6. Analysis of the Discrepancies Between the A1 Method Data and Theory

[23]  On of the first hypotheses aimed to agree the gas-kinetic and empirical estimates of the effective electron collision frequency for the F region was suggested by Setty [1972]. Because of the complication and low reliability of laboratory measurements of the electron scattering cross sections at atoms of oxygen Q_O he have made their correction (revision) in the direction of a considerable increase relative to the known values [Banks, 1966]. The hypothesis was checked by Setty [1972] in the following way. Using the empirical estimates of n_e for the maximum of the F region, known values of N, model values of the neutral concentration and electron temperature T_e, he reached (because of the increase of Q _O ) a complete agreement between the theoretical and experimental results. Since T_e was changing from one measurement to another, determination of the Q _O(T_e) became possible. It was found that on the whole the cross section should be increased by more than two orders of magnitude [Setty, 1972].

[24]  The Setty [1972] hypothesis was checked by Vodolazkin et al. [1983], who used the empirical model of n_e(h) for four seasons. For every season a complete agreement between the theoretical and experimental results at heights of 150-200 km was obtained. Because of T_e variations with height a determination of the Q _O(T_e) became possible. One could expect that (if the Setty hypothesis is correct) the Q _O(T_e) functions for all seasons would be similar and agree with the Setty [1972] estimates. However, it appeared that first, Q_O(T_e) varies from one season to another and, second, the character of the temperature dependence differs from the results of Setty [1972]. In the case of Vodolazkin et al. [1983], Q _O(T_e) increases with an increase of the electron temperature, whereas Setty [1972] obtained a decrease of Q _O(T_e) with T_e. Therefore one cannot reach a coincidence of the theory and experiment reconsidering the scattering cross sections.

[25]  It was found later that the use of the absorption of waves of different polarizations obtained at the same time to reconstruct n_e(h) profiles gives different results. The collision frequency estimated from the "o" waves absorption always is higher than analogous results obtained using "x" waves. [Denisenko et al., 1987a]. This fact initiated a modernization of the Vodolazkin et al. [1983] empirical model. On the basis of 392 simultaneous of simultaneous measurements of the absorption of "o" and "x" waves in the daytime in 1988 ( F_10.7=140 ) empirical models of n_e(h) at heights of 100-200 km were created for each month [Vodolazkin et al., 1993]. In the same way as in the Vodolazkin et al. [1983] model the experimental estimates exceeded the gas-kinetic ones above 150 km. The mean empirical values of n_e for the 150-200 km height interval are shown in Table 1.

[26]  Table 1 shows that for the conditions of Rostov on Don the collision frequencies in the F region measured using "o" waves are by a factor of 2-2.5 higher than the estimates obtained using "x" waves [Vodolazkin et al., 1993]. The latter are higher than the gas-kinetic ones by a factor of 5-6. This is confirmed by the results of independent measurements [Setty et al., 1970].

[27]  The dependence of the results of the n_e diagnostics on the polarization of the sounding signals led to the conclusion on the need for a search of collisionless mechanisms of HF wave attenuation. Therefore a hypothesis was suggested that the cause of the discrepancy between the "o" and "x" estimates lies in the anomalous absorption of "o" waves caused by their transformation into slow extraordinary ( z ) waves due to the scattering at irregularities of the electron concentration in the vicinity of the reflection level [Denisenko et al., 1987a, 1987b]. It was found that to explain the observed effects the relative fluctuations of the electron concentration should be (2-4) 10^-3 [Vodolazkin et al., 1989b]. The latter value does not exceed the observed fluctuations. Indirect confirmation of the transformation effect may be found in several publications [Denisenko et al., 1987b, 1987b; Korovin, 1984]. Korovin [1984] found a pronounced correlation between the observations of the enhanced derivative absorption of "o" waves during VS of the ionosphere in the vicinity of the F region critical frequency (the R condition) and appearance of field-aligned irregularities with the transverse dimensions of a few meters detected by the aspect scattering method in the VHF range. Denisenko et al. [1987a, 1987b, 1987a] explained the presence of diffusive traces in ionograms of topside sounding by the transformation effect of "o" waves into "z" waves (We discuss the frequency interval from f _max = max (f_ps, f_H) to f _UHF, where (f_ps and f_H) are the plasma frequency and gyrofrequency of electrons, respectively, and f _UHF is the upper hybrid frequency in the vicinity of the satellite.) The idea of attracting of collisionless absorption mechanisms appeared to be fruitful while interpreting the results of the HF absorption measurements by the A2 method in the high-latitude ionosphere. Gel'berg [1986] and Berezhko et al. [1987] showed that the extra attenuation is due mainly to the interaction of the waves to the plasma turbulent pulsations.