Showing papers on "Wave propagation published in 2011"

The Rock Physics Handbook: Tools for Seismic Analysis of Porous Media

Abstract: Preface 1. Basic tools 2. Elasticity and Hooke's law 3. Seismic wave propagation 4. Effective media 5. Granular media 6. Fluid effects on wave propagation 7. Empirical relations 8. Flow and diffusion 9. Electrical properties Appendices.

Edge and waveguide terahertz surface plasmon modes in graphene microribbons

Abstract: The authors acknowledge support from the Spanish MECD under Contract No. MAT2009-06609-C02 and Consolider Project “Nanolight.es.” A.Y.N. acknowledges the Juan de la Cierva Grant No. JCI-2008-3123

Tunable Unidirectional Sound Propagation through a Sonic-Crystal-Based Acoustic Diode

Abstract: Nonreciprocal wave propagation typically requires strong nonlinear materials to break time reversal symmetry. Here, we utilized a sonic-crystal-based acoustic diode that had broken spatial inversion symmetry and experimentally realized sound unidirectional transmission in this acoustic diode. These novel phenomena are attributed to different mode transitions as well as their associated different energy conversion efficiencies among different diffraction orders at two sides of the diode. This nonreciprocal sound transmission could be systematically controlled by simply mechanically rotating the square rods of the sonic crystal. Different from nonreciprocity due to the nonlinear acoustic effect and broken time reversal symmetry, this new model leads to a one-way effect with higher efficiency, broader bandwidth, and much less power consumption, showing promising applications in various sound devices.

Wave Fields in Real Media: Wave Propagation in Anisotropic, Anelastic and Porous Media

Abstract: This work examines the differences between an ideal and a real description of wave propagation, where ideal means an elastic (lossless), isotropic and single-phase medium, and real means an anelastic, anisotropic and multi-phase medium. The analysis starts by introducing the relevant stress-strain relation. This relation and the equations of momentum conservation are combined to give the equation of motion. The differential formulation is written in terms of memory variables, and Biot's theory is used to describe wave propagation in porous media. For each rheology, a plane-wave analysis is performed in order to understand the physics of wave propagation. The book contains a review of the main direct numerical methods for solving the equation of motion in the time and space domains. The emphasis is on geophysical applications for seismic exploration, but researchers in the fields of earthquake seismology, rock acoustics, and material science - including many branches of acoustics of fluids and solids - may also find this text useful.

An elastic metamaterial with simultaneously negative mass density and bulk modulus

Abstract: In this letter, an elasticmetamaterial which exhibits simultaneously negative effective mass density and bulk modulus is presented with a single unit structure made of solid materials. The double-negative properties are achieved through a chiralmicrostructure that is capable of producing simultaneous translational and rotational resonances. The negative effective mass density and effective bulk modulus are numerically determined and confirmed by the analysis of wave propagation. The left-handed wave propagation property of this metamaterial is demonstrated by the negative refraction of acoustic waves.

Pulse Wave Propagation in the Arterial Tree

Abstract: The beating heart creates blood pressure and flow pulsations that propagate as waves through the arterial tree that are reflected at transitions in arterial geometry and elasticity. Waves carry information about the matter in which they propagate. Therefore, modeling of arterial wave propagation extends our knowledge about the functioning of the cardiovascular system and provides a means to diagnose disorders and predict the outcome of medical interventions. In this review we focus on the physical and mathematical modeling of pulse wave propagation, based on general fluid dynamical principles. In addition we present potential applications in cardiovascular research and clinical practice. Models of short- and long-term adaptation of the arterial system and methods that deal with uncertainties in personalized model parameters and boundary conditions are briefly discussed, as they are believed to be major topics for further study and will boost the significance of arterial pulse wave modeling even more.

