[41] For interpretation of the obtained experimental results and confirmation of the assumptions presented, we performed imitation modeling on the basis of the ray tracing with the use of the imitation model of a wide-band ionospheric radio channel [Barabashov and Vertogradov, 1996; Vertogradov, 2003, 2004]. The modeling is based on the solution of the expanded system of characteristic equations in the ionosphere taken from the International Reference Ionosphere IRI 2001 [Bilitza, 2001, 2002]. The ionospheric collision losses were calculated on the basis of the Appleton approximation with the effective collision frequency found on the basis of the values of the ion and electron concentrations given by IRI 2001 with attraction of the neutral atmosphere model MSIS 90 with the utilization of known ratio [Gershman et al., 1984].
[42] The calculations of MUF of particular propagation modes have shown that the IRI 2001 model, as a rule, gives overestimated results, the discrepancy grows with the increase of the length path. Thus the IRI-2001 model needs an adaptation on the basis of the current geophysical information. Note that the IRI-2001 model assumes correction with respect to the sunspot numbers and the index of solar activity. The correction of the solar activity index influences only on the F region of the ionosphere and does not affect the lower ionosphere which determines the main losses caused by collisions of HF radio waves. As s consequence to provide the adaptation of the model to the experimental MOF values it is required only to correct the index of solar activity. As general geophysical information for correction of solar activity index we took the data on the effective sunspot number W eff [Secan and Wilkinson, 1997]. The results of the modeling showed that in this case one can obtain the predicted MUF values considerably closer to the measured values MOF at paths of all lengths and orientations for all seasons and solar activity levels considered (Figure 4, crosses). The latter result makes it possible to recommend using of predicted or current value of W eff as an index of solar activity in long-term or short-term forecasting of propagation characteristics on the basis of the IRI-2001 model. In that case the difference between the forecasted and monthly mean measured values does not exceed 12%. It is worth noting also that after the operation of correction of the IRI-2001 model by solar activity index, the forecasted values, as a rule, fall into the confidence interval with the width equal to the double root-mean-square value of daily changes of MOF.
[43] For interpretation of the observed features of the high-angle ray, ionograms were calculated taking into account TID [Vertogradov, 3, 2004]. TID were modeled by modulation of the average electron density N0(f, q, r, t) by a harmonic wave [MacDougall et al., 2001; Stocker et al., 2000]. The instant electron density in the disturbed ionosphere at the point with spherical coordinates f, q and r ( r is the distance from the center of the Earth, q is counted from the axis passing the North Pole, and f is the latitude) at the moment of time t is written in the form
| (2) |
where r0 is the Earth radius, dN is the relative amplitude of the disturbance, T is the TID period, F0 is the initial phase, k<undef>f=(2p/L) cosb sina, k<undef>q=(2p/L) cosb cosa, k<undef>g = |2p/L| sinb is the wave vector of TID with the wavelength L, a, and b provide the direction of the phase velocity of the wave disturbance, and Df and Dq are the changes of the spherical coordinates relative to the emitting point.
[44] The modeling of oblique LFM sounding of the ionosphere was performed using (2) in the situation in a maximum degree close both to the solar and geophysical conditions of HF propagation at real path and to the methods of processing of the residual signal. To do that, the structure-physical model of the HF radio channel was completed with a computer imitator of the wideband radio channel [Vertogradov, 3, 2004]. The modeling was performed in three stages.
[45] At the first stage, the radio channel is modeled for the given solar and geophysical conditions and the most important parameters of the complex transmission function of the HF channel in the given frequency band and in the given time interval of the modeling are calculated. In our case the important parameters of the transmission function were calculated within the 4-32 MHz band, the step of the parameters calculation being 0.05 MHz. The time interval of the modeling was chosen equal to 1800 s, the principal parameters being calculated with a step of 10 s within the entire band of modeling.
[46] At the second stage on the basis of the principal parameters of the HF channel, the complex transmission function of the HF channel in the 4-32 MHz band continuous in frequency and time is restored in the time interval 1800 s. A wideband digital signal supplied to the input of the computer imitator of the HF channel is brought through the channel with the found complex transmission function. In our case the LFM signal was supplied to the imitator input, the rate frequency sweeping of the signal being chosen equal to 100 kHz s -1. At the output of the computer imitator, a low-frequency complex residual signal similar in its structure to the real signal at the output of the LFM receiver was obtained. The signal from the imitator output was saved in the file. The file structure corresponded to the program of processing of real residual signal saved in the process of real LFM sounding.
[47] At the third stage a processing of the recorded residual signal by the program of processing of the signal at the output of a real LFM receiver was performed. The modeled residual signal was processed by the program of spectral processing of residual signal for obtaining of frequency-time and frequency-amplitude display ionograms.
[48] As a result, the procedure described makes it possible to make the modeling stage in the maximum degree similar to the conditions of the reception and processing of a LFM signal at real paths of the oblique sounding. In particular, the proposed algorithm of imitation modeling makes it possible to take into account peculiarities related to the fact that frequency-time and frequency-amplitude display ionograms at a real OS path are obtained not instantly but for the sounding interval of about 300 s. The channel properties can be changed during such time interval. Thus the main aim of the complete scheme of the imitation modeling corresponding to the conditions of processing of a complex (linearly frequency-modulated) signal is to take into account dynamical processes in the ionosphere during the sounding session and to reveal effects related to the processing of the LFM signal at the presence of wave disturbances.
