Dominant Wave Mode Research Articles

Nonlinear Development of Electron Heat Flux Instability: Particle in Cell Simulation

Abstract Finite heat flux often exists in space and astrophysical plasmas, which can be a free energy source for heat flux instability. The solar wind is a well-known example of such plasmas and a number of previous studies have investigated the characteristics of heat flux instability in the context of solar wind. In the literature there exists some uncertainties regarding the properties of heat flux instability. While some linear theories predict the association of the heat flux instability with right-hand polarized whistler waves, other studies argue for left-hand polarized unstable modes. The present study investigates the nonlinear development of initially unstable left-hand heat flux mode by means of particle-in-cell simulation. It is found that while the early phase is characterized by the left-hand polarization, in agreement with linear theory, as the wave amplitude becomes high and the instability enters the nonlinear phase, the dominant wave mode gradually switches over to the right-hand polarized waves. Such a behavior is related to the pitch angle scattering of the heat flux carrying electrons by nonlinear interaction with large-amplitude waves. The present study shows that the heat flux instability generally requires nonlinear treatment such that characterizing its behavior with linear theories may not always be adequate.

Medium- and High-Frequency Vibration Characteristics of a Box-Girder by the Waveguide Finite Element Method

An approach to predicting the vibration responses of a bridge and analyzing the wave propagation characteristics along the bridge is proposed based on the waveguide finite element (WFE) method. For verification, a field vibration test was performed on a 30[Formula: see text]m-long simply supported box-girder on the fourth line of Guangzhou Metro in China. The numerical results were shown to agree well with the test results, demonstrating the accuracy of the proposed approach. The advantages of the WFE approach are discussed by comparing the WFE with finite element (FE) analyses. The dispersion characteristics and mode shapes of waves propagating in the box-girder are calculated by using the WFE method, from which the dominant wave modes corresponding to the peaks of the medium- and high-frequency train-induced vibrations are identified. A vibration reduction measure is considered. Both the test and numerical results show that the medium- and high-frequency vibrations of the box-girder are predominately in the 1/3 octave center frequency range of 63–100[Formula: see text]Hz with maximum occurring at the center frequency of 80[Formula: see text]Hz. The WFE method has higher computational efficiency and requires smaller storage space than the FE counterpart, but provides similar predictions as the latter. In total, there are 14 wave modes propagating in the box-girder below 200[Formula: see text]Hz. The G1, A1 and D2 wave modes are the dominant wave modes generating vibration velocity peaks on the top, bottom and flange slabs, respectively. Adding a middle web slab to the center of the cross-section is an effective way to mitigate the vibration of the box-girder bridge.

Investigation of the Abnormal Quasi 2‐Day Wave Activities During the Sudden Stratospheric Warming Period of January 2006

AbstractA quasi 2‐day wave (QTDW) during the austral summer period usually coincides with a sudden stratospheric warming (SSW) event in the winter hemisphere, while the SSW influences on QTDWs are not totally understood. In this work, the hourly Navy Operational Global Atmospheric Prediction System‐Advanced Level Physics High Altitude reanalysis data sets during January/February 2006 are utilized to study the contribution of a major SSW on the anomalous QTDW activities during the same period. Our new findings are generalized as follows: (1) The summer easterly is enhanced during a SSW event due to the interhemispheric coupling, which is clearly indicated by the anomalous cross‐equator circulation from the winter to summer mesosphere. (2) The enhanced summer easterly could sustain critical layers for QTDWs with larger phase speeds (e.g., smaller zonal wave number or shortwave period) and strengthen the summer easterly barotropic/baroclinic instabilities, which are essential for the QTDW amplification through wave‐mean flow interactions. This is why a strong westward QTDW with zonal wave number 2 is identified besides the conventionally dominant wave mode of wave number 3, and their periods are only ~42–45 hr during January 2006. (3) The strong winter planetary waves during SSW periods facilitate the occurrence of the nonlinear interaction between QTDWs and stationary planetary waves, which is strongly suggested by the abnormal temporal variations of wave number 2 and wave number 3 QTDWs. We conclude that the anomalous QTDW behaviors in summer mesosphere during January 2006 are associated with the major SSW event in the winter stratosphere.

