Showing papers on "Wave propagation published in 1979"

Structure under Mount Rainier, Washington, inferred from teleseismic body waves

Abstract: Teleseismic long-period P waves recorded at the World-Wide Standard Seismograph Network station LON (Longmire, Washington) are shown to exhibit strong anomalous particle motion not attributable to instrument miscalibration or malfunction. In particular, a large and azimuthally smoothly varying tangential component is observed after vector rotation of horizontal P waves into the ray direction and after application of a deconvolution technique which equalizes effective source time functions and removes the instrument response. These tangential waves attain amplitudes comparable to the radial component and demonstrate wave form antisymmetry about a NNE azimuth. A model which contains a single high-contrast interface dipping toward the NNE at a depth of 15–20 km can explain most of the characteristics of the long-period P wave data, provided dips are greater than about 10° and only the interference of P and Ps generated at the interface is considered. The model breaks down for later arrivals which are presumably multiples or scattered waves. Examination of long-period S waves from several deep teleseisms shows a prominent Sp arrival 18 s before S. The timing of this phase conversion suggests an interface at about 145-km depth, and its sense of polarity suggests that the velocity contrast is from higher to lower velocities as depth decreases. This interface may correspond to the bottom of the upper mantle low-velocity zone in the area.

Constant Q-wave propagation and attenuation

Abstract: A linear model for attenuation of waves is presented, with Q, or the portion of energy lost during each cycle or wavelength, exactly independent of frequency. The wave propagation is completely specified by two parameters, e.g., Q and c0, a phase velocity at an arbitrary reference frequency ω0. A simple exact derivation leads to an expression for the phase velocity c as a function of frequency: c/c0 = (ω/ω0)γ, where γ = (1/π) tan−1 (1/Q). Scaling relationships for pulse propagation are derived and it is shown that for a material with a given value of Q, the risetime or the width of the pulse is exactly proportional to travel time. The travel time for a pulse resulting from a delta function source at x = 0 is proportional to xβ, where β = 1/(1 - γ). On the basis of this relation it is suggested that the velocity dispersion associated with anelasticity may be less ambiguously observed in the time domain than in the frequency domain. A steepest descent approximation derived by Strick gives a good time domain representation for the impulse response. The scaling relations are applied to field observations from the Pierre shale formation in Colorado, published by Ricker, who interpreted his data in terms of a Voigt solid with Q inversely proportional to frequency, and McDonal et al., who interpreted their data in terms of nonlinear friction. The constant Q theory fits both sets of data.

Electromagnetic propagation in birefringent layered media

Abstract: The propagation of electromagnetic radiation in birefringent layered media is considered. A general formulation of the plane-wave propagation in an arbitrarily birefringent layered medium is presented. The concepts of dynamical matrix and propagation matrix are introduced. A 4 × 4 transfer matrix method is used to relate the field amplitudes in different layers. Our general theory is then applied to the special case of periodic birefringent layered media, especially the Solc birefringent layered media [ I. Solc , Cesk. Casopis Fẏs.3, 366 ( 1953);Cesk. Casopis Fẏs.10, 16 ( 1960)]. The unit cell translation operator is derived. The band structures as well as the Bloch waves are obtained by diagonalizing the translation operator. Coupled mode theory is extended to the case of birefringent periodic perturbation to explain the exchange Bragg scattering. A general mode dispersion relation for guided waves is also obtained in terms of the transfer matrix elements.

On electromagnetic waves in chiral media

Abstract: We analyze the propagation of electromagnetic waves through chiral media, i.e., through composite media consisting of macroscopic chiral objects randomly embedded in a dielectric. The peculiar effects that such media have on the polarization properties of the waves are placed in evidence. To demonstrate the physical basis of these effects, a specific example, chosen for its analytical simplicity, is worked out from first principles.

The effect of orbital motions on synthetic aperture radar imagery of ocean waves

Abstract: The formation of wave-like patterns in synthetic aperture radar (SAR) images of the ocean surface caused by orbital motions is investigated. Furthermore, the degradation in azimuthal resolution due to these motions is calculated by applying a least square fit to the phase history. Formulas are given which describe the variation of intensity in azimuthal direction in the image plane as well as the degradation in azimuthal resolution as a function of ocean wave amplitude, wave frequency, direction of wave propagation, and radar wavelength, incidence angle, and integration time.

