Showing papers on "Plane wave published in 1978"

Wave propagation and scattering in random media

Abstract: A volume in the IEEE/OUP Series on Electromagnetic Wave Theory Donald G. Dudley, Series Editor This IEEE Classic Reissue presents a unified introduction to the fundamental theories and applications of wave propagation and scattering in random media. Now for the first time, the two volumes of Wave Propagation and Scattering in Random Media previously published by Academic Press in 1978 are combined into one comprehensive volume. This book presents a clear picture of how waves interact with the atmosphere, terrain, ocean, turbulence, aerosols, rain, snow, biological tissues, composite material, and other media. The theories presented will enable you to solve a variety of problems relating to clutter, interference, imaging, object detection, and communication theory for various media. This book is expressly designed for engineers and scientists who have an interest in optical, microwave, or acoustic wave propagation and scattering. Topics covered include:

Nonlinear phenomena in the ionosphere

Abstract: 1. Introduction.- 1.1 Data on the Structure of the Ionosphere.- 1.2 Features of Nonlinear Phenomena in the Ionosphere.- 1.2.1. Nonlinearity Mechanisms.- 1.2.2. Qualitative Character of Nonlinear Phenomena.- 1.2.3. Brief Historical Review.- 2. Plasma Kinetics in an Alternating Electric Field.- 2.1. Homogeneous Alternating Field in a Plasma (Elementary Theory).- 2.1.1.Electron Current-Electronic Conductivity and Dielectric Constant.- 2.1.2.Electron Temperature.- 2.1.3.Ion Current-Heating of Electrons and Ions.- 2.2. The Kinetic Equation.- 2.2.1. Simplification of the Kinetic Equation for Electrons.- 2.2.2. Transformation of the Electron Collision Integral.- 2.2.3. Inelastic Collisions.- 2.3. Electron Distribution Function.- 2.3.1. Strongly Ionized Plasma.- 2.3.2. Weakly Ionized Plasma.- 2.3.3. Arbitrary Degree of lonization-Concerning the Elementary Theory.- 2.4. Ion Distribution Function.- 2.4.1. Simplification of the Kinetic Equation.- 2.4.2. Distribution Function.- 2.4.3. Ion Temperature, Ion Current.- 2.5. Action of Radio Waves on the Ionosphere.- 2.5.1. lonization Balance in the Ionosphere.- 2.5.2. Effective Frequency of Electron and Ion Collisions-Fraction of Lost Energy.- 2.5.3. Electron and Ion Temperatures in the Ionosphere.- 2.5.4. Heating of the Ionosphere in an Alternating Electric Field.- 2.5.5.Perturbations of the Electron and Ion Concentrations.- 2.5.6. Artificial lonization of the Ionosphere-Heating of Neutral Gas.- 3. Self-Action of Plane Radio Waves.- 3.1. Simplification of Initial Equations.- 3.1.1. Nonlinear Wave Equation.- 3.1.2. Nonlinear Geometrical Optics of a Plane Wave.- 3.2. Effect of Nonlinearity on the Amplitude and Phase of the Wave.- 3.2.1. Self-Action of a Weak Wave.- 3.2.2. Self-Action of a Strong Wave.- 3.2.3. Self-Action of Waves in the Case of Artificial lionization.- 3.3. Change of Wave Modulation.- 3.3.1. Weak Wave.- 3.3.2. Change of Amplitude Modulation of Strong Wave.- 3.3.3. Phase Modulation.- 3.3.4. Nonlinear Distortion of Pulse Waveform.- 3.4. Generation of Harmonic Waves and Nonlinear Detection.- 3.4.1. Frequency Tripling.- 3.4.2. Nonlinear Detection.- 3.5. Self-Action of Radio Waves in the Lower Ionosphere.- 4. Interaction of Plane Radio Waves.- 4.1. Cross Modulation.- 4.1.1. Weak Waves.- 4.1.2. Strong Perturbing Wave.- 4.1.3. Resonance Effects near the Electron Gyrofrequency.- 4.2. Interaction of Unmodulated Waves.- 4.2.1. Interaction of Short Pulses.- 4.2.2. Change in the Absorption of a Wave Propagating in a Perturbed Plasma Region.- 4.2.3. Generation of Waves with Combination Frequencies.- 4.3. Radio Wave Interaction in the Lower Ionosphere.- 4.3.1. Cross Modulation.- 4.3.2. Fejer's Method.- 4.3.3. Nonstationary Processes in the Interaction of Strong Radio Waves.- 5. Self-Action and Interaction of Radio Waves in an Inhomogeneous Plasma.- 5.1. Inhomogeneous Electric Field in a Plasma.- 5.1.1. Fundamental Equations.- 5.1.2. Distribution of Density and Temperatures in Plasma.- 5.2. Kinetics of Inhomogeneous Plasma.- 5.2.1. Kinetic Coefficients. Elementary Theory.- 5.2.2. Kinetic Theory.- 5.2.3. Fully Ionized Plasma.- 5.3. Modification of the F Region of the Ionosphere by Radio Waves.- 5.3.1. Modification of the Electron Temperature and of the Plasma Concentration.- 5.3.2. Radio Wave Reflection Region.- 5.3.3. Growth and Relaxation of the Perturbations.- 5.4. Focusing and Defocusing of Radio Wave Beams.- 5.4.1. Nonlinear Geometrical Optics.- 5.4.2. Defocusing of Narrow Beams.- 5.4.3. Mutual Defocusing.- 5.4.4. Thermal Focusing in the Lower Ionosphere.- 6. Excitation of Ionosphere Instability.- 6.1. Self-Focusing Instability.- 6.1.1. Spatial Instability of a Homogeneous Plasma.- 6.1.2. Instability in the Wave-Reflection Region.- 6.2. Resonant Absorption and Resonance Instability.- 6.2.1. Langmuir Oscillations in an Inhomogeneous Plasma.- 6.2.2. Excitation of Plasma Waves.- 6.2.3. Resonance Instability.- 6.2.4. Absorption of Ordinary Radio Waves.- 6.3. Parametric Instability.- 6.3.1. Langmuir Oscillations of a Plasma in an Alternating Field.- 6.3.2. Parametric Excitation of Langmuir Oscillations.- 6.3.3. Parametric Instability in the Ionosphere.- 6.3.4. Dissipative Parametric Instability.

