foF2 Variability Research Articles

The effect of mixing EUV proxies on the correlation with foF2 and on long-term trends estimations

Solar proxies are a valuable tool used in most atmospheric models to simulate or predict the state and variation of the different layers linked to the solar source. In this work, we analyze the performance, through correlation analysis, of six solar EUV proxies, and different combination of them, to reproduce annual ionospheric F2 critical frequency (foF2) from ten different stations. We find that certain combinations of traditional proxies outperform the capacity to reproduce interannual ionospheric foF2 variability, resulting in better statistical correlation and time stability, these new proxies could be a valuable tool to predict the state of the atmosphere and for interpolation in case of missing data. Different combinations of proxies such as F30, MgII, Sn or Lya, creates better indicators than using them alone. These new EUV proxies are also used to estimate long-term trends along the ten stations. As we increase the correlation, the estimated trend decrease as expected by definition. However, a key point is that the average trends, after filtering solar activity, result negative as was predicted by the increase of the greenhouse gases.

Dependence of long-term trends in foF2 at middle latitudes on different solar activity proxies

The most studied ionospheric parameter for long-term trends is foF2. The dominant factor of foF2 variability is the solar cycle, which is much stronger than the long-term trends. Therefore its effect in data must be removed. However, several decade long homogeneous measurements of the solar EUV fluxes are not available, so various solar activity proxies (solar activity indices) must be used. The aim of this paper is to study the impact of selection of different solar activity proxies on foF2 long-term trends and to find the best solar activity proxy for foF2 trends at middle latitudes. The results based on yearly average data of six midlatitude stations from four continents (1976–2014) and of six solar activity proxies show that the long-term trends in foF2 depend substantially on the solar activity proxy used, and the only solar proxy, which provides trends of the same sign for all stations and both sub-periods, is F30. Based on results of this paper and that of Laštovička and Burešová (2023), I can recommend F30 as the best solar proxy for studying long-term trends of foF2 at middle latitudes (at least for yearly average values).

Assessment of improvement of the IRI model for foF2 variability over three latitudes in different hemispheres during low and high solar activities

This paper discusses the diurnal and seasonal variations of the F2 layer critical frequency (foF2) and the improvement of performance of the IRI-2016 model in predicting foF2 over three latitudes in different hemispheres during low and high solar activities. We extracted the foF2 data from six ionosonde stations which are Manila (14.7oN, 121.1oE), Yamagawa (31.2oN, 130.6oE), Yakutsk (62.0oN,129.6oE), Townsville (19.6oS, 146.8oE), Hobart (42.9oS, 147.3oE) and Terre Adelie (66.6oS, 140.0oE). The data of both low solar activity (LSA) period and high solar activity (HSA) periods were divided into three seasons as Northern Summer (May, June, July and August), Equinoxes (March, April, September and October) and Northern Winter (November, December, January and February). The present study showed that the IRI-2016 performance is strongly dependent on the solar activity, latitude, season, local time and hemisphere. For both hemispheres, the foF2 values at low latitude station are larger than those at middle latitude station, whereas the foF2 values at middle latitude station are larger than those at high latitude station. The agreement between IRI2016-modelled foF2 and foF2 measurements on all stations selected in the northern hemisphere is best for North Summer and worst for North Winter. For northern hemisphere, the values of relative deviations during both solar activities are largest in high latitudes and smallest in middle latitudes. As for southern hemisphere, the values of relative deviations during LSA are largest in middle latitudes and smallest in high latitudes, whereas the values of relative deviations during HSA are largest in low latitudes and smallest in high latitudes. It is thought that the relative deviations in the observed foF2 values are caused by solar activity that strongly alter chemical and electromagnetic processes in the ionosphere. These results are important for future improvements depending on solar activity and seasons in the IRI model for foF2 values over three latitudes in different hemispheres.

