Timing of development the first amplitude minimum, SR1 is in good correlation with time when sunrise terminator reaches the middle part of the propagation path. The phase transition from nighttime to daytime condition passes through the peak, while the amplitude drops to lower values. In the process of changing daytime to nighttime condition, when sunset terminator reaches middle part of path there is occurrence of the last amplitude minimum, SS2. Development of the amplitude minimum SS2 is followed with well recognized peak in phase values.
During daytime condition over path, two amplitude minima labeled as SR2 (morning) and SS1 (afternoon) are observed. The lowest values of these amplitude minima are recorded under solar zenith angle χ=-74° and χ=71°, respectively. The amplitude minimum SR2 is followed with suddenly decreasing of phase values, while the amplitude minimum SS1 is in correlation with increasing phase values.
DHO-BEL, ICV-BEL and NSC-BEL paths and great segment of GQD-BEL path are in the same time zone. Middle of GQD-BEL and NSC-BEL propagation paths are far apart in longitude for 7°. All the paths are similarly illuminated during daytime condition while there are differences in the level of illumination during dawn and dusk in accordance to geographic coordinates of transmitter. Based on these facts, our results are as follows:
1.
The process of ionization in the D-region begins when solar zenith angle reaches value χ=-99°, and sunrise terminator reaches the height h=95 km. When this process starts in the middle of the path, it causes the changes of altitude profile of ionospheric conductivity and the appearance of the first amplitude minimum.
2.
Our results based on the simulation of the VLF/LF propagation over short paths (D