## 4 Conclusions and Perspectives  
Ionospheric conditions highly differ depending on the time of day when sudden disturbances occur, with a transition period between stable daytime and stable night-time ionospheric conditions, i.e., during dawn and sunset being especially challenging for modelling. Regular night-time ionospheric conditions are described with parameter pairs (\(\beta\), \(H^{\prime}\)) within a range of values, that is 0.47-0.50 km\({}^{-1}\) and 50-87 km, respectively, while in unperturbed daytime ionospheric conditions, this pair is defined only by one pair (0.3 km\({}^{-1}\), 74 km). In the case of perturbation presence, ionospheric conditions are changed affecting VLF transmission within the Earth-ionosphere waveguide, inducing signal's amplitude and phase delay to deviate from their regular values. In such disturbed environment, parameter pairs (\(\beta\), \(H^{\prime}\)) also deviate from their regular values, depicting the change in electron density that takes place, following the causative agent's behaviour. In the case of the examined LEP event example, parameter pairs (\(\beta\), \(H^{\prime}\)) changed according to perturbed nocturnal waveguides. Specifically, in the case of the NAA signal parameter, \(H^{\prime}\) went through a decrease from 86.5 km (characteristic value for normal unperturbed nighttime ionospheric conditions in pre-LEP state) to 86 km at LEP's peak and returned to its regular value of 86.5 km in post-LEP conditions, while parameter reflection edge sharpness \(\beta\) decreased from 0.46 km\({}^{-1}\) (characteristic value for normal unperturbed night-time ionospheric conditions before LEP occurrence) to 0.36 km\({}^{-1}\) at LEP's peak, which is the value characteristic for daytime ionospheric conditions and returned to 0.46 km\({}^{-1}\), its regular value after the influence of the LEP event. Similar behaviour is also present in the case of GQD trace, where parameter \(H^{\prime}\) went through a decrease from 86.5 km to 85.8 km at LEP's peak and returned to 86.6 km, while reflection edge sharpness \(\beta\) decreased from 0.46 km\({}^{-1}\) to 0.38 km\({}^{-1}\) and returned to 0.46 km\({}^{-1}\). It can be concluded that after the influence of the analysed LEP event, the ionosphere is fully recovered in both waveguides, still remaining slightly higher, reflecting the edge height in the case of the GQD signal. Electron density change is one order of magnitude below regular values. Regarding measured \(\Delta A\) (dB) and \(\Delta P\) (\({}^{\circ}\)), in the case of the LEP event, they are in a range of \(-1\) to \(-2\) dB and \(+9\) to \(+13^{\circ}\) (where the minus sign denotes decrease and plus sign increase in signal values compared to pre-LEP state). In the case of mid- and strong flare events, \(\Delta A\) (dB) and \(\Delta P\) (\({}^{\circ}\)) are usually of higher values [38, 62, 63], and especially in X-class flare events such as the one analysed here, where they took values of about +4 dB and \(+159^{\circ}\), respectively. At the maximum X-ray irradiance during X1.3 class flare event that occurred on 7 September 2017, as the response within lower ionosphere, the reflecting edge height decreased by 13 km to the height 63 km, and the reflection edge sharpness increased to the value 0.42 km\({}^{-1}\), while the induced electron density increase reached almost two orders of magnitude compared to the regular value. The analysed flare event lasted for almost two hours, while it took much longer for the lower ionosphere to fully recover. Data related to the ionospheric research and the results obtained are of great use for various Earth observations and especially for telecommunications, while they also can be significant to other applications in modern society. Modelling ionospheric parameters is crucial for validation of proposed models. Results presented in this paper are related to modelling the plasma response of the ionospheric lowest region, to some high energy events, as recorded by radio signals.