FoF2 variability at a southern low-latitude station and the performance of IRI-2016 model during ascending phase of solar cycle-24

In this research, we have analyzed the data of the critical frequency of the F2 region (foF2) ionosphere over the southern low-latitude station COCO (Keeling) Island (Geog. Lat. 12.20°S; Geomag. Lat. 22.83°S; Geog. Long. 96.80°E) for the period 2009–2013 and thereof have compared the results with those obtained with the IRI-2016 model. Our analysis shows a good correlation between variations in foF2 and solar flux at an F10.7 cm wavelength. With regard to geomagnetic activity during 2009–2013, it has been found that the variability in the monthly mean Ap index remained below 20 nT throughout the period 2009–2013. Concerning quiet time F2 region variability, our results explicated solar cycle, semiannual, and seasonal/annual variations in foF2. Similar oscillations in foF2 have been explored using the Lomb-Scargle periodogram technique. We have also observed the fundamental mode (27 days) and its overtones (9- and 13-day) in the geomagnetic activity parameter (Ap). Analysis of foF2 showed the consistent presence of the semiannual anomaly and absence of the winter anomaly during the ascending phase of solar cycle-24. The presence of an annual component in normalized foF2 has been examined using regression analysis. A comparative study of ionosonde-observed foF2 with IRI-2016-modeled foF2 showed that the general ionospheric trends in IRI predictions are consistent with the observations for the diurnal, seasonal, and solar cycle variations. However, a deviation in the amplitude of foF2 up to the order of 5 MHz depending on local time, seasons, and phases of the solar cycle, respectively, has been observed.

Seismoionospheric anomalies associated with earthquakes from the analysis of the ionosonde data

Previous researchers have reported the earthquakes (EQs) induced ionospheric abnormalities, such as electron density depletion and/or enhancement, which they claimed were related to the impending EQs. In this paper, the peak plasma frequency of the ionosonde data from the Islamabad station was studied related to the July 24, 2015 (M5.1), October 26, 2015 (M7.5) and April 25, 2015 (M7.8) EQs in Pakistan and Nepal, respectively. The baseline of the running 30-day median (commonly used in other studies) was deployed on the foF2 data to detect the abnormality in the long term data before the EQs. A detailed analysis of the foF2 variability in a month before all the EQs of this study is then undertaken, where two out of three EQs showed simultaneous deviation in peak plasma frequency relative to the confidence intervals. There is no percentage deviation in foF2 data for the days associated with M7.8, proved no ionospheric signatures in the peak plasma frequency. We believe that the reason is the distance between the epicenter of M7.8 and the ionosonde station, which was approximately 12° longitudinal extension. On the other hand, the negative deviation beyond the lower confidence interval in critical frequency was detected for the mega thrust of M5.1. Similarly, positive foF2 anomalies were recorded within 10 days before the main shock of M7.5, where there was no high intensity geomagnetic storm. These results suggested that in order to detect reliable seismoionospheric precursor, the ionosonde station must operate within the range of seismic breeding zone.

COMPARISON OF DIURNAL AND SEASONAL VARIABILITY OF THE CRITICAL FREQUENCY OF THE F2 LAYER OVER ILORIN, JICAMARCA AND OKINAWA DURING MINIMUM SOLAR ACTIVITY

The ionosphere displays a wide range of variations ranging from diurnal, seasonal, annual and solar cycle variation. In this paper we present a study of the comparison of diurnal, seasonal and semiannual variation of the variability (VR) of the critical frequency of the F2 layer (foF2) over Ilorin (Lat. 8.47oN, 4.6oE, dip 4.1oS) in the African sector, Jicamarca (Lat. 11.9oS, Long.76.8oW, dip 1oN) in the American sector and Okinawa (26.3oN, 127.8oE, dip 36.8oN) in the Asian sector during solar minimum period. These stations lie within the equatorial anomaly region of the ionosphere. Diurnal analysis revealed that the critical frequency of the F2 layer is more prone to variability (VR) during the nighttime than the day time at these stations, with two characteristics peaks, post-midnight peak and pre-midnight peak. The peaks at Ilorin (20 - 43%; 16 - 25%) are observed to be higher in values than those at Jicamarca (17 - 27%; 15 - 22%) and Okinawa (22 – 39%; 15 - 30%). Seasonally, December solstice maximum was noticed at Ilorin. Semiannual analysis showed that foF2 VR is highest at Ilorin station, followed by Okinawa station and least by Jicamarca station during the nighttime. Diurnal curves of Mean foF2 revealed pre-noon and post-noon peaks at all stations, with Jicamarca having highest values for both peaks while the semiannual curves of mean foF2 values showed almost equal values for all stations. Overall, foF2 variability is highest at Ilorin and lowest at Jicamarca while for F2 layer ionization the reverse is the case.