An instability of finite amplitude circularly polarized Alfven waves

Abstract: A demonstration is presented that a finite-amplitude circularly-polarized Alfven wave is generally unstable in a MHD fluid. The wave decays by a four-wave coupling process in which the daughter waves are forward propagating random density and magnetic fluctuations and a backward-propagating magnetic wave. For parameters typical of the solar corona and the solar wind (thermal to magnetic energy density ratios between 0.1 and 1, and values between 0.1 and 0.9 for the ratio of magnetic energy density of the initial Alfven wave to that of the background magnetic field), large decay rates are found.

Modified spectrum of atmospheric temperature fluctuations and its application to optical propagation

Abstract: Recent experiments reveal the high-wave-number form of the power spectrum of temperature fluctuations in turbulent flow. It is precisely this high-wave-number portion of the temperature spectrum that strongly affects optical propagation in the atmosphere. An accurate model of the spectra of advected quantities, such as temperature, has been developed and is applied here to optical propagation. An outstanding feature of the model and the observed temperature spectrum is a “bump” at high wave numbers. The accurate model of the temperature spectrum is used to compute the temperature structure function, the variance of log intensity as a function of Fresnel-zone size, the covariance function of log amplitude, the structure function of phase, as well as the phase coherence length. These results are compared with the predictions of Tatarskii’s spectrum. The bump in the temperature spectrum produces a corresponding bump in the temperature structure function, the variance of log intensity, and the structure function of phase. The accurate model is also used to determine the shape of the structure function of aerosol concentration fluctuations; it is found that this structure function varies as the logarithm of the separation distance for small separations.

Modulational instability of finite-amplitude, circularly polarized Alfven waves

Abstract: The simple theory of the decay instability of Alfven waves is strictly applicable only to a small-amplitude parent wave in a low-beta plasma, but, if the parent wave is circularly polarized, it is possible to analyze the situation without either of these restrictions. Results show that a large-amplitude circularly polarized wave is unstable with respect to decay into three waves, one longitudinal and one transverse wave propagating parallel to the parent wave and one transverse wave propagating antiparallel. The transverse decay products appear at frequencies which are the sum and difference of the frequencies of the parent wave and the longitudinal wave. The decay products are not familiar MHD modes except in the limit of small beta and small amplitude of the parent wave, in which case the decay products are a forward-propagating sound wave and a backward-propagating circularly polarized wave. In this limit the other transverse wave disappears. The effect of finite beta is to reduce the linear growth rate of the instability from the value suggested by the simple theory. Possible applications of these results to the theory of the solar wind are briefly touched upon.

