High Phase Velocity Research Articles

Fully relativistic form factor for Thomson scattering

We derive a fully relativistic form factor for Thomson scattering in unmagnetized plasmas valid to all orders in the normalized electron velocity, beta[over ]=v[over ]/c. The form factor is compared to a previously derived expression where the lowest order electron velocity, beta[over], corrections are included [J. Sheffield, (Academic Press, New York, 1975)]. The beta[over ] expansion approach is sufficient for electrostatic waves with small phase velocities such as ion-acoustic waves, but for electron-plasma waves the phase velocities can be near luminal. At high phase velocities, the electron motion acquires relativistic corrections including effective electron mass, relative motion of the electrons and electromagnetic wave, and polarization rotation. These relativistic corrections alter the scattered emission of thermal plasma waves, which manifest as changes in both the peak power and width of the observed Thomson-scattered spectra.

Lithospheric Deformation of Continental China from Rayleigh Wave Azimuthal Anisotropy

AbstractThe tectonic history of continental China is complicated and the associated lithospheric deformation is large; the use of the anisotropic propagation of surface waves is an important way to investigate the history of lithospheric deformation pattern. We measured inter‐station Rayleigh‐wave phase velocity dispersion from cross‐correlation of narrow band‐pass filtered surface wave records and an image analysis technique. The dispersion data were then used to invert for the distribution of azimuthal anisotropy at periods 20~120 s of continental China and adjacent regions. Checkerboard tests show that the lateral resolution of phase velocity azimuthal anisotropy is about 5° (~500 km) in most area of continental China. Our results show that the pattern of azimuthal anisotropy in the study area displays clear spatial variation. The tectonic deformation of the crust and upper‐mantle east of 104°E is generally weaker than that to the west as inferred from the relative magnitude of azimuthal anisotropy, and strong tectonic deformation had occurred in the crust and upper‐mantle beneath Qinghai‐Tibet Plateau and its eastern margin. In eastern China relatively large amplitude of azimuthal anisotropy appears in South China block and Pearl River Estuary region, southwestern margin of Ordos basin and Qinling‐Dabie orogenic belt, indicating strong tectonic deformation occurred there in the geological history. At short and intermediate periods the fast propagation direction shows a rotational pattern from the west to the east within the Qinghai‐Tibet block consistent with the clockwise extrusion (or escape) of the crustal material in the block after the Eurasian and Indian collision. The deformation pattern in the mid‐lower crust of western Qinghai‐Tibet block is similar to that of the upper mantle, inferring possible vertically coherent deformation across the Moho interface. However, the central and eastern Qinghai‐Tibet block and its eastern margin show different deformation patterns in the crust and upper mantle, which suggests that deformation is not vertically coherent in the crust and upper mantle in eastern plateau region. Strong tectonic deformation had occurred within Tarim and Qaidam basins as well as Qilian fold belt north of Qinghai‐Tibet block. The fast propagation direction at the middle crustal depth is clearly different from those both in the lower crust and upper mantle underlying the upper Yangtze block including Sichuan basin. This suggests different tectonic deformation processes between the middle crust and lower crust beneath this block, whereas the lower crust and upper mantle appears to be coupled as inferred from the similar fast propagation direction at periods above 30 s. The Longmen Shan tectonic zone can be divided into southwestern and northeastern segments at about 103°E. The southwestern segment has relatively lower phase velocity while the northeastern area has higher phase velocity and stronger anisotropy. This suggests that the NE striking unilateral rupture propagation of the WenchuanMs8.0 earthquake on May 12, 2008, may be related not only to the cumulated high stress of the northeastern section but also to the underlying medium property along the segment with high phase velocity suitable for strain energy accumulation and concentrated release.