Upgrading CCIR's [formula omitted] maps using available ionosondes and genetic algorithms

We have developed a new approach towards a new database of the ionospheric parameter foF2. This parameter, being the frequency of the maximum of the ionospheric electronic density profile and its main modeller, is of great interest not only in atmospheric studies but also in the realm of radio propagation. The current databases, generated by CCIR (Committee Consultative for Ionospheric Radiowave propagation) and URSI (International Union of Radio Science), and used by the IRI (International Reference Ionosphere) model, are based on Fourier expansions and have been built in the 60s from the available ionosondes at that time. The main goal of this work is to upgrade the databases by using new available ionosonde data. To this end we used the IRI diurnal/spherical expansions to represent the foF2 variability, and computed its coefficients by means of a genetic algorithm (GA). In order to test the performance of the proposed methodology, we applied it to the South American region with data obtained by RAPEAS (Red Argentina para el Estudio de la Atmósfera Superior, i.e. Argentine Network for the Study of the Upper Atmosphere) during the years 1958–2009. The new GA coefficients provide a global better fit of the IRI model to the observed foF2 than the CCIR coefficients. Since the same formulae and the same number of coefficients were used, the overall integrity of IRI’s typical ionospheric feature representation was preserved. The best improvements with respect to CCIR are obtained at low solar activities, at large (in absolute value) modip latitudes, and at night-time. The new method is flexible in the sense that can be applied either globally or regionally. It is also very easy to recompute the coefficients when new data is available. The computation of a third set of coefficients corresponding to days of medium solar activity in order to avoid the interpolation between low and high activities is suggested. The same procedure as for foF2 can be perfomed to obtain the ionospheric parameter M(3000)F2.

A comparison of neural network-based predictions of foF2 with the IRI-2012 model at conjugate points in Southeast Asia

This paper presents the development of Neural Network (NN) model for the prediction of the F2 layer critical frequency (foF2) at three ionosonde stations near the magnetic equator of Southeast Asia. Two of these stations including Chiang Mai (18.76°N, 98.93°E, dip angle 12.7°N) and Kototabang (0.2°S, 100.3°E, dip angle 10.1°S) are at the conjugate points while Chumphon (10.72°N, 99.37°E, dip angle 3.0°N) station is near the equator. To produce the model, the feed forward network with backpropagation algorithm is applied. The NN is trained with the daily hourly values of foF2 during 2004–2012, except 2009, and the selected input parameters, which affect the foF2 variability, include day number (DN), hour number (HR), solar zenith angle (C), geographic latitude (θ), magnetic inclination (I), magnetic declination (D) and angle of meridian (M) relative to the sub-solar point, the 7-day mean of F10.7 (F10.7_7), the 81-day mean of SSN (SSN_81) and the 2-day mean of Ap (Ap_2). The foF2 data of 2009 and 2013 are then used for testing the NN model during the foF2 interpolation and extrapolation, respectively. To examine the performance of the proposed NN, the root mean square error (RMSE) of the observed foF2, the proposed NN model and the IRI-2012 (CCIR and URSI options) model are compared. In general, the results show the same trends in foF2 variation between the models (NN and IRI-2012) and the observations in that they are higher during the day and lower at night. Besides, the results demonstrate that the proposed NN model can predict the foF2 values more closely during daytime than during nighttime as supported by the lower RMSE values during daytime (0.5≤RMSE≤1.0 for Chumphon and Kototabang, 0.7≤RMSE≤1.2 at Chiang Mai) and with the highest levels during nighttime (0.8≤RMSE≤1.5 for Chumphon and Kototabang, 1.2≤RMSE≤2.0 at Chiang Mai). Furthermore, the NN model predicts the foF2 values more accurately than the IRI model at the three sites on average, as clearly seen on the yearly RMSE averages. The RMSE values of NN model are lower than those of both CCIR and URSI options, and in terms of the yearly percentage improvements, the NN model gives improvement of around 10–15% in 2009 and 10% in 2013 for Chiang Mai, 20–25% in 2009 and 5–10% in 2013 for Chumphon, and around 18–25% in 2009 and 20–30% in 2013 at Kototabang. Although the NN model predicts the foF2 values closely to the observed data and produces more accurate prediction than the IRI models, in some cases, the IRI model performs better than the NN model. Hence, there is still room for further improvement of the proposed NN model.