Plane-Wave Scattering-Matrix Theory of Antennas and Antenna-Antenna Interactions

Abstract: From Abstract: "This monograph is distinguished by the use of plane-wave spectra for the representation of fields in space and by the consideration of antenna-antenna (antenna-scatterer) interactions at arbitrary separation distances." From Preface: "The primary objective of this monograph is to facilitate the critical acceptance and proper application of antenna and field measurement techniques deriving more or less directly from the plane-wave scattering matrix (PWSM) theory of antennas and antenna-antenna interactions. A second objective is to present some recent and some new theoretical results based on this theory."

Nonlinear phenomena in the ionosphere

Abstract: 1. Introduction.- 1.1 Data on the Structure of the Ionosphere.- 1.2 Features of Nonlinear Phenomena in the Ionosphere.- 1.2.1. Nonlinearity Mechanisms.- 1.2.2. Qualitative Character of Nonlinear Phenomena.- 1.2.3. Brief Historical Review.- 2. Plasma Kinetics in an Alternating Electric Field.- 2.1. Homogeneous Alternating Field in a Plasma (Elementary Theory).- 2.1.1.Electron Current-Electronic Conductivity and Dielectric Constant.- 2.1.2.Electron Temperature.- 2.1.3.Ion Current-Heating of Electrons and Ions.- 2.2. The Kinetic Equation.- 2.2.1. Simplification of the Kinetic Equation for Electrons.- 2.2.2. Transformation of the Electron Collision Integral.- 2.2.3. Inelastic Collisions.- 2.3. Electron Distribution Function.- 2.3.1. Strongly Ionized Plasma.- 2.3.2. Weakly Ionized Plasma.- 2.3.3. Arbitrary Degree of lonization-Concerning the Elementary Theory.- 2.4. Ion Distribution Function.- 2.4.1. Simplification of the Kinetic Equation.- 2.4.2. Distribution Function.- 2.4.3. Ion Temperature, Ion Current.- 2.5. Action of Radio Waves on the Ionosphere.- 2.5.1. lonization Balance in the Ionosphere.- 2.5.2. Effective Frequency of Electron and Ion Collisions-Fraction of Lost Energy.- 2.5.3. Electron and Ion Temperatures in the Ionosphere.- 2.5.4. Heating of the Ionosphere in an Alternating Electric Field.- 2.5.5.Perturbations of the Electron and Ion Concentrations.- 2.5.6. Artificial lonization of the Ionosphere-Heating of Neutral Gas.- 3. Self-Action of Plane Radio Waves.- 3.1. Simplification of Initial Equations.- 3.1.1. Nonlinear Wave Equation.- 3.1.2. Nonlinear Geometrical Optics of a Plane Wave.- 3.2. Effect of Nonlinearity on the Amplitude and Phase of the Wave.- 3.2.1. Self-Action of a Weak Wave.- 3.2.2. Self-Action of a Strong Wave.- 3.2.3. Self-Action of Waves in the Case of Artificial lionization.- 3.3. Change of Wave Modulation.- 3.3.1. Weak Wave.- 3.3.2. Change of Amplitude Modulation of Strong Wave.- 3.3.3. Phase Modulation.- 3.3.4. Nonlinear Distortion of Pulse Waveform.- 3.4. Generation of Harmonic Waves and Nonlinear Detection.- 3.4.1. Frequency Tripling.- 3.4.2. Nonlinear Detection.- 3.5. Self-Action of Radio Waves in the Lower Ionosphere.- 4. Interaction of Plane Radio Waves.- 4.1. Cross Modulation.- 4.1.1. Weak Waves.- 4.1.2. Strong Perturbing Wave.- 4.1.3. Resonance Effects near the Electron Gyrofrequency.- 4.2. Interaction of Unmodulated Waves.- 4.2.1. Interaction of Short Pulses.- 4.2.2. Change in the Absorption of a Wave Propagating in a Perturbed Plasma Region.- 4.2.3. Generation of Waves with Combination Frequencies.- 4.3. Radio Wave Interaction in the Lower Ionosphere.- 4.3.1. Cross Modulation.- 4.3.2. Fejer's Method.- 4.3.3. Nonstationary Processes in the Interaction of Strong Radio Waves.- 5. Self-Action and Interaction of Radio Waves in an Inhomogeneous Plasma.- 5.1. Inhomogeneous Electric Field in a Plasma.- 5.1.1. Fundamental Equations.- 5.1.2. Distribution of Density and Temperatures in Plasma.- 5.2. Kinetics of Inhomogeneous Plasma.- 5.2.1. Kinetic Coefficients. Elementary Theory.- 5.2.2. Kinetic Theory.- 5.2.3. Fully Ionized Plasma.- 5.3. Modification of the F Region of the Ionosphere by Radio Waves.- 5.3.1. Modification of the Electron Temperature and of the Plasma Concentration.- 5.3.2. Radio Wave Reflection Region.- 5.3.3. Growth and Relaxation of the Perturbations.- 5.4. Focusing and Defocusing of Radio Wave Beams.- 5.4.1. Nonlinear Geometrical Optics.- 5.4.2. Defocusing of Narrow Beams.- 5.4.3. Mutual Defocusing.- 5.4.4. Thermal Focusing in the Lower Ionosphere.- 6. Excitation of Ionosphere Instability.- 6.1. Self-Focusing Instability.- 6.1.1. Spatial Instability of a Homogeneous Plasma.- 6.1.2. Instability in the Wave-Reflection Region.- 6.2. Resonant Absorption and Resonance Instability.- 6.2.1. Langmuir Oscillations in an Inhomogeneous Plasma.- 6.2.2. Excitation of Plasma Waves.- 6.2.3. Resonance Instability.- 6.2.4. Absorption of Ordinary Radio Waves.- 6.3. Parametric Instability.- 6.3.1. Langmuir Oscillations of a Plasma in an Alternating Field.- 6.3.2. Parametric Excitation of Langmuir Oscillations.- 6.3.3. Parametric Instability in the Ionosphere.- 6.3.4. Dissipative Parametric Instability.

