Short-wavelength Waves Research Articles

Identification of 1-MW infrared radiation in an electron-cyclotron maser as free-electron two-quantum Stark radiation

We find that the transition from a virtual state to another virtual state is forbidden. Because the intermediate state in two-quantum Stark (TQS) radiation is virtual, TQS radiation is an effective one-photon radiation. We find that the ratio of the power of TQS radiation to the power of the corresponding one-photon radiation in the electron beam is many orders of magnitude larger than that in atomic radiation. We attribute this phenomenon to the fact that in free-electron TQS radiation, the free electron is forced to emit radiation by an electric wiggler over a length longer than the wavelength of the electric wiggler while in bound-electron TQS radiation, the bound electron only experiences the wiggler potential within a length on the order of the atomic size. We find that plasma waves with potential amplitudes on the order of 1 kV and wavelengths on the order of 1 cm can be generated due to the \(\mathcal{E}_r \hat r \times B\hat z\) rotational-drift and the helical (or rippled) magnetic field in an electron-cyclotron maser (ECM). We reason that these plasma waves decompose into shorterwavelength plasma waves such that the wavenumber (K) spectrum of the potential amplitude scales as 1/K2 in the region of K ≪ K i = 2π/l i , where l i is the mean inter-electron distance. We find that the wavelength and the power of free-electron TQS radiation driven by these short-wavelength plasma waves under the influence of the axial uniform magnetic field are in good agreement with those of the measured 1-MW infrared radiation.

Numerical bifurcation and Hopf continuation of Lorenz–Stenflo system

A graphical overview of the bifurcation structures and their behaviour of the Lorenz–Stenflo system (Stenflo 1996 Phys. Scr. 53 83) for describing nonlinear low-periodic short-wavelength acoustic gravity waves are investigated. We utilize the MATCONT2.5.1 package to analyse the detailed bifurcation scenarios as the physical and control parameters are varied. The variety of patterns revealed here has not been studied before. Using forward and backward bifurcations, the Hopf bifurcation and their continuation curves of the system are presented in the rotation and Rayleigh number plane, Prandtl number and rotation plane, and also in the rotation and geometric parameter plane. These bifurcation phenomena and their behaviour may be useful for better prediction of existence and stability of large-scale quasi-stationary structures amid chaotic motion in the atmosphere.

Omnidirectional spin-wave nanograting coupler

Magnonics as an emerging nanotechnology offers functionalities beyond current semiconductor technology. Spin waves used in cellular nonlinear networks are expected to speed up technologically, demanding tasks such as image processing and speech recognition at low power consumption. However, efficient coupling to microelectronics poses a vital challenge. Previously developed techniques for spin-wave excitation (for example, by using parametric pumping in a cavity) may not allow for the relevant downscaling or provide only individual point-like sources. Here we demonstrate that a grating coupler of periodically nanostructured magnets provokes multidirectional emission of short-wavelength spin waves with giantly enhanced amplitude compared with a bare microwave antenna. Exploring the dependence on ferromagnetic materials, lattice constants and the applied magnetic field, we find the magnonic grating coupler to be more versatile compared with gratings in photonics and plasmonics. Our results allow one to convert, in particular, straight microwave antennas into omnidirectional emitters for short-wavelength spin waves, which are key to cellular nonlinear networks and integrated magnonics.

Spin Pumping and Enhanced Gilbert Damping in Thin Magnetic Insulator Films

Precessing magnetization in a thin-film magnetic insulator pumps spins into adjacent metals; however, this phenomenon is not quantitatively understood. We present a theory for the dependence of spin pumping on the transverse mode number and in-plane wave vector. For long-wavelength spin waves, the enhanced Gilbert damping for the transverse mode volume waves is twice that of the macrospin mode, and for surface modes, the enhancement can be ten or more times stronger. Spin pumping is negligible for short-wavelength exchange spin waves. We corroborate our analytical theory with numerical calculations in agreement with recent experimental results.

