Showing papers on "Phase velocity published in 2016"

The MJO as a Dispersive, Convectively Coupled Moisture Wave: Theory and Observations

Abstract: A linear wave theory for the Madden–Julian oscillation (MJO), previously developed by Sobel and Maloney, is extended upon in this study. In this treatment, column moisture is the only prognostic variable and the horizontal wind is diagnosed as the forced Kelvin and Rossby wave responses to an equatorial heat source/sink. Unlike the original framework, the meridional and vertical structure of the basic equations is treated explicitly, and values of several key model parameters are adjusted, based on observations. A dispersion relation is derived that adequately describes the MJO’s signal in the wavenumber–frequency spectrum and defines the MJO as a dispersive equatorial moist wave with a westward group velocity. On the basis of linear regression analysis of satellite and reanalysis data, it is estimated that the MJO’s group velocity is ~40% as large as its phase speed. This dispersion is the result of the anomalous winds in the wave modulating the mean distribution of moisture such that the moistur...

Wave propagation in viscoelastic single-walled carbon nanotubes with surface effect under magnetic field based on nonlocal strain gradient theory

Abstract: The governing equation of wave motion of viscoelastic SWCNTs (single-walled carbon nanotubes) with surface effect under magnetic field is formulated on the basis of the nonlocal strain gradient theory. Based on the formulated equation of wave motion, the closed-form dispersion relation between the wave frequency (or phase velocity) and the wave number is derived. It is found that the size-dependent effects on the phase velocity may be ignored at low wave numbers, however, is significant at high wave numbers. Phase velocity can increase by decreasing damping or increasing the intensity of magnetic field. The damping ratio considering surface effect is larger than that without considering surface effect. Damping ratio can increase by increasing damping, increasing wave number, or decreasing the intensity of magnetic field.

A global view of velocity fluctuations in the corona below 1.3 $R_\odot$ with CoMP

Abstract: The Coronal Multi-channel Polarimeter (CoMP) has previously demonstrated the presence of Doppler velocity fluctuations in the solar corona. The observed fluctuations are thought to be transverse waves, i.e. highly incompressible motions whose restoring force is dominated by the magnetic tension, some of which demonstrate clear periodicity. We aim to exploit CoMP's ability to provide high cadence observations of the off-limb corona to investigate the properties of velocity fluctuations in a range of coronal features, providing insight into how(if) the properties of the waves are influenced by the varying magnetic topology in active regions, quiet Sun and open fields regions. An analysis of Doppler velocity time-series of the solar corona from the $10,747$~A Iron XIII line is performed, determining the velocity power spectra and using it as a tool to probe wave behaviour. Further, the average phase speed and density for each region are estimated and used to compute the spectra for energy density and energy flux. In addition, we assess the noise levels associated with the CoMP data, deriving analytic formulae for the uncertainty on Doppler velocity measurements and providing a comparison by estimating the noise from the data. It is found that the entire corona is replete with transverse wave behaviour. The corresponding power spectra indicates that the observed velocity fluctuations are predominately generated by stochastic processes, with the spectral slope of the power varying between the different magnetic regions. Most strikingly, all power spectra reveal the presence of enhanced power occurring at $ \sim3$~mHz, potentially implying that the excitation of coronal transverse waves by $p$-modes is a global phenomenon.

A global view of velocity fluctuations in the corona below 1.3r⊙ with comp

Abstract: The Coronal Multi-channel Polarimeter (CoMP) has previously demonstrated the presence of Doppler velocity fluctuations in the solar corona. The observed fluctuations are thought to be transverse waves, i.e., highly incompressible motions whose restoring force is dominated by the magnetic tension, some of which demonstrate clear periodicity. We aim to exploit CoMP’s ability to provide high cadence observations of the off-limb corona to investigate the properties of velocity fluctuations in a range of coronal features, providing insight into how (whether) the properties of the waves are influenced by the varying magnetic topology in active regions, quiet Sun and open field regions. An analysis of Doppler velocity time-series of the solar corona from the 10747 A Iron XIII line is performed, determining the velocity power spectrum and using it as a tool to probe wave behavior. Further, the average phase speed and density for each region are estimated and used to compute the spectra for energy density and energy flux. In addition, we assess the noise levels associated with the CoMP data, deriving analytic formulae for the uncertainty on Doppler velocity measurements and providing a comparison by estimating the noise from the data. It is found that the entire corona is replete with transverse wave behavior. The corresponding power spectra indicate that the observed velocity fluctuations are predominately generated by stochastic processes,with the spectral slope of the power varying between the different magnetic regions. Most strikingly, all power spectra reveal the presence of enhanced power occurring at ∼3 mHz, potentially implying that the excitation of coronal transverse waves by p-modes is a global phenomenon.