Inverse source problems in optics

Abstract: 1. Introduction.- 1.1 Direct and Inverse Problems in Optical Physics.- 1.2 Role of Prior Knowledge.- 1.3 Survey of Specific Inverse Problems.- 1.4 Notati on i n Coherence Theory.- References.- 2. The Phase Reconstruction Problem for Wave Amplitudes and Coherence Functions.- 2.1 Phase Reconstruction for Wave Ampl i tudes.- 2.1.1 Relevance of the Phase Problem for Object Structure Determi nation.- 2.1.2 Derivation of the Basic Equations Governing the Phase Probl em.- 2.1.3 General Considerations on the Phase Problem.- 2.1.4 Greenaway's Proposal for Phase Recovery from a Single Intensity Distribution.- 2.1.5 The Method of Half-Plane Apertures for Semi-Weak Objects.- 2.1.6 The Logarithmic Hilbert Transform: Methods for Circumventing Complications Due to Zeros.- 2.1.7 Phase Retrieval for Strong Objects from Two Defocused Images.- 2.1.8 Phase Retrieval from the Intensity Distributions in Exit Pupil and Image Plane.- 2.1.9 Phase Retrieval from Two Defocused Images for Semi-Weak Objects.- 2.2 Phase Reconstruction for Coherence Functions.- 2.2.1 Phase Determination of Optical Coherence Functions.- 2.2.2 Determination of the Phase of the Spatial Coherence Function with an Incoherent Reference Point Source.- 2.2.3 Determination of the Phase of the Spatial Coherence Function with an Exponential Filter.- 2.2.4 Determination of the Phase of the Spatial Coherence Function from the Intensity in the Fraunhofer Plane.- References.- 3. The Uniqueness of Inverse Problems.- 3.1 Summary of Inverse Problems.- 3.1.1 Inverse Sturm-Liouville Problems.- 3.1.2 Reconstruction Problems.- 3.1.3 Three-Dimensional Reconstruction from Projections.- 3.2 Inverse Diffraction.- 3.2.1 Inverse Diffraction from Far-Field Data.- 3.2.2 Inverse Diffraction from Spherical Surface to Spherical Surface.- 3.2.3 Inverse Diffraction from Plane to Plane.- 3.2.4 Generalization to Arbitrary Surfaces.- 3.2.5 The Determination of the Shape of a Scatterer from Far-Field Data.- 3.3 Non-Radiating Sources.- 3.3.1 Early Results and Special Cases.- 3.3.2 General Theory.- 3.3.3 Integral Equations and Uniqueness by Prior Knowledge.- 3.4 The Determination of an Object from Scattering Data.- 3.4.1 Examples of Nonuniqueness.- 3.4.2 Phase Shift Analysis and the Reconstruction of a Potential.- 3.4.3 The Determination of a Potential or Index of Refraction from the Scattered Fields Generated by a Set of Monochromati c PIane Waves.- 3.4.4 The Unique Determination of an Object from Scattering Data.- 3.4.5 The Analytical Continuation of the Electromagnetic Field from the Exterior to the Interior of a Scatterer and Its Physical Implications.- References.- 4. Spatial Resolution of Subwavelength Sources from Optical Far-Zone Data.- 4.1 Approaches to Superresolution.- 4.1.1 Array of Sources with Known Radiation Pattern.- 4.1.2 Superresolution Using Evanescent Waves.- 4.1.3 x-Locali zed Sources.- 4.2 Partial Waves Associated with Complex Spatial Frequencies.- 4.3 Representations and Expansions of the EM Field.- 4.3.1 Integral Representations.- 4.3.2 Partial-Wave Representation of Exterior Field.- 4.3.3 Multipole Waves.- 4.3.4 Plane Waves.- 4.4 Band-Limiting at Variance with X-Localized Sources.- 4.5 High-Frequency Information in the Far Zone Given a X-Localized Source.- 4.6 X-Localized Sources Reconstructed from Far-Zone Data.- 4.7 Measurement of Phase and Magnitude of the Optical Radiation Pattern.- 4.8 Discussion.- References.- 5. Radiometry and Coherence.- 5.1 The Development of Radiometry.- 5.1.1 The Classical Period.- 5.1.2 The Baroque Period.- 5.1.3 The Modern Period.- 5.2 Coherence of Blackbody Radiation.- 5.2.1 Temporal Coherence.- 5.2.2 Spatial Coherence.- 5.3 First-Order Radiometry.- 5.3.1 Energy Flow in Scalar Fields.- 5.3.2 Coherence Theory and the Radiometrie Quantities.- 5.3.3 The Van Cittert-Zernike Theorem.- 5.3.4 An Example: Quasi stationary Sources.- 5.4 Radiant Intensity and Angular Coherence.- 5.4.1 Source Models.- 5.4.2 Inverse Relations.- 5.4.3 Bessel-Correlated Sources.- 5.4.4 Gauss-Correlated Sources.- 5.4.5 An Application: Coherence of Thermionic Sources.- 5.5 Radiation Efficiency.- 5.5.1 Radiance of Model Sources.- 5.5.2 Emittance and Radiation Efficiency.- 5.5.3 Exampl es.- 5.6 Second-Order Radiometry.- 5.6.1 Radiant Intensity Fluctuation and Autocorrelation.- 5.6.2 Second-Order Radiometric Quantities.- 5.6.3 An Example: Gauss-Correlated Chaotic Source.- References.- 6. Statistical Features of Phase Screens from Scattering Data.- 6.1 Basic Formulation of the Statistical Problem.- 6.1.1 Physical Models.- 6.1.2 Characteristic Functional of the Scattered Light.- 6.1.3 Correlation Functions.- 6.1.4 Gaussian Limit.- 6.2 More General Detection and Coherence Conditions.- 6.2.1 Gaussian Scattered Field.- 6.2.2 Polychromatic Speckle Patterns.- 6.3 Amplitude and Intensity Correlations.- 6.3.1 Information Contained in Amplitude Correlations.- 6.3.2 Information Contained in Intensity Correlations.- 6.3.3 Moving Diffusers.- 6.4 Number-Dependent Effects.- 6.4.1 Moments and Probability Distribution of Intensity.- 6.4.2 Examples.- 6.4.3 Applications.- 6.5 Concluding Remarks.- References.- Additional References with Titles.

