## 3 Results
### Comparing the Rioneter Data with D-RAP  
On 06 September, the X9.3 flare caused \(\sim\)1.6-2.6 dB changes in the ionospheric absorption according to the riometer data. The largest changes (2.6 dB) occurred at the ABI station, while the smallest one (1.6 dB) was recorded at the JYV station. Nevertheless, no clear latitudinal (or solar zenith angle) dependence can be recognized based on the results (Figure 8, Table 6). The flare was accompanied by an energetic particle event as it can be seen in the 30 and 60 MeV proton flux measured by the GOES satellite (Figure 8b). The energetic particles caused \(\sim\)0.3-1.2 dB variation in the absorption data. Clear latitude dependence can be recognized in the ionospheric response in this case, the larger the latitude, the greater the detected absorption changes were. The D-RAP model predicted \(\sim\)1-2.4 dB for the ionospheric absorption during and after the X9.3 flare, with clear latitudinal dependence, the larger the latitude, the greater the predicted values. However, it determined smaller absorption almost in all cases than was detected by the riometers at the same frequency and the same location (Table 6). The absorption varied between \(\sim\)0.7 and 2.2 dB with clear latitudinal dependence (from North to South) after the increased proton flux based on the D-RAP model. However, it is very striking from Figure 8 that the model overestimates the absorption generated by the energetic particles at every latitude compared with the values recorded at the riometer stations. The difference is larger at higher latitudes than at lower latitudes. Figure 8: X-Ray (panel (**a**)) and proton flux (panel (**b**)) on 06 September, and the changes measured by the Finnish Rioneter Chain (black curve) and determined by the D-RAP model at the same frequencies (30 and 32 MHz) and at the same locations (red curves) (panels (**c**-**g**)). The X-Ray flux values are from 0.1 to 0.8 nm wavelength band measured by the GOES spacecrafts. The magenta dashed line indicates the peak time of the X-class flare. Similar results were obtained during the X8.2 solar flare that occurred at \(\sim\)16 UT on 10 September. The X-class flare caused \(\sim\)0.4-1 dB of changes in the ionospheric absorption according to the riometer data (Figure 9, Table 6). Larger detected changes seem to be occurred at the higher latitudes (ABI, SOD \(\sim\)1 dB; IVA is an exception with \(\sim\)0.4 dB) than at the lower latitudes (OUL, JYV, \(\sim\)0.6-0.7 dB). This flare was also accompanied by an energetic particle event. Proton flux (at 10, 30 and 60 MeV as well) enhanced \(\sim\)20 min after the peak time of the flare. The induced absorption values recorded at the riometer stations as a result of the energetic particles varied between \(\sim\)0.2 and 2.3 dB. The observed changes showed a clear latitude dependence, at higher latitudes (ABI) reached 2.3 dB and it decreased with latitude until it became negligible at the OUL and JYV stations. The D-RAP model underestimated the effect caused by the increased X-Ray flux at every station, since it determined only \(\sim\)0.15-0.26 dB values (Figure 9 and Table 6). On the contrary, the model overestimated the impact of the energetic particles, especially at higher latitudes (ABI, IVA) where the predicted values for 18 UT reached \(\sim\)2.6-3.4 dB, which is  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Date**} & \multicolumn{1}{c}{**Class**} & \multicolumn{1}{c}{**Peak Time (UT)**} & \multicolumn{1}{c}{**X-Ray Flux [Wm\({}^{-2}\)]**} & \multicolumn{1}{c}{**ABI**} & \multicolumn{1}{c}{**IVA**} & \multicolumn{1}{c}{**SOD**} & \multicolumn{1}{c}{**ROV**} & \multicolumn{1}{c}{**OUL**} & \multicolumn{1}{c}{**JYV**} \\ \hline
2017-9-4 & C7.7 & 12:22 & 7.79 \(\times\) 10\({}^{-6}\) & 0.14 & 0.13 & - & 0.24 & 0.14 & 0.1 \\
2017-9-4 & M1.5 & 15:30 & 1.53 \(\times\) 10\({}^{  
\(\sim\)2 times higher than the observed values at the same time.