Analytical and numerical studies of approximate phase velocity matching based nonlinear S0 mode Lamb waves for the detection of evenly distributed microstructural changes

Abstract: Most previous studies on nonlinear Lamb waves are conducted using mode pairs that satisfying strict phase velocity matching and non-zero power flux criteria. However, there are some limitations in existence. First, strict phase velocity matching is not existed in the whole frequency bandwidth; Second, excited center frequency is not always exactly equal to the true phase-velocity-matching frequency; Third, mode pairs are isolated and quite limited in number; Fourth, exciting a single desired primary mode is extremely difficult in practice and the received signal is quite difficult to process and interpret. And few attention has been paid to solving these shortcomings. In this paper, nonlinear S0 mode Lamb waves at low-frequency range satisfying approximate phase velocity matching is proposed for the purpose of overcoming these limitations. In analytical studies, the secondary amplitudes with the propagation distance considering the fundamental frequency, the maximum cumulative propagation distance (MCPD) with the fundamental frequency and the maximum linear cumulative propagation distance (MLCPD) using linear regression analysis are investigated. Based on analytical results, approximate phase velocity matching is quantitatively characterized as the relative phase velocity deviation less than a threshold value of 1%. Numerical studies are also conducted using tone burst as the excitation signal. The influences of center frequency and frequency bandwidth on the secondary amplitudes and MCPD are investigated. S1–S2 mode with the fundamental frequency at 1.8 MHz, the primary S0 mode at the center frequencies of 100 and 200 kHz are used respectively to calculate the ratios of nonlinear parameter of Al 6061-T6 to Al 7075-T651. The close agreement of the computed ratios to the actual value verifies the effectiveness of nonlinear S0 mode Lamb waves satisfying approximate phase velocity matching for characterizing the material nonlinearity. Moreover, the ratios derived from the primary and secondary horizontal displacements generated from nonlinear S0 mode Lamb waves are closest to the real value, which indicates that using horizontal displacements is more suitable for detecting evenly distributed microstructural changes in large thin plate-like structure. Successful application to evaluating material at different levels of evenly distributed fatigue damage is also numerically conducted.

Imaging the lithospheric structure beneath the Indian continent

Abstract: We present a high-resolution 3-D lithospheric model of the Indian plate region down to 300 km depth, obtained by inverting a new massive database of surface wave observations, using classical tomographic methods. Data are collected from more than 550 seismic broadband stations spanning the Indian subcontinent and surrounding regions. The Rayleigh wave dispersion measurements along ~14,000 paths are made in a broad frequency range (16–250 s). Our regionalized surface wave (group and phase) dispersion data are inverted at depth in two steps: first an isotropic inversion and next an anisotropic inversion of the phase velocity including the SV wave velocity and azimuthal anisotropy, based on the perturbation theory. We are able to recover most of the known geological structures in the region, such as the slow velocities associated with the thick crust in the Himalaya and Tibetan plateau and the fast velocities associated with the Indian Precambrian shield. Our estimates of the depth to the Lithosphere-Asthenosphere boundary (LAB) derived from seismic velocity Vsv reductions at depth reveal large variations (120–250 km) beneath the different cratonic blocks. The lithospheric thickness is ~120 km in the eastern Dharwar, ~160 km in the western Dharwar, ~140–200 km in Bastar, and ~160–200 km in the Singhbhum Craton. The thickest (200–250 km) cratonic roots are present beneath central India. A low velocity layer associated with the midlithospheric discontinuity is present when the root of the lithosphere is deep.

