## 2 Methods and Data
### Ionosonde Measurements--Amplitude Method  
Amplitude data from three European ionosonde stations operating with DPS-4D Digisondes (a novel type of ionosonde) were used to calculate the ionospheric absorption. The three stations are (more-or-less) located along a meridional line: Juliusruh (JR, Germany, 54.63\({}^{\circ}\)N, 13.37\({}^{\circ}\)E), PrRhonice (PQ, Czechia, 49.98\({}^{\circ}\)N, 14.55\({}^{\circ}\)E) and San Vito (VT, Italy, 40.6\({}^{\circ}\)N, 17.8\({}^{\circ}\)E) (marked by green stars on Figure 2). The amplitudes (in dB) of the reflected echoes emitted at different sounding frequencies (typically between 1.2 and 12 MHz) and reflected at different heights are received and stored in the Digisonde systems. An ionogram was made every 5 and 15 min in the case of the JR and VT stations, respectively. The sampling rate was 2 and 15 min in the case of the PQ station during the investigated period. The denser, 2 min campaign took place on 08-09 September.  
The amplitude data recorded by the Digisondes at the three investigated stations (JR, PQ, VT) were used to calculate the relative ionospheric absorption of the emitted radio waves following the method proposed by [PERSON] [24]. This method is called the Sales-absorption method or amplitude method and it is based on the Friis-transmission equation [25]. From the Friis-equation, one can derive the loss of a given signal transmitted from an antenna at a certain wavelength which travels along a certain propagation path to arrive at the receiver station, as:  
\[10 log\ (L)=10 log\ (P_{t}G_{t}G_{r})+20 log\ (\frac{\lambda}{4\pi 2h})-10 log\ (P_{r}) \tag{1}\]  
where \(L\) is the loss term, \(P_{t}\) is the power of the transmitted signal, \(G_{t}\) and \(G_{r}\) are the gain of the transmitter and receiver, respectively, \(\lambda\) is the wavelength of the transmitted signal, \(h\) is the height of the reflection, and \(P_{r}\) is the power of the transmitter.  
However, in order to obtain the loss term, one must determine the \(P_{t}G_{t}G_{r}\) product first as it is not known a priori. To do this, each of the Digisonde systems was calibrated  
Figure 2: The geographical locations of the used riometer (blue stars) and ionosonde (green stars) stations, with the abbreviated names (codes) of the observation sites.  
based on quiet (i.e., solar X-Ray flux \(<\) 10 \(\times\) 10\({}^{6}\) Wm\({}^{-2}\)), nighttime (18-3 UT, which is 19-4 and 20-5 in the local time of the Digisonde measurements during winter and summer, respectively) data. In total, 173 quiet days from the years 2017 to 2018 were selected to this end. Assuming that the loss term was (relatively to the investigated disturbed period) zero during these quiet periods, one can calculate the \(P_{t}G_{t}G_{r}\) product from Equation (1). (please, refer to Figure 3 in [18]). More details about the calibration are described in chapter 2.1 in [PERSON] et al. [18].  
Ultimately, after the \(P_{t}G_{t}G_{r}\) product was quantified in the case of every Digisonde system and at different frequency ranges (namely at 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, and 5.5 MHz \(\pm\) 200 kHz, respectively), the loss term in Equation (1), which relates to the (relative) absorption, could be calculated in dB for the investigated time period.