Fast Surface Waves Research Articles

A semi-implicit finite volume method for the Exner model of sediment transport

The aim of this work is to construct efficient finite volume schemes for the numerical study of sediment transport in shallow water, in the framework of the Exner model [7,10]. In most cases, the velocity related to the sediment is much lower that the fluid velocity, which, in turn, may be much lower that the free-surface wave speed. Explicit methods that resolve all waves require small time steps due to the CFL stability restriction because of fast surface waves. Furthermore, if Rusanov flux is adopted, slow sediment waves may be affected by the large numerical diffusion. The objective of the present work is to drastically improve the efficiency in the computation of the evolution of the sediment by treating water waves implicitly, thus allowing much larger time steps than the one required by fully explicit schemes. The goal is reached by suitably semi-implicit schemes obtained by the use of implicit-explicit Runge-Kutta methods.

Observation of Analog Flatland Cherenkov Radiations on Metasurfaces

AbstractRecent theoretical studies have suggested the existence of a lower‐dimensional version of the Cherenkov radiation, known as “flatland Cherenkov radiation” (FCR). FCR is generated by leakage from a surface‐wave waveguide while being entirely confined in a plane. Here, a metasurface platform is proposed for generating and observing FCR, which is characterized by alternated regions of capacitive and inductive surface impedance, supporting transverse electric (TE) and magnetic (TM) surface waves (SWs), respectively. For experimental demonstration, a microwave setup is presented, and measured results are compared with numerical simulations and theoretical predictions. The study shows that the phase velocities of different SWs can be controlled by structural designs, and suitably fast TE SWs can excite FCR in the form of slower TM SWs. FCR has unique properties such as directionality, tunability, and subwavelength confinement. The proposed platform has important potential to enable nanophotonic applications based on 2D optics, such as agile waveguiding and scanning.

Synchrotron mechanism of X-ray and gamma-ray emissions in lightning and spark discharges

X-ray and γ-ray emissions observed in lightning and long sparks are usually connected with the bremsstrahlung of high-energy runaway electrons. Here, an alternative physical mechanism for producing X-ray and gamma-ray emissions caused by the polarization current and associated electromagnetic field moving with relativistic velocity along a curved discharge channel has been proposed. The existence of fast electromagnetic surface waves propagating along the lightning discharge channel at a speed close to the speed of light in vacuum is shown. The possibility of the production of microwave, X-ray and gamma-ray emissions by a polarization current pulse moving along a curved path via synchrotron radiation mechanism is pointed out. The existence of long tails in the power spectrum is shown, which explains observations of photon energies in the range of 10–100 MeV in the terrestrial gamma-ray flashes, as well as measured power spectrum of laboratory spark discharge.

A Backward Zenneck Wave Propagating along a Plane Interface between Media

We consider a surface Zenneck wave that propagates along a plane interface between a vacuum and a medium that is described by an isotropic dielectric permittivity and a surface Zenneck wave that propagates along a plane interface between a vacuum and a medium that is described by isotropic dielectric permittivity and magnetic permeability. In the latter case, we also consider an H wave and the possibility that the real parts of the permittivity and permeability are negative. Forward and backward waves at the boundary of a material with inhomogeneous profiles of the dielectric permittivity and magnetic permeability are considered. The conditions of the existence of forward and backward surface and volume waves, as well as of fast and slow surface waves are investigated.

Обратная волна Ценнека вдоль плоской границы раздела сред

We consider the surface Zennek wave along the plane boundaries of the vacuum-medium interface with isotropic permittivity, and the vacuum-medium described by isotropic dielectric and magnetic permittivity. In the latter case, we also consider the H-wave and the possibility of negative real parts of the permeability. Forward and backward waves at the boundary of a material with an inhomogeneous permittivity profile are considered. The conditions for the existence of forward and backward surface and volume waves, as well as fast and slow surface waves, are studied.