On the Variability of the Ionospheric F2-Layer During the Quietest Days in December 2009

December 2009 was one of the quietest (monthly Ap=2) months over the last eight decades. It provided an excellent opportunity to study the day-to-day variability of the F2 layer with the smallest contribution due to geomagnetic activity. With this aim, we analyze hourly values of the F2-layer critical frequency (foF2) recorded at 18 ionosonde stations during the magnetically quietest (Ap=0) days of the month. The foF2 variability is quantified as the relative standard deviation of foF2 about the mean of all the “zero-Ap” days of December 2009. This case study may contribute to a more clear vision of the F2-layer variability caused by sources not linked to geomagnetic activity. In accord with previous studies, we find that there is considerable “zero-Ap” variability of foF2 all over the world. At most locations, foF2 variability is presumably affected by the passage of the solar terminator. The patterns of foF2 variability are different at different stations. Possible causes of the observed diurnal foF2 variability may be related to “meteorological” disturbances transmitted from the lower atmosphere or/and effects of the intrinsic turbulence of the ionosphere-atmosphere system.

GNSS derived TEC data ingestion into IRI 2012

Experimental vertical total electron content (VTEC) data given by Global Ionospheric Maps (GIM) has been ingested into the IRI version 2012, aiming to obtain grids of effective input parameter values that allow to minimize the difference between the experimental and modeled vertical TEC.Making use of the experience gained with the technique of model adaptation applied to NeQuick (Nava et al., 2005), it has been found possible to compute IRI world grids of effective ionosphere index parameters (IG). The IG grids thus obtained can be interpolated in space and time to calculate with IRI the 3D electron density at any location and also the TEC along any ground-to-satellite ray-path for a given epoch. In this study, the ingestion technique is presented and a posteriori validation, along with an assessment of the capability of the ‘ingested’ IRI to reproduce the ionosphere day-to-day foF2 variability during disturbed and quiet periods. The foF2 values retrieved are compared with data from about 20 worldwide ionosondes for selected periods of high (year 2000) and moderate to low solar activity (year 2006). It was found that the use of the ingestion scheme enhances the performance of the model when compared with its standard use based on solar activity drivers (R12 and F10.7), especially for high solar activity. As an example, the mean and standard deviation of the differences between experimental and reconstructed F2-peak values for April of year 2000 is 0.09 and 1.28MHz for ingested IRI, compared to −0.81 and 1.27MHz (IRI with R12 input) and −0.02 and 1.46MHz (IRI with F10.7 input).

Ionospheric foF2 morphology and response of F2 layer height over Jicamarca during different solar epochs and comparison with IRI-2012 model

Diurnal, seasonal and annual foF2 variability and the response of the F2-layer height over Jicamarca (11.9 °S, 76.8 °W, 1 °N dip) during periods of low (LSA), moderate (MSA) and high (HSA) solar activities was investigated. The relative standard deviation (V R ) was used for the analysis. The F2-layer critical frequency pre-noon peak increases by a factor of 2 more than the post-noon peak as the solar activity increases. The variability coefficient (V R ) is lowest during the day (7–16%) for the three solar epochs; increases during nighttime (20–26%, 14–26%, and 10–20%, respectively for the LSA, MSA and HSA years); and attained highest magnitude during sunrise (21–27%, 24–27%, and 19–30%, respectively in similar order). Two major peaks were observed in V R – the pre-sunrise peak, which is higher, and the post-sunset peak. Generally, the variability increases as the solar activity decreases. Annually, V R peaks within 23–24%, 19–24% and 15–24% for the LSA, MSA, and HSA periods, respectively. The ionospheric F2-layer height rises to the higher level with increasing solar activity. The foF2 comparison results revealed that Jicamarca is well represented on the IRI-2012 model, with an improvement on the URSI option. The importance of vertical plasma drift and photochemistry in the F2-layer was emphasized.