Hawking spectrum for a fiber-optical analog of the event horizon

Abstract: Hawking radiation has been regarded as a more general phenomenon than in gravitational physics, in particular in laboratory analogs of the event horizon. Here we consider the fiber-optical analog of the event horizon, where intense light pulses in fibers establish horizons for probe light. Then, we calculate the Hawking spectrum in an experimentally realizable system. We found that the Hawking radiation is peaked around group-velocity horizons in which the speed of the pulse matches the group velocity of the probe light. The radiation nearly vanishes at the phase horizon where the speed of the pulse matches the phase velocity of light.

Two-receiver measurements of phase velocity: cross-validation of ambient-noise and earthquake-based observations

Abstract: Phase velocities derived from ambient-noise cross-correlation are compared with phase velocities calculated from cross-correlations of waveform recordings of teleseismic earthquakes whose epicentres are approximately on the station–station great circle. The comparison is conducted both for Rayleigh and Love waves using over 1000 station pairs in central Europe. We describe in detail our signal-processing method which allows for automated processing of large amounts of data. Ambient-noise data are collected in the 5–80 s period range, whereas teleseismic data are available between about 8 and 250 s, resulting in a broad common period range between 8 and 80 s. At intermediate periods around 30 s and for shorter interstation distances, phase velocities measured from ambient noise are on average between 0.5 per cent and 1.5 per cent lower than those observed via the earthquake-based method. This discrepancy is small compared to typical phase-velocity heterogeneities (10 per cent peak-to-peak or more) observed in this period range.We nevertheless conduct a suite of synthetic tests to evaluate whether known biases in ambient-noise cross-correlation measurements could account for this discrepancy; we specifically evaluate the effects of heterogeneities in source distribution, of azimuthal anisotropy in surface-wave velocity and of the presence of near-field, rather than far-field only, sources of seismic noise. We find that these effects can be quite important comparing individual station pairs. The systematic discrepancy is presumably due to a combination of factors, related to differences in sensitivity of earthquake versus noise data to lateral heterogeneity. The data sets from both methods are used to create some preliminary tomographic maps that are characterized by velocity heterogeneities of similar amplitude and pattern, confirming the overall agreement between the two measurement methods.

Wave propagation in a generalized thermoelastic plate using eigenvalue approach

Abstract: In the present work, we obtain a dispersion relation for Rayleigh–Lamb wave propagation in a plate of thermoelastic material. For this aim, we consider the theory of generalized thermoelasticity with one relaxation time. The thickness of the plate is taken to be finite and the faces of the plate are assumed to be isothermal and free from stresses. We obtain the analytical solution for the temperature, displacement components, and stresses using an eigenvalue approach. Finally, we derive a dispersion relation for the plate in closed form taking into account isothermal boundary conditions for wave mode propagation. To obtain the phase velocity and attenuation coefficients of propagating wave mode, we use the function iteration numerical scheme to solve the complex dispersion relation. The phase velocity and attenuation coefficients for the first five modes of waves are represented graphically for Lord–Shulman and classical coupled dynamical theories.

Universal scalings for laser acceleration of electrons in ion channels

Abstract: We analytically investigate the acceleration of electrons undergoing betatron oscillations in an ion channel, driven by a laser beam propagating with superluminal (or luminal) phase velocity. The universal scalings for the maximum attainable electron energy are found for arbitrary laser and plasma parameters by deriving a set of dimensionless equations for paraxial ultra-relativistic electron motion. One of our analytic predictions is the emergence of forbidden zones in the electrons' phase space. For an individual electron, these give rise to a threshold-type dependence of the final energy gain on the laser intensity. The universal scalings are also generalized to the resonant laser interaction with the third harmonic of betatron motion and to the case when the laser beam is circularly polarized.