Effect of Dynamical Phase on the Resonant Interaction Among Tsunami Edge Wave Modes

Different modes of tsunami edge waves can interact through nonlinear resonance. During this process, edge waves that have very small initial amplitude can grow to be as large or larger than the initially dominant edge wave modes. In this study, the effects of dynamical phase are established for a single triad of edge waves that participate in resonant interactions. In previous studies, Jacobi elliptic functions were used to describe the slow variation in amplitude associated with the interaction. This analytical approach assumes that one of the edge waves in the triad has zero initial amplitude and that the combined phase of the three waves φ = θ1 + θ2 − θ3 is constant at the value for maximum energy exchange (φ = 0). To obtain a more general solution, dynamical phase effects and non-zero initial amplitudes for all three waves are incorporated using numerical methods for the governing differential equations. Results were obtained using initial conditions calculated from a subduction zone, inter-plate thrust fault geometry and a stochastic earthquake slip model. The effect of dynamical phase is most apparent when the initial amplitudes and frequencies of the three waves are within an order of magnitude. In this case, non-zero initial phase results in a marked decrease in energy exchange and a slight decrease in the period of the interaction. When there are large differences in frequency and/or initial amplitude, dynamical phase has less of an effect and typically one wave of the triad has very little energy exchange with the other two waves. Results from this study help elucidate under what conditions edge waves might be implicated in late, large-amplitude arrivals.

Hydrodynamic characteristics of internal waves induced by a heaving body in a two-layer fluid

The stratification of fluid caused by the change of water densities can generate two water waves on each fluid boundary. One is the surface wave on the free surface, and the other is the internal wave on the interface between the fluid layers. Two wave modes exist in the two-layer fluid: barotropic mode and baroclinic mode. This study develops a numerical model to simulate the surface and internal waves in the time domain. The time histories of the water waves on both surface boundaries are simulated using the numerical wave tank (NWT) technique by an oscillating circular body located on the free surface or underwater in the lower fluid domain. The NWT technique is based on the boundary element method with properly defined boundary conditions, including interface boundaries. The leapfrog method is used for the time integration of the water surface boundary conditions. The time-varying generated waves are disassembled through the Fast Fourier Transform (FFT). Each wave component is compared with analytic solutions. The relative magnitudes of the generated waves on both fluid layers are examined according to each wave mode. The dominant wave mode varies according to the oscillating body frequencies. The amplitude ratios of the two waves are compared for various density ratios and fluid depths.

Stratosphere and lower mesosphere wind observation and gravity wave activities of the wind field in China using a mobile Rayleigh Doppler lidar

AbstractSince the mobile Rayleigh Doppler lidar of the University of Science and Technology of China was developed in 2013, more than 100 days of valid nighttime wind data from 15 to 60 km altitude were obtained during recent 3 years. The observation locations cover the northwest (midlatitude) of China: Delingha (37.371°N, 97.374°E), Xinzhou (38.425°N, 112.729°E), and Jiuquan (39.741°N, 98.495°E). Recently, we have extracted perturbations of the wind profiles from the wind field measurements and we have found that inertia gravity waves and mountain waves existed at the same time. The results of wind field and several gravity waves cases are shown in this paper. Typical characteristics of the gravity waves are analyzed in this midlatitude area of China. A 2‐D fast Fourier transform of the wind perturbation shows that a dominant stationary wave mode and a downward wave mode exist simultaneously in the spectrum. A band‐pass 2‐D filter was applied to the spectrum followed by inverse fast Fourier transform to separate inertia gravity waves from stationary mountain waves. The horizontal wavelength is retrieved using hodograph methods, indicating that the inertia waves are generated thousands of kilometers away. Observed mountain waves from a combination of vertical wind and leaned line of sight wind measurements show a small‐angle leaned wave front from the horizontal direction. This kind of gravity wave observation of the stratospheric wind field and its wave patterns is rare and significant for the study of atmospheric dynamics.

