7. Oblique Sounding Experiments

[28]  The explanation of the anomalous values of the "o" waves absorption measured by the A1 method was able to solve only part of the problem. It was necessary to find the cause of the discrepancy between the gas-kinetic values of the collision frequencies and the "x" evaluations and also the results of the oblique diagnostics [Anyutin et al., 1985; Baulch and Butcher, 1988]. In this case the waves do not reach the transformation region. Special experiments were conducted on simultaneous diagnostics of n_e by "x" waves using the methods of vertical and oblique (OS) sounding of the ionospheric F region [Beley et al., 1990]. The essence of the experiments was the following.

[29]  The midlatitude path Vladikavkaz-Kharkov about 940 km long was used for OS. Monochromatic waves at frequencies close to 10 and 13.5 MHz were emitted at Vladikavkaz. The signals were received near Kharkov by the decameter radiotelescope UTP-2. The interference structure of the field at the passing of the dead zone boundary over the reception point was registered. The depth of its modulation (the electron concentration vertical profile being known) was used to find the effective collision frequency in the F region. The necessary conditions were realized in the dawn and dusk hours when the D and E regions were weakly pronounced. In Rostov on Don, approximately at a distance of 130 km from the middle point, a vertical diagnostics of the collision frequency by the A1 method was conducted using waves of ordinary and extraordinary polarization. The experiments were carried out during 10 days in wintertime and gave the following results.

[30]  The empirical estimates of the collision frequency according to the data of OS diagnostics did not coincide with the results of the VS diagnostics. Moreover, it was found that the OS estimates increase with an increase of the sounding frequency and that the attenuation of OS signals varies proportionally to the frequency squared and exceeds the theoretical evaluations by a factor of about 10-20. Thus the idea on the existence of "hidden" parameters not taken into account by the gas-kinetic theory was finally buried. The character of the frequency dependence of the attenuation led to the conclusion that at propagation of radio waves with frequencies close to MUF the main input into the energetic losses in the F region is provided by the multiple scattering of the waves into the topside ionosphere at irregularities of the electron concentration with the dimensions much larger than the wavelength [Beley et al., 1990; Bronin et al., 1991, 1993].

[31]  Besides the experiments on determination of the attenuation of HF waves from relative measurements of the signal levels at vertical and oblique sounding of the ionosphere, numerous measurements of the absolute values of the field magnitude of HF transmitters were conducted in the Rostov State University. The experiment was carried out at five midlatitude paths from 15 to 500 km long. Different orientation of the paths relative the field-aligned irregularities provided different conditions for the scattering. The effect of the transformation of "o" waves into "z" waves was possible only at the shortest path. In all other cases the effect was impossible. The experiment covered periods of both low and high solar activity. The working frequencies were in the 0.5-0.95 MUF range. This provided reflection of the entire emission (including the scattered component) back to the Earth surface. The choice of such small path length and working frequencies provided signal penetration up to the heights of the F -region maximum. In this very condition the collision absorption, which depends on the profile, is manifested in the maximum degree.

[32]  The experiment included measurements of the mean values of the field strength ( E ) with temporary separation of the rays at synchronous pulse oblique sounding in the conditions of the checking of the emitted power and state of the ionosphere. The checking of the state of the ionosphere was provided by the height-frequency characteristics of VS in the point of emission. The standard derivative of the equipment determined by the accuracy of the checking of the emitted power and measurements of the field strength did not exceed 2 dB.

[33]  The experiment included 12 stages and was conducted both in the daytime and at night. The measurements at each stage were conducted every day at the same time at 8 fixed frequencies, not less than 15 min at each frequency. The duration of one stage was 10-15 days and was determined by the condition that the statistical error of the averaging of E over all days should not exceed the equipment error. On the whole, 2600 15-min series were conducted and processed. This makes it possible to consider the measurement results statistically significant. The information on the paths and stages, solar activity ( W ), frequency ranges, and the number of the 15-min series n is presented in Table 2. The detailed information on the experiments was published by Barabashov et al. [1997].