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| Figure 8 |
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| Figure 9 |
[50] 1. Ionograms obtained on the basis of the model residual signal agree qualitatively to the ionograms obtained in the process of real oblique sounding of the ionosphere (see Figures 2 and 3 and Figures 8 and 9). In particular, at the presence of TID, the effect of amplitude-frequency modulation of the high-angle ray is observed in the model ionograms calculated with the complete scheme (see Figure 9d) in the same way as in the experiment (see Figure 3b (top), x mode for high-angle ray). In the same time, simple modeling of the ionospheric HF propagation does not reveal this effect. This fact should be taken into account at interpretation of experimental data.
[51] 2. A good agreement of the experimental and calculated ionograms from the point of view of the TID influence on both, the 1F and 2F modes is obtained (see Figures 3c, 3d, 8a, 8d, 9a, 9d, and 9f).
[52] 3. The z-type features at frequency-time display are actually caused by the TID motion in the ionospheric plasma. Taking into account of the finite time of the sounding does not change this conclusion.
[53] 4. The modeling showed that the conditions of formation of z-type features at frequency-time display are tough enough and depend on the relative amplitude of TID, the wavelength of the disturbance, and the direction of its propagation relative to the radio-path orientation. Such traces in frequency-time display appear only under particular relations between the amplitude and wavelength of TID under not very small TID propagation angles to the horizon. For example, for the cases presented in Figure 8, (a-c) the TID parameters were the following: the relative electron concentration dN = 20%, the wavelength L = 150 km, the period T = 15 min, the angle from the horizon b = -30o. Nevertheless, during the whole 15 min period of modeling (3 ionograms) for 1F mode no z-type disturbances with appreciable amplitude were observed. At the same time the increase of the angle b up to -60o caused the appearance of the z-type features at frequency-time display (see Figure 8, (d-f)). As a result, attraction of model calculations makes it possible to estimate TID parameters on the basis of the results of oblique LFM sounding of the ionosphere. It follows from the calculated ionograms that the z-type features are formed only under relative amplitudes dN of medium-scale disturbances exceeding 15% and under the TID propagation angles to the horizon plane from -60o to -30o. In other words, for appearance of characteristic z-type features in oblique sounding ionograms, the vertical scale of the wave disturbance should not cover the entire F region of the ionosphere. In the opposite case, the propagating wave disturbance would lead only to quasiperiodic time variations of MOF of the path, but not to local formations at the frequency-time ionogram.
[54] 5. As far as at the real OS at the Cyprus—Rostov-on-Don path z-type disturbances are observed quite often (especially in the dawn-dusk periods of the day), one can conclude that they are generated by medium-scale TID with relative amplitudes not less than 15-20%, this fact agreeing well with the results presented by Stocker et al. [2000].
[55] As for the registration in the OS ionograms of several high-angle rays in the form of a "comb'' (see Figure 3a), we think that such effect can be caused by a quasi-regular stratification of the electron concentration in the vicinity of the F-layer maximum with a vertical scale of ~10-20 km. In that case, in the vicinity of the F-layer maximum, a comb-like structure is formed with several local maximums of the electron concentration, high-angle rays propagating along these maximums. One can assume that the diffuse background accompanying the comb of the high-angle rays is determined by the rays propagating in a combined way, that is part of the way the high-angle ray propagates along one ridge of the electron concentration and then transits (for example, due to a scattering) to another ridge etc. These assumptions are of a preliminary discussion character. Additional studies are needed for a more substantiated conclusion.
[56] Thus, according to the results of continuous observations during several months of the conditions of ionospheric HF propagation at the oblique sounding paths of various length and orientation, it is found that wave disturbances are present almost permanently. The typical periods of wave disturbances lie within the interval from 15 min to 1.5 hours.
[57] The variety of physical mechanisms of wave disturbance formation of both natural and artificial origin (including passage of the terminator, magnetic storms, volcano eruptions, hurricanes, thunderstorm activity, explosions and other impacts on the near-Earth medium) makes the picture of wave-like disturbances appearance complicated and nonstationary. It is evident that this appearance is caused by the presence of various relations in the solar wind-magnetosphere-ionosphere-atmosphere-Earth system. The physical basis of these relations is provided by generation, propagation, and dissipation of atmospheric waves of various scales including planetary, tidal, acoustic-gravity, and infrasonic waves. The dissipation of the energy of these waves determines the essential part of the energetic balance of the upper atmosphere. At the presence in such complicated system of nonlinear interactions, an important role could be played by the trigger effect of a release of the accumulated energy, when local sources of a small power are able to cause considerable large-scale disturbances. For example, according to Chernogor [2003a] in some cases the coefficient describing the trigger effect of the energy release can reach values of 105 - 1010.
[58] In conclusion, we note that study of wave disturbances is far for completing and requires systematic observations with attraction of various methods at the boundary of geophysics, physics of solar-terrestrial relations, seismology, and radio physics. In this aspect the results of systematic observations of HF propagation conditions at the network of paths of the oblique LFM sounding present obvious scientific and practical interest and can be applied to studies of fine effects related to the stratification of the ionosphere under impact of various types of wave processes caused by geomagnetic and seismic activity, as well as by other phenomena.
[59] Later on we plan to perform the detailed analysis of separate events when the wave disturbances manifest themselves in ionograms of oblique sounding. These manifestations should be compared with the geophysical and seismic data, the information concerning the rocket launches, and some other peculiarities of the performed experiments. The aim of such analysis is to try to single out on the background of the dominated sources (like terminator and geophysical disturbances) some other sources and to estimate their input to the observed effects.
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