Dynamic Behavior of Spicules Inferred from Perpendicular Velocity Components

Abstract Understanding the dynamic behavior of spicules, e.g., in terms of magnetohydrodynamic (MHD) wave mode(s), is key to unveiling their role in energy and mass transfer from the photosphere to corona. The transverse, torsional, and field-aligned motions of spicules have previously been observed in imaging spectroscopy and analyzed separately for embedded wave-mode identification. Similarities in the Doppler signatures of spicular structures for both kink and torsional Alfvén wave modes have led to the misinterpretation of the dominant wave mode in these structures and is a subject of debate. Here, we aim to combine line- of-sight (LOS) and plane-of-sky (POS) velocity components using the high spatial/temporal resolution Hα imaging-spectroscopy data from the CRisp Imaging SpectroPolarimeter based at the Swedish Solar Telescope to achieve better insight into the underlying nature of these motions as a whole. The resultant three-dimensional velocity vectors and the other derived quantities (e.g., magnetic pressure perturbations) are used to identify the MHD wave mode(s) responsible for the observed spicule motion. We find a number of independent examples where the bulk transverse motion of the spicule is dominant either in the POS or along the LOS. It is shown that the counterstreaming action of the displaced external plasma due to spicular bulk transverse motion has a similar Doppler profile to that of the m = 0 torsional Alfvén wave when this motion is predominantly perpendicular to the LOS. Furthermore, the inferred magnetic pressure perturbations support the kink wave interpretation of observed spicular bulk transverse motion rather than any purely incompressible MHD wave mode, e.g., the m = 0 torsional Alfvén wave.

The Active Element Patterns of a Finite Array-Space Wave and Leaky Wave Perspective

The active element patterns (AEPs) of a finite array of parallel-plate waveguides are studied. It is shown that an excited element in array environment produces a number of space and leaky wave modes (but no surface wave mode) that can be expressed in terms of branch-cut integrals and residues, respectively, in the complex wavenumber plane. The dominant space wave mode has a very sharp radiation pattern with large phase variation near its peak causing constructive and destructive interferences with the primary pattern of the excited element within a small angular region. As a result, the AEP exhibits sharp dips that are very close to the local peaks. The number and the extent of excited modes that cause interference depend on the location of the excited element in a finite array. Consequently, the shape of the AEP changes with the element location. It is found that an array generates forward and backward waves on its aperture, and for element spacing less than one wavelength, the backward wave dominates. As a result, the AEP for the left-edge element has a dip at the left-hand side of the boresight and the beam tilts toward the left. For large element spacing, multiple dips may occur on both sides. The present analysis explains several interesting characteristics of finite and infinite arrays and very closely agrees with the plane wave spectrum model.

Wave and Jet Maintenance in Different Flow Regimes

Abstract The wave spectrum and zonal-mean-flow maintenance in different flow regimes of the jet stream are studied using a two-layer modified quasigeostrophic (QG) model. As the wave energy is increased by varying the model parameters, the flow transitions from a subtropical jet regime to a merged jet regime and then to an eddy-driven jet regime. The subtropical jet is maintained at the Hadley cell edge by zonal-mean advection of momentum, while eddy heat flux and eddy momentum flux convergence (EMFC) are weak and concentrated far poleward. The merged jet is narrow and persistent and is maintained by EMFC from a narrow wave spectrum, dominated by zonal wavenumber 5. The eddy-driven jet is wide and fluctuating and is maintained by EMFC from a wide wave spectrum. The wave–mean flow feedback mechanisms that maintain each regime are explained qualitatively. The regime transitions are related to transitions in the wave spectrum. An analysis of the wave energy spectrum budget and a comparison with a quasi-linear version of the model show that the balance maintaining the spectrum in the merged and subtropical jet regimes is mainly a quasi-linear balance, whereas in the eddy-driven jet regime nonlinear inverse energy cascade takes place. The amplitude and wavenumber of the dominant wave mode in the merged and subtropical jet regimes are determined by the constraint that this mode would produce the wave fluxes necessary for maintaining a mean flow that is close to neutrality. In contrast, the equilibrated mean flow in the eddy-driven jet regime is weakly unstable.

