J.O. Adeniyi

EQUATORIAL F2-PEAK PARAMETERS,IN THE IRI MODEL

We have used measurements of an ionosonde station near the magnetic equator in Ouagadougou, Burkina Faso to evaluate the ability of the International Reference Ionosphere (IRI) model to correctly represent ionospheric F2 peak parameters in this region. The data represent conditions of high and low solar activity. Comparing the URSI and CCIR option for the F2 plasma frequency, foF2, we find that for low solar activity both options agree quite well with the ionosonde foF2 values and overall the CCIR maps show a slightly better fit. During high solar activity discrepancies are found during nighttime and the URSI maps providing the overall better fit. The measured F2 peak height values, hmF2, are compared on one hand with IRI predictions that are obtained based on the CCIR model for the propagation factor M(3OOO)F2 and on the other hand with the ionosonde-measured M(3OOO)F2 values. As expected using the measured values results in more accurate predictions. It is important to note that with the measured M(3OOO)F2 values IRI predicts the characteristic post-sunset that is seen in the measurements but not in the IRI predictions with the CCIR-M(3OOO)F2 model. 0 2003 COSPAR. Published by Elsevier Science Ltd. All

New B0 and B1 models for IRI

Abstract The electron density profile in the F region bottomside is described in the International Reference Ionosphere (IRI) by two parameters: a thickness parameter B 0 and a shape parameter B 1. The models used for B 0 and B 1 in IRI are based on ionosonde data from magnetic mid-latitude stations. Comparisons with ionosonde data from several stations close to the magnetic equator show large discrepancies between the model and the data. We propose new models for B 0 and B 1 based on data from several ionosondes including low and mid latitude stations. Close to the magnetic dip equator the new B 0 model provides an improvement over the current IRI model by a factor of up to 1.5.

EQUATORIAL F2-LAYER PEAK HEIGHT AND CORRELATION WITH VERTICAL ION DRIFT AND M(3000)F2

Ionosonde data recorded at Korhogo, Ce Longitude - 5.4, Dip -0.67) during a year of low solar activity (1995) were used to investigate ways of improving the representation of equatorial F2 peak height (hmF2) in the International Reference Ionosphere (IRI). For this purpose we have studied the correlation between hmF2 and the equatorial F region vertical drift as given by the model of Scherliess and Fejer (1999). The positive correlation found during nighttime could be helpful in representing the post-sunset peak of hmF2 that is currently not represented by IRI. We have also investigated the reliability of the CCIR model for the propagation factor M(3OOO)F2 model since the IRI hmF2 model is based on the strong anti-correlation between hmF2 and M(3OOO)F2. Overall the CCIR model represents the diurnal variation of M(3OOO)F2 quite well but does not represent small-scale features. With the M(3OOO)F2 values deduced from the Korhogo ionograms as input, the IRI hmF2 model provides an excellent representation of the observed diurnal structure including the post-sunset peak.

Magnetic storm effects on the morphology of the equatorial F2-layer

Abstract Magnetic storm effects on the F2-Layer at Ibadan (lat. 7.4°N, long. 3.9°E, magnetic dip 6°S) were observed over a solar cycle period (1956–1966). During the period of high solar activity day-time decreases in NmF2 occur during the initial phase of storms. The main and early part of the recovery phases of storms cause either increases or no significant changes in the build-up interval (0500–0900 h L.T.) of the F2 layer usually, increases being most frequent during the March equinox season. This same part of the storm causes increases in NmF2 in the day-time (0900–1800 h L.T.) and night-time (1800-0800 h L.T.) intervals usually, at all seasons of the year. Similar effects occur during the period of low solar activity, but on a reduced scale.

Diurnal variation of ionospheric profile parameters B0 and B1 for an equatorial station at low solar activity

Abstract Parameters B 0 and B 1 are compared with those of the International Reference Ionosphere (IRI), using the Gulyaeva option. The data used are those of Ouagadougou, Burkina Faso, (latitude 12.4°N, longitude 1.5°W, dip 5.9°N) for 1994, a year of low solar activity. Night-time and early daytime B 0 values agree well with those of the IRI. The average value of B 0 for night-time is 73. Major differences occur during the daytime (09:00–18:00 LT) and these are dependent on season. Daytime B 0 shows a diurnal variation which depends on the solar zenith angle χ. The cos(χ) exponents were found to be 0.45, 0.67, 0.56, and 0.75 for winter (January), spring (April), summer (July), and autumn (October), respectively. For B 1 , IRI assumes a constant value of 3 most of the time and this appears to overestimate the experimental B 1 values. The experimental values of B 1 are fairly constant during daytime hours (08:00–15:00 LT), with an average value of 1.7. They are between 2.6 to 3.8 during the evening and night-time period up to 07:00 h.

