The geomagnetic storm time responses of the TEC, foF2, and hmF2 in different solar activity during solar cycle 24 and 25

This paper mainly examines the response of variation of the TEC, foF2, and hmF2 obtained from observations (GPS and digisondes) and models (IRI 2016 and IRI-Plas 2017) across low-to-high latitudes during various geomagnetic storm time conditions in different solar activity years. The 19 February 2014, 17 March 2015, and 4 November 2021 geomagnetic storm cases caused positive storm effects (particularly at low latitudes), while the 8 September 2017, and 26 August 2018 geomagnetic storm cases resulted in negative storm effects, especially at mid and high latitudes. Furthermore, during the 19 February 2014 storm, the sharp increase (peak) diurnal digisondes TEC values are observed, on average, when the hmF2 values reach about 360, 282, and 312 km, in the low, mid and high latitudes, respectively. During the 26 August 2018 storm, the peak TEC values are observed, on average, when the hmF2 values reach about 313, 258, and 268 km in the low, mid and high latitudes, respectively. Hence, the digisonde-derived peak TEC in mid latitudes typically coincides with a decrease in hmF2, while in low latitudes, it is associated with an increase in hmF2. Additionally, during low solar activity periods, digisonde-derived peak TEC values were observed when hmF2 decreased, contrasting with patterns seen during high solar activity. Both the IRI 2016 and IRI-Plas 2017 models performed well, with the models peak TEC values being observed when the hmF2 variations attain similar values with the observations, reinforcing the models' reliability in capturing ionospheric responses during geomagnetic storms.