Based on the observed and presented data on Fig. 7b it is easily to summarize the following:
1.
On GQD-BEL path amplitude decreases during occurrences of four C class solar flares (Fig. 7a, panel number 2). The size of amplitude perturbations is proportional to the intensity of solar X-ray flux. A phase change on this path is complicated, displaying increase and also decrease during the occurrence of C7.53 (7.53·10-6 Wm−2) class solar X-ray flare.
2.
On DHO-BEL path the amplitude and phase increase and the size of amplitude and phase perturbations are in correlation with the intensity of solar X-ray flux.
3.
On ICV-BEL path both amplitude and phase increase with the changes of intensity of X-ray flux. The shape of curves of amplitude variation with time for DHO-BEL and ICV-BEL paths are very similar to each other. The size of amplitude perturbations on DHO-BEL and ICV-BEL paths caused by same solar X-ray flare are similar to each other (Table 5).
4.
Phase perturbations on NSC-BEL path display more sensitivity to the changes on intensity of solar X-ray flux than amplitude. During the occurrence of C2.65 (2.65·10-6 Wm−2) class solar X-ray flare there is no visible development of amplitude perturbation, while the phase increase is significant.
During March 2011 we examined thirty events of solar X-ray flares that occurred in the time sector for solar zenith angle -60°⩽χ⩽60°. Intensity of those X-ray flares was in range from C1 to M5.3 (1·10-6 to 5.3×10-5 Wm−2) class. For each flare we measured amplitude and phase perturbations on four VLF/LF paths, except when the transmitter was off-air. In Fig. 8b there are data points of the observed GQD/22.10 kHz, DHO/23.40 kHz, ICV/20.27 kHz and NSC/45.90 kHz amplitude and phase perturbations as a function of solar X-ray flare intensity, respectively.