Jupiter plasma wave observations: an initial voyager 1 overview.

Abstract: The Voyager I plasma wave instrument detected low-frequency radio emissions, ion acoustic waves, and electron plasma oscillations for a period of months before encountering Jupiter's bow shock. In the outer magnetosphere, measurements of trapped radio waves were used to derive an electron density profile. Near and within the Io plasma torus the instrument detected high-frequency electrostatic waves, strong whistler mode turbulence, and discrete whistlers, apparently associated with lightning. Some strong emissions in the tail region and some impulsive signals have not yet been positively identified.

Compressional and shear waves in saturated rock during water-steam transition

Abstract: Compressional and shear wave velocities were measured in water-filled Berea sandstone as a function of pore pressure, with a constant confining pressure of 300 bars. The measurements were made at room temperature, 145°C, and 198°C. At 145°C, compressional velocity increased from vapor-saturated (low pore pressure) to liquid-saturated (high pore pressure) conditions, whereas shear wave velocity decreased. For Compressional waves there was a velocity minimum and increased attenuation near the liquid-vapor transition. The results at 198°C show decreases of both compressional and shear velocities and a small velocity minimum for compressional velocity without marked attenuation. At both temperatures, Vp/Vs and Poisson's ratios increased from steam- to water-saturated rock. The results are compatible with the mechanical effects of mixing steam and water in the pore space near the phase transition and may be applicable to in situ geothermal field evaluation.

On the origin of plasmaspheric hiss: The importance of wave propagation and the plasmapause

Abstract: The presence of a turbulent band of ELF hiss throughout the plasmasphere has been modeled by following unducted whistler mode waves from a restricted region of cyclotron resonant growth in the outer plasmasphere. The most unstable waves originate near the equatorial plane within a modest cone of wave normal angles aligned along the ambient magnetic field. As the waves propagate to higher latitude, their wave normal direction becomes progressively oblique, and ray paths generally migrate outward until they reach the plasmapause. A major fraction of the incident waves internally reflect at the steep plasmapause density gradient and subsequently propagate inward to populate most of the plasmasphere. Only a minor part of the wave energy is able to escape from the plasmasphere, primarily at high latitude. A detailed study of the internally trapped ray paths shows that certain waves can return to the equatorial growth region with field-aligned propagation vectors and thus experience further amplification. This is particularly important for the maintenance of quiet time hiss when a single transit of the growth region is insufficient to amplify the background incoherent cyclotron noise to detectable levels. The model calculations demonstrate that the observed frequency spectrum and polarization of plasmaspheric hiss not only are controlled by the relative rates of cyclotron growth and Landau damping but also are strongly influenced by the wave propagation characteristics.

Planetary Circulations: 2. The Jovian Quasi-Geostrophic Regime

Abstract: The characteristics of the two-level quasi-geostrophic model are evaluated for a wide range of parameter values in the Jovian domain. The results support the hypothesis that baroclinic instability energizes the circulation of Jupiter and Saturn and that the blocking effect of planetary wave propagation on quasi-geostrophic turbulent cascades determines the width and zonality of the bands—the degree of zonality being higher in the absence of surface drag. The model circulations consist of multiple westerly jets, separated by strong easterly flows—the result of momentum partitioning by the Kuo vortex separation process. There are no large-scale vertical motions. A cyclic variation occurs (with a time scale of several years) during which phases with intense, large-scale baroclinic activity alternate with longer, more quiescent phases involving weak, small-scale baroclinic instability and neutral baroclinic waves. These neutral waves, generated by quasi-two-dimensional cascades and propagating at spe...

