Performance analysis of IRI 2016 model TEC predictions over Northern and Southern Hemispheric IGS stations during descending phase of solar cycle 24

Global Positioning System (GPS) is an efective tool for monitoring the Earth’s ionosphere. This paper concerns with temporal and spatial variations of ionospheric total electron content (TEC) at RAMO, Israel (geographic coordinates: 30.597o N, 34.76o E; geomagnetic coordinates: 27.17o N, 112.40o E), and ZAMB, Zambia (geographic coordinates: 15.42o S, 28.31o E; geomagnetic coordinates: 16.98o S, 98.67o E) for the descending phase of solar cycle-24. The VTEC estimated from GPS measurements and VTEC values modeled from the IRI-2016 model are obtained over both the GPS stations, i.e., RAMO station, in Northern Hemisphere (NH) and ZAMB station in Southern Hemisphere (SH). The diurnal, seasonal, annual, and solar cycle variations in TEC are investigated during 2016–2018. Also, a comparative study is performed between VTEC derived from GPS observations and International Reference Ionosphere-2016 (IRI-2016) model using the statistical analysis. It has been observed that the observed and modeled maximum VTEC decreases with the declining phase of solar cycle-24 over both the stations. The semiannual patterns are noticed in VTEC values of both the IRI-2016 model and GPS observations for all the years, i.e., 2016–2018. At RAMO station, seasonal analysis depicted a year-wise decrease in maximum TEC as follows March Equinox (Mar-Equ), September Equinox (Sep-Equ), December Solstice (Dec-Sol), and June Solstice (Jun Sol). It is observed from the monthly average estimations of the IRI-2016 model that it has relatively more overestimations of VTEC values over RAMO station in NH than over ZAMB in SH during 2016–2018. However, the IRI-2016 model has underestimated the GPS-VTEC values from June–September 2018 over NH, RAMO station. The root-mean-square error (RMSE) values of the IRI-2016 model delineate that the model has more RMSE during March Equinox than September Equinox, whereas these RMSEs are recorded high over NH (RAMO) than SH (ZAMB). At RAMO, the IRI-2016 model has shown high RMSE values during the June solstice compared to the December solstice. On the other hand, at ZAMB, the highest RMSE values are observed during the December solstice than June solstice. Ionolab-TEC and GIM-TEC also considered over both the stations for the analysis. The IRI-2016 model predictions are in good agreement with GPS-VTEC values over SH (ZAMB) compared to NH (RAMO).