## 3 Results  
### Lower Ionosphere during SF Event  
The monitoring and investigation of VLF data has been carried out simultaneously with the examination of the correlative incoming solar X-ray fluxes collected from the Geostationary Operational Environmental Satellite (GOES).  
In the presence of SIDs, a standard numerical procedure for the estimation of plasma parameters is based on comparison of the recorded changes of amplitude and phase with the matching values acquired in simulations by the LWPC numerical software package as explained in [9; 44; 46; 47].  
We studied data for 7 September 2017, as an example of a day with strong X1.3 class SFs that caused SID and seriously affected the VLF signal. It should be noted that the development to the flux of X-ray lasted almost one hour.  
The monitored and simulated changes of amplitude (\(\Delta A\)) and phase delay (\(\Delta P\)) compared to normal day-time levels for a signal emitted on frequency 24 kHz from Cutler (44.65\({}^{\circ}\) N, 67.30\({}^{\circ}\) W), ME, USA, with codename NAA measured at the BEL site is presented in Figure 1. Besides the good agreement between simulated and monitored data, X-ray flux peak and VLF signal perturbation happened synchronously i.e., energetic radiation perturbed the ionosphere which, in turn, caused the activation of disturbances in radio signal.  
Figure 1: Measured and simulated amplitude (**a**), and phase delay (**b**) excesses of NAA radio signal during X1.3 class SF on 7 September 2017.  
Figure 2 presents obtained values of effective reflection height \(H^{\prime}\) and the sharpness \(\beta\) during almost two hours, i.e., during occurrences of SFs on 7 September 2017. During X1.3 class SFs, the shape of reflection height is in anticorrelation and the sharpness is in correlation with X-ray flux.  
The change of the reflection height is a normal behaviour, i.e., after the beginning of SFs the reflection height decreases to a minimum and after peaks of X-ray flux it rises to the preflare value (see Figure 2a). The shape of sharpness is correlated with the shape of recorded X-ray flux increase. During the X1.3 SF, the sharpness increases to a maximum and after the SF peak (14:36 UT) it decreases to the pre-flare value (Figure 2b). At peak time (14:36 UT), the X-ray flux is \(I\)x = 1.3 \(\times\) 10\({}^{-4}\) Wm\({}^{-2}\), and the \(H^{\prime}\) lowers by 13 km to a value \(H^{\prime}\) -63 km. The sharpness values increase to 0.42 km\({}^{-1}\) at peak time 14:36 UT on 7 September 2017.  
Electron density profile changes can demonstrate the variation in distribution of ionization at the D-layer due to SFs. Figure 3a presents the simultaneous change of X-ray flux, and the corresponding \(Ne\) (Figure 3b) obtained at reference height 74 km versus UT during occurrence of X1.3 class SF on 7 September 2017. It can be seen that the electron density is in correlation with X-ray flux and, \(Ne\) increased by almost two orders of magnitude during the X1.3 class SF.  
The current study is important because most results concerning investigation in the D-region are scattered and vary in order of magnitude (see [9; 23; 52; 53]), thus it is necessary to take into account all the data and correct the existing data. Sudden events induce changes in the ionosphere and, consequently, the electron density height profile. It is useful to obtain new results by analysing the similarities and differences, especially for case studies, i.e., when solar flares last significantly longer and perturb the ionosphere in a specific way. An additional aim is the simultaneous analysis of the lower ionospheric response and electron density height profile variations in the lower ionosphere induced by sudden events during daytime solar flares and during night-time lightning-induced electron precipitation.  
Figure 2: Parameters \(H^{\prime}\) (**a**), and \(\beta\) (**b**) and X-ray flux (right axis) during occurrence of X1.3 class SF on 7 September 2017.