Kinerja dan Verifikasi Teknik Seismik Gelombang Permukaan Terintegrasi untuk Pengukuran Modulus Elastisitas Tanah Dasar

An integrated-spectral analysis of surface waves (ISASW) test has been recently proposed as an alternative technique in pavement evaluation and monitoring. ISASW technique is developed from the spectral analysis of surface wave (SASW) method which has been a well known nondestructive testing (NDT) for geotechnical and pavement structures. Surface wave propagation between a set of receivers were transformed to frequency domain using fast Fourier transform (FFT). Therefore, a phase spectrum was produced for measuring the time lag between receivers. A wavelet filtering on impulse response was used for constructing an enhanced phase spectrum. Using phase different method, an experimental dispersion curve was generated. Inversion analysis based on the 3-D stiffness matrix method was then performed in order to produce the shear wave velocity profile. The elastic modulus of pavement layer was calculated based on the linear elastic theory. In this paper, the performance of ISA SW measurement for determining the elastic modulus of pavement subgrade is presented in this paper. Verification on its elastic modulus obtained from ISASW technique was also conducted by using falling weight deflectometer (FWD) test. The result shows that the dispersion curves obtained in the ISASW test are sensitive to indicate the stiffness change in each layer of a pavement structure. Stiffer layer, i.e., pavement surface layer, can be indetified by the parameter of high frequency and phase velocity range; and vice versa. ISASW technique is also able to comprehensively investigate the elastic modulus of subgrade layer in exisiting pavement without any destruction. A good agreement of elastic modulus of subgrade layer from ISASW and FWD test was also pres ented.

Novel features of non-linear Raman instability in a laser plasma

The electron phase space evolution in a non-relativistic and homogeneous laser plasma generated by a nanosecond laser in a near infrared region in the presence of stimulated Raman scattering is investigated by a numerical simulation. The mechanism of electron acceleration in the potential wells of the plasma wave accompanying the Raman back-scattering is analyzed in a 1D Vlasov-Maxwell model. The dominant wave modes are both the backward and the forward propagating Raman waves, each accompanied by a daughter electrostatic wave. In addition to a strong interaction of plasma electrons with the primary electrostatic wave in the case of back-scattering, a cascading is observed consisting in a secondary scattering of the primary Raman back-scattered wave. This phenomenon reduces the Raman reflectivity and causes an acceleration of electrons against the direction of the heating laser beam. Moreover, the strong trapping in the primary electrostatic wave generated by the Raman back-scattering leads due to the trapped particle instability to a significant spectral broadening of the original plasma wave and a subsequent intermittent behaviour of the scattering process. The high phase velocity electrostatic daughter wave of the forward Raman scattering cannot trap the electrons directly, but there is an indication of non-resonant quasi-modes combined of this wave and of the simultaneously existing electrostatic daughter wave accompanying the Raman back-scattering. The transform method is used for a solution of the set of partial differential equations, which consists of the Vlasov equation and of the full set of Maxwell equations in a 1D approximation. A simplified Fokker-Planck collision term is added to overcome the numerical instabilities during the simulation. The model has relevance to a long scale plasma geometry, such as occurring in the indirect drive experiments near the light entrance holes of target hohlraum.

Practical comparison of 2.7 μm fused-core silica particles and porous sub-2 μm particles for fast separations in pharmaceutical process development

Fused-core silica stationary phases represent a key technological advancement in the arena of fast HPLC separations. These phases are made by fusing a 0.5 μm porous silica layer onto 1.7 μm nonporous silica cores. The reduced intra-particle flow path of the fused particles provides superior mass transfer kinetics and better performance at high mobile phase velocities, while the fused-core particles provide lower pressure than sub-2 μm particles. In this work, chromatographic performance of the fused-core particles (Ascentis Express) was investigated and compared to that of sub-2 μm porous particles (1.8 μm Zorbax Eclipse Plus C18 and 1.7 μm Acquity BEH C18). Specifically, retention, selectivity, and loading capacity were systematically compared for these two types of columns. Other chromatographic parameters such as efficiency and pressure drop were also studied. Although the fused-core column was found to provide better analyte shape selectivity, both columns had similar hydrophobic, hydrogen bonding, total ion-exchange, and acidic ion-exchange selectivities. As expected, the retention factors and sample loading capacity on the fused-core particle column were slightly lower than those for the sub-2 μm particle column. However, the most dramatic observation was that similar efficiency separations to the sub-2 μm particles could be achieved using the fused-core particles, without the expense of high column back pressure. The low pressure of the fused-core column allows fast separations to be performed routinely on a conventional LC system without significant loss in efficiency or resolution. Applications to the HPLC impurity profiling of drug substance candidates were performed using both types of columns to validate this last point.

