(a) Locations of observation equipment used over Europe. The blue dashed line is the imaginary boundary where the pattern of the ionosphere storm changes. (b) and (c) Variations in hmF2 and foF2 values from ionosondes. The black solid lines represent the IQD reference day and the red ones are those on the event days. (d) Variations in VTEC values from GNSS receivers. The (1)–(4) and R markers indicated at the top of (b)–(d), along with the storm phases used in Figure 2 illustrate the state of geomagnetic storms.