HIGHLIGHTING SEASONAL ANOMALY IN IONOSPHERE DURING MINIMUM AND MAXIMUM SOLAR CYCLE PHASES

Ionosphere is the part of atmosphere where waves reflect for telecommunication because of its concentration in particles. The morphology of ionosphere layer is not uniform. The density in particles in this layer varies from daytime to nighttime, from winter to summer, via spring and autumn. Time parameter and solar activity also influence this variation. This study is focused on the electron density in ionosphere layer during minimum and maximum of solar cycle 22, at Ouagadougou station. It uses International Reference Ionosphere model. The seasonal peak of electron density and total electron content parameters are carried out by running the 2012-version of the model. Comparison from winter to summer shows out a decrease of the peak of electron density and total electron content in ionosphere. Winter anomaly is highlighted by the time profiles of these two parameters. This correlates with previous results found by other authors. causes ionization of ionosphere constituents by hitting the molecules and atoms. Solar radiations intensity is also closely linked to solar activity and seasons. On winter, the concentration in electrons (volume or surface) in ionosphere is 10% superior to its value on summer during solar minimum. During solar maximum, this value grows up to more than 18% between winter and summer. Analyzing time variation of these parameters enables to highlight the winter anomaly phenomenon on the peak of electron density and the total electron content at minimum and maximum solar cycle phases.

Ionosphere is the part of atmosphere where waves reflect for telecommunication because of its concentration in particles. The morphology of ionosphere layer is not uniform. The density in particles in this layer varies from daytime to nighttime, from winter to summer, via spring and autumn. Time parameter and solar activity also influence this variation. This study is focused on the electron density in ionosphere layer during minimum and maximum of solar cycle 22, at Ouagadougou station. It uses International Reference Ionosphere model. The seasonal peak of electron density and total electron content parameters are carried out by running the 2012-version of the model. Comparison from winter to summer shows out a decrease of the peak of electron density and total electron content in ionosphere. Winter anomaly is highlighted by the time profiles of these two parameters. This correlates with previous results found by other authors.

…………………………………………………………………………………………………….... Introduction:-
The major constituents of ionosphere layer are N2, O2 and O (Nier et al., 1964;Schaefer and Brown, 1964;Reber and Nicolet, 1965). Solar radiations coming from the Sun hit these particles and cause their ionization. This ionization phenomenon releases electrons and ions (Nicolet and Mange, 1954). The density in particles (ions, electrons, atoms, molecules) gives to ionosphere layer high or low reflectivity to waves emitted from stations on the Earth. This layer behaves as an obstacle for waves diffusion. The reflectivity of ionosphere layer to waves transmission is dependent on its concentration in particles. Below a given limit of waves frequency value, pointing out the critical frequency, waves are reflected in ionosphere layer. This is used for data signals (telecommunication, navigation, etc.) transmission. This limit value is variable. It is dependent on time, solar cycle phase, season, solar activity. Thus, ionosphere layer is important for telecommunication and must be well known for a good prediction of waves behavior. Many models are developed for ionosphere investigation (Rishbeth, 1964 (Rishbett and Gariott, 1969). This study also considers the quiet periods for minimum and maximum of solar cycle phase 22 and low latitudes for running the model.

Material and Methods:-Fundamentals
The study is based on the four following core principles (Nanéma et al., 2018): (i) Minimum and maximum phases of solar cycle. The minimum and maximum phases are respectively determined by the sunspot number Rz ≤ 20 and Rz ≥ 100. (ii) Quiet days. The quiet days are given by Aa index inferior to 20 nT. In this range, the five quietest days are considered for the study. So, the five lowest values in the characteristic month are used. (iii) Integrating the values. The average value on the five quietest days of each parameter at the same time is the hourly value for this parameter. (iv) Characteristic month. Each season in the year is well described by its characteristic month (March for spring, June for summer, September for autumn and December for winter). Ouagadougou station (latitude: 12.5°N, longitude: 258.5°E), located in West Africa is considered to run the model. Table 1 is derived from the principles (i), (ii) and (iv). 1985 and 1990 are respectively minimum and maximum of solar cycle 22. Running the model for each day in Table 1 gives the hourly values of the parameters. Then, the use of the principle (iii) leads to the equations (1) and (2) as follows.

Results And Discussion:-
The peak of electron density is expressed by: And total electron content expression is: In equations (1) and (2), "i" points out the time all daylong while "j" designates the quietest day. So, I ∈ [0, 24], NmF2i and TECi are respectively the average hourly value (calculated on the five quietest days) of peak of electron density and total electron content. NmF2 i,j and TEC i,j are respectively the hourly value at each quietest day.
Peak of electron density and total electron content values calculated above with equations (1) and (2)      The table 2 shows that on winter during maximum solar cycle phase, peak of electron density and total electron content values are higher than those on summer. This is also shown on comparison between the other hourly values of these parameters. This means that on winter, the concentration in electrons is higher than that on summer. The ionization of ionosphere layer is more intensive on winter than that on summer. However, solar radiations are more intensive on summer than that on winter. This is winter anomaly that has already been found (Rishbeth et

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TEC ∑ NmF2 24 (4) Figure 3 presents the seasonal mean values of peak of electron density and total electron content calculated by equations (3) and (4).   a and 3.b present the comparison between peak of electron density average values on winter and summer and total electron content during these seasons at minimum phase. The average values of these parameters on winter are higher than those on summer. The concentration of electrons in ionosphere layer is then higher on winter than that on summer. This concentration is 116.57% for the peak of electron density and 112.87% for the total electron content. This approach highlights the winter anomaly on minimum solar cycle phase. Figures 3.c and 3.d shows the same phenomenon on solar maximum (126.72% for the peak of electron density and 118.70% for the total electron content). This has been found previously.

Conclusion:-
In this study, IRI model is used to carry out peak of electron density and total electron content in ionosphere layer. The intensity of solar radiations, growing up from night to day, and from minimum to maximum solar cycle phases, 56 causes ionization of ionosphere constituents by hitting the molecules and atoms. Solar radiations intensity is also closely linked to solar activity and seasons. On winter, the concentration in electrons (volume or surface) in ionosphere is 10% superior to its value on summer during solar minimum. During solar maximum, this value grows up to more than 18% between winter and summer. Analyzing time variation of these parameters enables to highlight the winter anomaly phenomenon on the peak of electron density and the total electron content at minimum and maximum solar cycle phases.