.6 | 70.4 | 105.3 | 52 | ||
21 | 1975-1976 | 1977-1978 | 1979–1982 | 1983-1984 | Years |
14.1 | 60 | 141.6 | 56.3 | ||
22 | 1985-1986 | 1987 | 1988-1989 | 1992–1994 | Years |
15.7 | 29.4 | 136.5 | 59.6 |
Figure
The analysis of Figure
Table
At Dakar (right panels) the lines for the increasing and decreasing phases are similar through a whole day for solar-cycle 21 but only at daytime for cycle 22. foF2 at this station also increases from solar minimum to solar maximum. It can be mentioned from this figure that an enhancement of foF2 occurs around midnight during solar maximum and that the foF2 enhancement can be seen before sunrise during solar decreasing phase. All solar-activity phases foF2 increase before sunrise (before 0600 LT) except during solar-activity maximum and decreasing phases of solar-cycle 22 where foF2 increases after sunrise (between 0600 LT and 0800 LT). The right panel (b) of Figure
The Ouagadougou data (left panels) show no difference of foF2 between the solar-activity decreasing and increasing phases. At Dakar (right panels); on the other hand, difference of foF2 between the increasing and decreasing phases is seen. The difference is larger during cycle 21 than that of cycle 22. For solar-cycle 22, foF2 is higher during the decreasing phase than during the increasing phase. This result has been pointed out by Özgüç et al. [
The results of Figure
The difference between the Dakar and Ouagadougou electron densities shows the necessity to study separately the data from these kinds of stations, as foF2 shows longitudinal effects.
Figures
Percentage deviations of monthly average data from one solar-cycle to another with respect to month and solar-cycle phase. The left panels are for Ouagadougou and the right panels are for Dakar.
Percentage deviations (with sign reversed) of diurnal variations with respect to solar minimum conditions for three solar-cycle phases: increasing (blue), maximum (green), and decreasing (red).
In Figure
Figure
The phase-to-phase variability of the percentage variations shows the necessity to take into account the different solar-cycle phases in the study of ionosphere. The different variability of the percentage deviation values between the March and September equinox periods is related to the equinoctial asymmetry previously noted in the qualitative analysis. The qualitative analysis shows the phase-to-phase variability of foF2.
Figure
In Figure
In Figure
In Figure
This study shows the correlation between foF2 and
Authors thank the Ecole Nationale de Télécomunication de Bretagne (ENST-Bretagne) and, SPIDR webmaster for providing data. Authors also thank Dr. Rolland Fleury and Dr. Patrick Lassudrie Duchesne from the ENST-Bretagne for their collaboration and Dr. Arthur Richmond from HAO at NCAR for his proofreading and advices. Thanks to International Journal of Geophysics editor and reviewers for their kindly remarks, suggestions, and propositions which allow them to improve the paper.