Velocity estimation and downward continuation by wavefront synthesis

Abstract: A “wave stack” is any stack over a common shot or geophone gather in which the moveout is independent of time. It synthesizes a particular wavefront by superposition of the many spherical wavefronts of raw data. Unlike the common midpoint stack, wave stacks retain the important property of being the sampling of a wave field and, as such, permit wave‐equation treatment of formerly difficult or impossible problems. Seismic sections of field data generated by wave stacks that synthesized slanted downgoing plane waves showed a similarity in appearance to the common midpoint stacks. In signal‐to‐noise ratio they lay between the single offset section and the midpoint stack. The angle selectivity of the slanted plane‐wave stacks permitted detection of a reflector that was not visible on either the midpoint stack or the raw gathers. Simple velocity estimation in slant frame coordinates differs only in detail from standard frame coordinates. Because of the wave field character of data that have been slant plane‐wa...

The application of reciprocity theory to scattering of acoustic waves by flaws

Abstract: The reciprocity theorem and the scattering matrix formalism of electromagnetic theory have been adapted to obtain formulas for scattering of acoustic waves from flaws. The technique has been applied, in the Born approximation, to determine scattering of plane waves by small flaws, and with an elastostatic approximation to find the scattering from a flat elliptical crack. A third example is a treatment of the optical approximation to scattering from a curved interface between two media. In all cases results are obtained which are applicable for flaws placed either in the Fresnel region or Fraunofer region of unfocused transducers, or near the focal point of a focused transducer. A fourth example, given in the Appendix, uses the reciprocity theory to derive the axial field of a piston transducer in contact with the surface of a solid.

