Performance of International Reference Ionosphere-2016 and NeQuick-2 Models in Estimating Global Positioning System Total Electron Content Over a Nigerian Equatorial Station during Two Phases of Solar Cycle 24

Abstract

Aim: This study examines the temporal behaviour of GPS-derived total electron content (GPS-TEC) and evaluates the capability of the International Reference Ionosphere-2016 (IRI-2016) and NeQuick-2 models to reproduce observed ionospheric conditions over Birnin Kebbi, Nigeria (12.64° N, 4.22° E; dip latitude 2.68° N). Methods: The analysis considers 2011 and 2014, representing the ascending and maximum phases of Solar Cycle 24, respectively. Dual-frequency GNSS observations were processed to vertical TEC (VTEC), and corresponding model estimates were obtained from IRI-2016 and NeQuick-2. Monthly mean diurnal data were grouped into four seasons: March equinox, June solstice, September equinox and December solstice. Model disagreement was quantified using mean absolute error (MAE). Results: GPS-TEC increased from pre-sunrise values to daytime maxima and decreased after sunset. Observed maxima generally occurred at 13:00–15:00 LT and had a dome-shaped profile, whereas the models frequently produced an earlier maximum and a noon-depression or double-peaked structure. June-solstice TEC was consistently lower than the other seasonal groupings. IRI-2016 had the lower MAE in most seasonal comparisons; NeQuick-2 showed a slightly lower MAE only during the December solstice of 2011. Conclusion: IRI-2016 provided the closer climatological representation for most of the cases examined, but neither model reproduced all features of the observed local-time and seasonal behaviour. The findings support the need for regional validation of empirical ionospheric models over the Nigerian equatorial sector.