On the nature of middle-latitude F-spread

The results of a special experiment to study the statistics of short-wave signals reflected from the ionosphere during F-spread conducted in the first half of 1994 at the proving ground of the Scientific-Research Radio-Physics Institute at Zimenki of the Nizhny Novgorod region are presented. The dependence of the index of amplitude fluctuations on the reception bandwidth was studied. The experiment results are compared with existing theoretical concepts of the origin of F-spread. The experiment favors the hypothesis that middle-latitude F-spread is result of the multiple-ray nature of short-wave signals reflected from the large-scale quasi-regular inhomogeneous structure of the ionosphere.Scientific-Research Radio-Physics Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 11, pp. 1447–1451, November, 1994.     

Behavior characteristics of the index of middle-latitude F-spread

We present the results of analysis of data on ionospheric vertical sounding in Zimenki (Nizhny Novgorod) over the period from 1964 to 1992. The January ionograms are used for constructing the plots of daily indices of F-spread for an ionospheric layer. The dependence of the average monthly (January) index of F-spread on the solar activity is obtained. The index of F-spread is found to grow with time. The technogenic nature of this increase is assumed. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 3, pp. 322–327, March, 1997.     

On the role of large-scale ionospheric inhomogeneities in the formation of middle-latitude F-spread

We present the results of a special experiment carried out in the settlement of Zimenki, Nizhniy Novgorod province, on ionospheric vertical sounding by HF signals under F-spread using practically synchronous measurements of the multiple-ray nature of normal waves reflected from magnetoactive ionospheric plasma with ordinary and extraordinary polarizations at frequencies shifted by 0.7MHz. The experiment confirms the dominating role of large-scale electron-density inhomogeneities of the moving ionospheric disturbance type in the formation of middle-latitude F-spread.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 5, pp. 564–567, May, 1996.This work was supported by the Russian Foundation for Fundamental Research (project N. 95-02-03716 RFFI).     

On the propagation of electromagnetic pulses under the conditions of quasi-resonance

A theory of quasi-resonance for the model of atoms with an arbitrary number of levels is developed by the method of unitary transformation without recourse to Bloch’s equations. Corrections to the results known from the theory of adiabatic following, which refine these results and take into account dispersion of the nonlinear response of the resonant system, are obtained. The condition for quasi-resonance with a single atomic transition is shown to impose stringent limitations on the form of the nonlinear evolution equation for the electric field envelope of the pulse with dispersion of the nonlinear response taken into account, which prohibit the reduction of the equation indicated to familiar, fully integrable systems.     

Ultrashort pulse propagation under manipulated resonance conditions

The propagation of a polarized ultrashort laser pulse is analyzed by the inverse scattering method under initial conditions including a spatial pulse profile, a state of the medium, and a “switched-on” resonant atom-field interaction. Magnetic degeneracy of atomic levels is taken into account. The Maxwell-Bloch equations are rewritten in Hamiltonian form without redefining the spatial and temporal variables. The inverse scattering method is based on an analysis of a new spectral problem. Gelfand-Levitan-Marchenko-type equations are derived, a soliton solution is obtained, and the changes in parameters of two solitons after their collision are calculated. A possible experimental setup for implementing the system under analysis is discussed.     

On a model of artificial F-spread in the ionosphere

The results of a complex experiment on the synchronous observation of geostationary-satellite signals and of reflected HF signals of the vertical-sounding station in the course of ionosphere modification by powerful short waves are analyzed using the method of refractive scattering of radio waves. We show that the main statements of the known stochastic theory of F-spread in the ionosphere can be used in the development of a model of artificial F-spread. In particular, the artificial F-spread can be described as multiple-ray propagation of short radio waves in the ionosphere modified by high-power radio waves, the electron-density distribution of which is a random process locally stationary in space and time.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 7, pp. 876–881, July, 1996.This paper was supported by the Russian Foundation for Fundamental Research (project 95-02-03716).     

On one feature of the F-spread phenomenon in the midlatitude ionosphere

We report on the results of observations of reflected SW signals by vertical sounding (VS) of the ionosphere in Zimenki, Nizhny Novgorod region, in the summers of 1994 and 1995. We discovered an anomalous increase in the multipath propagation of the received SW signals during ionospheric F-spread: The duration of a multipath reflected signal increased considerably when we used a highly directional antenna compared to the case of a weakly directional antenna. The fast amplitude fluctuation index increased with increasing delay of the reflected SW signals. The observations are interpreted in terms of a stochastic model of midlatitude F-spread as the phenomenon of multipath radio wave scattering (reflection) from large-scale electron density inhomogeneities under conditions of a strongly developed inhomogeneous structure of the ionosphere.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 10, pp. 1064–1070, October, 1995.     

Fluctuations of the ultra-short-wavelength wave in tropospheric propagation under extremal climatic conditions

Institute of Natural Sciences of the Buryatskii Affiliate, Siberian Branch, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 33, No. 9, pp. 1027–1032, September, 1990.     

On the possibility of soliton-like double-frequency propagation of femtosecond pulses in an optical fiber under conditions of second harmonic generation

A soliton-like mode of double-frequency propagation of femtosecond pulses during second harmonic generation (SHG) in optical fibers is predicted on the basis of computer simulation. The essence of this mode is that, under certain conditions, the central part of the pulses of both the fundamental and the second harmonic waves propagate without a change in the intensity of either wave. This mode can be realized by either of two schemes of the SHG process. In the first scheme, the second harmonic enters a nonlinear medium with some definite input amplitude and definite phase shift with respect to the fundamental wave. In the other scheme, the second harmonic has a zero input amplitude and a phase shift is introduced into the interacting waves in particular cross sections of the medium.     

