Showing papers on "Group velocity published in 1969"

Slot Line on a Dielectric Substrate

Abstract: Slot line consists of a narrow gap in a conductive coating on one side of a dielectric substrate, the other side of the substrate being bare. If the substrate's permittivity is sufficiently high, such as /spl epsiv//sub r/ = 10 to 30, the slot-mode wavelength will be much smaller than free-space wavelength, and the fields will be closely confined near the slot. Possible applications of slot line to filters, couplers, ferrite devices, and circuits containing semiconductor elements are discussed. Slot line can be used either alone or with microstrip line on the opposite side of the substrate. A "second-order" analysis yields formulas for slot-line wavelength, phase velocity, group velocity, characteristic impedance, and effect of adjacent electric and magnetic walls.

Group velocity of spiral waves in galactic disks.

Abstract: Group velocity of spiral density waves propagating in disk galaxies implying self destruction, discussing possible replenishment sources

Recent Improvements in the Analysis of Surface Wave Observations

Abstract: Surface wave study techniques, discussing digital moving window analysis of group and phase velocity and use of time variable filters

Microseisms: Coastal and pelagic sources

Abstract: Wave-number frequency spectra of seismic background recordings from the large aperture seismic array (LASA) in eastern Montana have been used to study the source locations of different wave types in the frequency band from 40 to 500 mHz. Microseisms in this band consist of surface waves of the Rayleigh and Love type and compressional body, waves. The peak power band near 140 mHz (7-sec microseisms) and the lower frequency band near 70 mHz consist of fundamental Rayleigh waves, which often come from the same direction. This is especially true for directions from coasts in the vicinity of large storms. The average directional properties of the two bands are similar, indicating coastal sources for both. Love waves and higher mode Rayleigh waves in some instances come from the same coastal directions as the fundamental mode. Compressional body wave sources, pinpointed by using horizontal phase velocity to measure range, occur near storms both in coastal and pelagic regions. Pelagic storm sources were found only at frequencies that were high compared with double the frequency of ocean waves having a group velocity equal to the storm velocity. Located in the wake of a moving storm, such sources appear to be due to the oppositely traveling waves set up when a storm moves faster than its waves.

Higher mode interference and observed anomalous apparent Love wave phase velocities

Abstract: For a representative selection of spherical earth models compatible with seismic observations, there is a significant frequency range over which the fundamental and first higher Love mode group velocity curves approach each other closely or actually overlap. Higher Love modes can be excited comparably to the fundamental mode for both shallow and deep sources under a variety of circumstances, and thus higher mode interference is an important factor to be taken into account in the proper interpretation and analysis of Love waves. Simple theoretical computations reveal the nature of the effect of mode contamination on measured phase velocities, and biases in the selection and analysis of surface wave data make it appear likely that fundamental mode Love waves experiencing higher mode interference will exhibit anomalously high apparent phase velocities, as observed in the United States midcontinent and in Japan. Thus it is suggested that it may not be necessary to resort to complex or anisotropic models to explain these observations. Consideration of the effects of mode interference, as well as knowledge of source parameters (type, depth, orientation), are important in determining dependable fundamental and higher Love mode phase velocity dispersion over a broad frequency band. Phase velocity filtering across a large array could effectively separate the modes. Love waves traversing continental regions from the source will suffer little or no contamination from higher modes for wave periods of less than about 60 sec and thus are preferred to oceanic paths in obtaining reliable fundamental mode data in this restricted frequency range.

Crustal structure of the Mediterranean Sea Part II. Phase velocity and travel times

Abstract: A large number of observations of phase velocity of Love and Rayleigh waves have been used in order to determine the crust-mantle structure of the Mediterranean Sea. These velocities have been measured by using records from the Standard Stations located in the Mediterranean borders. The empirical dispersion curves have been compared with those of several models computed for this purpose. Travel-time curves of body waves for paths crossing the Mediterranean region, making use of all possible coastal stations and local earthquakes, have been found to substantiate the results from the surface wave dispersion. Also the value of the Bouguer anomaly in different points of the region was considered in the determination of the crustal thickness, as well as some measurements of group velocity from clear observations of higher modes. The crust-mantle structure under the Mediterranean Sea was found to be formed by two clearly distinct zones which correspond roughly to the areas to the west and east of Italy. The western zone, between Italy and Spain, is of oceanic type with a thin crust (about 14 km) and a low-velocity channel in the Upper Mantle. In the eastern zone, south of Greece, the crust (some 23 km thick) shows a great thickness of the uppermost sedimentary layers, which gives rise to very low velocities of short-period surface waves in that region. Love waves with group velocities smaller than those of Rayleigh have been clearly observed in a large number of earthquakes in this region. The surface wave dispersion as well as the travel-time curves of P and S waves of this zone indicate the existence of a low-velocity channel in the Upper Mantle.