Phase Velocity Analysis of Projected Wave Motion Along Oblique Radar Beams - A Numerical Study of Type-1 Radar Echoes

The nonlinear features of type-1 radar echoes were studied by a two-dimensional (2D) simulation of saturation the Farley-Buneman (FB) wave. The behavior of this FB wave in a plane perpendicular to the Earth's magnetic field was simulated with a two-fluid code in which electron inertia was discounted while ion inertia was retained. It showed that the appearance of secondary waves propagating vertically and obliquely as the primary horizontal FB wave saturates. The secondary waves originating from nonlinear saturation process will construct the evolution of 2D modes which can be observed by oblique radar beams. We carried out the statistical analysis of projection phase velocities of 2D modes along oblique radar beam at different radar elevation angles. The result revealed that a likely density gradient effect of type-1 radar echoes for the wavelength dependence of phase velocity would appear at a larger radar elevation angle while short wavelength waves would approach isotropic speeds close to ion acoustic speed. This interesting result is primarily attributed to the spectral features of 2D modes.

Response spectra for differential motion of structures supports during earthquakes in Egypt

Differential motions of ground supports of stiff structures with large plan dimensions and separate foundations under earthquakes were studied by researchers during the last few decades. Such a type of structural response was previously underestimated. The importance of studying such a response comes up from the fact that usually the structures affected are of strategic importance such as bridges. During their expected life, structures may experience vibrations excited by ground waves of short wavelengths during near-source earthquakes, or during amplified earthquake signals, during explosions, or during vibrations induced from nearby strong vibration sources. This is the case when the differential motion of supports becomes considerable. This paper aims to review the effects of seismic signal variations along the structures dimensions with emphasis on Egypt as a case study. The paper shows some patterns of the damage imposed by such differential motion. A replication of the differential motion in the longitudinal direction is applied on a frame bridge model. The resulting straining actions show the necessity for considering the differential motion of supports in the design of special structures in Egypt. Finally, response spectra for the differential motion of supports, based on the available data from previous earthquakes in Egypt, is derived and proposed for designers to include in the design procedure when accounting for such type of structural response, and especially in long-span bridges.

Wave propagation in micropolar thermoelastic layer with two temperatures

The propagation of waves in micropolar thermoelastic homogeneous plate with two temperatures subjected to stress free thermally insulated and isothermal conditions is investigated. The secular equations for both symmetric and skew-symmetric wave mode of propagation have been derived. Helmholtz decomposition technique has been used to simplify mathematical model and frequency equations for different mechanical conditions. The regions of secular equations are obtained and some special cases such as thin plate and short wavelength waves of the secular equations are also discussed. The phase velocity and attenuation coefficient are computed numerically and depicted graphically. The amplitudes of stress components and thermodynamic temperature distribution for the symmetric and skew-symmetric wave modes are computed analytically and presented graphically. Results of some earlier workers have been deduced as particular cases.

Spin Pumping Driven by Bistable Exchange Spin Waves

Spin pumping driven by bistable exchange spin waves is demonstrated in a Pt/Y3Fe5O12 film under parametric excitation. In the Pt/Y3Fe5O12 film, the spin pumping driven by parametric excitation selectively enhances the relaxation of short-wavelength exchange spin waves, indicating strong coupling between the exchange spin waves and spin currents at the interface through efficient spin transfer. The parametric spin pumping, furthermore, allows direct access to nonlinear spin wave dynamics in combination with the inverse spin Hall effect, revealing unconventional bistability of the exchange spin waves.

Improved limits on short-wavelength gravitational waves from the cosmic microwave background

The cosmic microwave background (CMB) is affected by the total radiation density around the time of decoupling. At that epoch, neutrinos comprised a significant fraction of the radiative energy, but there could also be a contribution from primordial gravitational waves with frequencies greater than $\ensuremath{\sim}{10}^{\ensuremath{-}15}\text{ }\text{ }\mathrm{Hz}$. If this cosmological gravitational wave background were produced under adiabatic initial conditions, its effects on the CMB and matter power spectrum would mimic massless noninteracting neutrinos. However, with homogenous initial conditions---as one might expect from certain models of inflation, prebig bang models, phase transitions, and other scenarios---the effect on the CMB would be distinct. We present updated observational bounds for both initial conditions using the latest CMB data at small scales from the South Pole Telescope in combination with Wilkinson Microwave Anisotropy Probe, current measurements of the baryon acoustic oscillations, and the Hubble parameter. With the inclusion of the data from the South Pole Telescope, the adiabatic bound on the cosmological gravitational wave background density is improved by a factor of 1.7 to ${10}^{6}{\ensuremath{\Omega}}_{\mathrm{gw}}{h}^{2}\ensuremath{\lesssim}8.7$ at the 95% confidence level, with weak evidence in favor of an additional radiation component consistent with previous analyses. The constraint can be converted into an upper-limit on the tension of horizon-sized cosmic strings that could generate this gravitational wave component, with $G\ensuremath{\mu}\ensuremath{\lesssim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ at 95% C. L., for string tension $G\ensuremath{\mu}$. The homogeneous bound improves by a factor of 3.5 to ${10}^{6}{\ensuremath{\Omega}}_{\mathrm{gw}}{h}^{2}\ensuremath{\lesssim}1.0$ at 95% C. L., with no evidence for such a component from current data.

