Information about the altitudinal distribution of ionospheric electron density (Ne) in the African equatorial region is very scanty. Such information is vital in modeling of key ionospheric parameters and in solving problems associated with ionospheric effect on the applications that depend on space-based positioning and navigation systems. For the first time in the region, the entire Ne profile estimated by IRI is evaluated with profile deduced from Digisonde measurements at Ilorin (geog. 8.50°N, 4.50°E; dip. −7.9°) during low solar activity condition. Results obtained indicate marked discrepancies between the two profiles with magnitudes revealing local time and altitudinal and seasonal trends. The largest discrepancies are found between the post-sunset and sunrise hours regardless of the seasons. IRI underestimates the Ne around the F1 region and is most obvious in September equinox and December solstice with deviation up to −42%. Further, IRI overestimates the Ne around the F2 peak at daytime, mostly in solstices with maximum deviation of 26% at F2 peak. All the three IRI topside models generally underestimate Ne between ~F2 peak height and ~800 km in all seasons except in June solstice. Beyond 800 km, the Digisonde, IRI-NeQuick, and IRI-2001 Corr profiles tend to converge, whereas the IRI-2001 profiles still show marked divergence from the Digisonde profile in all seasons. Overall, the IRI-NeQuick gives the best topside representation at this station at daytime/nighttime. The comparative analysis of Digisonde-TEC and GPS-TEC suggests that the Digisonde alone may not give a reliable estimate of the nighttime topside altitudinal distribution of Ne at this location.

This paper compares the quiet time variation of the Total Electron Content (TEC) over four stations located at high and mid latitudes in the northern and southern hemispheres of the African-European longitudes. Five years Global Positioning System (GPS) data, from 2002 to 2006, representing the periods of high to low solar activities were used for the study. Generally, the maximum diurnal values of TEC are observed between 10:00 – 14:00 LT in all the stations during the periods investigated. The minimum values of TEC are observed during the pre-sunrise hours for the two mid latitude stations and around the pre-midnight/post-midnight for the high latitude stations. The maximum values of TEC, however vary with season, latitude and solar activity in all the stations. The values decrease with increase in latitudes and decrease in solar activity. The values range between 10 – 32 and 11 – 50 TECU respectively, for high and mid latitudes for all the years considered. Seasonally, the highest values of TEC are generally observed during the equinoxes in all the stations except at the southern mid latitude station where it can as well be observed in summer, particularly during the Moderate Solar Activity (MSA) and Low Solar Activity (LSA) periods. The lowest values of TEC are observed in winter in all the stations in the southern hemisphere and can be observed in both winter and summer for stations in the northern hemisphere depending on the latitude and solar activity period. TEC variation also exhibits (1) asymmetry in the equinoctial values in all the stations and the magnitude is most pronounced during the period of High Solar Activity (HSA); (2) winter ionosphere anomaly feature, observed only in the northern hemisphere stations; and (3) daytime minimum and nighttime maximum in the diurnal structures of TEC at high latitude in the northern hemisphere during the winter. The nighttime maximum value was observed around 21:00 LT with magnitude that decreases with decrease in solar activity. The annual maximum value of TEC decreases with solar activity at all the stations, with the highest/lowest peak observed in HSA/LSA periods.