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Scattering of Electromagnetic Waves in Random Media

#G-717


Propagation of Electromagnetic Waves and Optical Beams in a Randomly Inhomogeneous Nonstationary Absorptive Media

Tech Area / Field

  • PHY-RAW/Radiofrequency Waves/Physics

Status
8 Project completed

Registration date
04.05.2001

Completion date
11.01.2005

Senior Project Manager
Malakhov Yu I

Leading Institute
Georgian Technical University, Georgia, Tbilisi

Collaborators

  • University of Washington, USA, WA, Seattle

Project summary

The Earth's ionosphere is an inhomogeneous medium, the parameters of which randomly vary in space and time (turbulent medium). Nonlinear processes and dispersion lead to the creation of vortices in such a medium. Trapped particles execute rotation collective motion inside the vortices. Propagated electromagnetic waves and laser beams multiple scatter on inhomogeneities and vortical structures of random medium. Therefore, investigation of the effects of the creation of inhomogeneities and vortical structures at different altitudes of the ionosphere is a most topical problem. Complicated physical and chemical processes occur in the Earth's ionosphere. Simulation of these processes is necessary in order to investigate the possible creation of nonlinear vortices at an initial stage of turbulence. The theory of nonlinear waves is based on a set of nonlinear magnetohydrodynamic equations of motion. Nonlinearity and dispersion are taken into account. Some physical–mathematical models can be reduced to well–known classical nonlinear equations. However, other equations lead to more interesting equations, which have been little studied in mathematical physics. Investigation of statistical characteristics of scattered radiation on inhomogeneities of turbulent medium is a topical problem, particularly from an experimental point of view. This investigation is based on stochastic differential equations in view of boundary conditions. Medium parameters (dielectric permittivity, conductivity and so on) and scattered waves (electric and magnetic) are random functions of coordinate and time.
In this project we shall submit a new model of a turbulent medium of the Earth's ionosphere and investigate statistical characteristics of multiple scattered electromagnetic waves and laser beams in nonstationary unisotropic absorptive media. The main goal of the project will be achieved through analytical calculations and numerical simulation.
At the first stage of this project we shall investigate the creation of inhomogeneities and vortex structures in different layers of the Earth's ionosphere. It is necessary to simulate physical and chemical process in order to investigate the evolution of turbulence of nonlinear vortex structures. The original approach of investigation of the atmosphere is the novelty of this task, including nonlinear vortex formations as structural elements of turbulence. A strong structural turbulence model will be suggested for ionospheric plasma on the basis of solitary nonlinear vortices with account of geomagnetic field. This model has an advantage over other models as far as it includes mass transfer process. Different dynamical characteristics will be calculated for these structures. These investigations will be carried out on the basis of analytical calculations and numerical simulations of nonlinear dynamic equations. Some obtained results will experimentally explain observable perturbations in ionospheric layers; other results will stimulate new experimental investigations and their technical solutions.
Statistical characteristics of scattered radiation (angular power spectrum) of a source located in magnetized turbulent ionospheric plasma will be investigated at the next stage of this project, using both smooth perturbation and ray–(optics) methods. Plane–parallel layer of turbulent plasma is under the action of oblique external magnetic field at different orientations of the observation line connecting the source and the antenna. We shall consider the case when a source and the antenna are located on opposite boundaries of the layer and beyond them. The relationship between the spectral width and displacement of its maximum versus distance from a receiver to the layer boundaries will be considered. Angular power spectrum will be investigated through numerical simulation of radiation transfer equation using the Monte-Carlo method. Statistical moments of multiple scattered electromagnetic waves will be calculated and the evolution of the form of a power spectrum will be studied.
Analysis of the amplitude of scattered radiowaves allows us to investigate the structures and nature of ionospheric inhomogeneities. It is usually supposed that inhomoheneities are chaotically distributed and the amplitude is small. Hence, radiowave scattering is weak. However, observations show that fluctuations of signal amplitude passing through the ionosphere are periodically modulated. This testifies that ionospheric inhomogeneities have a periodical wavy structure. It was established by observations that small-scale inhomogeneities have an ellipse form and their axes are oriented mainly towards the Earth's magnetic field. Inhomogeneities of ionospheric plasma have spatial scales from ten to several hundred meters. Large-scale inhomogeneities also have a prolate nature and have a spatial scale of an order of magnitude equal to ten to a hundred kilometers. Signal propagation at great distances scatter on turbulent inhomogeneities of low layers of the ionosphere (troposphere). In many papers multiple scattering on dielectric permittivity fluctuations of the troposphere is assumed to be weak and, therefore, theoretical results are not in good agreement with experimental data at long–haul tropospheric propagation. Prolate inhomogeneities stretching towards the external magnetic field will be considered in this project. Dependence of this spectrum versus: distance between antenna and source for different spectra of random prolate inhomogeneities (including stretching degree); asymmetry of absorption coefficient; steepness of the slope of the layer and external magnetic field will be investigated. We shall study the mechanism of formation of directivity diagram of the source in medium with prolate random inhomogeneities.
Turbulent fluctuations of dielectric permittivity lead to distortion of laser beam radiation. Investigation of fluctuation disturbances at spatial limited light beam propagation in the atmosphere and different inhomogeneous media is a topical problem for optical communication systems. Medium inhomogeneities and absorption disturb the coherence of a laser beam and limit the scope of equipment application. Determination of random wandering of a beam, owing to turbulence, has great potential for practical application (particularly in medicine). In a number of practical applications it is necessary to transfer optical radiation simultaneously over several channels. In this case, knowledge of mutual statistical characteristics of the fields of wavy beams passing through a turbulent layer of the atmosphere is important. The influence of absorption on statistical characteristics of scattered laser beam radiation in unisotropic randomly inhomogeneous medium will be investigated using smooth perturbation. This method takes into account multiple scattering of waves. Displacement of the "center of gravity" of spatial limited light beams attracts great attention, owing to wide-ranging practical applications (communication, geodetic measurements, optical location and so on). Different statistical characteristics (particularly, correlation and structural functions of phase and amplitude fluctuations) for plane and spherical waves, convergent, collimating and focused laser beams will be calculated at the final stage of this project. Numerical simulation of both intensity variation and normalized structure functions for different spectra which characterize turbulent medium and beam geometry, will be carried out.
Obtained results have a wide range of practical applications. Analytical investigation and numerical simulation of a new relationship between statistical parameters of the received signal, the features of scattered absorptive media and geometry of the task will be utilized for modification of remote diagnostics algorithms of natural and artificial objects. Some results may be utilized to describe X-ray scattering on molecules of thermotropic and liotropic liquid crystals, ultraviolet light scattering by chloroplast and light scattering by polymer macromolecules oriented to the direction of liquid stream. Others may be used to develop atmospheric probing methods to investigate parameters, design communication systems for millimeter and sub–millimeter wave bands, describe electromagnetic wave propagation in the ionosphere, cosmic and laboratory plasma. Another important application of the obtained results is investigation of the statistical characteristics of receiving signals, to detect flying objects, optimize radiometeorology systems, and to select the dimension and wave band of antenna systems. Investigation of the influence of nonisotropic absorption of turbulent medium on statistical characteristics of scattered laser beam radiation is a topical problem for both fundamental investigation and practical application, particularly in medicine.


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