## 3 Results
### Analysis of the LEP Event  
In perturbed ionospheric conditions at the Belgrade receiver site (44.85\({}^{\circ}\) N, 20.38\({}^{\circ}\) E), an increase in phase delay and decrease in amplitude for both analysed VLF signals, are registered (Figures 4 and 10). Such behaviour can be explained by lowering the VLF signal reflection height (delay in the phase is less) and, respectively, by reducing the sharpness of the ionized environment lower edge. In perturbed waveguides, modal minima are mitigated compared to the unperturbed ionospheric state. In the perturbed NAA waveguide, reflection height decreased from 86.5 km (characteristic value for normal unperturbed nighttime ionospheric conditions) to 86 km, while reflection edge sharpness decreased from 0.46 km\({}^{-1}\) (characteristic value for normal unperturbed night-time ionospheric conditions) to 0.36 km\({}^{-1}\), which is a value characteristic for daytime ionospheric conditions. Changes in GQD propagation are similar: in perturbed waveguide, reflection height decreased from 86.5 km to 85.8 km, while reflection edge sharpness decreased from 0.46 km\({}^{-1}\) to 0.38 km\({}^{-1}\) (value characteristic for daytime ionospheric conditions). A less sharp lower edge of the ionized environment enables the VLF signal to penetrate into the ionized environment, and at the same time, VLF signal deviant energy absorption takes place. It is well known that \(\beta\) is a function of \(\omega\). After an LEP event, an NAA signal pair (\(\beta/H^{\prime}\)) have values (0.46 km\({}^{-1}\)/86.5 km) and GQD have values (0.46 km\({}^{-1}\)/86.6 km), so it can be said that the ionosphere is fully recovered. Same ionospheric conditions ((\(\beta/H^{\prime}\)) have values of 0.46 km\({}^{-1}\)/86.5 km) that apply for areas outside of the irregularity, too. Obtained values of (\(\beta/H^{\prime}\)) pairs are in agreement with [48], who reported \(\beta\) in the range 0.48-0.5 km\({}^{-1}\) and \(H^{\prime}\) in the range 85.3-87 km for cases of short signals with a mid-latitudinal transmission, also with [57] who reported \(\beta\) in the range 0.46-0.5 km\({}^{-1}\) and \(H^{\prime}\) in the range 84-87 km for cases of short and moderate length signals in most cases with mid-latitudinal transmission, and [54], who reported \(\beta\) in the range 0.307-0.42 km\({}^{-1}\) and \(H^{\prime}\) in the range 80.3-87 km for cases of short length signals in most cases with mid-latitudinal transmission. An obtained relative change in reflection height during analysed perturbation of 0.5 km and  
Figure 10: Electron density \(N_{e}(z)\) height profiles for GQD (at reflection height 85.8 km) and NAA (at reflection height 86 km) signals, calculated for altitude range 60–90 km, for 16 November 2004, at 03:37 UT, in the area of the most prominent electron density change along GCPs at a location approximately 1100 km looking from Belgrade towards both transmitters. The unperturbed state for GQD signal is given in red hollow squares, the perturbed state for GQD signal is given in blue hollow squares, unperturbed state for NAA signal is given in red solid circles and perturbed state for NAA signal is given in blue solid circles. 0.7 km in cases of NAA and GQD signals, respectively, are in agreement with the results reported by [54; 55] who reported ranges of 7-10 km and 3.7-6.7 km, respectively. Measured and simulated relative change in amplitude and phase delay values (Table 1) are in line with [18] and reference therein, an amplitude difference \(\sim\)6 dB and \(\sim\)20 deg of phase shift and [48; 54] who reported relative measured and modelled amplitude and phase change of 1.4 and 1.5 dB and 9 and 8 deg and up to 2.01 and 1.74 dB and 3.63 and 3.15 deg, respectively.