GPS derived TEC and foF2 variability at an equatorial station and the performance of IRI-model

The ionosphere induces a time delay in transionospheric radio signals such as the Global Positioning System (GPS) signal. The Total Electron Content (TEC) is a key parameter in the mitigation of ionospheric effects on transionospheric signals. The delay in GPS signal induced by the ionosphere is proportional to TEC along the path from the GPS satellite to a receiver. The diurnal monthly and seasonal variations of ionospheric electron content were studied during the year 2010, a year of extreme solar minimum (F10.7=81 solar flux unit), with data from the GPS receiver and the Digisonde Portable Sounder (DPS) collocated at Ilorin (Geog. Lat. 8.50°N, Long. 4.50°E, dip −7.9°). The diurnal monthly variation shows steady increases in TEC and F2-layer critical frequency (foF2) from pre-dawn minimum to afternoon maximum and then decreases after sunset. TEC show significant seasonal variation during the daytime between 0900 and 1900UT (LT=UT+1h) with a maximum during the March equinox (about 35 TECU) and minimum during the June solstice (about 24 TECU). The GPS-TEC and foF2 values reveal a weak seasonal anomaly and equinoctial asymmetry during the daytime. The variations observed find their explanations in the amount of solar radiation and neutral gas composition. The measured TEC and foF2 values were compared with last two versions of the International Reference Ionosphere (IRI-2007 and IRI-2012) model predictions using the NeQuick and CCIR (International Radio Consultative Committee) options respectively in the model. In general, the two models give foF2 close to the experimental values, whereas significant discrepancies are found in the predictions of TEC from the models especially during the daytime. The error in height dependent thickness parameter, daytime underestimation of equatorial drift and contributions of electrons from altitudes above 2000km have been suggested as the possible causes.

FoF2 Diurnal Variability at African Equatorial Stations: Dip Equator Secular Displacement Effect

The paper goal is to analyze the variability of foF2 at African equatorial stations and the effect of dip angle on this variability. The gap between the dip angle of Dakar and Ouagadougou is superior to that between Djibouti and Ouagadougou. The trend of the dip angle at Ouagadougou and Dakar decreases while that of Djibouti increases. The relative position of the station with respect to the equator and the trend sign explains the difference observed in foF2 variability at Dakar station and at the two other stations. At Djibouti and Ouagadougou, foF2 exhibits noon bite out profile during all solar cycle phases while at Dakar observed profile is dome or plateau during the maximum and the predominance afternoon peak for the other solar cycle phases.

Does the Equatorial Ionospheric Peak Electron Density Really Record the Lowest During the Recent Deep Solar Minimum?

AbstractThe solar activity in 2008–2009, located at the minimum phase of solar cycle 23/24, was unusually low, which attracts ionospheric physics scientists to explore the plasma behaviors in the ionosphere during the deep solar minimum. Some investigators reported a reduction in ionospheric electron density during this period, while others found a marginal and minor change. In this study we collected the critical frequency of the F2‐layer ionosphere (foF2) data retrieved from ionogram records observed by an ionosonde at Jicamarca (12°S, 76.9°W), an American station located near the dip equator, to examine the changes in electron density in equatorial regions and clarify what caused the inconsistence between the differences of solar minimum foF2 in published work. We determined the foF2 differences in moving yearly, seasonal and monthly medians and the Fourier series analysis. The picture of ionospheric changes is found to be related with the data analysis methods used. The seasonal median and moving yearly values of foF2 were smaller in 2008–2009 than in 1996–1997, both in the daytime and nighttime. In contrast, the monthly median foF2 is found to be of varying solar minimum‐to‐minimum differences. Although greater values in the last minimum are prevailing in the monthly median case at most local times, opposite changes are also found at some time intervals. Further analysis reveals that the reduction in foF2 during the current solar cycle minimum is certainly presented in the longer time scale variations of foF2. Therefore, the inconsistent changes in the published investigations reflect the impact of foF2 variability on the solar minimum‐to‐minimum difference over different time scales.