Synthetic aperture radar imaging of moving ocean waves

Abstract: A theory for the radar imaging of ocean waves is presented under the assumptions that a swell propagates through an ensemble of Bragg scatterers and that the integration time of the synthetic aperture radar (SAR) is small compared to the angular velocity of the swell. Results are prsented which show image development and distortions caused by the radial velocities and accelerations of the swell. Neglecting small wave bunching and tilts due to the longer underlying waves, and considering only one-dimensional geometries, the mechanism of wave motions are considered and their efforts on the production of the usual intensity Pattern representing the wave image are studied. The analysis shows that in certain situations a processed image can appear which has twice the spatial period of the actual long wave on the ocean, which can confuse the interpretation of ocean wave analysis.

Hydromagnetic surface waves

Abstract: Plane and filamentary structures aligned with a magnetic field abound on the Sun and in both interplanetary and interstellar space. When the Alfven speed changes across such boundaries, hydromagnetic surface waves can travel along them, carry energy, and provide heating. This paper surveys the nature of such surface waves, with emphasis on the dispersion relations, the spatial extent of the waves, the degree of gas compression, and the possibility of coupling to ordinary hydromagnetic waves. Explicit results are provided for the cases when the gas pressure is either much smaller or much larger than the magnetic pressure on either side of the surface. The waves are shear waves wherever p< or =B/sup 2//8..pi... All surface waves involve finite gas compression, but this compression is negligible when kxBvery-much-less-thankB.

The reflection of impulses from a nonlinear random sea

Abstract: A model of the reflection of radar impulses from the sea at near-vertical incidence is used to account for non-Gaussian ocean waves statistics. The joint probability density function (pdf), of wave height and slope, is calculated according to the theory of Longuet-Higgins (1963) on the distribution of variables in a 'weakly nonlinear' random era. The long-crested approximation is made, a Phillips wave spectrum is assumed, and the Gram-Charlier series is truncated after skewness terms. It is found that the height and height-slope skewness coefficients bear the ratio 1:2 and that the derived impulse response and conditional cross section versus wave height are in excellent agreement with previous observations. Finally, it is suggested that the empirically determined and theoretically predicted sea state bias be corrected for in the routine processing of satellite radar altimeter data.

Third-order approximation to short-crested waves

Abstract: Short-crested wave systems, as produced by two progressive waves propagating at an oblique angle to each other, have an extremely important effect on a sedimentary bed. The complex water-particle motions are conducive to lifting material into suspension and sustaining it in motion. In order to study this phenomenon rigorously, the variables of this wave system are derived to a third-order approximation by a perturbation method. The case of waves reflecting obliquely from a vertical wall is examined under the assumptions of full reflexion, uniform finite depth and an inviscid incompressible fluid. The new formulation reduces to standing or Stokes waves at the limiting angles of approach. Expressions for kinematic quantities are also presented.

Direct Solution of Wave Dispersion Equation

Abstract: Approximate solutions for wave speed given by equations reduce the number of arithmetic operations required during a refraction calculation. The precise saving depends on the algorithms presently in use, but a time saving of about half appears typical.

Modal propagation angles in a cylindrical duct with flow and their relation to sound radiation

Abstract: The main emphasis is upon the propagation angle with respect to the duct axis and its relation to the far-field acoustic radiation pattern. When the steady flow Mach number is accounted for in the duct, the propagation angle in the duct is shown to be coincident with the angle of the principal lobe of far-field radiation obtained using the Wiener-Hopf technique. Different Mach numbers are allowed within the duct and in the external field. For static tests with a steady flow in an inlet but with no external Mach number the far-field radiation pattern is shifted considerably toward the inlet axis when compared to zero Mach number radiation theory. As the external Mach number is increased the noise radiation pattern is shifted away from the inlet axis. The theory is developed using approximations for sound propagation in circular ducts. An exact analysis using Hankel function solutions for the zero Mach number case is given to provide a check of the simpler approximate theory.

A Mechanistic Model of Ozone Transport by Planetary Waves in the Stratosphere

Abstract: The role of planetary-scale waves in the transport of ozone in middle and high latitudes is studied with a linearized model which takes into account the coupling between radiation, chemistry and dynamics. The governing equations are a quasi-geostrophic potential vorticity equation and a continuity equation for perturbations of the O3 mixing ratio. These equations include the effects of absorption of solar radiation by O3, temperature-dependent O3 photochemistry and advection of O3 by wave motions. Model runs show that, with a basic state representative of winter conditions, ultralong waves can produce O3 perturbation which exhibit a phase shift of nearly 180° between the lower stratosphere (where the O3 perturbations are controlled by advection) and the upper stratosphere (where they are controlled by photochemical processes). This phase shift brings the O3 perturbation into phase with the eddy meridional velocity (and 180° out of phase with the eddy vertical velocity) at ∼45 km, giving rise to l...