Mathematical model of light propagation through liquid crystal display working under real conditions

The main aim of our work is to present a theory of light propagation through a liquid crystal display worked out in our University. This theory takes into account real conditions of a display operation such as temporal coherence of light source, interference phenomena, spectral characteristics of refractive coefficients of individual layers and their complex forms, real directions of ordinary and extraordinary wavevectors into each layer, real directions of light polarization into anisotropic or dichroic layers etc. Therefore, our theory can be used to determine optical parameters of a liquid crystal display operating under real or at least quasi-real conditions. Contrary to other theories (e.g., Beremann or geometric optics approximation), this model is characterized by a small number of simplifications. Thus, it can be used to analyse a display operating in special conditions, such as military or out-door applications. In these cases, very high optical parameters (luminance and contrast ratio) aare needed. Application of even small simplifications conducts to significant changes of calculated parameters.     

Features of propagation of HF signals on mid-latitude paths under conditions of geomagnetic disturbances

We present the results of experimental studies of the features of HF-signal propagation on oblique-sounding paths in the Eurasian longitude sector between England and Magadan during geomagnetic disturbances. The joint analysis of the satellite data and the data of vertical and oblique sounding of the ionosphere shows that the appearance of additional signals during magnetic disturbances can be stipulated by refraction of radio waves in the region of the auroral oval and the main ionospheric trough as well as by scattering from small-scale magnetic-field-aligned irregularities near the equatorial boundary of the auroral oval. Based on calculations and comparison with experimental data of oblique sounding of the mid-latitude ionosphere, we identify the lateral spreading signals registered on the Magadan — Irkutsk and Inskip (England) — Rostov-on-Don paths as signals scattered by small-scale field-aligned irregularities whose location coincides with the southern boundary of the auroral oval. We show that the Russian and global chirp-ionosonde networks are promising for studying the dynamics of the main ionospheric trough and the auroral oval during geomagnetic disturbances as a manifestation of space weather in the near-Earth environment.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 47, No. 12, pp. 1041–1056, December, 2004.     

Diffusive liquid propagation in porous and elastic materials: the case of foams under microgravity conditions

We report the results of fluid transport experiments in aqueous foams under microgravity. Using optical and electrical methods, the capillary motion of the foam fluid and the local liquid fractions are monitored. We show that foams can be continuously wetted up to high liquid fractions ( approximately 0.3), without any bubble motion instabilities. Data are compared to drainage models: For liquid fractions above 0.2, discrepancies are found and identified. These new results on foam hydrodynamics and structure can be useful for other poroelastic materials, such as plants and biological tissues.     

A localized thickened flame model for simulations of flame propagation and autoignition under elevated pressure conditions

The present study proposes a localized thickened flame (LTF) model for the accurate prediction of flame propagation and autoignition timing. The unresolved-scale terms appeared in spatially-filtered governing equations due to thin flame structures are constructed under a physical constraint in which laminar flame speed is maintained. A high-order derivative is introduced to dynamically localize the effects of the LTF in the regions of unresolved propagating flame. The model is also designed such that the thickened flame is resolved by the same number of grid points for any grid size used. Therefore, a user-specified constant in the model does not need to be adjusted depending on the employed grid size. Laminar flame propagation problems are used to validate the performance of the proposed LTF model and determine the appropriate value of the user-specified constant. The results using a one-dimensional constant-volume reactor demonstrate that the LTF successfully captures the accurate flame propagation behaviors under elevated pressure conditions, while not affecting the end-gas autoignition timing, even on relatively coarse grid resolutions. The high-order derivative in the LTF serves as a dynamic parameter for detecting the thinning flame under elevated pressure conditions.     

A stochastic model of the F-spread phenomenon in the ionosphere

A stochastic model of the F-spread phenomenon in the ionosphere is proposed. The model is based on the idea that under F-spred conditions the electron-density distribution in the F-region of the ionosphere is a random process that is locally stationary in space and time and assumes multiple-ray propagation of short waves with independent phase fluctuations in individual rays. Some peculiarities of F-spred ionograms are analyzed. The problem of intensity fluctuations of short radiowaves during F-spred in the ionosphere is considered.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 12, pp. 1227–1240, December, 1995.     

Seasonal behavior of middle-latitude short-wave mesospheric radio echoes

Using the radar of the “Sura” facility, we studied the diurnal and seasonal behavior of mesospheric middle-latitude short-wave radio echoes. It was, found that the diurnal dependence of the intensity of reflected signals is mainly determined by the solar illumination of the mesosphere, i.e., by photoionization intensity. We revealed an almost permanent and season-independent presence of a thin turbulent layer at altitudes of 82 to 84 km whose radio echoes are maximal in summer and winter and much weaker in spring and fall. Sporadic summer radio echoes with increased intensity, which are observed at somewhat greater (by 2 to 4 km) altitudes, apparently correspond to the phenomenon of mesospheric summer echo (MSE) observed in the VHF range. The material of the article has been reported at the IIIrd International School on Space Plasma Physics. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebmykh Zavedenii, Radiofizika, Vol. 41, No. 10, pp. 1248–1257, October 1998.     

On anomalous diffusion of fractal clusters under certain realistic physical conditions

Summary Diffusion of a fractal cluster of dimensiond _f in a three-dimensional space is investigated. The diffusion process is assumed to be modelled by a standard parabolic diffusion equation, although a more general case represented by the Fokker-Planck-Kolmogoroff equation is also introduced. The mean-square displacement of the cluster mass centre is analysed and its anomalous behaviour is presented and critically discussed. The results obtained can be applied to describe some effects which may occur during the diffusion-limited cluster-cluster aggregation process, especially when the viscosity of the solvent is changed in time and/or a directed transport of clusters is present in the system. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.