Statistical Study of foF 2 Diurnal Variation at Dakar Station from 1971 to 1996 : Effect of Geomagnetic Classes of Activity on Seasonal Variation at Solar Minimum and Maximum

The statistical study of F2 layer critical frequency at Dakar station from 1971 to 1996 is carried out. This paper shows foF2 statistical diurnal for all geomagnetic activities and all seasons and that during solar maximum and minimum phases. It emerges that foF2 diurnal variation graphs at Dakar station exhibits the different types of foF2 profiles in African EIA regions. The type of profile depends on solar activity, season and solar phase. During solar minimum and under quiet time condition, data show the signature of a strength electrojet that is coupled with intense counter electrojet in the afternoon. Under disturbed conditions, mean intense electrojet is observed in winter during fluctuating and recurrent activities. Intense counter electrojet is seen under fluctuating and shock activities in all seasons coupled with strength electrojet in autumn. In summer and spring under all geomagnetic activity condition, there is intense counter electrojet. During solar maximum, in summer and spring there is no electrojet under geomagnetic activity conditions. Winter shows a mean intense electrojet. Winter and autumn are marked by the signature of the reversal electric field.


Introduction
Geomagnetic activity, defined as the disturbance of the geomagnetic field due to external source [1], has been divided into four classes of activity [2]- [5] These classes are: quiet activity due to slow solar wind, shock activity due to CMEs (Coronal Mass Ejections), recurrent activity provoked by high speed solar wind coming from coronal hole and fluctuating activity constituted by the other activities which are not clear [4].The impact of each class of activity in the critical frequency of F2 layer (foF2) has been study through Ouagadougou station (Lat: 12.4˚N; Long: 358.5˚E; dip: 1.43˚) foF2 variability [6]- [9] and Dakar (Lat: 14.8˚N; Long: 342.6˚E, dip: 8.44˚) station foF2 variability [10].The first foF2 statistical variation for an African Equatorial Ionization Anomaly (EIA) station has been performed by Ouattara and Amory Mazaudier [9] at Ouagadougou station located at the trough of ionization.This paper, the second one that focuses on foF2 statistical variation concerns Dakar station situates between the crest and the trough of the EIA region of African sector from 1971 to 1996.Our objective in statistical works is, on the one hand, to investigate existing models predictions by comparing the statistical experimental results coming from long series data base with models' results.On the second hand, to contribute to space weather study by determining ionosphere variability under different types of solar activity and that for different seasons and solar cycle phases.The present study goal is to contribute to space weather study.Dakar station is used because foF2 variation comparative study through its long term trend variation and diurnal variation shows the particularity of Dakar station (see [11]- [13]).For this paper, our attention is only focused on foF2 statistical diurnal variation during solar minimum and maximum phases.

Materials and Methods
The data used in this study concern Dakar station (Lat: 14.8˚N; Long: 342.6˚E, dip: 8.44˚) in Sénégal provided by Télécom Bretagne.We analyze foF2 statistical diurnal variation from 1971 to 1996.That period involved three solar minimum and two solar maximum.In this paper, we neglect the impact of long term trend in data because Gnabahou et al. [12] at Dakar station showed that on the one hand there is no difference between disturbed and quiet time periods for foF2 yearly variation and on the other hand the long term trend slop at Dakar station is weak.
For the determination of solar cycle phases, we use sunspot number R Z and follow Ouattara et al. [8] method.According to their method solar minimum year is given by R Z < 20 and solar maximum year is obtained by taking R Z > 100.It is important to note that for small solar cycles (solar cycles with sunspot number maximum (Rz max) less than 100) the maximum phase is obtained by considering R z > 0.8 × R Z max.Solar increasing phase years is given by 20 ≤ R Z ≤ 100 and R z greater than the previous year's value.Solar decreasing phase is obtained by 100 ≥ R Z ≥ 20 and R Z less than the previous year's value.Keep in mind that we will only consider solar minimum and maximum in this work.
For the determination of geomagnetic conditions, we use pixel diagrams (Figure 1 example of pixel diagram; see Ouattara and Amory Mazaudier [14] for more details) built by using 1) Mayaud [15] [16] geomagnetic index aa by considering its daily mean value Aa, 2) the date of Sudden Storm Commencement (SSC) and 3) color code that allow us to identify the four classes of geomagnetic activity.In pixel diagram, quiet time condition is highlighted by white and blue colors and given by Aa ≤ 20 nT.The other conditions namely disturbed conditions that take together the three other conditions are given by Aa > 20 nT and highlighted by the other colors (green, yellow, orange, red and olive red).
For seasonal study, the following seasons are considered: winter (December, January and February), spring (March, April, May), summer (June, July, August) and autumn (September, October and November).
To perform the statistical hourly foF2 values we follow Ouattara and Amory Mazaudier [9] method.Therefore, the following equations are used:

Geomagnetic Activity Impact Evaluation
The hourly foF2 values for geomagnetic quiet conditions from 1971-1996 ( Hourly Geomag foF2 ) are determined by using Equation ( 1): In this equation, Hourly Cycle foF2 is the hourly mean foF2 value for the considered solar cycle phase and Nc the number of solar cycle phase involved in the period 1971-1996.
From Table 1, it can be seen for our period involved, three solar minimum phases, two solar maximum phases, three solar decreasing phases and two solar increasing phases.Only solar maximum and minimum phases will be considered as previously noted.By using Equation (1), we determine the 24 hours mean values of foF2 for each type of geomagnetic class of activity by considering the whole 9496 days of the 26 years involved (20 years with 365 days and 6 years with 366 days).Daily geomagnetic activity is evaluated by using pixel diagrams (see [2] [3] [14]).

