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[1] Accurate ionospheric specification is necessary for improving human activities such as radar detection, navigation, and Earth observation. This is of particular importance in Africa, where strong plasma density gradients exist due to the equatorial ionization anomaly. In this paper the accuracy of three-dimensional ionospheric images is assessed over a 2 week test period (2-16 December 2012). These images are produced using differential Global Positioning System (GPS) slant total electron content observations and a time-dependent tomography algorithm. The test period is selected to coincide with a period of increased GPS data availability from the African Geodetic Reference Frame (AFREF) project. A simulation approach that includes the addition of realistic errors is employed in order to provide a ground truth. Results show that the inclusion of observations from the AFREF archive significantly reduces ionospheric specification errors across the African sector, especially in regions that are poorly served by the permanent network of GPS receivers. The permanent network could be improved by adding extra sites and by reducing the number of service outages that affect the existing sites.

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This article presents the first results regarding the investigation of the response of the equatorial ionospheric F region in the African sector during geomagnetic storm periods between April 2000 and November 2007 using GPS‐derived vertical total electron content observed at Libreville, Gabon (0.35°N, 9.67°E, dip latitude −8.05°S). We performed a superposed epoch analysis of the storms by defining the start time of the epoch as the storm onset time. During geomagnetic storms, the altered electric fields contribute significantly to the occurrence of negative and positive ionospheric storm effects. Our results showed that the positive storm effects are more prevalent than the negative storm effects and generally last longer irrespective of storm onset times. Also, the positive storm effects are most pronounced in the daytime than in the premidnight and postmidnight periods.

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Abstract. The study of diurnal and seasonal variations in total electron content (TEC) over Nigeria has been prompted by the recent increase in the number of GPS continuously operating reference stations (CORSs) across Nigeria as well as the reduced costs of microcomputing. The GPS data engaged in this study were recorded in the year 2012 at nine stations in Nigeria located between geomagnetic latitudes -4.33 and 0.72 • N. The GPS data were used to derive GPS TEC, which was analysed for diurnal and seasonal variations. The results obtained were used to produce local GPS TEC maps and bar charts. The derived GPS TEC across all the stations demonstrates consistent minimum diurnal variations during the pre-sunrise hours 04:00 to 06:00 LT, increases with sharp gradient during the sunrise period (∼ 07:00 to 09:00 LT), attains postnoon maximum at about 14:00 LT, and then falls to a minimum just before sunset. Generally, daytime variations are found to be greater than nighttime variations, which range between 0 and 5 TECU. The seasonal variation depicts a semi-annual distribution with higher values (∼ 25-30 TECU) around equinoxes and lower values (∼ 20-25 TECU) around solstices. The December Solstice magnitude is slightly higher than the June Solstice magnitude at all stations, while March Equinox magnitude is also slightly higher than September Equinox magnitude at all stations. Thus, the seasonal variation shows an asymmetry in equinoxes and solstices, with the month of October displaying the highest values of GPS TEC across the latitudes.

In this paper we present the first results from measurements of scintillation and total electron content (TEC) from an equatorial station, Lagos (Latitude 6.5°N, Longitude 3.4°E, magnetic latitude 3.03°S), Nigeria, using a Novatel GSV4004B GPS ionospheric scintillation and TEC monitor. Details are presented for data collected between February 2010 and August 2010. The results show that the presence of some large scale depletions of TEC or plasma bubbles may be noted during the evening hours and that TEC depletions correspond to increased rate of change of TEC (ROT). This confirms that plasma bubbles are associated with large scale irregularities. It is also established that enhanced amplitude scintillation (S4) corresponds quite well with TEC depletions and increases in ROT. The diurnal and seasonal percentage occurrence for different levels of scintillation activity has peaks in the equinox months (March and April) at 23:00 LT.

The ionosphere is an indispensable medium for the propagation of communication and navigation signals. It has been extensively studied over the years to understand its properties and variability under different conditions. Spread F (SF) is a common feature of the ionosphere that is caused by irregularities in electron density distribution in the F-layer. Radio signals incident on these irregular plasma structures are scattered, thereby causing diffuse spreading in the F-layer trace of ionograms. Bottomside spread F was first reported by Booker and Wells (1938) and a number of mechanisms are understood to be responsible for its occurrence and development. Post-sunset SF is most commonly attributed to increased upward vertical plasma drift (also known as the pre-reversal enhancement [PRE]) in response to an enhanced eastward electric field. Due to the absence of a highly conducting E-layer at night, plasma instabilities can develop more easily at elevated F-layer heights and can subsequently grow via the Rayleigh-Taylor instability (RTI). This phenomenon is even more common at equatorial latitudes because the geometry of the geomagnetic field at these latitudes facilitates  E B plasma drifts (Sreehari et al., 2006). Apart from the PRE associated with post-sunset F-region electrodynamics, other mechanisms known to contribute to F-layer plasma instabilities include zonal winds, atmospheric gravity waves (AGWs) which manifest in the ionosphere as traveling ionospheric disturbances (TIDs), and trans-equatorial thermospheric winds (Abdu, 2012). The role of the RTI in the development of the equatorial spread F (ESF) was first suggested by Dungey (1956). The RTI develops when a heavy fluid rests on a lighter one. In such a state, gravity or other triggers can cause the heavier liquid to become unstable. Ossakow (1979) and references therein discussed the role of the F-layer peak and electron density scale length in the linear growth of the RTI. A higher F-layer peak and smaller electron density scale length are both favorable conditions for the linear growth of the RTI.In the mathematical formulation by Huang et al. (1993), it was shown that AGWs can directly seed the RTI. Huang et al. (1993) showed that AGWs with speed 5-20 ms −1 and wavelength in the order of 100 km Abstract Daytime equatorial spread F (ESF) is not as common as nighttime ESF due to the presence of a highly conducting E-layer during the daytime which counteracts the development of F-layer plasma irregularities. This study presents two rare daytime ESF-like events which occurred over an interval ∼2 h and were detected by the HF Doppler receiver located in Lagos (LAG: geographic: 3.27°E, 6.48°N; dip latitude −1.72°) and the Lowell Digisonde at Ilorin (ILR; 4.68°E, 8.50°N; dip latitude −1.25°), managed by Lowell GIRO Data Center (LGDC). Analysis of the first event revealed ∼30 min periodic oscillations in iso-heights of ionospheric electron density. Shorter period (∼15 min) oscillations appeared simultaneously in HF Doppler measurements and these oscillat...