Modeling of thermal processes in high pressure liquid chromatography: II. Thermal heterogeneity at very high pressures

Advanced instruments for liquid chromatography enables the operation of columns packed with sub-2 μm particles at the very high inlet pressures, up to 1000 bar, that are necessary to achieve the high column efficiency and the short analysis times that can be provided by the use of these columns. However, operating rather short columns at high mobile phase velocities, under high pressure gradients causes the production of a large amount of heat due to the viscous friction of the eluent percolating through the column bed. The evacuation of this heat causes the formation of significant axial and radial temperature gradients. Due to these thermal gradients, the retention factors of analytes and the mobile phase velocity are no longer constant throughout the column. The consequence of this heat production is a loss of column efficiency. We previously developed a model combining the heat and mass balance of the column, the equations of flow through porous media, and a linear isotherm model of the analyte. This model was solved and validated for conventional columns operated under moderate pressures. We report here on the results obtained when this model is applied to columns packed with very fine particles, operated under very high pressures. These results prove that our model accounts well for all the experimental results. The same column that elutes symmetrical, nearly Gaussian peaks at low flow rates, under relatively low pressure drops, provides strongly deformed, unsymmetrical peaks when operated at high flow rates, under high pressures, and under different thermal environments. The loss in column efficiency is particularly important when the column wall is kept at constant temperature, by immersing the column in a water bath.

Density gradients in packed columns: II. Effects of density gradients on efficiency in supercritical fluid separations

To investigate how fluid compressibility affects efficiency in supercritical fluid separations, band dispersion along a packed capillary column was measured from on-column elution rate profiles obtained under solvating gas chromatography (SGC) conditions; this allowed efficiency to be determined with respect to position along the column. Theoretical efficiency was also modeled. The model indicates that the primary cause of band broadening in SGC is high mobile phase velocity near the column outlet. However, the experimental results show that significant band broadening also occurs near the column inlet in a region that corresponds to high elution rates of the analyte. On-column detection also revealed spatial focusing of the analyte as it moves down the column density gradient.

Influence of Viscous Friction Heating on the Efficiency of Columns Operated under Very High Pressures

When columns packed with very fine particles are operated at high mobile phase velocities, the friction of the mobile phase percolating through the column bed generates heat. This heat dissipates along and across the column and axial and radial temperature gradients appear. The wall region of the column tends to be cooler than its center, and due to the influence of temperature on the mobile phase viscosity and on the equilibrium constant of analytes, the band velocity is not constant across the column. This radial heterogeneity of the temperature distribution across the column contributes to band broadening. This phenomenon was investigated assuming a cylindrically symmetrical column and using the general dispersion theory of Aris, which relates the height equivalent to the theoretical plate (HETP) contribution due to a radial heterogeneity of the column to the radial distribution of the linear velocities of a compound peak and to the radial distribution of its apparent dispersion coefficients in the column bed. The former is known from the temperature gradient across the column, the temperature dependencies of the mobile phase viscosity, and the retention factor of the compound. The latter is derived from the known expression of the transverse reduced HETP equation for the column. The values of the HETP calculated with the Aris model and a classical HETP equation were compared to those measured on a 2.1 x 50 mm Acquity BEH-C(18) column, run at flow rates of 0.6, 0.95, 1.30, and 1.65 mL/min, with pure acetonitrile as the mobile phase and naphtho[2,3-a]pyrene as the retained compound. These two sets of data are in generally good agreement, although the experimental values of the HETP tend to increase faster with increasing mobile phase velocity than the calculated values.