Day to day variability of h ′F and foF2 during some solar cycle epochs

Comparison of the diurnal and seasonal changes of the variability (VR) of the virtual height of reflection (h′F) and those of critical frequency of the F layer (foF2) is carried out at Ibadan (7.4°N, 3.9°E, 6°S dip) in the African sector. The effect of latitude on both characteristics is investigated by combining data from Singapore (1.3°N,103.8°E, 17.6°S dip) in the East Asian sector and Slough (51.5°N, 359.4°E, 66.5°S dip) in the European sector. The variability of foF2 is found greater than that of h′F except during high solar activity when night-time h′F VR is about the same as night-time foF2 VR possibly due to high post-sunset rise in F region heights at this epoch. Both characteristics have pre-midnight and post-midnight peaks with the latter being the greater one at all epochs with the exception of all the seasons of 1958 but June Solstice, very likely because of the same greater F region height during maximum solar activity. Night-time foF2 VR is greater in September Equinox and June Solstice at all epochs while night-time h′F VR is greater in June Solstice during the three epochs, on a general note. No seasonal trend is observed in the daytime variability of both characteristics during the three epochs except for h′F VR of December Solstice during high solar activity, which is greater than those of other seasons. VR of both parameters are found to increase and decrease alternately with Zurich sunspot number (Rz) at Ibadan and Singapore throughout the day. VR of foF2 at Slough is found to increase from midnight to 1300 h after which it alternates. Generally, post-midnight VR of both characteristics are greater than those of pre-midnight at low latitudes during all epochs while daytime VR are the least. At the mid-latitude station of Slough, foF2 VR is about the same throughout the hours of the day during 1968 and 1971, its post-midnight value is however greater than those of pre-midnight and daytime during 1970. Daytime h′F VR is greater than post- and pre-midnight values during 1968, but about the same throughout the hours of the day during 1971.

Anomalous enhancement of ionospheric F2 layer critical frequency and total electron content over low latitudes before three recent major earthquakes in China

This paper reports unusual variations in the ionospheric total electron content (TEC) and the critical frequency of the F2 layer (foF2) a few days before the main shock of three major earthquakes (M > 6). Epicenters of these earthquakes are distributed in China. Ionospheric data, recorded by an Indian network of ionosonde and GPS receivers at Delhi (28.6°N, 77.2°E), Bhopal (23.29°N, 77.46°E), and Trivandrum (8.4°N, 76.6°E) are analyzed to find seismoionospheric signatures of three recent major earthquakes in China. The results clearly indicate large enhancements in foF2 and TEC on geomagnetic quiet days, observed mainly at a low‐latitude station in Delhi, which is nearest to the earthquake epicenters. The anomalous behavior of foF2 and TEC appeared 1–4 days before the main shock and especially during 1100–1700 LT. However, the ionospheric foF2 and TEC variability at the equatorial station Trivandrum and equatorial anomaly station Bhopal did not show any significant changes, thereby indicating the localized nature of such unusual ionospheric variations. The observed results suggest that the unusual enhancements of ionospheric foF2 and TEC over Delhi a few days before the main shock of each earthquake are most likely due to a seismoionospheric link.

