# Low Ionosphere under Influence of Strong Solar Radiation: Diagnostics and Modeling
## 3 Results  
In this research we studied the amplitude (\(A\)) and phase (\(P\)) data, obtained by monitoring VLF signals emitted by worldwide transmitters during solar-induced SIDs. The VLF signals which we examined are so-called sky waves, which travel via Earth-ionosphere duct and fluctuate in real-time due to the physical conditions through duct [34]. On the contrary, so-called ground waves closely follow and interact with terrestrial surface, and are attenuating. This category of EM waves is not investigated here.  
All the data were registered by two receiver systems at the Belgrade site: AbsPAL and AWESOME. The stations can monitor VLF signals under different time resolutions. These kind of monitored data must be independently metabolized and differently processed. The receivers can simultaneously collect several signals emitted by different emitters (located at different countries and territories) at the fixed frequencies. This makes it possible to monitor huge regions of the Earth and enables detection of local and global perturbations. The quality of the monitored data depends on the characteristics of the emitters, i.e., the geophysical location and the emitted power. Furthermore, the quality of the recorded data depends on the length of the VLF route, physical properties of the Earth-ionosphere duct, electromagnetic pollution, etc.  
The technicalities and description of the Belgrade site are presented in [22]. Location of emitters and the receiver site and GCPs of subionospherically propagating VLF signals are provided in Figure 1 and also in Table S1 in online material. Here, we present our results of amplitude and phase perturbations on the GQD/22.10 kHz radio signal observed over a long time period. Behaviors of amplitude and phase data on VLF radio signals induced by different intensity of solar X-ray fluxes and observed on Belgrade site were examined. These results are presented.  
Figure 1: The geographic location of Belgrade system of VLF receivers (blue circle) and the GQD transmitter (red circles) (54.73\({}^{\circ}\) N, 2.88\({}^{\circ}\) W) Anthorn UK with GCP of sub-ionospheric propagating VLF signal.  
The signal propagates from the GQD transmitter to the receiver site over the great circle distance of 1982 km along a short and mostly operand great circle path that covers two time zones (propagating from Anthorn (54.72\({}^{\circ}\) N, 2.88\({}^{\circ}\) W), UK to Belgrade (44.85\({}^{\circ}\) N, 20.38\({}^{\circ}\) E)--see Figure 1. The GQD transmitter is located in the UK, but a great segment of the radio signal propagates over path in the same time zone where the receiver is.  
We investigated VLF signal changes and observed that the shape of the signal curve changes dramatically with time. Characteristic examples of SID event that happened during occurrence of strong SF is presented in Figure 2. SF X3.6-class on 9 October 2005 lasted from 09:42-10:08 UT with a peak at 09:59 UT. The dramatical changes of GQD amplitude and phase values at peak time can be observed.  
The monitoring and investigation of VLF data was carried out simultaneously with the examination of the corresponding incoming solar X-ray fluxes [36; 37]. The intensity of X-ray flux _lx_ is recorded by GOES satellite in the band 0.1-0.8 nm, available from [38] and [36]. SFs can be classified in categories based on the peak emission recorded by GOES satellite in the 0.1-0.8 nm band, namely from A-, B-, C-, M-, and X-classes.  
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 [27; 30] as explained in [26; 29; 39]. Furthermore, for details, see Section 2.