MODELING CRITICAL FREQUENCY OF IONOSPHERE D-LAYER AT MINIMUM AND MAXIMUM OF THE SOLAR CYCLE 22WITH IRI-2016

One of the interests of the study of the ionosphere lies in its importance for the transmission of radio waves in telecommunications. The ionospherebehaves as an obstacle to the passage of waves. Thus, the signals of short wavelengths are reflected by the F layer or the upper part of the sublayer E, while theD-layeris the seat of the reflection of low-frequencywaves. The presentstudyinvestigates the temporal variability of the criticalfrequency of the D-layer (for) using the 2016 version of the International Reference Ionosphere (IRI) model under quiet day conditions during at maximum and minimum phase of solar cycle 22. The workisconductedat the Ouagadougou station, located in West Africa. The methodology of the workadopted for the determination of the parameter foDisbased on the calculation of the monthlyhourlyaverages of this variable obtainedwith the help of the model during the monthsthatcharacterize the seasons. The resultsobtained for the parameter for as a function of time during the minimum and maximum of the solar cycle 22 have been presented. The seasonal and temporal variations of the criticalfrequency of the ionosphereD-layer show that the foD values are lower during a minimum of the solar cycle and present maximum values at the Zenith (1200 TL) at a minimum and maximum. Theseresultsalsorevealthatthisparameter varies with time, season, and geographical position. The results of thisstudy show a criticalfrequencybelow 1 MHz during both phases of the solar cycle.

One of the interests of the study of the ionosphere lies in its importance for the transmission of radio waves in telecommunications. The ionospherebehaves as an obstacle to the passage of waves. Thus, the signals of short wavelengths are reflected by the F layer or the upper part of the sublayer E, while theD-layeris the seat of the reflection of low-frequencywaves. The presentstudyinvestigates the temporal variability of the criticalfrequency of the D-layer (for) using the 2016 version of the International Reference Ionosphere (IRI) model under quiet day conditions during at maximum and minimum phase of solar cycle 22. The workisconductedat the Ouagadougou station, located in West Africa. The methodology of the workadopted for the determination of the parameter foDisbased on the calculation of the monthlyhourlyaverages of this variable obtainedwith the help of the model during the monthsthatcharacterize the seasons. The resultsobtained for the parameter for as a function of time during the minimum and maximum of the solar cycle 22 (Bilitza et al., 1993), It is a standard empirical model of the ionosphere, created in 1960 sponsored by the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI). It generates time and date-dependent averages of the parameters at an altitude ranging from 50 to 2000 km. It includes a FORTRAN program, subprograms (CCIR, URSI), index files, etc.,The model has a better representation of the ionosphere during very weak solar activity (Bilitza et al., 1993). It is executable on the Virtual Ionosphere Thermosphere Mesosphere Observatory (VITMO) site. To better understand the behavior of the ionosphere, many reports have highlighted the variability of the critical frequency (foF2) of the F-layer during various seasons, days, hours, solar events, and latitudes (Ouattara et  The methodology for determining the critical frequency is based on the calculation of the monthly hourly average of the foD parameter on the five quietest days of each characteristic month. Thus, equation (1) defines the critical frequency as follows: In relation (1), foD h denotes the critical frequency of the layer D at time h for the characteristic month considered, foDh , d is the value of the critical frequency at time h for day d. Thus, h ∈ [0.24], and j∈ [1,5].
The IRI model allows the extraction of different values of foDh , j . It then becomes possible to determine the value of the critical frequency at time h foDh by calculating the average value of the parameters foDh , j over the five quietest days for each characteristic month.
The table below shows the five quietest days of each characteristic month of each season for the solar cycle considered. The peak of the critical frequency of the D-layer (foD) is related to the electron density by the relation: In this expression, the critical frequency foD is in MHz and the peak electron density NmD is in m -3.

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Results:- Figures 1 and 2 show the temporal variability of the critical frequency foD during the minimum and maximum of solar cycle 22, respectively. Panels 1a, 1b, 1c, 1d are the hourly profiles of foD in spring, summer.
Autumn and winter during the minimum, respectively, and those in panels 2a, 2b, 2c, and 2d are the hourly profiles of foD at the maximum. Winter is the least sunny season. The ionization of particles in the ionosphere due to solar radiation is, therefore, lower in winter than in the other seasons. During the minimum phase, the critical frequency profile decreases between 12.00 TL and 17.00 TL, increases during the day between 08.00 TL and 12.00 TL, and has a maximum at 12.00 TL. During a maximum phase of the solar cycle, the critical frequency profile decreases between 12.00 TL to 17.00 TL, increases between 07.00 TL to about 12.00 TL, and a maximum at 12.00 TL. The electron density increases during the day and depends on the weather, the season, and the solar cycle phase. It varies with the intensity of the solar rays. At sunrise, the ionization is important and continues to ionize for a few hours after the position of the maximum (12.00TL) of production until the disappearance of the electron density. For this, we also observe a profile of the critical frequency during this period. The recombination is very fast, so that this layer remains only at certain times when the photoionization has stopped. This explains the absence of foD during these hours. These results (figures 1 and 2) show a low value of foD in winter; this could be explained by a ray of weak sunshine, which is due to the fast recombination of electrons in this layer. The maximum critical frequency obtained in this study is less than 1 MHz. This explains why this layer does not appear on the ionograms (traces of the layers obtained by sounding) because of the low critical frequency (the highest frequency likely to return to the ground after vertical incidence) which is 964 lower than the classic limit frequency of the ionosondes (sounding device) 1 MHZ. During the day, very long waves (kilometers) reflect at the bottom of the D-layer. The medium waves (hectometres)reflect higher and undergo a very strong absorption. The decametric waves cross entirely this layer with a weak absorption. The collision frequencies (Ven) electron-neutral and (Vin) ion-neutral are very high, resulting in a strong absorption of HF radio waves and an attenuation of waves of frequencies lower than 1 MHz. In addition, the collision frequency of electrons with neutral particles is inversely proportional to the square of the radio wave frequency and, therefore, the attenuation is greater. This is due to the fact that radio waves of lower frequency are too attenuated when passing through the D-layer, while those of higher frequencies will not be reflected because the maximum critical frequency is exceeded. These low frequencies are generally used in marine navigation and system LORAN C.

Conclusion:-
This present work highlights the variability of the critical frequency (foD) in the D-layer during the solar cycle 22 phase minimum and maximum using the IRI model in its 2016 version. The ionization causes the difference in the foD values of the solar cycle phase and between different seasons of the year. In winter, the values of the critical frequency are lower than during the other seasons. This work does not highlight the winter anomaly. Our study also shows a peak foD at 12.00TL for all seasons. From this study, it appears that the critical frequency does not affect the radio waves when the D layer disappears.