5. Conclusions

[60]  Almost during the entire 2005, we conducted systematic observations of the conditions of ionospheric propagation of HF radio waves at the paths of various length and orientation. The results may be formulated in the following way.

[61]  1. On the basis of the analysis of experimental and calculated data of the oblique sounding at midlatitude paths of various length and orientation, it is demonstrated that the best results on forecasting of MUF of particular propagation modes are obtained at correction of the IRI 2001 model by the effective solar activity index.

[62]  2. The standard mean deviations of MOF from their monthly mean mathematical expectations grow with an increase of the path length. Typical values of the standard deviation values in quiet geophysical conditions are estimated for paths of various length and orientation. In the daytime the typical standard deviations of MOF are: 1.2 MHz (Cyprus—Rostov-on-Don), 1.7 MHz (Inskip—Rostov-on-Don), 2.0 MHz (Norilsk—Rostov-on-Don), 2.2 MHz (Irkutsk—Rostov-on-Don), and 2.5 MHz (Magadan—Rostov-on-Don). At night the typical standard deviations of MOF are 1.0 MHz (Cyprus—Rostov-on-Don), 1.5 MHz (Inskip—Rostov-on-Don), 1.5 MHz (Norilsk—Rostov-on-Don), 1.5 MHz (Irkutsk—Rostov-on-Don), and 1.6 MHz (Magadan—Rostov-on-Don). No seasonal dependence of the standard deviation was found as a result of the measurements at the Cyprus—Rostov-on-Don and Inskip—Rostov-on-Don paths.

[63]  3. At all paths almost always, MOF undergo short-period variations with quasi-periods from 20 min to 1.5 hour. The fluctuation amplitude can reach 2 MHz in quiet ionospheric conditions at noon hours of the day. In separate days in dawn-dusk hours at the passage of the terminator, the fluctuations at the Cyprus—Rostov-on-Don path can increase up to 5-8 MHz.

[64]  4. For various seasons, the spectra of MOF fluctuations are of a well-pronounced linear structure. The power of spectral components of MOF fluctuations is concentrated in the 20-90 min range. The spectral composition of the MOF fluctuations varies from one day to another, though some quasi-harmonic components can be present in MOF fluctuations spectra during several days.

[65]  5. It is found that TID with periods 15-30 min correlate well with sunrise and sunset for the middle point of the Cyprus—Rostov-on-Don path. The latter fact manifests an important role of the mechanism of generation by the terminator of acoustic-gravity waves (AGW). TIDs are the ionospheric response of AGW.

[66]  6. The imitation modeling confirmed that the z-type features in ionograms are related to the traveling ionospheric irregularities. The modeling showed that the conditions for z-type features formation are rather tough and depend on the relative amplitude of TID, wavelength of the disturbance, and the direction of its propagation relative to the radio path orientation. On the basis of the comparison of the experimental and calculated data of oblique sounding, TID parameters are estimated. It is shown that TID with the relative amplitude of electron density disturbance dN 15 - 20%, wavelength of 150 km, and period of 15 min propagating downward at the angle of 30^o-60^o to the horizon can be responsible for the observed z-type features at the frequency-time ionogram registered at the Cyprus—Rostov-on-Don path.

[67]  7. The behavior of the measured frequency-amplitude ionograms can explain by the interference of nonseparated discrete rays, their phase not being a random value.

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