Radiative amplification of sound waves in the winds of O and B stars

Abstract: The velocity perturbation associated with an outwardly propagating sound wave in a radiation-driven stellar wind gives rise to a periodic Doppler shifting of absorption lines formed in the flow. A linearized theory applicable to optically thin waves is used to show that the resulting fluctuation in the absorption-line force can cause the wave amplitude to grow. Detailed calculations of the acceleration due to a large number of lines indicate that significant amplification can occur throughout the high-velocity portion of winds in which the dominant force-producing lines have appreciable optical depths. In the particular case of the wind of Zeta Pup (O4f), it is found that the e-folding distance for wave growth is considerably shorter than the scale lengths over which the physical properties of the flow vary. A qualitative estimate of the rate at which mechanical energy due to nonlinear waves can be dissipated suggests that this mechanism may be important in heating the supersonic portion of winds of early-type stars.

Electromagnetic wave propagation

Abstract: Methods in Electromagnetic Wave Propagation. By D.S. Jones. Pp. 887. (Clarendon/Oxford University Press: Oxford, 1979.) £22.50.

Elastic waves in bodies with initial stresses

Abstract: An analysis of research available on the problem under consideration shows that at this time: a) Wave propagation in unbounded, bounded, and composite laminar bodies with homogeneous initial states has been studied sufficiently extensively; b) methods to determine the third-order elastic constants have been developed on the basis of existing theories; c) an ultrasonic nondestructive method to determine the stress in solids has been developed, where the stresses averaged over the bulk of the body are determined. In light of the above, in the authors' opinion, the following should be considered the most urgent questions for further investigations on the problem: a) the investigation of wave propagation regularities in bodies with inhomogeneous initial states (it is hence important to execute quantitative and qualitative analyses of the phenomenon); b) development of an ultrasonic nondestructive method of determining the stress in the near surface layers of a solid, which will afford a possibility of determining not only the membrane stresses but also the bending stresses; c) an investigation of the wave propagation regularities in fibrous composites with initial stresses.

The acceleration of minor ion species in the solar wind

Abstract: This paper provides a comprehensive analysis of the dynamics of the flow of minor ion species in the solar wind under the combined influences of gravity, Coulomb friction (with protons), rotational forces (arising from the Sun's rotation and the interplanetary spiral magnetic field) and wave forces (induced in the minor ion flow by Alfven waves propagating in the solar wind). It is assumed that the solar wind can be considered as a proton-electron plasma which is, to a first approximation, unaffected by the presence of minor ions. In the dense hot region near the Sun Coulomb friction accelerates minor ions outwards against the gravitational force, part of which is cancelled by the charge-separation electric field. Once the initial acceleration has been achieved, wave and rotational forces assist Coulomb friction in further increasing the minor ion speed so that it becomes comparable with, or perhaps even exceeds, the solar wind speed. A characteristic feature of the non-resonant wave force is that it tends to bring the minor ion flow into an equilibrium where the radial speed matches the Alfven speed relative to the solar wind speed, whereas Coulomb friction and rotational forces tend to bring the flow into an equilibrium where the radial speed of the minor ions equals the solar wind speed. Therefore, provided that there is sufficient wave energy and Coulomb friction is weak, the minor ion speed can be ‘trapped’ between these two speeds. This inteststing result is in qualitative agreement with observational findings to the effect that the differential flow speed between helium ions and protons is controlled by the ratio of the solar wind expansion time to the ion-proton collision time. If the thermal speeds of the protons and minor ions are small compared to the Alfven speed, two stable equilibrium speeds can exist because the rapid decrease in the Coulomb cross-section with increasing differential flow speed allows the non-resonant wave force to balance Coulomb friction at more than one ion speed. However, it must be emphasized that resonant wave acceleration and/or strong ion partial pressure gradients are required to achieve radial speeds of minor ions in excess of the proton speed, since, as is shown in Section 4, the non-resonant wave acceleration on protons and minor ions are identical when their radial speeds are the same, with the result that, in the solar wind, non-resonant wave acceleration tends (asymptotically) to equalize minor ion and proton speeds.