Two-dimensional simulations of nonlinear beam-plasma interaction in isotropic and magnetized plasmas

Nonlinear interaction of a low density electron beam with an uniform plasma is studied using two-dimensional particle-in-cell simulations. We focus on formation of coherent phase space structures in the case, when a wide two-dimensional wave spectrum is driven unstable, and we also study how nonlinear evolution of these structures is affected by the external magnetic field. In the case of isotropic plasma, nonlinear buildup of filamentation modes due to the combined effects of two-stream and oblique instabilities is found to exist and growth mechanisms of secondary instabilities destroying the Bernstein-Green-Kruskal–type nonlinear wave are identified. In the weak magnetic field, the energy of beam-excited plasma waves at the nonlinear stage of beam-plasma interaction goes predominantly to the short-wavelength upper-hybrid waves propagating parallel to the magnetic field, whereas in the strong magnetic field, the spectral energy is transferred to the electrostatic whistlers with oblique propagation.

Horizontal coherence of low-frequency fixed-path sound in a continental shelf region with internal-wave activity

Sound at 85 to 450 Hz propagating in approximately 80-m depth water from fixed sources to a joint horizontal/vertical line array (HLA/VLA) is analyzed. The data are from a continental shelf area east of Delaware Bay (USA) populated with tidally generated long- and short-wavelength internal waves. Sound paths are 19 km in the along-shore (along internal-wave crest) direction and 30 km in the cross-shore direction. Spatial statistics of HLA arrivals are computed as functions of beam steering angle and time. These include array gain, horizontally lagged spatial correlation function, and coherent beam power. These quantities vary widely in magnitude, and vary over a broad range of time scales. For example, correlation scale can change rapidly from forty to five wavelengths, and correlation-scale behavior is anisotropic. In addition, the vertical array can be used to predict correlation expected for adiabatic propagation with cylindrical symmetry, forming a benchmark. Observed variations are in concert with internal-wave activity. Temporal variations of three coherence measures, horizontal correlation length, array gain, and ratio of actual correlation length to predicted adiabatic-mode correlation length, are very strong, varying by almost a factor of ten as internal waves pass.

Spin pumping by parametrically excited short-wavelength spin waves

We use both parallel and perpendicular parametric pumping techniques to excite short-wavelength spin waves in an yttrium iron garnet film and study the spin current generation from spin waves excited by these pumping methods with the help of the inverse spin-Hall effect in the adjacent Pt layer. We observed clear spin current generations for these pumping techniques and find that the efficiency is nearly independent of the magnitude and the direction of the wave vectors of excited spin waves. These experimental results are important for future spintronic devices operated by short-wavelength spin waves.

Degenerate mixing of plasma waves on cold, magnetized single-species plasmas

In the cold-fluid dispersion relation ω=ωp/[1+(k⊥/kz)2]1/2 for Trivelpiece-Gould waves on an infinitely long magnetized plasma cylinder, the transverse and axial wavenumbers appear only in the combination k⊥/kz. As a result, for any frequency ω<ωp, there are infinitely many degenerate waves, all having the same value of k⊥/kz. On a cold finite-length plasma column, these degenerate waves reflect into one another at the ends; thus, each standing-wave normal mode of the bounded plasma is a mixture of many degenerate waves, not a single standing wave as is often assumed. A striking feature of the many-wave modes is that the short-wavelength waves often add constructively along resonance cones given by dz/dr=±(ωp2/ω2-1)1/2. Also, the presence of short wavelengths in the admixture for a predominantly long-wavelength mode enhances the viscous damping beyond what the single-wave approximation would predict. Here, numerical solutions are obtained for modes of a cylindrical plasma column with rounded ends. Exploiting the fact that the modes of a spheroidal plasma are known analytically (the Dubin modes), a perturbation analysis is used to investigate the mixing of low-order, nearly degenerate Dubin modes caused by small deformations of a plasma spheroid.