Practical assessment of frictional heating effects and thermostat design on the performance of conventional (3 μm and 5 μm) columns in reversed-phase high-performance liquid chromatography

A practical investigation of frictional heating effects in conventional C18 columns was undertaken, to investigate whether problems found for sub-2 μm columns were also present for those of particle size 3 μm and 5 μm and different internal diameter. The influence of a water bath, a still air heater, and a forced air heater on performance was investigated. Heating effects were substantial, with a decrease in k of almost 15% for toluene over the flow rate range ∼0.4–2.3 mL/min with a 15 cm × 0.46 cm ID column packed with 3 μm particles. Heating effects on retention increased with increasing solute k, with increase in the column ID, with decrease in the column particle size, and with decrease in the set column oven temperature. While the water bath minimised axial temperature gradients and thus its effect on k, radial temperature gradients were potentially serious with this system, especially at high mobile phase velocity, even with columns containing 5 μm particles. In contrast to the effects of axial temperature gradients in 4.6 mm columns, very little difference in Van Deemter plots was noted between the three different thermostats with 2 mm ID columns, even when 3 μm particles were used. However, the efficiency of 2 mm columns for peaks of low or moderate k ( k < 4) can be compromised by the extra dead volume introduced by the heating systems, even with conventional HPLC systems with otherwise minimised extra column volume.

Rayleigh surface acoustic wave modes of (100) ZnO films on (111) diamond

(100) zinc oxide (ZnO) films were combined with (111) diamond to be a composite high velocity surface acoustic wave (SAW) substrate. Rayleigh SAW modes of (100) ZnO films on (111) diamond were theoretically analyzed. Those Rayleigh SAW modes have smaller films thickness ratios (h/λ), higher phase velocities, and larger electromechanical coupling coefficients (K2) than the ones of (002) ZnO films on (111) diamond. The research results provide a predictable and theoretical basis for further application on the high velocity SAW devices.

Model of a tunneling current in an anisotropic Si/Si1−xGex/Si heterostructure with a nanometer-thick barrier including the effect of parallel–perpendicular kinetic energy coupling

A theoretical model of an electron tunneling current in an anisotropic Si/Si1−xGex/Si heterostructure was developed. The parallel and perpendicular kinetic energies were coupled and the coupling was included in expressing the electron transmittance through the anisotropic heterostructure. The model was applied to the anisotropic Si(1 1 0)/Si0.5Ge0.5/Si(1 1 0) heterostructure with a 25 nm thick strained Si0.5Ge0.5 potential barrier, in which each layer of the heterostructure has three valleys (valleys 1, 2 and 3) with different inverse effective mass tensors and a conduction band discontinuity of 216 meV. The Si(1 1 0)/SiGe structure implies that only the four equivalent valleys (valleys 1 and 2) are considered in calculations. It was found that the transmittance for valley 1 is the same as that for valley 2 due to the same barrier height. The transmittance decreases as the electron phase velocity increases because the electron phase velocity enhances the barrier height. Moreover, the total tunneling current density for the phase velocity higher than 3 × 105 m s−1 differs significantly from that obtained without including the kinetic energy coupling. As the electron phase velocity gets higher, the total tunneling current density lowers. This implies that the coupling effect cannot be ignored for electrons with high phase velocity.

Rayleigh surface acoustic wave modes of interdigital transducer/(100) AlN/(111) diamond

In this research, Rayleigh surface acoustic wave (SAW) modes of interdigital transducer (IDT)/(100) AlN/(111) diamond were theoretically analyzed and exhibited some excellent acoustic properties. Those Rayleigh SAW modes have smaller film thickness ratios (h/λ), higher phase velocities, and larger electromechanical coupling coefficients (K2) than the ones of IDT/(002) AlN/(111) diamond. Especially for mode 1, the phase velocity is 10 474 m/s, the K2 is 2.31%, and the film thickness ratio (h/λ) is 0.3. The research results exhibit that IDT/(100) AlN/(111) diamond has some excellent properties and provides a predictable and theoretical basis for further application in high velocity SAW devices.