Attenuation, scattering, and absorption of ultrasound in the skull bone

Abstract: (Received 27 June 2011; revised 26 October 2011; accepted for publication 22 November 2011;published 21 December 2011)Purpose: Measured values of ultrasound attenuation in bone represent a combination of differentloss mechanisms. As a wave is transmitted from a fluid into bone, reflections occur at the interface.In the bone, mode conversion occurs between longitudinal and shear modes and the mechanicalwave is scattered by its complex internal microstructure. Finally, part of the wave energy isabsorbed by the bone and converted into heat. Due to the complexity of the wave propagation andthe difficulty in performing measurements that are capable of separating the various loss mecha-nisms, there are currently no estimates of the absorption in bone. The aim of this paper is, thus, toquantify the attenuation, scattering, and thermal absorption in bone.Methods: An attenuating model of wave propagation in bone is established and used to develop athree-dimensional finite difference time domain numerical algorithm. Hydrophone and optical het-erodyne interferometer measurements of the acoustic field as well as a x-ray microtomography ofthe bone sample are used to drive the simulations and to measure the attenuation. The acousticmeasurements are performed concurrently with an infrared camera that can measure the tempera-ture elevation during insonication. A link between the temperature and the absorption via a three-dimensional thermal simulation is then used to quantify the absorption coefficients for longitudinaland shear waves in cortical bone.Results: We demonstrate that only a small part of the attenuation is due to absorption in bone andthat the majority of the attenuation is due to reflection, scattering, and mode conversion. In the ninesamples of a human used for the study, the absorption time constant for cortical bone was deter-mined to be 1.04 ls628%. This corresponds to a longitudinal absorption of 2.7 dB/cm and a shearabsorption of 5.4 dB/cm. The experimentally measured attenuation across the approximately 8 mmthick samples was 13.360.97 dB/cm.Conclusions: This first measurement of ultrasound absorption in bone can be used to estimate theamount of heat deposition based on knowledge of the acoustic field.

Possible evidence of alfvén-cyclotron waves in the angle distribution of magnetic helicity of solar wind turbulence

Abstract: The fluctuating magnetic helicity is considered an important parameter in diagnosing the characteristic modes of solar wind turbulence. Among them is the Alfv?n-cyclotron wave, which is probably responsible for the solar wind plasma heating, but has not yet been identified from the magnetic helicity of solar wind turbulence. Here, we present the possible signatures of Alfv?n-cyclotron waves in the distribution of magnetic helicity as a function of ?VB, which is the angle between the solar wind velocity and local mean magnetic field. We use magnetic field data from the STEREO spacecraft to calculate the ?VB distribution of the normalized reduced fluctuating magnetic helicity ?m. We find a dominant negative ?m for 1 s 150? in the solar wind inward magnetic sector. These features of ?m appearing around the Doppler-shifted ion-cyclotron frequencies may be consistent with the existence of Alfv?n-cyclotron waves among the outward propagating fluctuations. Moreover, right-handed polarized waves at larger propagation angles, which might be kinetic Alfv?n waves or whistler waves, have also been identified on the basis of the ?m features in the angular range 40? < ?VB < 140?. Our findings suggest that Alfv?n-cyclotron waves (together with other wave modes) play a prominent role in turbulence cascading and plasma heating of the solar wind.

A global model of Love and Rayleigh surface wave dispersion and anisotropy, 25–250 s

Abstract: SUMMARY A large number of fundamental-mode Love and Rayleigh wave dispersion curves were determined from seismograms for 3330 earthquakes recorded on 258 globally distributed seismographic stations. The dispersion curves were sampled at periods between 25 and 250 s to determine propagation-phase anomalies with respect to a reference earth model. The data set of phase anomalies was first used to construct global isotropic phase-velocity maps at specific frequencies using spherical-spline basis functions with a nominal uniform resolution of 650 km. Azimuthal anisotropy was then included in the parametrization, and its importance for explaining the data explored. Only the addition of 2ζ azimuthal variations for Rayleigh waves was found to be resolved by the data. In the final stage of the analysis, the entire phase-anomaly data set was inverted to determine a global dispersion model for Love and Rayleigh waves parametrized horizontally using a spherical-spline basis, and with a set of B-splines to describe the slowness variations with respect to frequency. The new dispersion model, GDM52, can be used to calculate internally consistent global maps of phase and group velocity, as well as local and path-specific dispersion curves, between 25 and 250 s.

Plasma Jet Braking : Energy Dissipation and Nonadiabatic Electrons

Abstract: We report in situ observations by the Cluster spacecraft of wave-particle interactions in a magnetic flux pileup region created by a magnetic reconnection outflow jet in Earth's magnetotail. Two distinct regions of wave activity are identified: lower-hybrid drift waves at the front edge and whistler-mode waves inside the pileup region. The whistler-mode waves are locally generated by the electron temperature anisotropy, and provide evidence for ongoing betatron energization caused by magnetic flux pileup. The whistler-mode waves cause fast pitch-angle scattering of electrons and isotropization of the electron distribution, thus making the flow braking process nonadiabatic. The waves strongly affect the electron dynamics and thus play an important role in the energy conversion chain during plasma jet braking.

