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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.

Abstract. The ionosphere is the major error source for the signals of global positioning system (GPS) satellites. In the analysis of GPS measurements, ionospheric error is assumed to be somewhat of a nuisance. The error induced by the ionosphere is proportional to the number of electrons along the line of sight (LOS) from the satellite to receiver and can be determined in order to study the diurnal, seasonal, solar cycle and spatial variations in the ionosphere during quiet and disturbed conditions. In this study, we characterize the diurnal, seasonal and solar cycle variation in observed total electron content (OBS-TEC) and compare the results with the International Reference Ionosphere (IRI-2016) model. We obtained TEC from a dual-frequency GPS receiver located at Birnin Kebbi Federal Polytechnic (BKFP) in northern Nigeria (geographic location: 12.64∘ N, 4.22∘ E; 2.68∘ N dip) for the period 2011–2014. We observed differences between the diurnal variation in OBS-TEC and the IRI-2016 model for all hours of the day except during the post-midnight hours. Slight post-noon peaks in the daytime maximum and post-sunset decrease and enhancement are observed in the diurnal variation in OBS-TEC during the equinoxes. On a seasonal scale, we observed that OBS-TEC values were higher in the equinoxes than the solstices only in 2012. However, in 2011, the September equinox and December solstice recorded a higher magnitude, followed by the March equinox, and the magnitude was lowest in the June solstice. In 2013, the December solstice magnitude was highest, followed by the equinoxes, and it was lowest in the June solstice. In 2014, the March equinox and December solstice magnitudes were higher than the September equinox and June solstice magnitude. The June solstice consistently recorded the lowest values for all the years. OBS-TEC is found to increase from 2011 to 2014, thus revealing solar cycle dependence.