## 3 Results and Discussion
### Analyses of SF Events
Sharpness \(\beta\), effective reflection height \(H^{\prime}\), amplitude and phase of VLF signals (recorded and simulated), electron density at reference height, and X-ray flux during the presence of two M flares on 12 June 2014 are presented in Figure 6 on the upper to lower panels, respectively. For the period around SF on 12 June 2014, we have calculated the time-dependent effective reflection height and sharpness as shown in Figure 6b,a. Generally, as in previous example, the shape of \(H^{\prime}\) and \(\beta\) are in correlation with solar X-ray flux. During occurrences of M1.8- and M2.7-class SFs on 12 June 2014, the changes of \(H^{\prime}\) during time have a normal nature. \(H^{\prime}\) lowers in intensity to a minimum after SFs beginning and it rises to a preflare value after X-ray flux peaks. The structure of \(\beta\) is correlated with the amplitude shape. At the time of this SF, it rose quickly to a maximum after the SF peak time and then collapsed to preflare size. At peak time of the M1.8-class SF, (\(I_{x}\) = 1.81\(\cdot\)10\({}^{-5}\) Wm\({}^{-2}\)), \(H^{\prime}\) lowers to a value of 67 km and \(\beta\) rises to 0.42 km\({}^{-1}\). At peak time 10:21 UT during M2.7-class SF with flux \(I_{x}\) = 2.7\(\cdot\)10\({}^{-5}\) Wm\({}^{-2}\), the \(H^{\prime}\) decreases to 65 km and the sharpness increases to 0.46 km\({}^{-1}\). The \(N_{e}\) at reference height is also in correlation with the intensity and shape of X-ray flux during the analysed time period (see Figure 6e). Figure 7 presents the values of simulated amplitude and phase of the NAA signal along the GCP distance, obtained for quiet and disturbed ionospheric state during SFs on 12 June 2014. The simulations of propagation were performed using LWPC code. Amplitude and phase values of NAA signal proportionally increase with increasing X-ray intensity along the whole path. During the initial and main phase of the SID event, the signal variations are more frequent. SID events in the D-region are most noticeable as changes in the phase of VLF radio signals. The height profile, i.e., vertical \(N_{e}\) density profile before, after and during the M1.8- and M2.7-class solar flares on 12 June 2014 are shown in Figure 8. For unperturbed (preflare) conditions (black dashed line), one can see a slow increment of \(N_{e}\) (from 3.2\(\cdot\)10\({}^{7}\) m\({}^{-3}\) at 60 km height, to about 9.7\(\cdot\)10\({}^{8}\) m\({}^{-3}\) at 80 km height). Different slopes have a pink line and dashed red lines (at peak times of M-class SFs), with an increment from \(\sim\)6\(\cdot\)10\({}^{7}\) m\({}^{-3}\) and \(\sim\)1.2\(\cdot\)10\({}^{8}\) m\({}^{-3}\) at 60 km height to 1.45\(\cdot\)10\({}^{10}\) m\({}^{-3}\) and 5\(\cdot\)10\({}^{10}\) m\({}^{-3}\) at 80 km height, respectively. Figure 7: The phase (**a**) and amplitude (**b**) of the NAA/24.0 kHz signal along the GCP distance from Maine, USA, to BEL VLF station, Belgrade, obtained for quiet and disturbed ionospheric state during SFs on 12 June 2014. Strong SF: In order to cover the whole \(I_{x}\) spectral range needed for further analysis, we studied X-class solar flares with flux \(\geq\)10\({}^{-4}\) Wm\({}^{-2}\). Figure 9 presents the values of the monitored and simulated excess of amplitude and phase of the NAA signal simultaneously with X-ray flux during SF on 11 June 2014. Measured and simulated signal values are in good agreement, i.e., almost identical. Barely visible differences are at the right corner of the figure, i.e., during relaxation time and the end of SF. Additionally, amplitude and phase values of NAA signal proportionally increase with increasing X-ray intensity, and during relaxation time (right wings of signal) amplitude  
Figure 8: The altitude profile of electron density during two successive M-class SFs on 12 June 2014. Figure 2:0 Time variation of X-ray irradiance (on the right axes), NAA/24.00 kHz signal phase (**a**) and amplitude (**b**) enhancement during M1.0 (09:06 UT) events on 11 June 2014.  
values decrease more slowly than the flux due to ionosphere sluggishness.