## 3 Results
### On the Occurrence of the Ionospheric Storm Effects
[42] also reported that the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) model was not sufficiently able to reproduce the F2 peak electron density variations observed during the CIR events that occurred during the Whole Heliosphere Interval in 2008 (Day of Year 50-140). Such results may be considered to be suggestive of different ionospheric response to different levels of solar activity and/or different ionospheric response to geomagnetic storms driven by different solar/interplanetary conditions. The latter was also supported by the results obtained through the assessment of the Solar Wind-driven Autoregression Model for Ionospheric Forecast (SWIF) [43, 44, 45]. In particular, it was found that, while ionospheric disturbances related to the occurrence of CME-driven storms in the ascending phases of the solar cycles 23 and 24 were successfully predicted by the SWIF model, the performance of the model was significantly poorer during non-CME related ones which occurred in the same solar activity phase [46]. The work performed by [PERSON] et al. [47] supported an investigation of the long-term behavior of the ionospheric disturbances during the ascending phase of solar cycle 24 (i.e., for the time interval 2008-2014) with two objectives: the analysis of the ionospheric behavior in a wide range of geophysical conditions, and the determination of specific features that  
Figure 2: The ionospheric response over Europe to the intense geomagnetic storm (min Dst -223 nT) that occurred in the time interval 16–20 March 2015. This was the greatest geomagnetic storm of solar cycle 24. Top panel: The variation of the Dst index. Middle panel(s): the variation of foF2 and hmF2 ionospheric characteristics in comparison to their monthly medians over Chilton. Bottom panel(s): the variation of foF2 and hmF2 ionospheric characteristics in comparison to their monthly medians over Ebre. The two stations are located in the same local time sector. The F2 layer is uplifted at both ionospheric locations during the storm main phase. Impulse-like increases in the height of the peak electron density are related to similar increases in the ionospheric ionization (positive storm effects) on 17 March. The effects are more intense at lower latitudes (i.e., over Ebre). One may also note two qualitative differences in the response over the two distinct locations: (i) the positive storm penetrates the evening sector at lower latitudes, while at Chilton, it remains confined to daytime hours; (ii) Chilton experiences strong negative storm effects (foF2 is reduced by up to 50% with respect to the normal values) on the second storm day, most probably by entering the disturbance zone in the evening hours, while Ebre recovers to normal conditions. may differentiate the ionospheric response to different faces of space weather driving. By extending a key part of their analysis here, Figure 3 provides the annual distribution of the DfoF2 as a metric of the ionospheric activity for the years 2008-2020. The results verify that significant ionospheric disturbances (greater than 20% with respect to normal conditions) are recorded during all years, even in extremely low solar activity conditions mainly capturing the impact of geo-effective solar wind structures. Furthermore, the results indicate that there are qualitative differences in the ionospheric activity between different levels of solar activity: the intensity of the positive disturbances tends to be higher during the years of low solar activity and solar minima (i.e., 2008-2010 and 2017-2019) compared to the solar maximum years (2012-2014), while they clearly dominate numerically over the negative disturbances. More generally, it was suggested that during low solar activity and solar minimum, the solar wind impact on the geospace environment particularly favors the occurrence of ionization increases, both in frequency and intensity. Ionization depletions are still present, but they mainly appear in moderate intensity, in contrast with the situation in solar maximum.