ИСТИНА

Keywords: Venus, ionosphere, atmosphere, radio wave, occultation, invariant. Introduction: One of the advantages of radio occultation experiments for the study of ionosphere and atmosphere of Venus is a possibility of obtaining uniform long-term measurements of the environment parameters. One year of observation of Venera-15,-16 signals accumulated data which, when analyzed, showed that ionospheric anomalies correlate with solar activity and allowed selection of data for detailed analysis of ionospheric disturbances. This talk presents results of an analysis of occultation data obtained in various missions to Venus, including Mariner-5,-10, Venera-9,-10, Pioneer-Venus, Venera-15,-16, Magellan, Venus Express. Numeric modelling of experiments revealed the nature of influence of various disturbances on the reliability of determination of multilayer plasma structures in Venusian ionosphere observed in radio occultation data. Radio Occultation Experiments: Radio signal parameters are measured during spacecraft (SC) motion on an orbit around Venus, therefore a signal received at some point has limited energy, accumulated in the receiver in the period of measurement. The energy of the radio wave packet being registered determines the reaction of the receiver to the signal, therefore the increase in energy diminishes relative noise. The error of the measured ionospheric parameters depends on the ratio of the signal to the equipment noise and fluctuations along the ray path. It is the random fluctuations of power, frequency and phase which put a limit to the precision of obtained ionospheric and atmospheric parameters. Occultation data from Venera-15,-16 SC are an important part for the understanding of Venus ionosphere formation. The used decimeter range wavelengths (λ = 32 cm) proved to be a significant advantage of our occultation experiments over those in Western missions (i.e. compared to λ = 13 and 3.6 cm), as they ensured a unique possibility to study even low-density plasma in night ionosphere and in the lower ionospheric regions. Method for determining layered structures: We consider the ionosphere during the experiment spherically layered in a narrow probed region having less than a 5-degree width. If horizontal linear gradient of concentration exists in the layers in this limited area, then phase of radio wave varies in the same way as in probing a spherically symmetrical medium. The ray crosses each plasma layer twice relative to the ray perpendicular, therefore a linear increase in concentration in one part of the layer is completely compensated by the decrease in concentration in the second half of its path, i.e. a linear concentration gradient does not influence the result of calculation of an integral electronic concentration along the ray path. Therefore, a vertical profile of electron concentration characterizes local concentrations averaged over approximately 200km along the ray path. Can the absence of non-linear gradients be verified? It so happens the spherical layered structure of the medium possesses an important quality. In VENERA-D VENUS MODELLING WORKSHOP occultation experiments in the gaseous planetary atmosphere and ionosphere stratified by gravity, the change in the energy flow density is proportional to the gradient of the deviation angle of the wave vector if geometric optic applicability conditions are satisfied. Therefore, the registered wave power variations are proportional to the speed of change of its frequency, while random fluctuations between them are uncorrelated. This invariant for the occultation experiment is observable only in spherically symmetric layered mediums, if the equipment noise levels are low, there are no diffraction effects, and there is no wave absorption and scattering. The existence of this criterion allowed us to confirm the reality of discovered periodic structures in electronic concentration profiles in the lower parts of day ionosphere on Venus, as well as the existence of both one-layer and two-layer structures in night ionosphere. Modelling of occultation experiments: For studying fine structure of ionosphere, decimeter waves, for which parameter variations during propagation in planetary plasma layers exceed considerably instrument fluctuations of signal phase and power, caused by the instability of SC equipment. However, it should be remembered that the influence of plasma on lower-frequency wave may lead to the violation of ray approximation conditions, which are fundamental for the occultation experiment analysis. The conditions of radio wave focusing were studied with the Venera-15,-16 data and it was shown that signal parameter variations for wavelengths over 50cm may be caused not only by refraction, but also by diffraction of radio waves on the layer boundaries in the day ionosphere on Venus. Modelling results demonstrate/confirm linearity between signal power and the velocity of its frequency change when probing spherically symmetrical ionosphere with a monochromatic high-frequency radio signal under conditions of ray approximation. Violation of linearity (or of the “occultation invariant”) may be caused by the influence of random noise, by ionosphere asymmetry, by the appearance of diffraction effects, and/or by absorption of radio waves in the probed medium. Conclusion: It was shown that precision of interpretation of occultation data depends on the selections of equipment and experiment parameters. Quality of obtained information depends on the interrelations between radio physical effects, determined by the studied medium, and the hindering factors. How informative the radio science occultation experiments are depending on the energy ratios along the signal path and on the stability of the transceiver using coherent monochromatic radio waves. Proper selection of frequencies is an important task in the planning of the occultation experiments for the coming Venus-D mission [2]. Under optimal conditions there appears a possibility to acquire information about the atmosphere and ionosphere of the planet, not obtainable before, which is important for the study of anomalous properties of the gaseous atmosphere on Venus and a study of propagation of gravity waves from the atmosphere into the ionosphere. Acqnowlegenments: The work is partially supported by the RAS Presidium Program № 1.7P. References: [1] Gavrik A.L. et al. Oscillations detected near the lower boundary of the Venus ionosphere from the radio occultation measurements of the Venera-15 and Venera-16 satellites // Journal of Communications Technology and Electronics. 2013. V. 58. No. 10. P. 985-995. [2] Vorontsov V.A. et al. Prospective spacecraft for Venus research: Venera-D design // Solar System Research. 2011. V. 45. No. 7. P. 710-714.