The Earth's hum: the excitation of Earth normal modes by ocean waves

SUMMARY The recent discovery that the seismic normal modes of the Earth are excited to a nearly constant level during seismically quiet days (‘the hum of the Earth’) has lead to much speculation as to what drives the observed background in the absence of large earthquakes. Other authors have shown that the hum cannot be explained by the many small earthquakes occurring each day and have also precluded excitation by ‘slow’ earthquakes as the source. The non-linear interaction of low frequency ocean waves (infragravity waves) generates high phase velocity components that excite the seismic normal modes of the Earth. I show the forcing of the normal modes of the Earth by infragravity waves interacting over the continental shelves and over the deep ocean basins is sufficient to explain the background seismic acceleration spectrum observed during seismically quiet days. Atmospheric turbulence is shown to be a negligible source of the Earth’s hum. Observations suggesting enhanced spectral levels for two seismic modes that couple strongly with atmospheric modes (0S27 and 0S39) have been cited as evidence that the source of the hum lies in the atmosphere. However, the wave interaction mechanism also couples energy into infrasound and thus may explain these observations. A calculation of the coupling between ocean waves and normal modes expected from ocean waves interacting over the shelves reproduces the vertical acceleration spectrum observed at quiet seismic sites from 2 to 40 mHz including the small ‘hump’ between 5 and 15 mHz and the observed rise at higher frequencies. The model diverges above 40 mHz because the ‘single frequency’ microseism peak is generated by a different mechanism. The shape of the hum spectrum is controlled primarily by the elastic properties of the Earth with the attenuation structure controlling mode amplitudes. A second calculation shows the excitation of Earth’s normal modes by infragravity waves interacting over the deep ocean basins can also explain the observed spectrum of the Earth’s hum. Infragravity waves are less energetic and the forcing mechanism weaker over deep water, but the integrative effect of forcing over a large area offsets the weaker forcing per unit area. It is likely that both pelagic and shelf ocean wave sources contribute significantly to the forcing of the Earth’s hum and both coastal and pelagic source regions have been identified using seismic techniques. Infragravity wave spectra in deep water over the Pacific basin vary little, thus explaining the small variation in the hum that is observed. However, time varying source regions are also observed in the hum, and may be best explained by intense near shore sources. The great similarity between the spectra predicted from continental shelf forcing and from forcing over the deep ocean basins precludes discriminating between coastal and pelagic sources based on the shape of the spectrum. A more complete knowledge of the climatology of infragravity waves coupled with better seismic observations are needed to resolve this question.

Plasma heating by intense electron beams in fast ignition

Collisionless electron beam–plasma instabilities are expected to play an important role in fast ignition. Such beams are produced by the short high power ignition laser interacting with long scale length plasmas. Here we present results from a one-dimensional Vlasov–Poisson code used to investigate different electron beam temperatures and background plasma conditions. The simulations demonstrate that the beam–plasma instabilities drive large amplitude electrostatic waves that undergo the parametric decay instability driving backwards propagating electrostatic waves and much lower frequency ion acoustic waves. Saturation of the beam–plasma instability creates a plateau in the electron distribution function consistent with quasi-linear theory. We observe the creation of high energy tails in the electron and ion distribution functions, formed by the trapping of particles in the waves formed during the collapse of the beam. The high energy tails of the ion distribution are found to account for up to one-half of the energy gained by the ion population from the beam collapse. Furthermore, at the highest electron beam temperatures we observe the formation of long-lived coherent phase–space structures. These phase–space structures are a direct consequence of the cascade nature of the parametric instability driving up lower wavenumber modes that have higher phase velocities that can in turn accelerate electrons to energies in excess of the initial beam energy. A quasi-linear treatment also shows similar effects but the simulations are clearly beyond a simple quasi-linear treatment and demonstrate the transfer of energy from an incident beam to the ion population via collisionless effects. The implications of these mechanisms for the fast ignition scheme will be discussed.