Self‐modulated VLF wave‐electron interactions in the magnetosphere: A cause of auroral pulsations

Abstract: A model is developed for self-modulated pulsations in the outer trapped electron belts The pulsations are a consequence of growth of VLF waves and removal of trapped electrons following an electron injection event If too many electrons are removed by wave interactions, the system restores itself by building up the flux again to values exceeding the equilibrium flux, repeated through many cycles Pulsations are shown to be possible for both broadband VLF wave generation and narrowband wave generation The resulting electron orecipitation pulses are consistent with observations of pulsating aurorae on the morningside following a substorm injection event The conditions for pulsations are quite stringent and favor trapped particle fluxes near the stable trapping limit Individual pulses may be in or near strong diffusion

Molecular Dynamics of Shock Waves in Three-Dimensional Solids: Transition from Nonsteady to Steady Waves in Perfect Crystals and Implications for the Rankine-Hugoniot Conditions

Abstract: Molecular-dynamics calculations of shock waves in perfect three-dimensional solids at nonzero initial temperatures reveal a transition in the nature of the asymptotic shockwave structure as a function of shock strength. The key to this transition from nonsteady to steady waves where the Rankine-Hugoniot relations are obeyed is the partial relaxation of compressive shear stress behind the shock front which accompanies small, but permanent, transverse strains in atomic positions.

Shear damping of two-dimensional drift waves in a large-aspect-ratio tokamak

Abstract: In a uniform plane slab, with shear in the magnetic field, damping of drift waves is due to the outward convection of energy by the wave. It is known, however, that the inclusion of two-dimensional effects, such as toroidal modulation of shear or magnetic field, can inhibit propagation of the wave and so reduce shear damping. This effect is investigated by using a two-dimensional model representing long-wavelength drift waves in a large-aspect-ratio tokamak. It is shown that this two-dimensional problem can be reduced to a one-dimensional eigenvalue equation from which the shear damping can readily be computed. It is confirmed that toroidal effects can annul the shear damping, and some examples are given.

Interaction of interplanetary shock waves with the bow shock-magnetopause system

Abstract: The collision of interplanetary shock waves with the bow-shock-wave-magnetopause system is considered both in the gas-dynamic and magnetohydrodynamic approximations. It is shown that the shock wave is reflected from the magnetopause as a rarefaction wave which, in turn, is reflected from the rearward side of the bow shock. This secondary rarefaction wave arrives at the magnetosphere after a time interval of 3--5 min after the interplanetary shock wave's arrival. The rarefaction wave decreases the flow pressure on the magnetosphere and causes the reverse (i.e., outward) motion of the magnetopause. With the help of an approximate solution of the differential equations the problem of rarefaction wave reflection from the magnetosphere is considered. The law of the subsolar point motion of the magnetosphere during the abrupt shock-like perturbation is obtained. The experimental data confirm the theoretical results.

Coherent electromagnetic wave propagation through randomly distributed dielectric scatterers

Abstract: We present a vector multiple-scattering analysis of the coherent wave propagation through an inhomogeneous media consisting of a random distribution of identical, oriented, nonspherical, dielectric scatterers. The single-scattering aspect of the problem is dealt with through application of the transition or $T$ matrix. Configurational averaging techniques are employed to determine the "hole" correction integrals which are subsequently solved to yield the dispersion relations characterizing the bulk or effective properties of the medium. Closed-form solutions in the Rayleigh limit are derived for both spherical and spheroidal scatterer geometries. These solutions, together with the $T$ matrix, form the basis of our computational method for determining the coherent wave phase velocity and attenuation as a function of frequency ($\mathrm{ka}$) and scatterer concentration. Numerical results are presented for spherical and oblate spheroidal geometries over a range of $\mathrm{ka}$ values (0.05-2.0) and scatterer concentrations (0.05-0.20).