Effect of uniaxial stress on the propagation of higher-order Lamb wave modes

Abstract: On the basis of the non-linear theory of elasticity and the invariant based formulation developed by Ogden, we analyse the effect of homogeneous stress on the propagation of Lamb waves. Using the theory of incremental deformations superimposed on large deformations, we derive the equations governing the propagation of small amplitude waves in a pre-stressed plate. By enforcing traction-free boundary conditions at the surfaces of the plate, we further obtain the characteristic equations for symmetric and anti-symmetric Lamb wave modes and investigate the effect of stress on the phase velocity, i.e. the acoustoelastic effect. A comparison with experimental data exhibits a better correlation than previously published results. The outcomes of this study can be utilised in the development of new techniques for the measurement of applied stresses based on the acoustoelastic effect. In particular, a strong sensitivity of the phase velocity to the applied stress near the cut-off frequencies of higher-order Lamb wave modes is a very promising option, which seems to have been overlooked in previous studies.

Constraining depth range of S‐wave velocity decrease after large earthquakes near Parkfield, California.

Abstract: We use noise correlation and surface wave inversion to measure the S wave velocity changes at different depths near Parkfield, California, after the 2003 San Simeon and 2004 Parkfield earthquakes. We process continuous seismic recordings from 13 stations to obtain the noise cross-correlation functions and measure the Rayleigh wave phase velocity changes over six frequency bands. We then invert the Rayleigh wave phase velocity changes using a series of sensitivity kernels to obtain the S wave velocity changes at different depths. Our results indicate that the S wave velocity decreases caused by the San Simeon earthquake are relatively small (similar to 0.02%) and access depths of at least 2.3 km. The S wave velocity decreases caused by the Parkfield earthquake are larger (similar to 0.2%), and access depths of at least 1.2 km. Our observations can be best explained by material damage and healing resulting mainly from the dynamic stress perturbations of the two large earthquakes.

Optical Cherenkov radiation in overmoded microresonators.

Abstract: We show that an optical analog of Cherenkov radiation (dispersive wave) is observable in a nonlinear microring resonator generating Kerr frequency comb and containing linearly interacting families of equidistant modes. The radiation results from disruptions in the frequency dependent group velocity dispersion of the pumped cavity modes and is emitted into different mode families of the resonator. This effect reveals itself as a dispersive shaped structure in the spectral envelope of the frequency comb. We found that the dips in the comb spectrum correspond to peaks of the emission of the power in the other mode families of the resonator. The spectrum of the combs that includes both mode families does not have any dips, but peaks and resembles the Cherenkov radiation spectra frequently observed in Kerr comb systems. This Letter shows that a correct description of the Kerr comb in presence of mode anti-crossings should take into account not only the pumped mode family with modified dispersion parameter, but also the modes of the interacting families.

Surface Waveguides Supporting Both TM Mode and TE Mode With the Same Phase Velocity

Abstract: Two kinds of surface-wave waveguide (SWG) topologies are proposed in this paper with the objective to achieve the property of supporting both transverse magnetic (TM) and transverse electric (TE) modes with the same phase velocity. The first type is composed of two frequency-selective surfaces (FSSs) as layers whose dominant modes are TM mode and TE mode, respectively. For illustrationC the combination of loop-type FSS and wire-grid-type FSS is analyzed and its dispersion characteristics are examined as well. The second class also consists of two layers. For the top layer, there are gaps in one direction and continuous conducting strips in the orthogonal direction. The bottom layer is created from a 90° rotation of the top layer. As a particular illustration, a modified bow-tie-like SWG structure is investigated. The simulated results show that the two proposed SWG structures exhibit the property of supporting both TM mode and TE mode with the same phase velocity over a broad bandwidth. In addition, the effects of lattice types on dispersion diagrams are discussed in this paper. Near field measurements are also carried out to validate the simulations and good agreements are achieved.

Analysis of the zero-crossing technique in relation to measurements of phase velocities of the Lamb waves

Abstract: The delay time estimation using zero-crossing technique is widely used in ultrasonic measurements. The measurements of the phase velocity of guided waves are more complicated due to the dispersion. Application of zero-crossing technique for the measurements of the phase velocities is more complicated due to changes of the waveform of the signals and limited ranges of the measurements base. The objective of the work presented was to investigate in more details the influence of different parameters of the zero – crossing technique on measurement of the phase velocity of A 0 mode of the Lamb waves. Using the signals obtained from the finite element modelling of Lamb waves in a 2 mm thickness aluminium plate it was demonstrated that an insufficient sampling frequency can lead to the errors in the phase velocity and corresponding frequency estimations. On the other hand it was shown also, that in order to obtain a higher equivalent sampling frequency it is reasonable to exploit interpolation. The optimal parameters of the zero-crossing technique and necessary steps for phase velocity measurement are presented also.

