Contribution of the International Reference Ionosphere-2016 Model in the Evidence of the Winter Anomaly

Moustapha Konaté^1,2*, Raoul Ilboudo¹, Kadidia Nonlo Drabo¹, Emmanuel Nanéma^1,3, Frédéric Ouattara¹

¹Unité de Recherche et de Formation en Sciences et Technologie (UFR-ST), Laboratoire de Recherche en Energétique et Météorologie de l’Espace (LAREME), Université Norbert Zongo, Koudougou, Burkina Faso.
²Centre Universitaire de Gaoua, Université Nazi BONI, Bobo Dioulasso, Burkina Faso.
³Centre National de la Recherche Scientifique et Technologique (CNRST), Institut de Recherche en Sciences Appliquées et Technologies (IRSAT), Ouagadougou, Burkina Faso.

DOI: 10.4236/ojapps.2022.1211120                

Abstract

This study is a contribution to the estimation of the winter anomaly in the F2 layer of the ionosphere at low latitudes. The aim is to study the variability of the virtual height (hmF2) of the F2 region of the ionosphere through the predictions of the latest International Reference Ionosphere model (IRI-2016). The present work allows analyzing the temporal evolution of hmF2 according to the different phases of three (3) solar cycles during the quiet geomagnetic activity to estimate the seasonal anomaly at the Ouagadougou station. The analysis of the seasonal profiles shows that the variability of hmF2 is: 1) strongly linked to the solar cycle activity, 2) dependent on the season and 3) variable from one cycle to the next for the same phase. It appears that hmF2 increases during the ascending phase to reach its maximum value at the phase maximum. During the descending phase, it decreases until the phase minimum where it finds its minimum value. The difference between winter and summer on the hmF2 values for each phase of the cycle is obtained at the phase minimum and is estimated to be at least 16 km. In low latitudes, solar irradiation is greater in summer than in winter. From this study, hmF2 values are larger in winter compared to summer indicating an anomaly in the virtual height of the F2 layer of the ionosphere through the predictions of IRI-2016 at the Ouagadougou station.

Keywords

Winter Anomaly, IRI, hmF2, Quiet Day, Solar Cycle Phases

1. Introduction

The ionosphere is the part of the Earth’s atmosphere where high-energy solar radiation causes the ionization of the particles present in this zone [1]. This layer, because of its particle composition and its location, is important for navigation and satellite communication. Many models exist to study the dynamics of the ionospheric layer [2] [3] [4]. Some are based mainly on purely mathematical models, others on the other hand, on in situ measurements recorded on the various stations. These models allow either to validate dynamo theories [5], or to reproduce the characteristics of the ionosphere [6] [7]. The International Reference Ionosphere (IRI) model is a semi-empirical model which allows reproducing the characteristic parameters of the ionosphere. Numerous studies have been conducted to understand the morphology of the variation of ionospheric parameters especially in low latitudes as a function of the model, solar cycles, solar cycle phases, seasons and the impact of different classes of geomagnetic activities [8] [9] [10] [11] through various data sources. The present study focuses on the IRI-2016 predictions on the virtual height parameter of the F2 layer of the ionosphere (hmF2) to analyze the seasonal variation at the Ouagadougou station (Latitude 12.5˚N and Longitude 358.5˚E) during the different phases of the solar cycle C21, C22 and C23. The objective is to compare the height of this layer between summer, which is a period characterized by high solar irradiation, and winter by low sunshine at low altitudes. It allows to contribute to the estimation of the seasonal anomaly in low latitudes, whose phenomenon has been studied by the authors [12] [13] [14] through different parameters and ionospheric data sources.

2. Materials and Methods

The methodology of this study on the variation of seasonal mean values is based on the following assumptions: 1) the quiet day is determined by the index Aa ≤ 20 nT [15]; 2) the season is characterized by its characteristic month namely: March for spring, June for summer, September for autumn, and December for winter [16]; and 3) the phases of the solar cycle are determined by the annual average of the number of spots (or Wolf number) Rz. For the minimum phase Rz < 20, the rising and falling phase 20 < Rz < 100 and the maximum phase Rz > 100 [17]. From one maximum to the next, the solar cycle lasts on average 11 years and is related to the activity of spot cycle [18]. The solar cycles 21, 22 and 23 last respectively: 1976-1985, 1985-1996 and 1996-2006. The study of the variation of the average ionospheric values [19] is carried out according to the four phases of the solar cycle (minimum, ascending, maximum and descending) [20]. From this subdivision of the solar cycle, we can determine the years that belong to each phase of the three (3) solar cycles studied. The quietest day describes the whole season and the Ouagadougou station is determined by the geographical coordinates: latitude 12.4˚N and longitude 358.5˚E. The IRI model is run under conditions (1), (2) and (3) to determine the hourly average values of the virtual height. The data is then exported to an Excel file for the calculation of mean values, variation and curve plots. Equation (1) gives the seasonal average value of the virtual height.