Ionospheric foF2 variability over the Southeast Asian sector

This paper presents the diurnal, seasonal, and solar activity impacts on the variability of ionospheric foF2 over the Southeast Asian sector. Data from three ionosonde stations, namely, Manila (14.7°N, 121.1°E, dip 14.7°N), Okinawa (26.3°N, 127.8°E, dip 36.8°N), and Vanimo (2.7°S, 141.3°E, dip 22.5°S), were used for the investigation via the standard deviation approach. The diurnal analysis revealed that foF2 is more susceptible to variability during the nighttime than the daytime. The observed variability is characterized by two peaks: a postsunset peak (15%–50%; corresponding to 3–14 MHz for the mean foF2 values) and a postmidnight peak (18%–68%; corresponding to 2–9 MHz for the mean foF2 values). Seasonally, the trend of the foF2 variability maximum does not follow any typical pattern (no particular season has a leading edge over others) for all levels of solar activity. On average the foF2 variability maximum was observed at the Vanimo station during low‐ and high‐solar‐activity years (1986 and 1989, respectively). During the moderate‐solar‐activity year (1983), the foF2 variability maximum was observed at the Manila station. With the exception of the Okinawa data, where little or no difference in the foF2 variability peaks during the low‐ and moderate‐solar‐activity years was noticed, foF2 variability increases with decreasing solar activity at all stations.

Variability of foF2 in the African equatorial ionosphere

This paper presents the impact of diurnal, seasonal and solar activity effects on the variability of ionospheric foF2 in the African equatorial latitude. Three African ionospheric stations; Dakar (14.8°N, 17.4°W, dip: 11.4°N), Ouagadougou (12.4°N, 1.5°W, dip: 2.8°N) and Djibouti (11.5°N, 42.8°E, dip: 7.2°N) were considered for the investigation. The overall aim is to provide African inputs that will be of assistance at improving existing forecasting models. The diurnal analysis revealed that the ionospheric critical frequency (foF2) is more susceptible to variability during the night-time than the day-time, with two peaks in the range; 18–38% during post-sunset hours and 35–55% during post-midnight hours. The seasonal and solar activity analyses showed a post-sunset September Equinox maximum and June Solstice maximum of foF2 variability in all the stations for all seasons. At all the stations, foF2 variability was high for low solar activity year. Overall, we concluded that equatorial foF2 variability increases with decreasing solar activity during night-time.

The response of African equatorial foF2 to geomagnetic storms: Comparison between observations and IRI-2007 predictions

This study examines the response of the African equatorial ionospheric foF2 to different levels of geomagnetic storms, using the foF2 hourly data for the year 1989 from Ouagadougou (12.4°N, 1.5°W, dip: 2.8°N). The study also compares the observed data for the selected storm periods with the latest IRI model (IRI-2007). The foF2 values (both observed and predicted) show typical features of daytime peak and post-midnight minimum peak. The response of the ionospheric foF2 over Ouagadougou to storms events, during the night-time and post-midnight hours indicates negative responses of the ionospheric foF2, while that of the daytime hours indicates positive responses. For the investigation on the variability of the observed foF2 with respect to IRI-2007 model, with the exception of the analysis of the 20–22, October, 1989 data, where a midday peak was also observed on the first day, this study reveals two characteristic daily foF2 variability peaks: post-midnight and evening peaks. In addition, for all the geomagnetic storms considered, the URSI and CCIR coefficients of the IRI-2007 model show reasonable correspondence with each other, except for some few discrepancies. For instance, the event of 28–30 August, 1989 shows comparatively higher variability for the URSI coefficient, and at the foF2 peak values, the event of 20–22 October, 1989 shows that the CCIR coefficient is more susceptible to foF2 variability than the URSI coefficient. This study is aimed at providing African inputs for the future improvement of the IRI model.

Regular changes in the critical frequency of the F2 layer of the quiet midlatitude ionosphere

A method for constructing the empirical model of the F2 layer critical frequency (foF2) under magnetically quiet conditions, aimed at analyzing disturbances of any nature, is proposed. This method has been analyzed, and typical features of regular changes in foF2 of the quiet ionosphere and day-to-day foF2 variability are analyzed using the data from Irkutsk and Slough stations as an example. In particular, it has been obtained that this model differs from the international IRI model, and this difference is mainly caused by the fact that the foF2 values in the IRI model do not correspond to quiet conditions. Therefore, this model gives a larger amplitude of the annual and semiannual variations in foF2 than the IRI model. In addition, this model more accurately reproduces the rate of foF2 annual variations at a fixed local time, especially in equinoxes, when foF2 variations can exceed 1 MHz within one month.