Electromagnetic wave propagation path and electromagnetic wave propagation device

Abstract: An electromagnetic wave propagation device includes multiple planar propagation media each formed by laminating at least one planar conductor and at least one planar dielectric, multiple transceivers for transmitting and receiving information among electronic apparatuses, and a first interface for transmitting and receiving the electromagnetic wave between the transceivers and the planar propagation media. Planar dielectric spacers are provided for isolating the multiple planar propagation media from one another. The planar propagation medium is disposed to have an overlapped part with at least the other of the planar propagation media so that an obverse face of the medium and a reverse face of the other medium are at least partially overlapped with each other. The planar conductor is provided with an electromagnetic wave linking unit at the overlapped part that transmits and receives the electromagnetic wave between the planar propagation media.

On the effects of reflected waves in transient shear wave elastography

Abstract: In recent years, novel quantitative techniques have been developed to provide noninvasive and quantitative stiffness images based on shear wave propagation. Using radiation force and ultrafast ultrasound imaging, the supersonic shear imaging technique allows one to remotely generate and follow a transient plane shear wave propagating in vivo in real time. The tissue shear modulus, i.e., its stiffness, can then be estimated from the shear wave local velocity. However, because the local shear wave velocity is estimated using a time-of-flight approach, reflected shear waves can cause artifacts in the estimated shear velocity because the incident and reflected waves propagate in opposite directions. Such effects have been reported in the literature as a potential drawback of elastography techniques based on shear wave speed, particularly in the case of high stiffness contrasts, such as in atherosclerotic plaque or stiff lesions. In this letter, we present our implementation of a simple directional filter, previously used for magnetic resonance elastography, which separates the forward- and backward-propagating waves to solve this problem. Such a directional filter could be applied to many elastography techniques based on the local estimation of shear wave speed propagation, such as acoustic radiation force imaging (ARFI), shearwave dispersion ultrasound vibrometry (SDUV), needle-based elastography, harmonic motion imaging, or crawling waves when the local propagation direction is known and high-resolution spatial and temporal data are acquired.

Asymmetric wave propagation in nonlinear systems.

Abstract: A mechanism for asymmetric (nonreciprocal) wave transmission is presented. As a reference system, we consider a layered nonlinear, nonmirror-symmetric model described by the one-dimensional discrete nonlinear Schrodinger equation with spatially varying coefficients embedded in an otherwise linear lattice. We construct a class of exact extended solutions such that waves with the same frequency and incident amplitude impinging from left and right directions have very different transmission coefficients. This effect arises already for the simplest case of two nonlinear layers and is associated with the shift of nonlinear resonances. Increasing the number of layers considerably increases the complexity of the family of solutions. Finally, numerical simulations of asymmetric wave packet transmission are presented which beautifully display the rectifying effect.

A parametrization study for surface seismic full waveform inversion in an acoustic vertical transversely isotropic medium

Abstract: SUMMARY Full waveform inversion (FWI) of surface seismic data requires low-frequency and long-offset data, namely diving waves or post-critical reflections, to update the long wavelength features of the parameters. In most geological settings, short and long offsets cannot be interpreted simultaneously without accounting for anisotropy. FWI therefore may require anisotropic modelling, which leads to the question of the parametrization of the inversion, since it is not possible to retrieve all the earth models from pressure (or vertical displacement) data, even in the simple vertical transversely isotropic (VTI) case. With a change of variables in the acoustic VTI wave equations, it can be shown that, in the context of macromodel building, namely background estimation, acoustic VTI FWI mainly depends on the normal moveout (NMO) velocity and the horizontal velocity (or equivalently a combination of these two velocities). An analysis of the eigenvalue/eigenvector decomposition of the Hessian confirms the relevance of the parametrization of the VTI wave equations with the NMO and horizontal velocities. It also points out a possible ambiguity when one inverts for two parameters. This ambiguity is further illustrated in synthetic examples. The trade-off between velocity heterogeneities and anisotropy also complicates the recovery of the second anisotropic parameter η. Nevertheless, FWI successfully interprets the kinematics of the data.