Solar Cycle Phase Impact Evaluation
The mean hourly foF2 value of a solar cycle phase ( Hourly Cycle

foF2
) is carried out through Equation ( 2): In this equation, Hourly Year foF2 corresponds to the mean hourly foF2 value of the considered year and Ny the number of year involved in the considered solar cycle phase.The number of year per solar cycle phase can be obtained by counting the number of year of each solar cycle phase shown in Table 1.
is given by Equation (3): In Equation (3), Hourly Month foF2 corresponds to the monthly hourly mean value of foF2 and Nm the number of available month involved in the considered year.In the previous equation, Hourly Day foF2 is an hourly foF2 value for a geomagnetic class day and Nd the number of geomagnetic class day involved in the month considered.

Results
In the present paper, we present foF2 diurnal variation during solar minimum and maximum phases under the four geomagnetic conditions and for different seasons.
Figure 2 and Figure 3 show seasonal variability of diurnal variation of foF2 during all geomagnetic activities for solar cycle minimum phase and solar maximum phase, respectively.In these figures, blue graph corresponds to winter, red curve to spring; black graph is devoted to autumn and green curve to summer.
Figure 2(a) shows the first peak at 1000 LT and the second at 1800 LT.The highest morning peak and afternoon peak is seen in autumn graph with 9.29 MHz and 10.64 MHz as foF2 value respectively.Winter foF2 values are superior to summer foF2 values; this shows winter anomaly.There is equinoctial asymmetry during daytime.There is no night time peak during this period.
In   For summarizing, during solar minimum only under autumn, foF2 diurnal variation graph show noon bite out profile with predominance afternoon peak.During all time geomagnetic conditions, summer and spring graph exhibit afternoon profile.Winter curve shows morning peak (panels a, b and d) except shock activity where one observe a plateau profile at 06h00 -14h00 (panel c).
F2 layer density is the highest under recurrent condition and the lowest during quiet condition.It can be retained that quiet foF2 density < fluctuating foF2 density < shock foF2 density < recurrent foF2 density.
Except the profile type, seasonal effect is non-perceptible under shock condition because graphs are merged.The night peak is seen in autumn graph under disturbed periods (panels b, c and d).Shock activity exhibits this peak at all seasons.anomaly is shown by the values of winter foF2 superior to those of summer.Double peak is seen in autumn curve with 14 MHz as peak values and located at 1000 LT and 1700 LT for morning peak and afternoon peak, respectively.Winter curve presents morning peak profile where the peak is situated at 1000 LT with 14.16 MHz.Spring profile is that plateau with 13.44 MHz as foF2 maximum value.Summer curve highlights dome profile with the maximum located at 1700 LT with 12.69 MHz.Autumn values are superior to those of the other seasons.Except autumn graph the others are merged and have the same behavior.There is equinoctial asymmetry during this activity.Winter graph shows morning peak profile where the peak is situated at 1000 LT with 14 MHz; autumn exhibits the noon bite out while spring curve is plateau profile and dome profile is seen at summer.
Night time peak is seen at 2000 LT for autumn curve with 12.69 MHz and winter graphs at 2100 with 11.34 MHz as foF2 values.
For summarizing at solar maximum, under all geomagnetic activities conditions, dome profile is shown in summer, plateau profile is observed in spring graph and morning peak profile is seen in winter.Except shock activity, autumn presents a noon bit out profile.Winter and autumn curves present night time peak at all geomagnetic activities.All graphs, under shock activity show night time peak.Except in summer, all graphs are merged under shock and recurrent activities.

