Pattern of variation of TEC, hmF2 and foF2, and their correlation during the geomagnetic storm time conditions

This paper mainly examines the diurnal variation of the Total Electron Content (TEC) and critical frequency of the F2-layer (foF2) and their correlation with the height of the peak electron density (hmF2). This is carried out employing the observations (co-located Global Positioning System (GPS) and digisonde) and empirical models (International Reference Ionosphere, IRI, 2016 and IRI-extended to the Plasmasphere, IRI-Plas, 2017) in the low- to-high latitudes during relatively similar intense level geomagnetic storms that occurred during the high solar activity (February 19, 2014) and low solar activity (September 08, 2017). The GPS-derived TEC and digisonde-- derived TEC, hmF2 and foF2 variabilities show large fluctuations on most of the stations when compared to the IRI 2016 and IRI-Plas 2017 variations. Moreover, the highest GPS-derived TEC values are observed when the hmF2 values reach in the ranges of about 270-309 km (low latitude), 203-266 km (mid latitude) and 259-311 km (high latitude) regions. The highest digisonde-derived TEC values are also depicted at the height ranges of about 256-451 km (low latitude), 250-326 km (mid latitude) and 309-388 km (high latitude) regions. In addition, the highest digisonde-derived foF2 values are observed when the hmF2 values reach about 253-384, 217-311 and 259-281 km in the low, mid and high latitudes, respectively. The model-derived TEC and foF2 variations also reveal that the highest values are generally observed at relatively similar height ranges with the observations. Moreover, the highest TEC and foF2 values are observed relatively at lower altitudes in the mid latitudes when compared to the low and high latitudes. The highest values also tend to move to the lower altitudes in shifting from the high to the low solar activity during similar intense level geomagnetic storms.