Stability analysis of split-explicit free surface ocean models: Implication of the depth-independent barotropic mode approximation

The evolution of the oceanic free-surface is responsible for the propagation of fast surface gravity waves, which approximatively propagate at speed gH (with g the gravity and H the local water depth). In the deep ocean, this phase speed is roughly two orders of magnitude faster than the fastest internal gravity waves. The very strong stability constraint imposed by those fast surface waves on the time-step of numerical models is handled using a mode splitting between slow (internal/baroclinic) and fast (external/barotropic) motions to allow the possibility to adopt specific numerical treatments in each component. The barotropic mode is traditionally approximated by the vertically integrated flow because it has only slight vertical variations. However the implications of this assumption on the stability of the splitting are not well documented. In this paper, we describe a stability analysis of the mode splitting technique based on an eigenvector decomposition using the true (depth-dependent) barotropic mode. This allows us to quantify the amount of dissipation required to stabilize the approximative splitting. We show that, to achieve stable integrations, the dissipation usually applied through averaging filters can be drastically reduced when incorporated at the level of the barotropic time stepping. The benefits are illustrated by numerical experiments. In addition, the formulation of a new mode splitting algorithm using the depth-dependent barotropic mode is introduced.

Analysis of Two-Media Structures Using Longitudinal Spectrum: Space, Surface, and Zenneck Waves

The longitudinal spectral expansion method is extended to simplify the electromagnetic field analysis in a two-media structure with and without ground plane. A nonconventional Riemann sheet is invoked that clarifies some important characteristics of the fields. For instance, it is straightforward to prove that in a two-media half-space, no surface wave exists for lossless media. However, if a medium is lossy, then a fast surface wave, known as a Zenneck surface wave ( ZSW ), exists that dominates within a limited radial distance.

Wave modes excited by photosphericp-modes and mode conversion in a multi-loop system

Context. Waves are ubiquitous in the solar corona and there are indications that they are excited by photosphericp-modes. However, it is unclear howp-modes in coronal loops are converted to sausage modes and transverse (kink) modes, which are observed in the corona.Aims. We aim to investigate how those wave modes are excited in the lower corona by photospheric acoustic waves.Methods. We built 3D magnetohydrostatic loop systems with multiple inclinations spanning from the photosphere to the lower corona. We then simulated these atmospheres with the MANCHA code, in which we perturb the equilibrium with ap-mode driver at the bottom of the domain. By splitting the velocity perturbation into components longitudinal, normal, and azimuthal to the magnetic flux surfaces we can study wave behavior.Results. In vertical flux tubes, we find that deformed fast sausage surface waves and slow sausage body waves are excited. In inclined flux tubes fast kink surface waves, slow sausage body waves, and either a fast sausage surface wave or a plane wave are excited. In addition, we calculate a wave conversion factor (0 ≤ C ≤ 1) from acoustic to magnetic wave behavior by taking the ratio of the mean magnetic energy flux to the sum of the mean magnetic and acoustic energy flux and compare it to a commonly used theoretical conversion factor. We find that between magnetic field inclinations of 10° to 30° those two methods lie within 40%. For smaller inclinations the absolute deviation is smaller than 0.1.

On the Condition for Transformation of a Fast Surface Wave into a Slow Surface Wave

Surface plasmons (polaritons) along the boundary between vacuum and a dissipative half-space with given complex permittivity, including the case of placing an electrically thin dielectric layer (system of layers) on the boundary, are considered. Conditions for transformation of a fast gliding Zenneck surface wave (polariton) into a slow wave (slow surface plasmon) are considered. The behavior of the group velocity and the energy velocity of motion (transfer) in such structures are analyzed. The reflection coefficient of the Zenneck wave along the sea for the ice pack of a given thickness is determined. It has been shown that rigorous solution of the Sommerfeld problem by means of numerical calculation of integrals in the near zone allows obtaining the solution of the inverse problem of the structure of the surface region on the basis of redundant experimental data on components of the electromagnetic field on the surface at different ranges from the antenna.

On Slow and Fast Surface Waves Propagating on Plane Boundaries of Conducting Media with High Conductance. The Zenneck Wave

The regularities of transition of phase and group velocities from slow surface waves (whose velocities are below the light speed) to fast surface waves (whose velocities are above the light speed) on the plane boundaries of highly conducting media are investigated when the parameters of media vary. It is shown that the structure of waves does not change qualitatively in the case of this transition: the waves remain to be surface ones in contrast to surface waves of dielectric waveguides. The latter ones are transformed into leaking waves or their structure changes in a more complicated way in the case of such transition. A result relating, in particular, to the Zenneck wave is obtained.

