# Low Ionosphere under Influence of Strong Solar Radiation: Diagnostics and Modeling
## 4 Discussion and Conclusions  
In this paper we analyzed around 200 solar X-ray flares events during the 2003-2017 period. The perturbed daytime VLF amplitude and phase data are obtained by recording the GQD/22.10 kHz radio signal for a GCP = 1982 km between the transmitter in Anthorn, UK and the receiver in Belgrade, Serbia. We inspect the magnitude of impact of these \(\sim\)200 SFs on Earth atmosphere and consequences of these explosive events. The VLF data and important Wait's parameters--during the enhancements of X-ray flux due to these SFs--are presented and discussed. We would like to emphasize that, for the first time, we analyzed the D-region in detail; we obtained ionospheric parameters and electron density profile during consecutive huge SFs which continuously perturbed this region for a few hours. The results show that the impact of subsequent flares can be higher than expected and cannot be analyzed as a stand-alone case without analysis of the state of the ionosphere in the moments before their starts. Furthermore, the results of the ionospheric parameters presented in this paper are compared with the results of other authors in literature and we conclude that they are in good agreement with the findings of most authors. It can be noticed that the intense solar radiation, namely solar extreme events, lead to an increased electron production rate and can increase electron density up to a few orders of magnitude, depending on the flare intensity with distortion of the amplitude and phase VLF signal. In addition, we give a simple approximative formula for altitude electron density profile as a function of X-ray flux which is valid for nonperturbed and also for the perturbed D-region. The results confirmed the advantageous usage of the presented method for investigation of solar-terrestrial coupling processes as well as for detection and analysis of space weather phenomena such as solar explosive events. The analyzed data can be used in studies of the radiation characteristics before its impact on the atmosphere, modeling of VLF radio propagation in the lower ionosphere, modeling of disturbed ionosphere, etc. We note that the research requires multidisciplinary approach together with application of various models [45; 46]. The following are available online at [[https://www.mdpi.com/article/10.3390/app11167194/s1](https://www.mdpi.com/article/10.3390/app11167194/s1)]([https://www.mdpi.com/article/10.3390/app11167194/s1](https://www.mdpi.com/article/10.3390/app11167194/s1)), Figure S1: X-ray flux, amplitude and phase of GQD signals during occurrence of M3.7 class solar flare on 09 Sep 2017, Figure S2: Schematic presentation of a method for atmosphere electron density modeling. Figure S3: X-ray flux, perturbation of amplitudes and perturbation of phases and the corresponding electron density at reference height on 28 Oct 2003 during X17.2 class solar flare, Figure S4: The electron density at reference height calculated for this study together with the results of other authors, Table S1: Data of amplitude and phase perturbations of VLF signal and ionospheric parameters induced by different SF events analyzed in this study, Table S2: VLF receiving and transmitting sites, Table S3: Data needed for approximative function. Conceptualization, V.A.S. and D.M.S. ; Formal analysis, D.M.S., V.V. and V.A.S. ; Validation, V.A.S., L.I., V.V. and D.M.S. ; Visualization, V.A.S., D.M.S. and V.V. ; Writing--original draft, V.A.S. and D.M.S. ; Writing--review & editing, V.A.S., L.I. and D.M.S. All authors contributed equally to this work. All authors have read and agreed to the published version of the manuscript. This work was funded by the Institute of Physics Belgrade and Astronomical Observatory Belgrade (contract 451-03-68/2020-14/200002), through the grant by the Ministry of Education and Science of the Republic of Serbia.