Extensional and Transversal Wave Motion in Transversely Isotropic Thermoelastic Plates by Using Asymptotic Method

The present investigation is concerned with the study of extensional and transversal wave motions in an infinite homogenous transversely isotropic, thermoelastic plate by using asymptotic method in the context of coupled thermoelasticity, Lord and Shulman (1967, “The Generalized Dynamical Theory of Thermoelasticity,” J. Mech. Phys. Solids, 15, pp. 299–309), and Green and Lindsay (1972, “Thermoelasticity,” J. Elast., 2, pp. 1–7) theories of generalized thermoelasticity. The governing equations for extensional, transversal, and flexural motions have been derived from the system of three-dimensional dynamical equations of linear thermoelasticity. The asymptotic operator plate model for extensional motion in a homogeneous transversely isotropic thermoelastic plate leads to sixth degree polynomial secular equation that governs frequency and phase velocity of various possible modes of wave propagation at all wavelengths. It is shown that the purely transverse motion (SH mode), which is not affected by thermal variations, gets decoupled from rest of the motion. The Rayleigh–Lamb frequency equation for the plate is expanded in power series in order to obtain polynomial frequency equation and velocity dispersion relations. Their validation has been established with that of asymptotic method. The special cases of short and long wavelength waves are also discussed. The expressions for group velocity of extensional and transversal modes have been derived. Finally, the numerical solution is carried out for homogeneous transversely isotropic plate of single crystal of zinc material. The dispersion curves of phase velocity and attenuation coefficient are presented graphically.

Excitation of short-wavelength spin waves in magnonic waveguides

By using phase-resolved micro-focus Brillouin light scattering spectroscopy, we demonstrate experimentally a phenomenon of wavelength conversion of spin waves propagating in tapered Permalloy waveguides. We show that this phenomenon enables efficient excitation of spin waves with sub-micrometer wavelengths being much smaller than the width of the microstrip antenna used for the excitation. The proposed excitation mechanism removes restrictions on the spin-wave wavelength imposed by the size of the antenna and enables improvement of performances of integrated magnonic devices.

Hydromagnetic waves in weakly-ionized media - I. Basic theory, and application to interstellar molecular clouds

We present a comprehensive study of magnetohydrodynamic (MHD) waves and instabilities in a weakly-ionized system, such as an interstellar molecular cloud. We determine all the critical wavelengths of perturbations across which the sustainable wave modes can change radically (and so can their decay rates), and various instabilities are present or absent. Hence, these critical wavelengths are essential for understanding the effects of MHD waves (or turbulence) on the structure and evolution of molecular clouds. Depending on the angle of propagation relative to the zeroth-order magnetic field and the physical parameters of a model cloud, there are wavelength ranges in which no wave can be sustained as such. Yet, for other directions of propagation or different properties of a model cloud, there may always exist some wave mode(s) at all wavelengths (smaller than the size of the model cloud). For a typical model cloud, magnetically-driven ambipolar diffusion leads to removal of any support against gravity that most short-wavelength waves (or turbulence) may have had, and gravitationally-driven ambipolar diffusion sets in and leads to cloud fragmentation into stellar-size masses, as first suggested by Mouschovias more than three decades ago - a single-stage fragmentation theory of star formation, distinct from the then prevailing hierarchical fragmentation picture. The phase velocities, decay times and eigenvectors (e.g. the densities and velocities of neutral particles and the plasma, and the three components of the magnetic field) are determined as functions of the wavelength of the disturbances in a mathematically transparent way and are explained physically. Comparison of the results with those of nonlinear analytical or numerical calculations is also presented where appropriate, excellent agreement is found, and confidence in the analytical, linear approach is gained to explore phenomena difficult to study through numerical simulations. Mode splitting (or bifurcation) and mode merging, which are impossible in single-fluid systems for linear perturbations (hence, the term 'normal mode' and the principle of superposition), occur naturally in multifluid systems (as do transitions between wave modes without bifurcation) and have profound consequences in the evolution of such systems.