Effect of Higher Mode Contamination on Measured Love Wave Phase Velocities

Abstract: In some circumstances the first higher mode of the Love surface wave can be significantly excited in the period range 30–90 sec by earthquakes and may travel at a group velocity comparable to that of the fundamental mode. This difficult-to-separate contamination will produce a large scatter, but no uniform bias, in phase velocities measured from an ensemble of events. The phase velocities measured from a single event may show a bias over a limited range of periods, but this perturbation may be positive or negative. These results argue against an explanation of observed inconsistencies of Love and Rayleigh wave phase velocities as being due to higher mode contamination. The results are also applicable to the effect of multipath interference of two similar-mode waves traveling at an angle with respect to one another.

Properties of dielectric-tube waveguides

Abstract: Two aspects of dielectric-tube waveguide properties are considered. First, a mode designation is given which is consistent with one previously adopted for the dielectric rod, based on the behaviour of field components as well as on the sequence of solutions. Computed 2- and 3-dimensional field configurations are shown for the HE11 and EH11 modes. The second aspect is concerned with the propagation characteristics for all modes. Expressions are derived for power flow, energy storage and power loss using a perturbation method, giving expressions for group velocity and attenuation coefficient. It is indicated that a certain configuration of the dielectric-tube waveguide may be practicable at the higher microwave frequencies

Magnetoelastic Waves in Time‐Varying Magnetic Fields. I. Theory

Abstract: The propagation of microwave magnetoelastic waves in a ferromagnet subjected to pulsed magnetic fields is investigated. In the first part of the theoretical analysis, which is restricted to spatially uniform bias fields, the interaction between the magnetic and elastic systems is neglected. Using a coupled‐mode approach, it is found that plane spin waves propagate in magnetic field transients with negligible reflections, constant momentum and wavenumber, but variable frequency and power flow. These results are further elucidated by the study of a simple step transient case. In the presence of magnetoelastic interaction, the theory indicates that besides changes in frequency and in power flow, the field gradient causes group velocity modulation and changes in the time duration of wave packets. In addition there is an exchange of momentum between the different branches of the dispersion relation; this exchange is calculated in terms of critical field gradients, The analysis reveals that negligible coupling ...

Π-pulse propagation in the presence of host dispersion

Abstract: A steady-state solution describing \Pi -pulse propagation in a homogeneously broadened laser amplifier embedded in a dispersive lossy host is found. For a given model of dispersion, the solution satisfies exactly the second-order wave equation together with the equations of motion for the density matrix of the two-level resonant system. The salient feature of the solution is that a small amount of dispersion in the host group velocity causes a large monotonic frequency sweep during the pulse. The effects of this chirp on the pulse properties are described in detail, and the relation of the theory to experiments is discussed.

Energy transport velocity of electromagnetic propagation in dispersive media

Abstract: For electromagnetic propagation in linear dispersive media, two definitions of energy transport velocity may be introduced. The first is the transport velocity of total stored energy density, and it was first discussed by Brillouin. This concept does not distinguish different forms of energy, but considers only the total. It is equal to the group velocity for the case of a lossless dispersive medium. Its significance is not fully understood for a maser medium where it is either greater than the velocity of light or negative, or for a dissipative dispersive medium where it does not seem to offer a simple description of the propagation process. The second is the transport velocity of electromagnetic energy. It is based on the distinction of propagating energy which is just electric and magnetic field energy, from localized energy which is stored in the degrees of freedom of the dispersive medium. Thus propagation involves electromagnetic propagation per se, plus a transfer of energy between the propagating and localized forms. This transport velocity is always less than the velocity of light, for absorbing and maser media alike. It may be determined through a gain or attenuation measurement since it enters the gain equation of electromagnetic transmission structures.

The motion generated by a body moving along the axis of a uniformly rotating fluid

Abstract: Experiments have been made to investigate the motion generated by a body moving along the axis of a uniformly rotating fluid.Part of the investigation concerns the motion generated in a cylinder whose radial dimensions are much greater than those of the body. Measurements have been made of the velocities of particles on the axis of rotation both ahead of and behind the body, and the results indicate that there is a significant axial motion generated by the body over a wide range of Rossby numbers. A measurement of the instantaneous velocity profile ahead of the body, determined as a function of the radius, agrees fairly well with a low Rossby number calculation of the flow due to a circular disk (Morgan 1951). In addition, the forward influence of the body has been measured as a function of the Rossby number and from these results it is suggested that the body has a finite influence far upstream at all Rossby numbers and that the blocking phenomenon first reported by Taylor (1922) probably occurs for all values of the Rossby number (UΩa) less than a critical value which is about 0·7.Experiments have also been made in a long cylindrical tube which acts as a wave guide. At large distances from the body the separate effects of the various modes can be observed and hence it is possible to measure the flow corresponding to an individual wave-number: these measurements show that, as a result of the Doppler effect, the motion a large distance ahead of the body is different from that far behind (see Lighthill 1967). Moreover, the experiments indicate that no disturbances propagate ahead of the body when its velocity exceeds the maximum group velocity in the fluid, but that disturbances trail behind the body when its velocity is far in excess of the maximum group velocity. Measurements of the maximum group velocity are in good agreement with the theoretical value.