$hmF{2}_{mean}=\frac{{{\displaystyle \sum }}_{h=0}^{i}hmF{2}_{j,d}}{i}$ (1)

In this equation, hmF2_mean denotes the seasonal mean value of hmF2 of the considered characteristic month of the season; hmF2_j,h is the hourly mean value of the virtual height at time h for day d. Under these conditions, the index h Î [0, 24], the index d Î [1, 31], and i = 25. Equation (2) is used to calculate the annual average value of the solar cycle phase.

$hmF{2}_{mean.ann}=\frac{{{\displaystyle \sum }}_{s=0}^{n}hmF{2}_{mean}}{n}$ (2)

From Equation (2), hmF2_mean.annl denotes the annual average value of the minimum, rising, maximum or falling phase of hmF2 and n the number of years of the phase considered.

Equation (3) allows us to estimate the variation of hmF2 between winter and summer for the different phases

$\Delta hmF{2}_{mean.ann}=hmF{2}_{mean.ann}\left(winter\right)-hmF{2}_{mean.ann}\left(summer\right)$ (3)

In this equation, $\Delta hmF{2}_{mean.ann}$ is the change in height between winter and summer for a given phase. $hmF{2}_{mean.ann}\left(winter\right)$ and $hmF{2}_{mean.ann}\left(summer\right)$ are the mean hmF2 values in winter and summer.

Equation (4) estimates the change in virtual height between winter and summer as a function of the percentage of hmF2 in winter.

$\%P_{hmF2\left(winter\right)}=\frac{\Delta hmF{2}_{mean.ann}}{hmF{2}_{mean.ann}\left(winter\right)}\times 100$ (4)

$\%P_{hmF2\left(winter\right)}$ : Percentage difference between winter and summer expressed as a function of hmF2 in winter.

3. Results and Discussion

Using assumption (2), June defines summer and December defines winter. According to the principle (3) for the solar cycle: solar cycle 21: the minimum phase is 1976, the ascending phase goes from 1977 to 1979, the maximum phase is 1980 and the descending phase goes from 1981 to 1984; solar cycle 22: the minimum phase is 1985, the ascending phase goes from 1986 to 1989, the maximum phase is 1990 and the descending phase goes from 1991 to 1995 for the solar cycle; solar cycle 23: the minimum phase is 1996, the ascending phase goes from 1997 to 2000, the maximum phase is 2001 and the descending phase goes from 2002 to 2005.

Table 1 is obtained using conditions (1), (2), and (3). It shows the quietest day of summer and winter during the different phases of solar cycles 21, 22 and 23.

For each characteristic day of the season, the IRI model is run under conditions

Table 1. Distribution of the quietest days during the phases of solar cycles 21, 22 and 23.

(1) for the determination of hmF2 and the use of Equation 1 generates the daily average value of hmF2 during the characteristic year of the cycle phase ( $hmF{2}_{mean}$ ). The annual average virtual height values ( $hmF{2}_{mean.ann}$ ) are obtained using Equation (2) during the phases.

Figures 1-3 represent the variation of hmF2 during the phases of solar cycles 21, 22 and 23 in summer and winter. This study takes into account the different phases of the solar cycle and allows us to analyze the evolution of the virtual heights. From the right to the left, we have the minimum phase, the rising phase, the maximum phase, the falling phase and the minimum phase of the next cycle.

Figure 1 shows the evolution of hmF2 during the summer and winter phases of solar cycle 21. The analysis of this figure shows that hmF2 is related to the solar cycle activity. The height shows minimum values at the phase minimum namely 327 km for winter and 310 km for summer. During the rising phase, hmF2 increases to reach its maximum value at 467 km for winter and 405 km for summer at the phase maximum. Then, it decreases during the descending phase until the next phase minimum. The difference between the maximum and minimum value during cycle 21 and for the same season is 140 km for winter and 95 km for summer. Examination of the figure also indicates that there is a remarkable seasonal variation between winter and summer in hmF2. For all phases of cycle 21, the virtual height in summer is lower than in winter. The variation in virtual height between the two seasons is estimated to be: 17.02 km (5.20% winter hmF2), 58.41 km (13.92% winter hmF2), 62.43 km (13.35% winter hmF2), and 22.28 km (5.74% winter hmF2) for phases: minimum, rising, maximum, and falling, respectively.

The seasonal variations of hmF2 in summer and winter for the solar cycle 22

Figure 1. Variation of hmF2 in summer and winter according to solar cycle phases 21.