Discussion and Conclusions
The five types of foF2 profile observed at Ouagadougou station (Long: 358.5˚E;Lat: 12.4˚N; dip: +1.45˚) by Fayot and Vila [17] are also seen here at Dakar station (Long: 342.6˚E;Lat: 14.8˚N; dip: +5.53˚): dome or D profile, noon bite out or B profile, morning peak or M profile, afternoon peak or reversed or R profile and plateau or P profile.Based on the fact that foF2 is able to describe the distribution of E layer electric current through the study of ground recorded magnetic field variations, Dunford [18], Vassal [19], Vassal [20], Acharya [21] and Acharya [22] show that it is possible to link ionosphere variability to the nature, the force or the absence of E region electric currents.Therefore, Vassal [20] relied the above different types of foF2 profiles of West Africa equatorial region on different types of E layer electric current (electrojet and counter electrojet).He attributed to D and P profiles the absence of electrojet, to R profile the presence of intense counter electrojet, to M profile the presence mean intensity electrojet and to B profile the presence of strength electrojet.
By taking into account the above relationship between E region electric current and foF2 diurnal variation, one can assert that: During solar minimum and: 1) under quiet time condition there is a strength electrojet at daytime coupled with intense counter electrojet at the afternoon; 2) under fluctuating condition we observe the presence of mean intense electrojet in winter, intense counter electrojet for the other seasons coupled with strength electrojet in autumn; 3) under shock activity, we note the presence of an intense counter electrojet for the seasons involved coupled with a strength electrojet in autumn; 4) under recurrent condition, mean intense electrojet is observed in winter, strength electrojet in autumn and intense counter electrojet in the others seasons.As in foF2 diurnal profile the presence of trough around midday observed in noon bite out profile expresses the signature of the vertical drift ExB (see [23]- [26]), it can be retained that only during quiet time condition and in autumn foF2 profiles express the signature of the vertical drift ExB.Fairly under all geomagnetic activities and in autumn this signature is observed.Moreover, it is well-known that a night time peak in foF2 diurnal variation graph is the signature of the reversal electric field in equatorial region (see [23]- [26]).Therefore, this signature is observed during the disturbed condition in autumn graph under all geomagnetic conditions and also in winter except under fluctuating activity.
During solar maximum and in: 1) summer and spring, the absence of an electrojet is seen under the all geomagnetic activities; 2) winter, we observe presence of mean intense electrojet; and autumn we note presence strength electrojet except during the shock activity; 3) winter and autumn under all geomagnetic conditions, we observe the signature of the reversal electric field; this signature is also observed for all seasons under shock activity; 4) all seasons, there is no seasonal effect under shock activity except in summer.

Figure 1 .
Figure 1.An example of pixel diagram (2003 year pixel diagram).Circle indicates the date of SSC.

Figure 2 (
b), one can observed the same foF2 diurnal variation as for quiet time (panel a).The highest morning peak value is 9.27 MHz for winter curve; that of afternoon peak is 11.14 MHz for autumn graph.Night time peak is observed in autumn graph at 2100 LT with 7.29 MHz.The principal difference between quiet time curves (panel a) and fluctuating time curves (panel b) is the presence of night time peak.One can see the presence of equinoctial asymmetry (autumn foF2 values are superior to those of spring) i.e.September/October peak superior to March/April.

Figure 2 (
c) highlights the absence of shock activity during summer.A fairly double peak is observed in autumn curves at 1000 LT and 1700 LT with 10.25 MHz and 11.8 MHz, respectively.It can be seen that at daytime spring values are superior to those of autumn.This observation points out the presence of equinoctial asymmetry.Night peak is exhibited by all recorded season.

Figure 2 (
d) shows at daytime equinoctial anomaly with autumn peak superior to that of spring.Morning peak is shown at (1) 1000 LT with 9.89 MHz during winter and at (2) 1200 LT with 10.1 MHz during autumn.Autumn graph highlights night peak at 2100 LT with 8.43 MHz while winter curve shows this peak at 2000 LT with 8.25 MHz.

Figure 3 (
a) shows equinoctial asymmetry as autumn foF2 values are superior to those of spring.As winter foF2 values are superior to those of summer data highlight winter anomaly.Autumn curve present noon bite out profile with morning peak at 1000 LT with 13.99 MHz and Afternoon peak is situated at 1700 LT with foF2 values 13.66 MHz.Winter curve show a morning peak profile with 14.29 MHz as foF2 values.Spring graph shows plateau profile with 13.05 MHz as foF2 value and the curve of summer exhibits dome profile with 12.16 MHz at 1700 LT.

Figure 3 (Figure 2 .
Figure 2. Seasonal and diurnal foF2 variation under quiet time (panel a), Fluctuating activity (panel b) shock activity (panel c) and recurrent activity (panel d) conditions for solar minimum.

Figure 3 (
c) concerns shock activity graphs.Winter anomaly is shown by data profiles.Equinoctial anomaly shown here is characterized by autumn data values superior to those of spring.Winter graph shows morning peak located at 1000 LT with 14.36 MHz.Spring and autumn curves show fairly plateau profile with 13.40 MHz as foF2 value.Dome profile is presented by summer graph with the maximum located at 1600 LT with 12.88 MHz as foF2 value.

Figure 3 .
Figure 3.The same as Figure 2 but for solar maximum.

Figure 3 (
Figure 3(d) is devoted to recurrent activity.It can be seen that winter values are superior to those of summer.This observation shows winter anomaly.Autumn values are superior to those of the other seasons.Except autumn graph the others are merged and have the same behavior.There is equinoctial asymmetry during this activity.Winter graph shows morning peak profile where the peak is situated at 1000 LT with 14 MHz; autumn exhibits the noon bite out while spring curve is plateau profile and dome profile is seen at summer.Night time peak is seen at 2000 LT for autumn curve with 12.69 MHz and winter graphs at 2100 with 11.34 MHz as foF2 values.For summarizing at solar maximum, under all geomagnetic activities conditions, dome profile is shown in summer, plateau profile is observed in spring graph and morning peak profile is seen in winter.Except shock activity, autumn presents a noon bit out profile.