Application of laser Doppler velocity instrumentation to the measurement of jet turbulence.

Abstract: Three dimensional laser Doppler velocity instrument for mean velocity and turbulence measurements in subsonic jet shear layer

Magnetoelastic Waves in Time-Varying Magnetic Fields. II. Experiments

Abstract: Experiments which confirm the theoretical predictions on the propagation of magnetoelastic waves in time varying magnetic fields are described. Frequency shifts of spin waves as high as 1 GHz have been observed at L‐band microwave frequencies, in axially magnetized rods of yttrium iron garnet (YIG). Experiments with magnetoelastic pulses were performed in cases where the signal is injected either as a phonon (piezoelectric excitation) or as a magnon (fine‐wire excitation). The first demonstrates frequency conversion and group velocity modulation. The latter scheme is suitable to observe pulse compression and expansion due to frequency modulation and time changes of the group velocity. It is also used to ``trap'' magnetoelastic pulses in the phonon state, by removal of the crossover point.

Radar Testing of the Relative Velocity of Light in Space

Abstract: Published interplanetary radar do to presents evidence that the relative velocity of light in space is c+v and not c.

Laser doppler system for measuring three dimensional vector velocity

Abstract: Describing laser Doppler velicometer for measuring mean velocity and turbulence of fluid flow

A Continuous Wave Technique for the Automatic Measurement of Ultrasonic Velocity Changes

Abstract: A technique is described for the automatic measurement of ultrasonic velocity changes in continuous wave experiments. An oscillator is locked to a mechanical resonance of the sample. Velocity changes appear as frequency changes of the oscillator and are easily recorded. Some typical results are given.

Measurement of Velocity Space Diffusion using the Plasma Wave Echo

Abstract: The diffusion of plasma electrons in velocity space has been measured using the plasma wave echo as an experimental tool. The measurements were made for velocity space diffusion due to noise impressed on the plasma and that due to background noise.

Upper mantle structure - Are the low velocity layers thin?

Abstract: Earth upper mantle wave velocity structure based on surface wave dispersion, noting low velocity layers thickness

The effect of bandwidth on the interpretation of the cross spectra of wave recordings from spatially separated sites

Abstract: It is argued that the coherence between records from a triangle of stations can be used to deduce not only the effective beamwidths of gravity wave patterns but can also be used to obtain their velocity bandwidth throughout the wave spectrum and its effect on the apparent velocity of propagation. Relationships between the apparent velocity of wave propagation and the velocity bandwidth are derived; they are analogous to the cross correlation approach of Briggs, Phillips, and Shinn but are herein generalized to include the frequency domain. The significance of the similarity hypothesis which is essential to the Briggs, Phillips and Shinn approach is discussed. The effect of the frequency bandwidth corresponding to the sampling rate and digital methods of analysis is considered in relation to the apparent phase velocity. The analytical results are applied to experimental data on gravity wave motions in the D region of the ionosphere.

Velocity dispersion in Brillouin scattering

Abstract: The dispersion relations for the velocity v for temporally absorbed hypersonic waves and for the velocity vB as determined from the positions of the Doppler-shifted Brillouin peaks are discussed fo...

Theory of the Axial Ray

Abstract: The theory of the ray that travels along or arbitrarily close to an axis of minimum velocity in the ocean is examined for an extremely wide class of velocity profiles in which the depth is expressed as a generalized power series in the velocity. Although the complete ray theory is intractable for this model, the range and travel‐time integrals can be evaluated for the axial ray. Derivatives of these integrals with respect to the ray parameter indicate whether the axial ray is faster or slower than near‐axial rays. The results for range, intensity anomaly, and the derivative of the group velocity with respect to the phase velocity are expressed as simple functions of σ1, the smallest exponent in the profile representation. Other ray‐theory results are expressed in terms of various order profile derivatives, evaluated at the axis, and the effect of discontinuities in the profile derivatives is examined. Certain range and travel‐time derivatives, although finite for symmetric profiles, are found to be infinite for nonsymmetric profiles. Thus, results for several symmetric profiles in common use differ considerably from the results for more‐realistic nonsymmetric profiles.