Infrared beam-steering using acoustically modulated surface plasmons over a graphene monolayer

We model and design a graphene-based infrared beamformer based on the concept of leaky-wave (fast traveling wave) antennas. The excitation of infrared surface plasmon polaritons (SPPs) over a ‘one-atom-thick’ graphene monolayer is typically associated with intrinsically ‘slow light’. By modulating the graphene with elastic vibrations based on flexural waves, a dynamic diffraction grating can be formed on the graphene surface, converting propagating SPPs into fast surface waves, able to radiate directive infrared beams into the background medium. This scheme allows fast on–off switching of infrared emission and dynamic tuning of its radiation pattern, beam angle and frequency of operation, by simply varying the acoustic frequency that controls the effective grating period. We envision that this graphene beamformer may be integrated into reconfigurable transmitter/receiver modules, switches and detectors for THz and infrared wireless communication, sensing, imaging and actuation systems.

A Mobile Acoustic Subsurface Sensing (MASS) System for Rapid Roadway Assessment

Surface waves are commonly used for vibration-based nondestructive testing for infrastructure. Spectral Analysis of Surface Waves (SASW) has been used to detect subsurface properties for geologic inspections. Recently, efforts were made to scale down these subsurface detection approaches to see how they perform on small-scale structures such as concrete slabs and pavements. Additional efforts have been made to replace the traditional surface-mounted transducers with non-contact acoustic transducers. Though some success has been achieved, most of these new approaches are inefficient because they require point-to-point measurements or off-line signal analysis. This article introduces a Mobile Acoustic Subsurface Sensing system as MASS, which is an improved surface wave based implementation for measuring the subsurface profile of roadways. The compact MASS system is a 3-wheeled cart outfitted with an electromagnetic impact source, distance register, non-contact acoustic sensors and data acquisition/processing equipment. The key advantage of the MASS system is the capability to collect measurements continuously at walking speed in an automatic way. The fast scan and real-time analysis advantages are based upon the non-contact acoustic sensing and fast air-coupled surface wave analysis program. This integration of hardware and software makes the MASS system an efficient mobile prototype for the field test.

On the existence conditions for a fast surface wave

Conditions are obtained for the existence of a fast-moving surface electromagnetic wave (with a speed close to the speed of light in the vacuum) on a flat interface between the vacuum and an isotropic dissipative medium with a permittivity . The interfaces considered include vacuum–seawater, vacuum–metal, vacuum–plasma, and vacuum–dielectric. Conditions for the existence of negligibly damped surface waves are considered for extremely high (vacuum–seawater, vacuum–metal) and very low (vacuum–plasma, vacuum–dielectric) values. It is shown that at least in these two limit cases, the phase wave velocity and the group wave velocity pass synchronously through the speed of light in the vacuum, which can be considered the reason why surface waves exist at the interface between vacuum and a collisionless plasma (with and ) and do not exist at the interface between the vacuum and a weakly absorbing dielectric (with and ). In the first limit case, it is shown that both the phase and group velocities pass at , implying that a surface wave exists at the vacuum-metal interface (with ), but that a surface wave (Zenneck's wave) cannot exist at the vacuum–seawater interface (with ).

Об условиях существования быстрой поверхностной волны

Об условиях существования быстрой поверхностной волны

Magnetoacoustic surface gravity waves at a spherical interface

Aims. The plasma structured by magnetic fields in the solar atmosphere is a perfect medium for the propagation of guided magnetic and magnetoacoustic waves. Geometrical restriction of wave propagation is known to confer a dispersive character for waves. In addition, waves propagating along discontinuities in the medium are known to remain localized. As an extension to theories of guided waves in magnetic slabs and cylinders under solar and stellar conditions, we aim to study the propagation of magnetoacoustic-gravity waves at a spherical interface in the low solar corona (considered here as a density discontinuity), modelling global waves recently observed in the corona in EUV wavelengths. Methods. Using conservation laws at the interface we derive the dispersion relation in spherical geometry with a radially expanding magnetic field in the presence of gravitational stratification. The obtained dispersion relation describing fast magnetoacoustic-gravity surface waves is derived using an approximative method taking into account that propagation takes place near the solar surface. Results. Theoretical results obtained in the present study are applied to investigate the propagation of EIT waves in the low corona. The frequency of waves is shown to increase with decreasing density contrast at the interface. We also show that, for a given azimuthal wavenumber, the magnetic field has a very small effect on the value of the frequency of waves. When plotted against the location of the interface (in the radial direction) the frequency varies inversely proportional to the distance, while for a fixed density ratio and location of the interface the frequency is obtained to be defined in a very narrow region.