Resonant excitation of whistler waves by a helical electron beam

AbstractChorus‐like whistler mode waves that are known to play a fundamental role in driving radiation belt dynamics are excited on the Large Plasma Device by the injection of a helical electron beam into a cold plasma. The mode structure of the excited whistler wave is identified using a phase correlation technique showing that the waves are excited through a combination of Landau resonance, cyclotron resonance, and anomalous cyclotron resonance. The dominant wave mode excited through cyclotron resonance is quasi‐parallel propagating, whereas wave modes excited through Landau resonance and anomalous cyclotron resonance propagate at oblique angles that are close to the resonance cone. An analysis of the linear wave growth rates captures the major observations in the experiment. The results have important implications for the generation process of whistler waves in the Earth's inner magnetosphere.

Low‐frequency waves in the Martian magnetosphere and their response to upstream solar wind driving conditions

AbstractWe characterize low‐frequency plasma waves in the Martian magnetosphere and in the upstream region by using transport ratios. To compute the transport ratios, we use Mars Atmosphere and Volatile EvolutioN mission's (MAVEN) solar wind ion analyzer and suprathermal and thermal ion composition instrument measurements of the ion moments and the magnetometer measurements of the magnetic field. We find that the Alfvén waves are the most dominant wave mode in the upstream region and the magnetosheath. Fast waves are found frequently near the bow shock and the magnetic pileup boundary. Mirror and slow waves, on the other hand, occur much less frequently. We also find that the Alfvén and fast wave occurrences vary dominantly near the bow shock in response to the solar wind dynamic pressure.

Identification of the dominant ULF wave mode and generation mechanism for obliquely propagating waves in the Earth's foreshock

AbstractWe discuss mechanisms of the generation of ultralow frequency (ULF) upstream waves in the terrestrial foreshock that are essential for the acceleration of ions in space plasmas. The analysis is based on global hybrid kinetic simulations of the magnetosphere that provide realistic environment for the growth of the ULF waves in a quasi‐radial configuration of the interplanetary magnetic field. We focus on a long‐debated problem of the generation mechanism of oblique and parallel ULF waves and provide quantitative arguments in favor of the ion/ion cyclotron resonant instability. We also show that parallel propagating waves are predominantly generated in this configuration, but geometrical effects related to the phase space density in wave vector space lead to apparent predominance of obliquely propagating waves. Correspondence between the results outlined above and previously published experimental claims is thoroughly discussed and our results are shown to be consistent with spacecraft measurements.

Benchmarking in a rotating annulus: a comparative experimental and numerical study of baroclinic wave dynamics

The differentially heated rotating annulus is a widely studied tabletop-size laboratory model of the general mid-latitude atmospheric circulation. The two most relevant factors of cyclogenesis, namely rotation and meridional temperature gradient are quite well captured in this simple arrangement. The radial temperature difference in the cylindrical tank and its rotation rate can be set so that the isothermal surfaces in the bulk tilt, leading to the formation of baroclinic waves. The signatures of these waves at the free water surface have been analyzed via infrared thermography in a wide range of rotation rates (keeping the radial temperature difference constant) and under different initial conditions. In parallel to the laboratory experiments, five groups of the MetStrom collaboration have conducted numerical simulations in the same parameter regime using different approaches and solvers, and applying different initial conditions and perturbations. The experimentally and numerically obtained baroclinic wave patterns have been evaluated and compared in terms of their dominant wave modes, spatio-temporal variance properties and drift rates. Thus certain “benchmarks” have been created that can later be used as test cases for atmospheric numerical model validation.

Investigation of dominant spin wave modes by domain walls collision

Spin wave emission due to field-driven domain wall (DW) collision has been investigated numerically and analytically in permalloy nanowires. The spin wave modes generated are diagonally symmetric with respect to the collision point. The non-propagating mode has the highest amplitude along the middle of the width. The frequency of this mode is strongly correlated to the nanowire geometrical dimensions and is independent of the strength of applied field within the range of 0.1 mT to 1 mT. For nanowire with film thickness below 5 nm, a second spin wave harmonic mode is observed. The decay coefficient of the spin wave power suggests that the DWs in a memory device should be at least 300 nm apart for them to be free of interference from the spin waves.