Ultrafast creation and control of population density gratings via ultraslow polarization waves

Abstract: In the regime of resonant coherent light-matter interaction, light pulses may interact with each other indirectly via a polarization wave created by the other pulse. We show that such interaction allows fast creation and erasing of high-contrast dynamic population density gratings, as well as control of their period in a few-cycle regime. Our scheme uses counter-propagating optical pulses, which do not cross each other in the medium. The mechanism is able to work with pulse durations up to the single-cycle limit. Somewhat surprisingly, ultrafast grating wave vector control requires the generation of polarization waves with the phase velocity much smaller than that of light.

Ion Acoustic Solitary Waves in Degenerate Electron-Ion Plasmas

Abstract: Employing the reductive perturbation method, a rigorous study of ion acoustic solitary waves (IASWs) in dense electron-ion plasmas is carried out leading to two evolution equations. Both the nonrelativistic and ultrarelativistic degenerate electron cases are considered. It is found that the relativistic parameter $R_{0}$ affects the IASW phase velocity. Stationary solutions of the derived evolution equations are estimated. The higher order contributions of the nonlinear and dispersion terms are calculated. It has been shown that it is more important to be included in the case of nonrelativistic electrons. These findings are devoted to explaining the observed solitary waves propagating in the outer periphery of compact dense stars, which mostly consists of hydrogen and helium ions with degenerate electrons.

Strongly Enhanced Stimulated Brillouin Backscattering in an Electron-Positron Plasma.

Abstract: Stimulated Brillouin backscattering of light is shown to be drastically enhanced in electron-positron plasmas, in contrast to the suppression of stimulated Raman scattering. A generalized theory of three-wave coupling between electromagnetic and plasma waves in two-species plasmas with arbitrary mass ratios, confirmed with a comprehensive set of particle-in-cell simulations, reveals violations of commonly held assumptions about the behavior of electron-positron plasmas. Specifically, in the electron-positron limit three-wave parametric interaction between light and the plasma acoustic wave can occur, and the acoustic wave phase velocity differs from its usually assumed value.

Toward a Single‐Station Approach for Microzonation: Using Vertical Rotation Rate to Estimate Love‐Wave Dispersion Curves and Direction Finding

Abstract: Microzonation, the estimation of (shear) wave velocity profiles of the upper few 100 m in dense 2D profiles, is one of the key methods for understanding the variation in seismic damage caused by ground‐shaking events and thus for mitigating the risk of damage in the future. In this article, we present a novel method for estimating the Love‐wave phase velocity dispersion using ambient noise recordings. We use the vertical component of rotational motions inherently present in ambient noise and the established relation to simultaneous recordings of transverse acceleration, in which the phase velocity of a plane SH (or Love)‐type wave acts as a proportionality factor. We demonstrate that the developed inversion technique shows comparable results to more classical, array‐based methods. Furthermore, we demonstrate that if portable weak‐motion rotational motion sensors are available and the installation of a seismic network or array is not possible, a single point, multicomponent approach for estimating the dominant direction of the incident wavefield and the local velocity structure will be feasible with similar performance compared to more classical techniques.

Ion gyroradius effects on particle trapping in kinetic Alfvén waves along auroral field lines

