## 4 Discussion
[16] demonstrated that the HF absorption took more than 3 h at Ebro station during the X9.3 class event. Based on their results, although the time interval when the X-Ray flux agreed with a category of that of an X-class lasted only for a few minutes, the radiation remained in the category of class M, consequently, the HF absorption is sensitive also to the fainter (M-class) solar flares. Our results also confirm this statement, since increased values of the fmin parameter (Table 3) and even 10-14 dB enhancement of the loss values determined by the amplitude data were found during the M-class flares of the investigated period. The riometers detected 1.1-1.5 dB of cosmic noise absorption during the X2.2 flare, which agrees well with the values reported by [PERSON] et al. [33], being 1-2 dB during X2 and X2.2 solar flares. The measured values varied between 1.6 and 2.6 dB in the case of the X9.3 flare event (Table 6). It has been extensively discussed in the literature that ionospheric absorption changes depend on the class of the flare and, thus, on the enhancement of the X-Ray flux that causes the increased ionization and attenuation of the electromagnetic waves in the ionospheric plasma. This was demonstrated by using a riometer (e.g., [33; 34]) and different ionosonde data (e.g., [5; 6]) as well. In the case of the ionosonde data, a slightly increasing trend with the X-Ray*cos (SZA) can be recognized in Figures 6 and 7, thus during the more intense flares the caused changes seem to be larger. Nevertheless, this trend is not that evident for these results from the amplitude method, like in the other cases reported in the literature [5; 6]. Our results regarding the absorption values recorded by the riometers also seem to agree with this, generally, the X-class solar flares had a larger impact (~1-2.6 dB) at most of the stations than the M1-M4-class flares (~0.1-0.7 dB, Table 6, Figure 10). Besides the X-Ray flux enhancement itself, the solar zenith angle of the observation site at the time of the flare events is also an important factor in determining the caused changes. The impact of 43 solar flares (37 M-class and 6 X-class) on the ionospheric absorption was investigated using riometer data from Abisko in the northern hemisphere [33]. According to their results, the solar zenith angle (SZA) plays a crucial role in the ionospheric response. The detected absorption was smaller in the case of higher SZA (e.g., 1 dB when SZA ~82.3\({}^{\circ}\) for X2 class) than under smaller SZA but with similar intensity (2 dB when SZA ~45.6\({}^{\circ}\) for X2.2 class). This agrees well with previous findings based on ionosonde data (fmin), that the larger the SZA, the larger the caused impact in the ionospheric absorption [5; 6]. Our present results in the case of the ionosonde data also confirm this SZA dependence (Figures 6 and 7). However, the role of the SZA for the caused changes in the ionospheric absorption as a result of the flares is not that clear based on the riometer data in the present case. For example, the impact of the X8.2 flare on 10 September seems to be higher at the higher latitude stations (e.g., 1 dB at ABI and SOD) than at the lower latitude stations (~0.6 dB at OUL and JYV, Figure 9 and Table 6). Nevertheless, [PERSON] et al. [33] discussed that other factors, like the latitudinal position of the station and background conditions, can also play an important role in cosmic radio noise absorption at high latitudes besides the flare intensity and the solar zenith angle. Some of the investigated flare events were accompanied by increased flux of energetic protons, the X9.3 flare (peak time at 12:02 UT) on 06 September and the X8.2 flare (peak time at 16:06 UT) on 10 September as is shown in Figures 5b, 8b and 9b. The absorption changes caused by the energetic protons on 10 September were even detectable at the sub-auroral region at 2.5 MHz at the Juliusruh station (Figure 5). This extra absorption related to the particle events was not determined by the D-RAP model.