Dispersion relation of longitudinal oscillation in relativistic plasmas with nonextensive distribution

The generalized dispersion equation for longitudinal oscillation in an unmagnetized, collisionless, isotropic and relativistic plasma is derived in the context of nonextensive q-distribution. The analytical expression for the dispersion relation is obtained in an ultra-relativistic regime, which is related to the q-parameter and temperature. In the limit q→1, the result based on the relativistic Maxwellian distribution is recovered. Using the numerical simulation method, we obtain the full dispersion curve which cannot be given by an analytic method. It is shown that the numerical solution is in good agreement with the analytical result in the long-wavelength, short-wavelength and near light wave region for ultra-relativistic plasmas.

Scale-dependent infrared radiative damping rates on Mars and their role in the deposition of gravity-wave momentum flux

Using a Curtis-matrix model of 15 μm CO 2 radiative cooling rates for the martian atmosphere, we have computed vertical scale-dependent IR radiative damping rates from 0 to 200 km altitude over a broad band of vertical wavenumbers ∣ m∣ = 2 π(1–500 km) −1 for representative meteorological conditions at 40°N and average levels of solar activity and dust loading. In the middle atmosphere, infrared (IR) radiative damping rates increase with decreasing vertical scale and peak in excess of 30 days −1 at ∼50–80 km altitude, before gradually transitioning to scale-independent rates above ∼100 km due to breakdown of local thermodynamic equilibrium. We incorporate these computed IR radiative damping rates into a linear anelastic gravity-wave model to assess the impact of IR radiative damping, relative to wave breaking and molecular viscosity, in the dissipation of gravity-wave momentum flux. The model results indicate that IR radiative damping is the dominant process in dissipating gravity-wave momentum fluxes at ∼0–50 km altitude, and is the dominant process at all altitudes for gravity waves with vertical wavelengths ≲10–15 km. Wave breaking becomes dominant at higher altitudes only for “fast” waves of short horizontal and long vertical wavelengths. Molecular viscosity plays a negligible role in overall momentum flux deposition. Our results provide compelling evidence that IR radiative damping is a major, and often dominant physical process controlling the dissipation of gravity-wave momentum fluxes on Mars, and therefore should be incorporated into future parameterizations of gravity-wave drag within Mars GCMs. Lookup tables for doing so, based on the current computations, are provided.

Numerical simulation of wave propagation along a discontinuity in depth in a rotating annulus

Coastal currents are strongly affected by the topography of the ocean bed close to the shore. In some locations, notably off the southern coast of Africa, the ocean depth drops off very sharply, forming a shelf break. Several authors have analysed the role of such sharply-varying topography in steering and stabilising coastal currents. The present work aims to resolve an apparent conflict between theoretical and experimental results for currents flowing along discontinuous topography in a rotating annulus. The existing analytical theory, based on a long-wave scaling, fails to explain the short-wavelength instabilities and wave breaking that arise in the laboratory experiments. We integrate the rigid-lid quasigeostrophic (QG) equations numerically in an annular domain using a combined finite-difference/pseudo-spectral approach. To replicate the inviscid theoretical conditions as closely as possible, we employ an additional piecewise-linear source term that sharpens fronts of potential vorticity. We find that initial configurations that vary slowly in azimuth rapidly develops features with large azimuthal variations. The numerical results also produce the same qualitative features as the experiments, suggesting that the experiments are well described by QG theory.

Numerical modeling of slug flow initiation in a horizontal channels using a two-fluid model

This paper presents a methodology for modeling slug initiation and growth in horizontal ducts. Transient two-fluid equations are solved numerically using a class of high-resolution shock capturing methods. The advantage of this method is that slug formation and growth in a stratified regime can be calculated directly from the solutions to the flow field differential equations. In addition, by using high-resolution shock capturing methods that do not contain numerical diffusion, the discontinuity generated by slugging in the flow field can be modeled with good accuracy. The two-fluid model is shown to be well-posed mathematically only under certain conditions. Under these circumstances, the two-fluid model is capable of correctly predicting and modeling the flow physics. When ill-posed, an unbounded instability occurs in the flow field solution, and the instability amplitude increases exponentially with decreasing mesh sizes. This work shows that there are three zones associated with slug formation. In addition, long wavelength slugs are shown to initiate from short wavelength waves. These short waves are generated at the interface of the two phases by the Kelvin–Helmholtz hydrodynamic instability. The results obtained through numerical modeling show good agreement with experimental results.