Abstract: In this study, a 2D self-consistent hybrid gyrofluid-kinetic electron model is used to investigate Alfven wave propagation along dipolar magnetic field lines for a range of ion to electron temperature ratios The focus of the investigation is on understanding the role of these effects on electron trapping in kinetic Alfven waves sourced in the plasma sheet and the role of this trapping in contributing to the overall electron energization at the ionosphere This work also builds on our previous effort [Damiano et al, 2015] by considering a similar system in the limit of fixed initial parallel current, rather than fixed initial perpendicular electric field It is found that the effects of particle trapping are strongest in the cold ion limit and the kinetic Alfven wave is able to carry trapped electrons a large distance along the field line yielding a relatively large net energization of the trapped electron population as the phase speed of the wave is increased However, as the ion temperature is increased, the ability of the kinetic Alfven wave to carry and energize trapped electrons is reduced by more significant wave energy dispersion perpendicular to the ambient magnetic field which reduces the amplitude of the wave This reduction of wave amplitude in-turn reduces both the parallel current and the extent of the high energy tails evident in the energized electron populations at the ionospheric boundary (which may serve to explain the limited extent of the broadband electron energization seen in observations) Even in the cold ion limit, trapping effects in kinetic Alfven waves lead to only modest electron energization for the parameters considered (on the order of tens of eV) and the primary energization of electrons to keV levels coincides with the arrival of the wave at the ionospheric boundary

Impact of layer and substrate properties on the surface acoustic wave velocity in scandium doped aluminum nitride based SAW devices on sapphire

Abstract: This paper investigates the performance of surface acoustic wave (SAW) devices consisting of reactively sputter deposited scandium doped aluminum nitride (ScxAl1-xN) thin films as piezoelectric layers on sapphire substrates for wireless sensor or for RF-MEMS applications. To investigate the influence of piezoelectric film thickness on the device properties, samples with thickness ranging from 500 nm up to 3000 nm are fabricated. S21 measurements and simulations demonstrate that the phase velocity is predominantly influenced by the mass density of the electrode material rather than by the thickness of the piezoelectric film. Additionally, the wave propagation direction is varied by rotating the interdigital transducer structures with respect to the crystal orientation of the substrate. The phase velocity is about 2.5% higher for a-direction compared to m-direction of the sapphire substrate, which is in excellent agreement with the difference in the anisotropic Young's modulus of the substrate corresponding...

A simple method for retrieving significant wave height from Dopplerized X-band radar

Abstract: . Retrieving spectral wave parameters such as the peak wave direction and wave period from marine radar backscatter intensity is very well developed. However, the retrieval of significant wave height is difficult because the radar image spectrum (a backscatter intensity variance spectrum) has to be transferred to a wave spectrum (a surface elevation variance spectrum) using a modulation transfer function (MTF) which requires extensive calibration for each individual radar setup. In contrast to the backscatter intensity, the Doppler velocity measured by a coherent radar is induced by the radial velocity (or line-of-sight velocity) of the surface scattering and its periodic component is mainly the contribution of surface waves. Therefore, the variance of the Doppler velocity can be utilized to retrieve the significant wave height. Analyzing approximately 100 days of Doppler velocity measurements of a coherent-on-receive radar operating at X-band with vertical polarization in transmit and receive, a simple relation was derived and validated to retrieve significant wave heights. Comparison to wave measurements of a wave rider buoy as well as an acoustic wave and current profiler resulted in a root mean square error of 0.24 m with a bias of 0.08 m. Furthermore, the different sources of error are discussed and investigated.

Multi-fluid Approach to High-frequency Waves in Plasmas. II. Small-amplitude Regime in Partially Ionized Media

Abstract: The presence of neutral species in a plasma has been shown to greatly affect the properties of magnetohydrodynamic waves. For instance, the interaction between ions and neutrals through momentum transfer collisions causes the damping of Alfven waves and alters their oscillation frequency and phase speed. When the collision frequencies are larger than the frequency of the waves, single-fluid magnetohydrodynamic approximations can accurately describe the effects of partial ionization, since there is a strong coupling between the various species. However, at higher frequencies, the single-fluid models are not applicable and more complex approaches are required. Here, we use a five-fluid model with three ionized and two neutral components, which takes into consideration Hall's current and Ohm's diffusion in addition to the friction due to collisions between different species. We apply our model to plasmas composed of hydrogen and helium, and allow the ionization degree to be arbitrary. By means of the analysis of the corresponding dispersion relation and numerical simulations, we study the properties of small-amplitude perturbations. We discuss the effect of momentum transfer collisions on the ion-cyclotron resonances and compare the importance of magnetic resistivity, ion-neutral and ion-ion collisions on the wave damping at various frequency ranges. Application to partially ionized plasmas of the solar atmosphere are performed.