Propagation of magnetoacoustic surface waves along static plasma slab surrounded by moving plasma and neutral gas

The existence and propagation of fast and slow magnetoacoustic surface waves (MASW) is investigated in our work by taking a theoretical model of a static plasma slab as the middle layer with a moving plasma region at the top and neutral gas medium as the bottom layer. Applying linear MHD, the dispersion relation is obtained and the propagation of magnetoacoustic surface waves, in the compressional limit for steady flow and for different values of dimensionless wave numbers, is analyzed. Steady flow of plasma along a structured atmosphere may cause enhancement of existing surface modes, disappearance of some modes and generation of new surface wave modes. The possible regions for the propagation of fast and slow surface and body waves for different mass density ratios and magnetic field ratios and with a small flow velocity are studied. Our discussion may help in analyzing more complicated cases.

Hydromagnetic surface waves on a tangential discontinuity

Propagation of hydromagnetic surface waves on an MHD tangential discontinuity, separating two uniform magnetic plasmas, across which the magnetic field changes both its direction and field-strength, is examined. The study is arbitrarily confined to discontinuities one side of which has a plasma-β value larger than unity. A classification scheme to identify the ‘fast’ and the ‘slow’ surface waves is employed thus enabling us to find new features in the modal structure of these waves. In the case of sufficiently small values for the density ratio between the higher and the lower Alfvén speed regions on opposite sides of the interface, this study reveals the presence of a ‘forbidden region’ for propagation of fast surface waves with values of their propagation angle being smaller than a certain critical value. The study also reveals the existence of two distinct slow surface waves with angles of propagation larger than a certain critical value but for two different ranges of values for the angle between magnetic field vectors across the interface. While revealing such new structures of the surface modes for intermediate parameter values, the present investigation also confirms earlier results regarding the asymptotic behaviour of the modes in the incompressible limit. §Present address: Physics and Applied Mathematics Unit, Indian Statistical Institute, 203 Barrackpore Trunk Road, Kolkata, India 700035.

Hydromagnetic surface waves along a compressible cylindrical plasmacolumn surrounded by neutral gas

The characteristics of hydromagnetic surface waves propagating along theinterface of a compressible cylindrical plasma column surrounded by neutralgas embedded in a steady magnetic field along the axis of the cylinder arestudied by varying the ratio of the magnetic field and the relative density ofthe two media. Symmetric and asymmetric modes are found to exist. When Alfvenspeed `vA' is less than the sound speed `s' in the neutral gas thesymmetric mode suffers lower cut-off in k for propagation. The cut-offwindow is narrowed when the magnetic field in the neutral gas is increased.For vA>s, the density of the neutral gas is found to suppress,restrict to a certain window, and allow the free propagation of both modes.There exists a critical magnetic field ratio for certain environments, inwhich both slow and fast surface waves are present simultaneously in symmetricand asymmetric modes. The findings for a special case are found to be inagreement with those obtained for an isolated magnetic flux tube studied earlier.

Propagation of fast leaky surface waves in a system of metal electrodes on a lithium tetraborate crystal

Dispersion curves are calculated for fast leaky surface acoustic waves in a periodic system of metal electrodes on a lithium tetraborate crystal. The periodic Green’s function analysis is applied to the piezoelectric halfspace. To allow for the mechanical properties of the electrodes, a perturbation method is developed that is accurate to within the first order of the electrode thickness-to-period ratio, and the finite-element method is used to investigate the higher-order effects. The reflection factor of a system of two open electrodes is shown to have a minimum at a certain electrode thickness. The fast leaky-to-Rayleigh wave conversion factor and the Rayleigh wave reflection factor are studied as functions of the electrode thickness.

Propagation of fast surface waves in an ideal Hall-magnetohydrodynamic plasma slab

The oblique and parallel propagation of fast sausage and kink magnetohydrodynamic (MHD) surface waves in an ideal magnetized plasma slab is studied taking into account the Hall term in the generalized Ohm’s law. It is found that, in the case of incompressible plasmas, the combining action of the Hall effect and the oblique wave propagation makes possible the existence of multivalued solutions to the dispersion relations of both MHD surface modes—some of them, corresponding to positive values of the transverse wave number (ky), undergo a “propagation stop” at specific (numerically found) full wave numbers. It is curious that such multivalued dispersion curves appear even at purely parallel propagation in low-β plasmas (β→0). Another peculiarity of these surface modes (in both cases) is that with the growing of the wave number they can change their nature—from bulk to pseudosurface (or pure surface) and leaky waves.