## 3 Results and Discussion
### Analyses of SF Events
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & & **SF Tim** & \multicolumn{4}{c}{**SID VLF Signatures**} & \multicolumn{2}{c}{**SF Data**} \\ \hline \begin{tabular}{c} **SF** \\ **(Class, Date)** \\ \end{tabular} & **Start [UT]** & **Peak [UT]** & **End [UT]** & \(\Delta\)\(\Delta\)**[dB]** & \(\Delta\)\(P\)**[deg]** & \(\Delta\)**t [min]** & \begin{tabular}{c} \(I_{x}\) **max XL** \\ **[Wm\({}^{-2}\)]** \\ \end{tabular} & \begin{tabular}{c} **Active** \\ **Region** \\ \end{tabular} \\ \hline \begin{tabular}{c} M1.3 \\ 10.05.2013 \\ \end{tabular} & 12:37 & 12:56 & 13:04 & 4.20 & 68.19 & 0 & \(1.36\times 10^{-5}\) & 1745 \\ \hline \begin{tabular}{c} C2.5 \\ 10.05.2013 \\ \end{tabular} & 14:30 & 14:37 & 14:42 & 1.06 & 19.20 & 1 & \(2.59\times 10^{-6}\) & 1745 \\ \hline \begin{tabular}{c} M1.8 \\ 12.06.2014 \\ \end{tabular} & 09:23 & 09:37 & 09:42 & 4.50 & 51.47 & 4 & \(1.81\times 10^{-5}\) & 2085 \\ \hline \begin{tabular}{c} M2.7 \\ 12.06.2014 \\ \end{tabular} & 10:14 & 10:21 & 10:27 & 5.72 & 83.01 & 2 & \(2.74\times 10^{-5}\) & 2087 \\ \hline \
\begin{tabular}{c} X1.0 \\ 11.06.2014 \\ \end{tabular} & 08:59 & 09:06 & 09:10 & 5.54 & 90.45 & 3 & \(1.00\times 10^{-4}\) & 2087 \\ \hline
\begin{tabular}{c} X1.3 \\ 07.09.2017 \\ \end{tabular} & 14:20 & 14:36 & 14:55 & 4.50 & 159 & 1 & \(1.39\times 10^{-4}\) & 2673 \\ \hline \end{tabular}
\end{table} 
Table 1: The data on SFs, amplitude and phase perturbations of VLF signals caused by different events analysed in this study. Figure 2: Measured variation of amplitude (**a**) and phase (**b**) on NAA signals for 9 May 2013 (quiet day) and 10 May 2013 with noticeable SF events. this region, i.e., at \(h\) = 80 km). These lines have almost the same behaviour. Completely different slope and behaviour have black and dashed dotted-dotted pink lines at peak times of M- and C-class SFs with increment from \(\sim\)7\(\cdot\)10\({}^{7}\) m\({}^{-3}\) at 60 km height to 1.6\(\cdot\)10\({}^{10}\) m\({}^{-3}\) and 4\(\cdot\)10\({}^{9}\) m\({}^{-3}\), respectively. It can be noted that the electron density profiles moved to higher electron densities with different slopes when compared to the density profile of the preflare ionospheric condition. These changes in altitude profile of electron density are important for the nature of VLF propagation. Figure 4: The altitude profile of electron density during two successive SFs on 10 May 2013. Figure 3: Simultaneous variations of the sharpness \(\beta\) (**a**), effective reflection height \(H^{\prime}\) (**b**), VLF amplitude (**c**) and phase (**d**) excess recorded and simulated, electron density at reference height (**e**) and X-ray flux (on the right axis (**e**)) during the occurrence of two successive SFs on 10 May 2013. X-ray flux, amplitude, and phase of NAA/24.00 kHz radio signal as a function of UT during strong M-class successive SF on 12 June 2014 is presented in Figure 5. The lower panel shows perturbation of amplitude and X-ray flux, and in the upper panel perturbation of phase and X-ray flux is presented. We analysed two successive M-class solar flares (M1.8 with peak time 09:37 UT and M2.7 with peak time 10:21 UT). There were visible changes in the VLF daily signal during the duration of M1.8 SF and M2.7 SF. SID amplitude and phase changes at the peak time of SF of M1.8 and M2.7 SF classes are \(\Delta A\) = 4.5 dB, \(\Delta P\) = 51.47 deg and \(\Delta A\) = 5.72 dB, \(\Delta P\) = 83.01 deg, respectively (see Table 1 and Figure 6c,d). Figure 5: X-ray flux, amplitude, and phase of NAA signal during strong successive flares on 12 June 2014. (Lower panel) Amplitude and X-ray flux variation; (Upper panel) phase and X-ray flux variation. Figure 6: Variations of the sharpness \(\beta\) (**a**), effective reflection height \(H^{\prime}\) (**b**), VLF amplitude (**c**) and phase (**d**) recorded and simulated, reference height electron density (**e**) and X-ray flux (on the right axis (**e**)), during the occurrence of two successive M-class SFs on 12 June 2014.