Helmholtz surface wave tomography for isotropic and azimuthally anisotropic structure

Abstract: SUMMARY The growth of the Earthscope/USArray Transportable Array (TA) has prompted the development of new methods in surface wave tomography that track phase fronts across the array and map the traveltime field for each earthquake or for each station from ambient noise. Directionally dependent phase velocities are determined locally by measuring the gradient of the observed traveltime field without the performance of a formal inversion. This method is based on the eikonal equation and is, therefore, referred to as ‘eikonal tomography’. Eikonal tomography is a bent-ray theoretic method, but does not account for finite frequency effects such as wave interference, wave front healing, or backward scattering. This shortcoming potentially may lead to both systematic bias and random error in the phase velocity measurements, which would be particularly important at the longer periods studied with earthquakes. It is shown here that eikonal tomography can be improved by using amplitude measurements to construct a geographically localized correction via the Helmholtz equation. This procedure should be thought of as a finite-frequency correction that does not require the construction of finite-frequency kernels and is referred to as ‘Helmholtz tomography’. We demonstrate the method with Rayleigh wave measurements following earthquakes between periods of 30 and 100 s in the western US using data from the TA. With Helmholtz tomography at long periods (>50 s): (1) resolution of small-scale isotropic structures, which correspond to known geological features, is improved, (2) uncertainties in the isotropic phase velocity maps are reduced, (3) the directionally dependent phase velocity measurements are less scattered, (4) spurious 1-psi azimuthal anisotropy near significant isotropic structural contrasts is reduced, and (5) estimates of 2-psi anisotropy are better correlated across periods.

Stable P-wave modeling for reverse-time migration in tilted TI media

Abstract: We present an approach for P-wave modeling in inhomogeneous transversely isotropic media with tilted symmetry axis (TTI media), suitable for anisotropic reverse-time migration. The proposed approach is based on wave equations derived from first principles — the equations of motion and Hooke's law — under the acoustic TI approximation. Consequently, no assumptions are made about the spatial variation of medium parameters. A rotation of the stress and strain tensors to a local coordinate system, aligned with the TI-symmetry axis, makes it possible to benefit from the simple and sparse form of the TI-elastic tensor in that system. The resulting wave equations can be formulated either as a set of five first-order or as a set of two second-order partial differential equations. For the constant-density case, the second-order TTI wave equations involve mixed and nonmixed second-order spatial derivatives with respect to global, nonrotated coordinates. We propose a numerical implementation of these equations using high-order centered finite differences. To minimize modeling artifacts related to the use of centered first-derivative operators, we use discrete second-derivative operators for the nonmixed second-order spatial derivatives and repeated discrete first-derivative operators for the mixed derivatives. Such a combination of finite-difference operators leads to a stable wave propagator, provided that the operators are designed properly. In practice, stability is achieved by slightly weighting down terms that contain mixed derivatives. This has a minor, practically negligible, effect on the kinematics of wave propagation. The stability of the presented scheme in inhomogeneous TTI models with rapidly varying anisotropic symmetry axis direction is demonstrated with numerical examples.

Interpretation of dark-field contrast and particle-size selectivity in grating interferometers.

Abstract: In grating-based x-ray phase sensitive imaging, dark-field contrast refers to the extinction of the interference fringes due to small-angle scattering. For configurations where the sample is placed before the beamsplitter grating, the dark-field contrast has been quantified with theoretical wave propagation models. Yet when the grating is placed before the sample, the dark-field contrast has only been modeled in the geometric optics regime. Here we attempt to quantify the dark-field effect in the grating-before-sample geometry with first-principle wave calculations and understand the associated particle-size selectivity. We obtain an expression for the dark-field effect in terms of the sample material’s complex refractive index, which can be verified experimentally without fitting parameters. A dark-field computed tomography experiment shows that the particle-size selectivity can be used to differentiate materials of identical x-ray absorption.

Biot-Rayleigh theory of wave propagation in double-porosity media

Abstract: [1] We derive the equations of motion of a double‐porosity medium based on Biot’s theory of poroelasticity and on a generalization of Rayleigh’s theory of fluid collapse to the porous case. Spherical inclusions are imbedded in an unbounded host medium having different porosity, permeability, and compressibility. Wave propagation induces local fluid flow between the inclusions and the host medium because of their dissimilar compressibilities. Following Biot’s approach, Lagrange’s equations are obtained on the basis of the strain and kinetic energies. In particular, the kinetic energy and the dissipation function associated with the local fluid flow motion are described by a generalization of Rayleigh’s theory of liquid collapse of a spherical cavity. We obtain explicit expressions of the six stiffnesses and five density coefficients involved in the equations of motion by performing “gedanken” experiments. A plane wave analysis yields four wave modes, namely, the fast P and S waves and two slow P waves. As an example, we consider a sandstone and compute the phase velocity and quality factor as a function of frequency, which illustrate the effects of the mesoscopic loss mechanism due to wave‐induced fluid flow.