Figure 2. Variation of hmF2 in summer and winter according to solar cycle phases 22.

phases are given in Figure 2. The morphological analysis of Figure 2 shows that hmF2 is also related to solar cycle activity. For this figure, hmF2 also has minimum values at the phase minimum at 330 km for winter and 313 km for summer. The virtual height increases during the rising phase to reach its maximum value at the phase maximum at 454 km for winter and 396 km for summer and decreases during the falling phase to the next phase minimum. The difference between the maximum and minimum value is estimated at 164 km for winter and 83 km for summer. Figure 2 shows a seasonal variation on the hmF2 parameter. During the phases of cycle 22, hmF2 in summer is lower than in winter. The variation in virtual height between the two seasons for cycle 22 is estimated to be: 16.52 km (5.00% of hmF2 in winter); 56.03 km (14.14% of hmF2 in winter); 57.39 km (12.63% of hmF2 in winter) and 22.73 km (6.13% of hmF2 in winter) for the phases: minimum, rising, maximum and falling, respectively.

Figure 3 shows the evolution of hmF2 during the summer and winter phases of solar cycle 23. The examination of Figure 3 is identical to that of Figure 1 and Figure 2. The minimum values of hmF2 during the phase minimum are obtained at 324 km for winter and 307 km for summer. At the phase maximum, the values of hmF2 are 424 km for winter and 373 km for summer. The difference

Figure 3. Variation of hmF2 in summer and winter as a function of solar cycle phases 23.

Figure 4. Variation of hmF2 in summer and winter as a function of the phases of solar cycles 21, 22 and 23. (a) Variation of hmF2 at the phase minimum of solar cycles 21, 22 and 23. (b) Variation of hmF2 during the increasing phases of solar cycles 21, 22 and 23. (c) Variation of hmF2 at the maximum of the phases of solar cycles 21, 22 and 23. (d) Variation of hmF2 during the decreasing phases of solar cycles 21, 22 and 23.

between the maximum and minimum value during solar cycle 23 and for the same season is 100 km for winter and 66 km for summer. The graphical analysis in Figure 3 shows that there is a seasonal variation on the mean values of hmF2 between winter and summer. For all phases of cycle 23, it is noticed that hmF2 in summer is lower than in winter. The variation in height between the two seasons is estimated to be about 17.69 km (5.44% of hmF2 in winter) for the minimum phase; 58.78 km (14.34% of hmF2 in winter) for the rising phase; 51.24 km (12.06% of hmF2) for the phase maximum and 19.81 km (5.46% of hmF2 in winter) for the falling phase.

Figure 4 shows the seasonal variation of hmF2 for quiet geomagnetic activity during the phase minimum (Figure 4(a)), the rising phase (Figure 4(b)), the phase maximum (Figure 4(c)) and the falling phase (Figure 4(d)) of solar cycles 21, 22, 23 at the Ouagadougou station.

The analysis of Figure 4 shows that the parameter hmF2 depends on the phase of the solar cycle. During the ascending phase, the phase maximum and the descending phase of the solar cycle, the height of the F2 region of the ionosphere presents values whose minimum is above 340 km. On the other hand, during the phase minimum, the values of hmF2 are below 340 km. Thus, during a phase maximum, the height of the F2 region of the ionosphere is higher than during other phases of the solar cycle. Moreover, for the same phase, the hmF2 profiles vary from one cycle to the next. The hmF2 profiles in winter for each phase of the cycle are higher than those in summer during the three (3) cycles studied. The percentage of winter hmF2 obtained for the different phases is at least: 5%, 13.92%, 12.06% and 5.46% for the phase minimum, rising phase, phase maximum and falling phase respectively. The anomaly is more accentuated during the ascending and maximum phases.

4. Conclusion

This study makes a comparison of the height of the F2 region of the ionosphere in summer and winter during periods of quiet geomagnetic activity in low latitudes using the IRI-2016 model. The study presented hmF2 profiles in summer and winter during solar cycles 21, 22 and 23 at the Ouagadougou station. The analysis of seasonal profiles of hmF2 variability during the three solar cycles shows that hmF2 is strongly related to the solar cycle activity and varies from one cycle to the next. Similarly, examination of the profiles indicates variations as a function of the solar cycle phase. Maximum and minimum values are obtained respectively during the maximum and minimum of the solar cycle phase. Also, for the same phase, the profiles vary from one cycle to the next. The influence of the season on the height of the F2 region of the ionosphere is highlighted. The values of hmF2 in winter for each phase of the cycle are 16 km higher (5% of hmF2 in winter) than those in summer during the three (3) cycles studied. The solar radiation intensity is higher in low latitudes in summer compared to other seasons of the year and especially in winter. From this study, the electron production process is more important in winter creating a higher virtual height than in summer. This work highlights the winter anomaly in the virtual height parameter and shows that the phenomenon is more accentuated during the ascending and maximum phases for solar cycles 21, 22 and 23 through the predictions of IRI-2016.

Acknowledgements

The authors thank the International Reference Ionosphere model for making the data available and express their appreciation to the journal for publishing the article.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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