Variation of bottomside profile parameters B0 and B1 at high solar activity for an equatorial station

Abstract Diurnal and seasonal variations in experimental profile parameters B0 and B1 are examined, at high solar activity, for Ouagadougou, Burkina Faso, an equatorial station in Africa (latitude 12.4°N, longitude 1.5°W, dip 5.9°N). The diurnal variations for B0 indicate a solar zenith angle dependence that can be described as B0 = A cosn (χ) with n = 0.40, 0.51, 0.46, and 0.71 for Winter, Spring, Summer and Autumn, respectively. The seasonal effect is most pronounced from 10:00–18:00 LT. Within this period, B0 is highest in Spring and lowest in Winter. A comparison of experimental B0 with the IRI B0 shows that the greatest discrepancy occur from about 11:00 to about 18:00 LT. Both are closer during most of the night time hours and in the early hours of the daytime. There is no obvious solar zenith angle dependence in B1. The range of variation for all seasons, during the day time (10:00–18:00 LT) is 0.8. For the remaining part of the 24 h period, the range is about 1.8. Generally, there is an overestimation of B1 by the IRI model.

Comparing the F2-layer model of IRI with observations at Ibadan

Abstract NmF2 and hmF2 of the IRI-90 model were compared with experimental data at a typical equatorial station, for high and low solar activity. For NmF2 agreement was quite generally found during 05 to 09 LT and for the other hours of the day during June solstice at low solar activity. For high activity good agreement occurs from 05 to 18 LT in the December solstice. Deviations were found at other seasons. As for hmF2 IRI gives larger values during the day at low solar activity while agreement is good in high activity. The post sunset peak that is normally seen in equatorial hmF2 is not shown in the IRI at any solar epoch.

Large‐scale ionospheric irregularities occurrence at Ilorin, Nigeria

Ionospheric irregularities are a regular occurrence at the equatorial latitude during the postsunset hours especially during high-solar activity. These irregularities could pose serious challenges to satellite-based navigation and positioning applications by causing fading and degradation of transionospheric signals passing through these irregularities. We have investigated large-scale ionospheric irregularity occurrence at Ilorin, Nigeria (latitude = 8.48°N, longitude = 4.67°W, dip = 4.1°S), a station located within the equatorial region in the African sector. The index used in this study is the rate of change of total electron content (rate of change) derived from 30 s receiver-independent exchange data obtained using a dual frequency GPS receiver (i.e., NovAtel GPStation-2). The study covers a period of 4 years (2009–2012). The results obtained showed that large-scale irregularities occur between March and November and are more pronounced between 1900 LT and 2400 LT. The irregularities were observed to show two peaks: one in March and the other in September. Solar activity trend was also observed. The irregularity level around the peaks seems to increase with solar activity. Although the study covered a period of 4 years, the period could be regarded as the increasing phase of the solar cycle 24.

Experimental equatorial ionospheric profiles and IRI model profiles

Abstract Ionospheric profiles below the F2 peak ionisation density are compared with those of the International Reference Ionosphere (IRI). The data used are those of Ibadan (latitude 7.4 ° N, longitude 3.9 ° E). The results show that, at low solar activity, the greatest difference between the model and experimental observations occurred in winter and in the September equinox, when the IRI model gives a thinner bottomside than the observations show. During the summer and March equinox of this solar epoch, the major difference occurs only around the F1 region, where the model gives a lower electron density than is observed experimentally. The difference in summer is not as great as that observed during the March equinox. At high solar activity, the model is close to the observed profile from the F2 peak down to a height around the point where the electron density is half of the F2 peak density (h 0.5). From h 0.5 to the height of the E layer peak, the IRI model gives a lower electron density than is observed experimentally. The discrepancy is smallest during the March equinox season. Some suggestions are made for the improvement of the model.

The equatorial electrojet and the profile parameters B0 and B1 around midday

This study presents the results of the comparison of B0, B1 and hmF2 with ΔH. B0 and B1 are parameters used in the international reference ionosphere model for the calculation of the F region bottom side profiles. The parameter ΔH obtained from the magnetic data recorded during the International Equatorial Electrojet Year (IEEY) in West Africa is used to describe the strength of the equatorial electrojet. This work covers the years 1993 and 1994, two years of low and moderate solar activity. The result shows that the electric field (E), which drives the equatorial electrojet, plays a major role in the variation of the thickness and the height of the F2 layer. However, the variation of the shape of the bottomside F2 is not sensitive to the electric field.