## 3 Results
### Comparing the Results of the Amplitude Method with D-RAP  
The most intense flare events, consequently the largest variations in the ionospheric absorption occurred during the X2.2 and X9.3 flares on 06 September. However, we do not show the occurred changes recorded at 2.5 and 4 MHz by the European ionosondes at midlatitude, because total radio fade-out was detected almost at every investigatedDigisonde station in most part of the day (see Table 3). Therefore, no measured values are available, especially at 2.5 MHz, for comparison with the D-RAP model on that day. Nevertheless, the X-class flares caused increased fmin values (Table 3) and increased amplitude data (Table 4) even after the long blackouts. Because of the quite long duration of the fade-outs at 2.5 and 4 MHz, we compared the measured values with the modeled ones at the time of the first record after the blackout (Table 4). In the case of the measured values at 4 MHz, it is clearly seen that one of the largest absorptions was detected after the X-class flares on 6 September (with 17-23 dB at the different stations). The absorption variation measured by the three ionosonde stations and determined by the D-RAP model on 08 September is seen in Figure 4. Two M-class flares occurred on that day (M8.1 and M3.0 with peak times of 7:49 and 15:47, respectively), as can be recognized in the X-Ray flux changes (Figure 4a). Total and partial radio blackouts occurred at every investigated station after the M8.1 flare (Table 3). and examples of ionograms are shown in Supplementary Figure S1). The time of the fade-out at 2.5 or 4 MHz was determined by a manual check of the ionograms. However, the ionograms from Juliusruh are quite noisy, thus, we have data points from the amplitude method during that period as well when the traces disappeared from the ionograms (some examples for the ionograms are presented in the Supplementary Materials, Supplementary Figure S3). The observed values varied between -10 and 25 dB in the periods before and after the investigated M8.1 and M3.0 class flares. The increased values of the absorption as a result of the flares are more remarkable at 2.5 MHz (Figure 4c-e), while it is difficult to recognise it at 4 MHz (Figure 4f-h). It is also worth mentioning here that the time resolution of the recorded data is also an important factor, e.g., PQ with 2 min resolution nicely follows the changes of the X-Ray curve during the M3.0 flare at 2.5 MHz, while the changes of the observed values at VT, with 15 min resolution, are not that remarkable, but still can be recognized. It is also important to note here, that there was a negative ionospheric storm in the F-region daytime on 08 September caused by the geomagnetic storm taking place then. The foF2 were below 4 MHz at some ionograms at JR and PQ stations in the afternoon hours leading to the increased values and large changes in the amplitude data there during those periods (Figure 4f,g, more details in [18]). Comparing the measured absorption values with the values determined by D-RAP, we can conclude that the recorded values exceed the estimated ones almost always during the day. Therefore, the model underestimates the ionospheric absorption, e.g., at the peak time of the M3.0 flare (which did not cause total radio blackouts) the D-RAP predicted 9-10 dB absorption at 2.5 MHz at the location of the three European Digisonde stations, while the observed values varied between -17 and 23 dB at 2.5 MHz (Figure 4, and Table 4). \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Peak Time and** & \begin{tabular}{c} **Duration of** \\ **Total Fade-Out** \\ **at JR [min]** \\ \end{tabular}  & \begin{tabular}{c} **Change in** \\ **fmin at JR [\%]** \\ \end{tabular}  & \begin{tabular}{c} **Duration of** \\ **Total Fade-Out** \\ **at PQ [min]** \\ \end{tabular}  & \begin{tabular}{c} **Change in** \\ **fmin at PQ [\%]** \\ \end{tabular}  & \begin{tabular}{c} **Duration of** \\ **Total Fade-Out** \\ **at VT [min]** \\ \end{tabular}  & \