A feasibility study on the application of microwaves for online biofilm monitoring in the pipelines

This study investigates the potential of microwave technique for online monitoring and evaluation of biofilms in the pipelines. A microwave vector network analyser and an in-house built transmitting and receiving coaxial-line transducer were employed to transmit microwave signals in the pipe. The brass pipe specimen was tested by adhering different volumes of polymeric tape layers onto its internal surface simulating the biofilm build-up. By taking the pipe as a circular waveguide of microwave, the frequency domain measurements were conducted in the 45–47 GHz range with TM01 dominant wave mode. The permittivity of the biofilm-contained area has been expressed as a function of the resonance frequency after establishing the resonance condition in the waveguide. It was realized that the resonance frequencies experience systematic shifts with the growth of biofilm layer length and thickness. The effects of dielectric material properties and the volume of the added biofilm layer on the resonance frequency records were then explained using the cavity perturbation theory which confirmed the experimental findings. Measurement results indicated a high degree of sensitivity to the small amounts of introduced biofilm which proves the potential of the microwave technique for online biofilm monitoring in both closed-end and open-end terminal conditions.

Impact of Asymmetric Dynamical Processes on the Structure and Intensity Change of Two-Dimensional Hurricane-Like Annular Vortices

AbstractIn this study, a simple two-dimensional (2D) unforced barotropic model is used to study the asymmetric dynamics of the hurricane inner-core region and to assess their impact on the structure and intensity change. Two sets of experiments are conducted, starting with stable and unstable annular vortices, to mimic intense mature hurricane-like vortices. The theory of empirical normal modes (ENM) and the Eliassen–Palm flux theorem are then applied to extract the dominant wave modes from the dataset and diagnose their kinematics, structure, and impact on the primary vortex.From the first experiment, it is found that the evolution and the lifetime of an elliptical eyewall, described by a stable annular vortex perturbed by an external wavenumber-2 impulse, may be controlled by the inviscid damping of sheared vortex Rossby waves (VRWs) or the decay of an excited quasimode. The critical radius and structure of the quasimode obtained by the ENM analysis are shown to be consistent with the predictions of a linear eigenmode analysis of small perturbations. From the second experiment, it is found that the outward-propagating VRWs that arise due to barotropic instability and the inward mixing of high vorticity in the unstable annular vortex affect the primary circulation and create a secondary ring of enhanced vorticity that contains a secondary wind maximum. Sensitivity tests performed on the spatial extent of the initial external impulse verifies the robustness of the results. That the secondary eyewall occurs close to the critical radius of some of the dominant modes emphasizes the important role played by the VRWs.

Moist Hadley Circulation: Possible Role of Wave–Convection Coupling in Aquaplanet Experiments

Abstract Aquaplanet simulations for a given sea surface temperature (SST) are conducted to elucidate possible roles of transient variability in the Hadley circulation and the intertropical convergence zone (ITCZ). Their roles are best illustrated with globally uniform SSTs. For such SSTs, an ITCZ and a Hadley circulation that are nearly equatorially symmetric emerge spontaneously. Their strength varies over a wide range from being faint to climatologically significant depending on a tunable parameter of the model’s cumulus parameterization. In some cases asymmetric Hadley circulations formed along with long-lived tropical cyclones. The tunable parameter affects the transient variability of tropical precipitation. In the runs in which well-defined near-symmetric ITCZs formed, tropical precipitation exhibited clear signatures of convectively coupled equatorial waves. The waves can explain the concentration of precipitation to the equatorial region, which induces the Hadley circulation. Also, the meridional structures of simulated ITCZs are consistent with the distribution of convergence/divergence associated with dominant equatorial wave modes. Even when the pole–equator temperature gradient is introduced, the dependence of the strength of the circulation to transient disturbances remains. Therefore, transient variability may have a broader impact on tropical climate and its numerical modeling than has been thought. The reason that a wide variety of circulation is possible when the SST gradient is weak is because the distribution of latent heating can be interactively adjusted while a circulation is formed. Angular momentum budget does not provide an effective thermodynamic constraint, since baroclinic instability redistributes the angular momentum.