Rayleigh‐Wave Phase Velocity Distribution in China Continent and its Adjacent Regions

AbstractUsing a two‐station analysis and vertical component records of Rayleigh waves from 102 seismograph stations in China and adjacent areas, we measured inter‐station fundamental mode Rayleigh‐wave phase velocity dispersion in the period band 20~120 s for 538 independent paths. The dispersion data were then used to invert for 21 phase velocity maps from 20 s to 120 s with 5 s interval in the continental China and its adjacent regions (70° ~ 140°E, 18° ~ 55°N). Checkerboard tests show that the lateral resolution is about 3° in central‐eastern China and about 5° in western China and adjacent regions. The spatial distribution of phase velocities in the study area is significantly different for the parts of China to the west and east of approximately 104°E. The phase velocity maps at shorter periods (20~35 s) are influenced by topography and crustal thickness. On these maps, the western part is characterized by low phase velocities and the eastern part shows relatively high phase velocities. The Tarim basin, the Qinghai‐Tibet plateau, and its eastern margin (the Songpan‐Garze block) form the most prominent low velocity structures in the study region. A large‐scale low velocity anomaly also appears in western Mongolia. The Sichuan basin, the Yangtze block, the south‐China block, the Songliao basin, the Sea of Japan, and eastern Mongolia are marked by relatively high phase velocities. At larger periods, the lateral size of the low velocity anomaly beneath central Qinghai‐Tibet becomes smaller whereas the phase velocities increase beneath the Himalayan thrust and Tarim Basin. This implies that the Qinghai‐Tibet low velocity area is surrounded by seismically fast structures to the south, northwest, and east, which may control a southeastern migration of mechanically weak, seismically slow material away from the central plateau. Meanwhile, a large area in the eastern part which appears fast at shorter periods shows relatively low phase velocity, suggesting a relatively thin lithosphere and a pronounced asthenosphere. As the low velocity anomaly beneath Qinghai‐Tibet plateau becomes weaker, the northern Indo‐China block and its adjacent off‐shore areas, East China Sea, Northeast China's deep‐earthquake region in Jilin Province, Sea of Japan, and the Sino‐Korean block to eastern Mongolia are marked by prominent low phase velocity anomalies at the period 120 s. However, high phase velocities appear in the upper Yangtze block (including the Sichuan Basin), which means that the ancient block is relatively stable. The North‐South Seismic Belt in China has relatively low phase velocity from periods 20 s to 120 s, and becomes a natural boundary separating the continental China into the eastern and western parts with different lithospheric phase velocity features.

Temporal coding with synchronization and acceleration as complementary mechanisms

Temporal coding requires an appropriate combination of synchronizing and desynchronizing mechanisms. We study temporal coding with a desynchronizing mechanism, referred to as acceleration, that causes the units of the network to oscillate with higher phase velocity in case of stronger and/or more coherent input. Recently, it was demonstrated that combining synchronization with acceleration implies a competition for coherence that has a profound and favorable effect on the segmentation of overlapping patterns. Here, we demonstrate, first, an effect of coupling non-identical oscillators. Second, we discuss how including inhibitory couplings enforces the segmenting effect of acceleration. Third, we show how stronger couplings between the units of a pattern may support it in winning this competition for coherence.

Simulations of Stimulated Raman Scattering in Low-Density Plasmas

Stimulated Raman scattering (SRS) in a low-density plasma slab isinvestigated by particle-in-cell (PIC) simulations. The backwardstimulated Raman scattering (B-SRS) dominates initially and erodesthe head of the pump wave, while the forward stimulated Ramanscattering (F-SRS) subsequently develops and is located at the rearpart of the slab. Two-stage electron acceleration may be moreefficient due to the coexistence of these two instabilities. TheB-SRS plasma wave with low phase velocities can accelerate thebackground electrons which may be further boosted to higher energiesby the F-SRS plasma wave with high phase velocities. The simulationsshow that the peaks of the main components in both the frequency andwave number spectra occur at the positions estimated from thephase-matching conditions.

