## 1 Introduction  
The ionosphere is a dynamic interface between the plasma space above and the neutral atmosphere below it. Its state is affected both from above, from the direction of outer space (mainly related to solar activity), and from below, from the neutral atmosphere (e.g., by tropospheric events, sudden stratospheric warmings). Radio and satellite communication and navigation highly depend on the state of the ionosphere, since the electromagnetic (EM) waves propagate through and/or reflect from it. The reduction of the electromagnetic waves traveling through the ionosphere is most pronounced in the high frequency (HF) range (3-30 MHz), which is crucial for radio communication and, by extension, for both commercial and military aircraft operations. Absorption happens when free electrons in the plasma, driven by the radio wave, lose energy through collisions with ions and neutral particles. The density of free electrons and the collision frequency are greatest in the D-region (approximately 2 \(\times\) 10\({}^{6}\) s\({}^{-1}\)), which is why radio waves below 10 MHz are primarily attenuated there [1]. In this region, the frequency of electron collisions is mainly influenced by the density and temperature of the neutral particles. The most intense external force causing further ionization and absorption in the ionosphere is related to solar flares, which release a substantial amount of electromagnetic energy across a wide range of wavelengths for a brief duration, typically lasting from about 30 min to 1 h [2]. According to the emitted X-Rays flux in the 0.1-0.8 nm band measured by the GOES spacecrafts, flares can be classified as large (X, \(>\)10 \(\times\) 10\({}^{-4}\) W/m\({}^{2}\)), medium-size (M, \(\sim\)10 \(\times\) 10\({}^{-5}\)-10 \(\times\) 10\({}^{-4}\) W/m\({}^{2}\)) and small (C, \(\sim\)10 \(\times\) 10\({}^{-5}\)-10 \(\times\) 10\({}^{-5}\) W/m\({}^{2}\)). Furthermore, with numbers from 0 to 9 within the above categories applying a finer logarithmic scale. Total radio fade-outs lasting for tens of minutes or even hours can occur during flare events as a result of the increased ionization and attenuation of the radio waves caused by the extra radiation [2; 3; 4; 5; 6; 7]. Enhancement of the D-region electron density and consequently increased radio wave absorption can also be caused by collision with energetic auroral electrons (Auroral Absorption [8]), or with solar energetic protons (Polar Cap Absorption--PCA). Generally, ionospheric absorption is measured by riometers (Relative Ionospheric Opacity meters), which record the cosmic radio noise at certain frequencies (\(\sim\)20-50 MHz frequency range) [9]. Nevertheless, these instruments are generally located at higher latitudes, thus they give information about the changes of the ionospheric absorption that occurred there [10]. Using data from the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjarvi (Finland) from solar cycle 23, [PERSON] et al. [11] identified that M4 class flares are the less intense ones that still can bear significant influence on cosmic noise absorption. They also presented the magnitude of absorption that the different classes of solar flares can produce. Data from imaging riometers were also used to investigate the frequency and spatial dependence of short-wave fadeouts (e.g., [12; 13]). The minimum frequency (fmin) recorded as the lowest frequency at which an o-mode echo is observed on the ionograms measured by the ionosonde is also a commonly used proxy for the \"non-deviative\" absorption of the ionosphere [3; 4; 5; 6; 7; 14; 15]. Another way to measure the ionospheric absorption variations by the ionosondes is the analysis of the signal-to-noise ratio of radio waves recorded on the ionograms (SNR method) during solar flares and its comparison with reference quiet days [16]. This method was further developed and used to study absorption changes caused by flare events by [PERSON] et al. [17].