Triggering Rogue Waves in Opposing Currents

Abstract: We show that rogue waves can be triggered naturally when a stable wave train enters a region of an opposing current flow. We demonstrate that the maximum amplitude of the rogue wave depends on the ratio between the current velocity U(0) and the wave group velocity c(g). We also reveal that an opposing current can force the development of rogue waves in random wave fields, resulting in a substantial change of the statistical properties of the surface elevation. The present results can be directly adopted in any field of physics in which the focusing nonlinear Schrodinger equation with nonconstant coefficient is applicable. In particular, nonlinear optics laboratory experiments are natural candidates for verifying experimentally our results.

Wigner functions in optics: describing beams as ray bundles and pulses as particle ensembles

Abstract: This tutorial gives an overview of the use of the Wigner function as a tool for modeling optical field propagation. Particular emphasis is placed on the spatial propagation of stationary fields, as well as on the propagation of pulses through dispersive media. In the first case, the Wigner function gives a representation of the field that is similar to a radiance or weight distribution for all the rays in the system, since its arguments are both position and direction. In cases in which the field is paraxial and where the system is described by a simple linear relation in the ray regime, the Wigner function is constant under propagation along rays. An equivalent property holds for optical pulse propagation in dispersive media under analogous assumptions. Several properties and applications of the Wigner function in these contexts are discussed, as is its connection with other common phase-space distributions like the ambiguity function, the spectrogram, and the Husimi, P, Q, and Kirkwood–Rihaczek functions. Also discussed are modifications to the definition of the Wigner function that allow extending the property of conservation along paths to a wider range of problems, including nonparaxial field propagation and pulse propagation within general transparent dispersive media.

Three‐dimensional, nonhydrostatic numerical simulation of nonlinear internal wave generation and propagation in the South China Sea

Abstract: [1] We present the results of three-dimensional, nonhydrostatic simulations of internal tides and waves in the South China Sea (SCS) using the SUNTANS model. Model results accurately predict the observed wave arrival times at two mooring locations in the SCS. Internal wave amplitudes are underpredicted which causes underprediction of internal wave speeds due to a lack of amplitude dispersion. We show that the well-known A and B waves arise from the steepening of semidiurnal internal tides that are generated due to strong barotropic flow over ridges in the Luzon Strait. A wave generation is stronger in the southern portion of the Luzon Strait because diurnal internal tidal beams augment the amplitude of the semidiurnal A waves. B wave generation is stronger in the northern portion where the distance between the eastern and western ridges is approximately equal to one internal tidal wavelength and leads to semidiurnal internal tidal resonance. The orientation of the ridges produces large A waves that propagate into the northern portion of the western SCS basin and stronger B waves that propagate into the southern portion. When traced back in time along linear characteristics, A waves consistently line up close to peak ebb (eastward) barotropic currents, while B waves consistently line up with peak flood (westward) barotropic currents. This reinforces the notion that the lee wave mechanism and associated hydraulic or nonlinear effects are weak, as demonstrated by a simple linear model relating the amplitude of the simulated waves to the excursion parameter at the ridges.

First evidence of coexisting eit wave and coronal moreton wave from sdo/aia observations

Abstract: “EIT waves” are a globally propagating wavelike phenomenon. They were often interpreted as fast-mode magnetoacoustic waves in the corona, despite various discrepancies between the fast-mode wave model and observations. To reconcile these discrepancies, we suggested that “EIT waves” are the apparent propagation of the plasma compression due to successive stretching of the magnetic field lines pushed by the erupting flux rope. According to this model, an EIT wave should be preceded by a fast-mode wave, which, however, had rarely been observed. With the unprecedented high cadence and sensitivity of the Solar Dynamics Observatory observations, we discern a fast-moving wave front with a speed of 560 km s −1 ahead of an EIT wave, which had a velocity of ∼190 km s −1 , in the “EIT wave” event on 2010 July 27. The results, suggesting that “EIT waves” are not fast-mode waves, confirm the prediction of our field-line stretching model for an EIT wave. In particular, it is found that the coronal Moreton wave was ∼3 times faster than the EIT wave, as predicted.