[34]  The values of the field strength of discrete rays obtained in the scope of the experiment were compared with the calculated values by the method proposed by Barabashov et al. [1983]. The method provides determination of the loss along the ray trajectories in the magneto-active spatially irregular ionosphere. A constant profile close to the gas-kinetic one was used in the calculations. The coincidence degree of the measured and calculated values of the field strength was estimated by the value

where E_e and E_t are the averaged over the stage experimental and calculated values of the field strength in dB. Here is the field strength derived from the hourly mean data of the current VS. The results of the D L evaluation by this method are shown in Table 2. The values of D L averaged over all measurement stages for solar maximum and minimum were 1.2 and 0.1 dB. They are less than the instrumental errors of the experiment (2 dB).

[35]  The experimental results were also compared to the calculation estimates obtained for the corresponding conditions by the Kazantsev-Smith [Aparina et al., 1972] and International Radio Consultative Committee (CCIR) [1982] methods. The calculations led to controversial results. The Kazantsev-Smith method gave the values of the field strength below the measured values on the average by 8.2 and 4.6 dB in the periods of maximum and minimum solar activity, respectively. In the CCIR method, vice versa, the calculated values stably exceed the observed values. For the maximum and minimum of solar activity the discrepancy was on the average 4.8 and 3.1 dB, respectively.

[36]  Thus a correct analysis of the experimental data showed that at midlatitude paths with the length less than 500 km, the main mechanism of attenuation of HF waves is the collision absorption which is fairly reliably evaluated using the gas-kinetic model of n_e(h).

[37]  The contradictions revealed in the VS and OS experiments (at different frequencies relative to the MUF) gave rise to the development of the theory of the electromagnetic emission transfer taking into account the multiple small-angle scattering in the randomly irregular magneto-active plasma. [Bronin and Zabotin, 1992]. Application of this theory to calculations of the reflected by the plane ionosphere electromagnetic emission of point-like source revealed an important feature. As a result of the scattering much more energy leaves the ray tube formed by the rays close to the vertical than comes in from the adjacent ray tubes. [Zabotin et al., 1998]. At measurements of the HF wave absorption by the A1 method this deficit is manifested in the additional collisionless attenuation. At a distance of about 100 km from the transmitter the effect becomes negligible. Currently, this point of view is confirmed by the results of the field strength measurements at frequencies below MUF and makes it possible to improve the agreement between the A1 experimental data and the theory [Bronin et al., 1999].

[38]  It is worth noting that the conclusions based on the experiment results provided a base for the creation of the HF channel model [Barabashov and Anishin, 2002; Barabashov and Vertogradov, 1996, 2000; Barabashov et al., 2001] and development of new approaches to their imitation simulation [Vertogradov, 2003; Vertogradov and Mineev, 2003]. During recent years the imitation model of the ionospheric HF channel was thoroughly tested experimentally at midlatitude paths of various orientation and length from 1000 to 6500 km [Vertogradov et al., 2004]. Modeling the radio signal propagation, the following principal statements are used. The mean field is determined as a result of incoherent summation of the fields of all possible rays in the reception point taking into account multiple propagation modes and antenna parameters.

[39]  The number of rays and their parameters are determined using the geometry-optical approximation on the basis of the solution of the eichannel equation solution and transfer with allowance for the magnetic field of the Earth.

[40]  The calculated signal characteristics at the reception point are: the field strength, arrival angles, group and phase paths, collision absorption, spatial attenuation, losses due to the reflection from the ionospheric E_s layer, losses at the reflection from the Earth for multiple propagation modes, and polarization discordance. The propagation medium is taken corrected by VS data using the IRI model of spatial distribution of the electron concentration. [Bilitza, 2001, 2002] and by the effective electron collision frequency. The partial collision frequencies with neutrals are calculated using the MSIS model of the neutral atmosphere and scattering cross sections [Banks, 1966; Gurevich and Shvartsburg, 1973]. The electron temperature and geomagnetic field parameters are also chosen according to the IRI model.