8. Conclusions

[41]  The gas-kinetic estimates of the effective electron collision frequency adequately describe the HF wave absorption in the midlatitude ionosphere [Barabashov et al., 1997; Benediktov and Tolmacheva, 1975; Skrebkova, 1975]. The estimates need improvement only in the sense of specification of the cross sections of electrons at atmospheric constituents and models of their concentration and electron temperature.

[42]  The discrepancy between the gas-kinetic values of the collision frequencies in the E region and empirical estimates obtained in the early experiments [Aggarwal et al., 1979; Setty, 1972; Thrane and Piggott, 1966] are caused by the errors in determination of the virtual heights [Vodolazkin et al., 1989a]. Similar discrepancies in the ionospheric F region manifest the presence of additional collisonless mechanisms of radio wave attenuation. The mechanisms may be dissipative or nondissipative.

[43]  An essential role is played by the nondissipative mechanism: small-angle scattering at electron concentration irregularities. At the ground-based sounding of the ionosphere this scattering leads to such redistribution of the reflected emission which gives an energy deficit in the vertical direction [Zabotin et al., 1998]. At the oblique propagation in the vicinity of MUF the scattering leads to outgoing of the emission into the topside ionosphere [Beley et al., 1990; Bronin et al., 1991]. The scattering does not play any significant role at the reception of the signals of space sources at middle latitudes when the coherent and incoherent components of the emission pass the ionosphere [Benediktov and Tolmacheva, 1975; Skrebkova, 1975]. Neither influence the scattering the intensity of the emission at the oblique sounding at frequencies below MUF when both emission components come to the observation point [Barabashov et al., 1997].

[44]  Two mechanisms of dissipative losses are revealed. One mechanism is observed in the A2 method data in the high-latitude ionosphere and is explained by the interaction of radio waves with the developed plasma turbulence [Berezhko et al., 1987; Gel'berg et al., 1985]. (The nature of the extra attenuation of the radioemission of space sources in the low-latitude ionosphere is not yet known.) The second mechanism is anomalous absorption of "o" waves caused by their statistical transformation into "z" waves [Denisenko et al., 1987a, 1987b; Vodolazkin et al., 1989a, 1989b]. As mentioned above, one of the arguments in favor of its existence is the observation of diffusive traces at topside ionograms in the frequency range from f _max to f _UHF [Denisenko et al., 1987a, 1987b, 1987a]. However, recently an alternate explanation (scattering of "z" waves) was proposed [Zabotin et al., 1997]. So direct observation of the transformation effect presents an obvious interest. This can be realized in special experiments on transionospheric sounding receiving pulse signals of ground-based transmitters on board a satellite orbiting below the ionospheric maximum [Denisenko et al., 2000].

[45]  On the basis of the results of the study a model of HF signal propagation in the ionospheric plasma was developed [Barabashov and Anishin, 2002; Barabashov and Vertogradov, 1996, 2000; Barabashov et al., 2001; Vertogradov, 2003; Vertogradov and Mineev, 2003].