## 5 Conclusions  
The most important results of the study can be summarized as follows:  
1. The X-class flares resulted in 1-2.6 dB of attenuation at 30-32.5 MHz according to the riometer data (Figures 8 and 9), whereas absorption changes derived from the Digisondes ranged from 10 to 15 dB in the 2.5-4.5 MHz frequency range (indicating a tenfold increase compared to previous cases, Figures 4 and 5). 2. The ionospheric response to X-Ray flux changes (flares) at midlatitude measured by the ionosondes shows a solar zenith angle dependence (Figures 4-7). This dependence is not evident for riometer measurements (Figures 8 and 9). A clear SZA dependence can not be determined in the modeled values at either frequency (Figures 6-9). 3. The effects of energetic particles following solar flares are evident in the riometer data (Figures 8 and 9), although they can only be detected at Juliusruh (\(\sim\)55\({}^{\circ}\)N) after some events (Figure 5) among the investigated Digisonde stations. Consequently, the absorption changes due to solar proton events are pronounced at high latitudes but diminish quickly as latitude decreases, becoming undetectable below the sub-auroral region. 4. The absorption changes measured by the riometers and determined by the D-RAP model as a result of the increased energetic particle flux seem to show a clear latitude dependence, the higher the latitude the larger the caused impact (Figures 8 and 9). 5. By comparing the results of the amplitude method with the D-RAP model, it is clear that the observed values almost always exceed the absorption values given by the model both at 2.5 MHz and at 4 MHz (Figures 4-7). 6. By comparing the riometer data with the modeled values, the model overestimates the impact of the solar flares at the highest latitude stations (IBA, AVI), while generally, it underestimates the caused effect at lower latitudes (Figure 10). However, it overestimates the caused impact during the particle events (Figures 8 and 9). 7. According to the current results, the amplitude method seems to be applicable for detecting the ionospheric absorption changes during flare events, taking into consideration its limitations. **Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/rs16213975/s1](https://www.mdpi.com/article/10.3390/rs16213975/s1)]([https://www.mdpi.com/article/10.3390/rs16213975/s1](https://www.mdpi.com/article/10.3390/rs16213975/s1)), Figure S1: Consecutive ionograms recorded at Pruhonice ionosonde station during and after an M8.1 class flare (between 07:45 and 08:40) on 08 September. The total radio fade-out is clearly seen on the ionograms. Figure S2: Consecutive ionograms recorded at VT ionosonde station during and after an X8.2 class flare (between 15:45 and 16:30 UT) on 10 September. The total radio fade-out is clearly seen on the ionograms. Figure S3: Consecutive ionograms recorded at Juliusruh ionosonde station during and after an M8.1 class flare (between 07:45 and 08:40) on 08 September. The total radio fade-out, thus the disappearance of the traces from the Ionograms can be seen, despite the fact that the Ionograms are very noisy. Table S1: Detailed data of the investigated flare and energetic particle events. Date, maximum X-Ray or proton flux (measured at different channels) and the exact time of the maximum. Table S2: The values measured by the Digisondes and determined by D-RAP at 2.5 and at 4 MHz at the peak time of the investigated flares. Table S3: The solar zenith angle (SZA) calculated at the peak time of the flare or, in the case of a fade-out, at the time of the first measurement after the fade-out (2.5 MHz measurements), and at the location of the Digisonde stations. The X-Ray flux values are from 0.1 to 0.8 nm wavelength band measured by the GOES spacecrafts.