# Low Ionosphere under Influence of Strong Solar Radiation: Diagnostics and Modeling
## 3 Results
### Intense Solar Radiation: Consecutive Strong SF  
We analyzed data for 10 June 2014 as an example of a day with several strong and successive SFs that induced SIDs and dramatically affected the VLF radio signal. Specifically, on 10 June 2014, the Sun released several consecutive SFs including two major X-class solar flares that occurred one after the other as shown in Table 1.  
Figure 6: The sharpness \(\beta\), effective reflection height \(H^{\prime}\), and electron density at reference height \(h\) = 74 km during occurrences of M3.7-class solar flare on 9 September 2017 from lower to upper panel, respectively. Figure 7 presents simultaneous variations of X-ray flux, amplitude, and phase of GQD radio signal against universal time during five successive flares on 10 June 2014. From Figure 7, one can see that VLF and X-ray flux peaks happened simultaneously, i.e., intense radiation instantly disturbed ionosphere, creating SIDs and resulting in perturbed VLF signal. During the time interval of 16 minutes, X-ray radiation of the second X-class flare reached a maximum. During the same interval, the shape of the phase changes is more noticeable with well-defined turning points on the graph of measured phase values as a function of time. Throughout this X1.5-class flare, the height of reflection dropped from 74 km to 60.85 km following the increase in \(\beta\) from 0.30 km\({}^{-1}\) to 0.54 km\({}^{-1}\)  
Figure 8 shows calculated values of time-dependent effective reflection height \(H^{\prime}\) and the sharpness \(\beta\) (with the time resolution of one minute) for the time period 08:00 to 14:30 UT during occurrences of five solar flare on 10 June 2014. During occurrences of X2.2- and X1.5-class SFs, the variation of \(H^{\prime}\)(t) with time has expected behavior: after SF starts, it drops rapidly to a minimum as expected, and after X-ray flux peaks, it keeps rising to a referent value (see Figure 8 middle panel). Behavior of \(\beta\)(t) is correlated with the shape of recorded increase in amplitude. Practically during this SF of X-class, it rises rapidly to a maximum as expected after the peak time of X-ray flux and drops to a referent value. At 11:42 UT, during peak time of X2.2-class SF, with the solar X-ray flux \(\mathrm{Ix}\) = 2.22 \(\times\) 10\({}^{-4}\) Wm\({}^{-2}\), the reflection height lowers by 14.9 km to a value of \(H^{\prime}\) = 59.1 km and the sharpness values rises to \(\beta\) = 0.53 km\({}^{-1}\). At peak time 12:52 UT, during X1.5-class SF with the solar X-ray flux \(\mathrm{Ix}\) = 1.55 \(\times\) 10\({}^{-4}\) Wm\({}^{-2}\), the reflection height lowers to \(H^{\prime}\) = 60.85 km and the sharpness values rises to a same value as for X2.2. During C-class SFs, the variation of reflection height \(H^{\prime}\) and the sharpness \(\beta\) are in correlation with X-ray flux. It can be noted that it is very difficult to analyze and model the D-region in the case of strong SFs. Therefore, the results related to ionospheric parameters of some authors have some deviations (see Section 3.5). An even more difficult case for modeling is when we have consecutive flares which are continuously perturbing the D-region. During consecutive huge SFs which repeatedly perturb this region for a few hours, the ionosphere lacks time for regeneration and returns to a normal condition. The flares follow each other, literally sticking together and amplifying their influence in the D-region. Therefore, we have the case that a weaker SF, i.e., X1.5-class, induces similar changes in amplitude as the stronger flare of class X2.2 (\(\Delta A\sim\) 7.4 dB and \(\Delta A\sim\) 7.3, respectively). It is also necessary to point out that the solar zenith angle and the duration of flare have an impact on the parameters of the D-region. Parameters \(H^{\prime}\) and \(\beta\) are used to determine the spatial structures of disturbances in the lower ionosphere.