Diffusion of a pulse in densely distributed scatterers

Abstract: This paper presents a theoretical study on propagation and scattering characteristics of a short optical pulse in a dense distribution of scatterers. Examples include pulse diffusion in whole blood and in a dense distribution of particulate matter in the atmosphere and the ocean. The parabolic equation technique is applicable to the forward-scatter region where the angular spread is confined within narrow forward angles. When the angular spread becomes comparable to the order of unit steradian, there is as much backscattering as forward scattering and diffusion phenomena take place. We start with the integral and differential equations for the two-frequency mutual coherence function under the first-order smoothing approximation, and a general diffusion equation and boundary conditions are obtained. As examples, we present solutions for diffusion of a pulse from a point source and a plane wave incident on a slab of scatterers.

Numerical computation of scattering from a perfectly conducting random surface

Abstract: A one-dimensionally rough random surface with known statistical properties was generated by digital computer. This surface was divided into many segments of equal length. The moments method was applied to each surface segment assuming perfect conductivity to compute the induced surface current and subsequently the backscattered field due to an impinging plane wave. The return power was then calculated and averaged over different segments. Unlike numerical computations of scattering from deterministic surfaces, problems of stability (as defined by Blackman and Turkey [11]) and convergence of the solution exist for random surface scattering. It is shown that the stability of the numerically computed estimate of the backscattered average power depends on N , the total number of disjoint surface segments averaged; \Delta x , the spacing between surface current points; D , the width of each surface segment; and g , the width of the window function. Relations are obtained which help to make an appropriate choice of these parameters. In general, choices of \Delta x, D , and g are quite sensitive to the incident wavelength and the angular scattering properties of the surface.

Nonlinear development of a wave in a boundary layer

Abstract: In recent years definite progress has been achieved in the construction of theoretical models of nonlinear wave processes which lead to a transition from laminar to turbulent flow [1, 2]. At the same time, there is a shortage of actual experimental material, especially for flows in a boundary layer. Fairly thorough experimental studies have been carried out only on the initial stage of the development of disturbances in a boundary layer, which is satisfactorily describable by the linear theory of hydrodynamic stability. In evaluating the theoretical models of subsequent stages of the transition, investigators have been forced to turn chiefly to much earlier experiments carried out by the United States National Bureau of Standards [3, 4], in which the main attention was concentrated on the three-dimensional structure of the transition region. The present investigation was undertaken for the purpose of obtaining detailed data on the structure of the flow in the transition region when there is disturbance in the laminar boundary layer of a two-dimensional wave. In order to make the two-dimensional nonlinear effects stand out more clearly, the amplitude of the wave was specified to be fairly large from the very outset. In contrast to earlier investigations, the main attention was centered on the study of the spectral composition of the disturbance field.

Interaction of electromagnetic waves with a moving ionization front

Abstract: The problem of an electromagnetic wave packet, incident on a relativistically moving plane ionization or recombination front in a stationary gas, is considered. The frequency of the reflected wave packet is found to obey the usual double Doppler shift relation. However, the reflection coefficients and the physics can differ significantly from the case of reflection from moving material objects. In the unmagnetized case, the ratio of the energy in the reflected wave packet to that in the incident wave packet is found to be er*/ei*=ωi*/ωr* for an oncoming overdense ionization front, for which ωi*<ωr* (where ωi* and ωr* are the incident and reflected wave frequencies in the laboratory frame). For an oncoming ionization front in the presence of an applied magnetic field normal to the front, er*/ei* can considerably exceed ωi*/ωr*. For a retreating recombination front (ωi*≳ωr*), in the overdense unmagnetized case, er*/ei*=ωr*/ωi*. These results have important implications for the production of sub‐millimeter w...

Stochastic acceleration by an obliquely propagating wave‐An example of overlapping resonances

Abstract: A simple problem exhibiting intrinsic stochasticity is treated: the motion of a charged particle in a uniform magnetic field and a single plane wave. Detailed studies of this wave‐particle interaction show the following features. An electrostatic wave propagating obliquely to the magnetic field causes stochastic motion if the wave amplitude exceeds a certain threshold. The overlap of cyclotron resonances then destroys a constant of the motion, allowing appreciable momentum transfer to the particles. A wave of large enough amplitude would thus suffer severe damping and lead to rapid heating of a particle distribution. The stochastic motion resembles a diffusion process even though the wave spectrum is monochromatic. The methods of this paper should be useful for other problems showing stochasticity such as superadiabaticity in mirror machines, destruction of magnetic surfaces in toroidal systems, and lower hybrid heating.