An experimental study of EMAT ultrasonic surface waves modes in railhead

A lab-based laser-EMAT (Electromagnetic acoustic transducers) system has been developed to observe the ultrasonic surface wave propagation and interaction with surface breaking defects on the sample rail head surface. The frequency- wavenumber dispersion curves of the dominant surface wave modes were determined using 2D-FFT method based on the measurements of the out-of-plane displacement of the surface wave. A complete picture of the ultrasonic surface wave on the sample surface over time was reconstructed with exceptionally high spatial and temporal resolution. Despite the curvature of the rail head, the ultrasonic surface guided wave propagating down the rail is found to have similar properties to Rayleigh waves by direct comparison to those observed on flat samples using the same technique. Rail breaks caused by rolling contact fatigue (RCF) cracks are of growing concern to the modern high speed railway industry (1). The fatigue crack initiates on or very close to the rail rolling surface, which is not related to any material defect. Its occurrence is increasing on high speed passenger lines, mixed and heavy haul railways and can lead to expensive rail grinding in the attempt to remove it, premature removal of the rails and complete rail failure. The rolling contact fatigue damage on rails can be divided in headchecks, squats and spalling. They are caused by a combination of high normal and tangential stresses between rail and wheel, which cause sever shearing of the rolling surface layer of rail head and either fatigue or exhaustion of ductility of material. The microscopic cracks produced propagations through the heavily deformed surface layer of railhead at a shallow angle to the rail running face (about 10 ◦ ∼15 ◦ ) until it reaches a depth where the steel retains its original isotropic properties (2,3). At this stage the crack is a few millimeters deep into the rail head, and usually the crack may simply lead to spalling of material from the rail surface. However, for reasons still not very clearly understood, isolated oblique crack can down into the rail, and if not to detect by ultrasonic testing car, it may cause the rail to break. RCF damages are much more severe from the point of view of the structure integrity for high speed railway track.

Wyrtki Jet dynamics: Seasonal variability

This study examines the dynamics of the Wyrtki jets, which are strong equatorial zonal flows that occur typically during boreal spring and fall in the Indian Ocean. Our diagnosis relies primarily on a continuously stratified linear longwave ocean model driven by QuikSCAT zonal winds. Model results, which compare well with satellite altimetry and in situ current observations, indicate that the zonal currents propagate westward along the equator at semiannual periods with an average speed of −1.5 m s−1. This propagation speed is three times faster than the propagation speed of the dominant wave mode in model zonal velocity, namely the first meridional, second baroclinic mode Rossby wave. We interpret this result in terms of a superposition of Rossby waves on a wind‐forced jet, with the jet stronger than the waves by a factor of 2. Sea surface height (SSH), on the other hand, shows propagating features that vary in both speed and direction from region to region. This contrasting behavior between SSH and zonal velocity results from differing influences of Kelvin and Rossby wave dynamics on the variability. These results are in many respects analogous to the distinction between SSH and zonal current behavior found in previous studies of the equatorial Pacific and Atlantic oceans on seasonal time scales.

Gravity waves propagating into an ice‐covered ocean: A viscoelastic model

A viscoelastic model is proposed to describe the propagation of gravity waves into various types of ice cover. The ice‐ocean system is modeled as a homogeneous viscoelastic fluid overlying an inviscid layer. Both layers have finite thickness. The viscosity is imagined to originate from the frazil ice or ice floes much smaller than the wavelength, and the elasticity from ice floes which are relatively large compared to the wavelength. A compact form of the dispersion relation is obtained. Under proper limiting conditions this dispersion relation can be reduced to several previously established models including the mass loading model, the viscous layer model and the thin elastic plate model. The full dispersion relation contains several propagating wave modes under the ice cover. The following two criteria are used to select the dominant wave mode: (1) wave number is the closest to the open water value and (2) attenuation rate is the least among all modes. The modes selected from those criteria coincide with the ones discussed in previous studies, which are shown to be limiting cases in small or large elasticity regimes of the present model. In the intermediate elasticity regime, however, it appears that there are three wave modes with similar wavelengths and attenuation rates. Implications of this intermediate elasticity range remain to be seen. The general viscoelastic model bridges the gap among existing models. It also provides a unified tool for wave‐ice modelers to parameterize the polar regions populated with various types of ice cover.