Numerical modeling of footpoint-driven magneto-acoustic wave propagation in a localized solar flux tube

Abstract: In this paper, we present and discuss results of two-dimensional simulations of linear and nonlinear magneto-acoustic wave propagation through an open magnetic flux tube embedded in the solar atmosphere expanding from the photosphere through to the transition region and into the low corona. Our aim is to model and analyze the response of such a magnetic structure to vertical and horizontal periodic motions originating in the photosphere. To carry out the simulations, we employed our MHD code SAC (Sheffield Advanced Code). A combination of the VALIIIC and McWhirter solar atmospheres and coronal density profiles were used as the background equilibrium model in the simulations. Vertical and horizontal harmonic sources, located at the footpoint region of the open magnetic flux tube, are incorporated in the calculations, to excite oscillations in the domain of interest. To perform the analysis we have constructed a series of time-distance diagrams of the vertical and perpendicular components of the velocity with respect to the magnetic field lines at each height of the computational domain. These time-distance diagrams are subject to spatio-temporal Fourier transforms allowing us to build ω-k dispersion diagrams for all of the simulated regions in the solar atmosphere. This approach makes it possible to compute the phase speeds of waves propagating throughout the various regions of the solar atmosphere model. We demonstrate the transformation of linear slow and fast magneto-acoustic wave modes into nonlinear ones, i.e., shock waves, and also show that magneto-acoustic waves with a range of frequencies efficiently leak through the transition region into the solar corona. It is found that the waves interact with the transition region and excite horizontally propagating surface waves along the transition region for both types of drivers. Finally, we estimate the phase speed of the oscillations in the solar corona and compare it with the phase speed derived from observations.

Transport through modes in random media

Abstract: The ease with which waves can travel through a disordered system is theoretically encapsulated in the separation and width of the energy levels — or modes — describing that system. But extracting this information is experimentally challenging because of the spectral overlap of these modes. Jing Wang and Azriel Genack now show how these modal properties can be reconstructed from measurements of the 'speckle' pattern of radiation transmitted through a disordered medium. The ease with which waves can travel through a disordered system is theoretically encapsulated in the separation and width of the energy levels, or modes, describing that system. However, extracting this information is experimentally challenging due to the spectral overlap of these modes. Here it is shown how these modal properties can be reconstructed from measurements of the 'speckle' pattern of radiation transmitted through a disordered medium. Excitations in complex media are superpositions of eigenstates that are referred to as ‘levels’ for quantum systems and ‘modes’ for classical waves. Although the Hamiltonian of a complex system may not be known or solvable, Wigner conjectured1 that the statistics of energy level spacings would be the same as for the eigenvalues of large random matrices. This has explained key characteristics of neutron scattering spectra2. Subsequently, Thouless and co-workers argued3,4 that the metal–insulator transition in disordered systems4,5,6 could be described by a single parameter, the ratio of the average width and spacing of electronic energy levels: when this dimensionless ratio falls below unity, conductivity is suppressed by Anderson localization5 of the electronic wavefunction. However, because of spectral congestion due to the overlap of modes7,8,9, even for localized waves, a comprehensive modal description of wave propagation has not been realized. Here we show that the field speckle pattern10 of transmitted radiation—in this case, a microwave field transmitted through randomly packed alumina spheres—can be decomposed into a sum of the patterns of the individual modes of the medium and the central frequency and linewidth of each mode can be found. We find strong correlation between modal field speckle patterns, which leads to destructive interference between modes. This allows us to explain complexities of steady state and pulsed transmission of localized waves and to harmonize wave and particle descriptions of diffusion.

Electromagnetic wave analogue of an electronic diode

Abstract: An electronic diode is a nonlinear semiconductor circuit component that allows conduction of electrical current in one direction only. A component with similar functionality for electromagnetic waves, an electromagnetic isolator, is based on the Faraday effect of rotation of the polarization state and is also a key component in optical and microwave systems. Here we demonstrate a chiral electromagnetic diode, which is a direct analogue of an electronic diode: its functionality is underpinned by an extraordinarily strong nonlinear wave propagation effect in the same way as the electronic diode function is provided by the nonlinear current characteristic of a semiconductor junction. The effect exploited in this new electromagnetic diode is an intensity-dependent polarization change in an artificial chiral metamolecule. This microwave effect exceeds a similar optical effect previously observed in natural crystals by more than 12 orders of magnitude and a direction-dependent transmission that differs by a factor of 65.