Disruption of wavefronts: statistics of dislocations in incoherent Gaussian random waves

Abstract: Wavefront dislocations-i.e. singularities of the phase of a wave psi in the form of moving lines in space where mod psi mod vanishes-are studied for initially plane waves that have passed through a random space and time-dependent phase-changing screen. For transmitted waves that are Gaussian random, incoherent, quasi-monochromatic and paraxial the following quantities are calculated in terms of the statistics of the phase screen: dislocation densities, i.e. the average number of dislocation lines piercing unit area of variously-oriented surfaces, and dislocation fluxes, i.e. the average number of dislocation lines crossing unit length of variously-directed lines in unit time.

Diffraction of an electromagnetic plane wave by a thick slit

Abstract: Diffraction of an electromagnetic plane wave by an infinite slit in a conducting screen of finite thickness is studied theoretically using Weber-Schafheitlin discontinuous integrals. The direction of the incident wave is assumed to be in a plane perpendicular to that of the screen, and both polarizations are treated. The problems are reduced to simulataneous equations with an infinite number of unknowns, which are truncated in actual numerical computation. Numerical results for transmission coefficients are compared with those calculated from the Mathieu function expansion, and fairly good agreement is obtained. Far-field patterns are measured experimentally for the E -Polarization using a parallel-plate waveguide. The measured results confirm the validity of the theoretical prediction.

Measurements of the Poynting vector of standing hydromagnetic waves at geosynchronous orbit

Abstract: The observation is discussed of a train of hydromagnetic waves with a period of about 150 sec seen at synchronous orbit by the ATS 6 spacecraft on June 27, 1974. The critical observation is a phase shift of 90 deg between east-west oscillations of the particle flow and the east-west component of magnetic field oscillations. This phase shift alone suggests a standing rather than a propagating hydromagnetic wave. Careful processing of the particle data makes it possible to determine the drift velocity and hence the electric field of the wave. The wave electric field together with the time-varying magnetic field reveals an oscillating Poynting vector with zero mean component aligned with the ambient magnetic field and nonzero azimuthal (westward) component.

Phase sensitive laser probe for high-frequency surface acoustic wave measurements

Abstract: Field measurements of surface acoustic waves can be performed by coherent light in several ways. A method is described where a laser beam is focused to a small spot on the surface to obtain phase and amplitude infomation. A modified knife-edge technique has been applied in combination with amplitude modulation of the laser beam and single-side-band modulation of the surface wave. The apparatus has proven to be a powerful and flexible analytical tool. It is able to retieve phase and amplitude information from a traveling wave, and a partially standing wave can be characterized in terms of phase and amplitude of its two haveling wave components. The apparatus has been used up to 450 MHz.

Scattering matrix for elastic waves. II. Application to elliptic cylinders

Abstract: Using the scattering‐matrix approach to elastic wave scattering, numerical results are presented for the scattering of P, SV, and SH waves from a cylinder of elliptic cross section for ratios of minor to major axis ranging from 0.25 to 1.0 and for nondimensional wave numbers in the range 0.1–3.2. Calculations were made for a tungsten cylinder embedded in aluminum and also for a cylindrical cavity in aluminum. The incident waves are taken to be plane waves incident obliquely with respect to the major axis of the ellipse.

Spectral analysis of high‐frequency diffraction of an arbitrary incident field by a half plane—Comparison with four asymptotic techniques

Abstract: The knowledge of high-frequency diffraction of an arbitrary wave incident on an edge is important in many applications, such as antennas mounted on aircraft and reflector antennas illuminated by complex feeds. In this paper the problem of a half plane illuminated by a nonplanar wave is investigated using the concept of the plane wave spectral representation. For large wave number k, a new higher-order asymptotic solution for the total field up to and including terms of order k−5/2 relative to the incident field is derived. The behavior of the solution for the observation points which coincide with shadow boundary directions of a multipole line source is discussed in detail. Furthermore, numerical solution of the field integral representation is constructed for the observation angles in the transition regions. The results are compared with those of the Geometrical Theory of Diffraction (GTD), the Uniform Asymptotic Theory (UAT), the Uniform Theory of Diffraction (UTD) and the Modified Slope Diffraction (MSD).

Modeling the presence of wave groups in a random wave field

Abstract: Field and laboratory observations of surface wave fields show that the propagation speed of harmonic wave components differs from the predictions of linear theory. Angular spread fails to account for the observed discrepancy. An explanation is that the wave field does not consist of independently propagating Fourier components only but also contains groups of Stokes waves, forming envelope solitons. A model field is proposed, and the power spectral density and wave number spectra, as well as cross spectra, are calculated and compared to observations. A method is also suggested for identifying the properties of solitons contained in an observed wave field from spectra and cross spectra.