Consequences of the radial heterogeneity of the column temperature at high mobile phase velocity

When a high velocity stream of mobile phase percolates through a chromatographic column, the bed cannot remain isothermal. Due to the mobile phase decompression, heat is generated along the column. Longitudinal and radial temperature gradients take place along and across its bed. The various consequences of this thermal heterogeneity are calculated and their effects on the column efficiency investigated for a 0.46cm×25cm stainless steel column packed with 5μm particles. The maximum pressure drop applied was varied from 0.1 to 2kbar. The amplitude of the longitudinal temperature gradient can be estimated on the basis of the integral heat balance equation applied to the whole column and of measurements of the eluent temperature at the column exit. Assuming that the radial gradient is parabolic and the longitudinal gradient linear, the amplitude of the radial gradient can be determined on the basis of the energy balance across the column and of direct measurements of the radial gradient at high inlet pressures. A radial temperature gradient causes a radial distribution of the eluent viscosity, hence of its local velocity. The result is that bands move faster in their center than along the wall, become warped, hence a radial concentration gradient, similar in origin to the one observed in open cylindrical tubes. Diffusion relaxes this gradient. If there is only a longitudinal temperature gradient, the column efficiency would be 30% smaller for a 2kbar pressure drop than if there is no longitudinal temperature gradient. However, when both a longitudinal and a radial temperature gradient coexist, there is a large loss of efficiency. If the influence of the diffusive relaxation of the radial concentration gradient is neglected, the peak shape would be broad and exhibit a marked shoulder.

An optical probe for liquid–liquid two-phase flows

A non-intrusive optical probe has been developed for the identification of flow patterns during liquid–liquid two-phase flow through a conduit. It is based on the difference in optical properties of the respective phases and works on the basis of the proportion of light attenuated and scattered by the two-phase mixture. The measuring system consists of a laser source, a light-dependent resistance (LDR) and a processing circuit. The source and the detector (LDR) are located on opposite sides of the test pipe to detect light after its passage through the test section containing the two-phase mixture. The LDR generates a variable resistance depending on the intensity of light incident on it. The voltage across the resistance is amplified by a three-stage amplifier circuit. The dc output of the circuit is recorded as time series signal and analysed for flow patterns during liquid–liquid flow through horizontal and vertical tubes. The probe provides similar signals for similar phase distributions in both vertical and horizontal pipes and could be applied successfully to differentiate between dispersed and separated flows in the two tube orientations. It has also been observed that the present method of detection is more effective as compared to a parallel wire conductivity probe in identifying flow patterns at high phase velocities.

The Earth’s ‘hum’ is driven by ocean waves over the continental shelves

Observations show that the seismic normal modes of the Earth at frequencies near 10 mHz are excited at a nearly constant level in the absence of large earthquakes. This background level of excitation has been called the 'hum' of the Earth, and is equivalent to the maximum excitation from a magnitude 5.75 earthquake. Its origin is debated, with most studies attributing the forcing to atmospheric turbulence, analogous to the forcing of solar oscillations by solar turbulence. Some reports also predicted that turbulence might excite the planetary modes of Mars to detectable levels. Recent observations on Earth, however, suggest that the predominant excitation source lies under the oceans. Here I show that turbulence is a very weak source, and instead it is interacting ocean waves over the shallow continental shelves that drive the hum of the Earth. Ocean waves couple into seismic waves through the quadratic nonlinearity of the surface boundary condition, which couples pairs of slowly propagating ocean waves of similar frequency to a high phase velocity component at approximately double the frequency. This is the process by which ocean waves generate the well known 'microseism peak' that dominates the seismic spectrum near 140 mHz (refs 11, 12), but at hum frequencies, the mechanism differs significantly in frequency and depth dependence. A calculation of the coupling between ocean waves and seismic modes reproduces the seismic spectrum observed. Measurements of the temporal correlation between ocean wave data and seismic data have confirmed that ocean waves, rather than atmospheric turbulence, are driving the modes of the Earth.