The dynamical theory of X-ray Bragg diffraction from a crystal with a uniform strain gradient. The Green Riemann functions

Abstract: The dynamical problem of X-ray Bragg diffraction from a thick (semi-infinite) crystal deformed by a uniform strain gradient (USG) is treated on the basis of the Green-Riemann function formalism. The rigorous solution of the problem is formulated by means of the Huygens-Fresnel principle. The exact Green functions are obtained in the form of the Laplace integrals suitable in physical applications. The quasi-classical and the Born (kinematical) asymptotic expansions of the Green functions are constructed as functions of the effective USG parameter B. Special attention is paid to the analysis of the wave-field propagation in a crystal with USG. The spatial harmonics Re(q) of the diffracted Green function, when Re(qB) 0 are damped exponentially in the bulk of the crystal. The Taupin problem of the Bragg dynamical diffraction of the X-ray incident plane wave from a thick crystal, the lattice spacing being a linear function of the coordinate z (along the inward normal to the entrance surface) only is solved exactly in analytical form. In the latter case the waveguide nature of the propagation of the spatial harmonics inside such a crystal, provided that Re(qB) < 0, is clearly revealed.

Heating of biological tissue in the induction field of VHF portable radio transmitters

Abstract: The results of a research project on the heating of simulated human tissue in the induction field of portable radios at VHF are summarized. The investigation was initiated because measurements made with commercially available field probes indicated that, in some cases, apparent power levels higher than 10 mW/cm2are incident on the operator. Two phantom models have been built for RF heating tests. The first is a parallelepiped of simulated muscle material 26 in long, 9 in wide, and 6.5 in high, topped by a 0.5 in layer of fat and bone composition. The other phantom is a human-size head and shoulders. This "dummy" is a 1/3-in thick shell of bone composition containing simulated brain material. The measurements of temperature increment due to radiation were performed with a digital thermometer having a sensitivity of 0.01°C. Temperature measurements on the parallelepipedal phantom have shown that the penetrating power densities in the simulated tissue are substantially lower than what could be expected from an incident plane wave with the same E-field intensity. The physical reason for this apparent discrepancy is that the strong fields of static nature emanating from a VHF helical antenna (commonly used with portable radios) are normally rather than tangentially directed to the surface of the phantom. These fields practically collapse at the air-body interface because of the high complex dielectric constant of human flesh. The results of the measurements performed on the head phantom have shown that a 6-W portable radio with a helical antenna held at 0.2 in from the operator's mouth causes very little heating of the simulated biological tissue (less than 0.1°C is highest temperature increase for one minute exposure). The maximum power density penetrating the dummy is less than 1 mW/cm2in the middle forehead. No detectable temperature increase is present in the immediate eye area. This is because in normal use, the eyes of the operator are exposed to the relatively low fields at the base of the antenna. A health hazard is present if the user places the tip of the antenna in the immediate vicinity of the eye (less than a 0.2-in distance) and then operates the transmitter. In this case, the possibility of damage is greatly reduced by a thick insulating cap at the tip of the antenna.

Simultaneous inversion of surface wave phase velocity and attenuation: Love waves in western North America

Abstract: A formalism is developed for simultaneous inversion of surface wave phase velocity and attenuation to determine shear wave velocity and Q−1 in the earth. A simultaneous inversion takes full account of the dependence of surface wave velocity and attenuation on both the elastic and the dissipative part of an earth structure and permits inclusion of the physical relationship between anelasticity and intrinsic dispersion that arises from linearity. The procedure is mathematically more complete than the approach of correcting phase velocity data for the intrinsic dispersion due to anelasticity and inverting the corrected velocity data alone, and it gives different results. The proposed formalism, including resolution analysis, weighted least squares inversion, and extremal inversion, is applied to Love waves in western North America. Various intrinsic dispersion-attenuation relations are tested, including Q independent of frequency, Q varying as a power of frequency, and Q specified by a sum of relaxation mechanisms. The results of the inversions confirm the coincidence of the low-velocity and low Q zones beneath western North America for frequencies in the surface wave band. Compared with previous inversion of Q−1 data alone, the simultaneous inversion results in improved depth resolution of Qs−1 and the elimination of an apparent incompatibility of low-attenuation data at 20- to 25-s periods. The Love wave data do not discriminate among the various dispersion-attenuation relations, though a constant Q leads to the removal of the requirement for a low-velocity zone at frequencies above 1 Hz. The predicted intrinsic dispersion within the low-velocity zone varies from 1% to 10% between 0.01 and 1 Hz for the various models; broadband measurements of body wave dispersion offer the greatest promise for choosing among the models.