Showing papers on "Amplitude published in 1998"

Evolution of Structure in Cold Dark Matter Universes

Abstract: We present an analysis of the clustering evolution of dark matter in four cold dark matter (CDM) cosmologies. We use a suite of high-resolution, 17 million particle, N-body simulations that sample volumes large enough to give clustering statistics with unprecedented accuracy. We investigate a flat model with Ω0 = 0.3, an open model also with Ω0 = 0.3, and two models with Ω = 1, one with the standard CDM power spectrum and the other with the same power spectrum as the Ω0 = 0.3 models. In all cases, the amplitude of primordial fluctuations is set so that the models reproduce the observed abundance of rich galaxy clusters by the present day. We compute mass two-point correlation functions and power spectra over 3 orders of magnitude in spatial scale and find that in all of our simulations they differ significantly from those of the observed galaxy distribution, in both shape and amplitude. Thus, for any of these models to provide an acceptable representation of reality, the distribution of galaxies must be biased relative to the mass in a nontrivial, scale-dependent fashion. In the Ω = 1 models, the required bias is always greater than unity, but in the Ω0 = 0.3 models, an "antibias" is required on scales smaller than ~5 h-1 Mpc. The mass correlation functions in the simulations are well fit by recently published analytic models. The velocity fields are remarkably similar in all the models, whether they are characterized as bulk flows, single-particle, or pairwise velocity dispersions. This similarity is a direct consequence of our adopted normalization and runs contrary to the common belief that the amplitude of the observed galaxy velocity fields can be used to constrain the value of Ω0. The small-scale pairwise velocity dispersion of the dark matter is somewhat larger than recent determinations from galaxy redshift surveys, but the bulk flows predicted by our models are broadly in agreement with most available data.

FAST satellite observations of large‐amplitude solitary structures

Abstract: We report observations of “fast solitary waves” that are ubiquitous in downward current regions of the mid-altitude auroral zone. The single-period structures have large amplitudes (up to 2.5 V/m), travel much faster than the ion acoustic speed, carry substantial potentials (up to ∼100 Volts), and are associated with strong modulations of energetic electron fluxes. The amplitude and speed of the structures distinguishes them from ion-acoustic solitary waves or weak double layers. The electromagnetic signature appears to be that of an positive charge (electron hole) traveling anti-earthward. We present evidence that the structures are in or near regions of magnetic-field-aligned electric fields and propose that these nonlinear structures play a key role in supporting parallel electric fields in the downward current region of the auroral zone.

Data analysis of gravitational-wave signals from spinning neutron stars. I. The signal and its detection

Abstract: We present a theoretical background for the data analysis of the gravitational-wave signals from spinning neutron stars for Earth-based laser interferometric detectors. We introduce a detailed model of the signal including both the frequency and the amplitude modulations. We include the effects of the intrinsic frequency changes and the modulation of the frequency at the detector due to Earth's motion. We estimate the effects of the star's proper motion and of relativistic corrections. Moreover we consider a signal consisting of two components corresponding to a frequency $f$ and twice that frequency. From the maximum likelihood principle we derive the detection statistics for the signal and we calculate the probability density function of the statistics. We obtain the data analysis procedure to detect the signal and to estimate its parameters. We show that for optimal detection of the amplitude modulated signal we need four linear filters instead of one linear filter needed for a constant amplitude signal. Searching for the doubled frequency signal increases further the number of linear filters by a factor of 2. We indicate how the fast Fourier transform algorithm and resampling methods commonly proposed in the analysis of periodic signals can be used to calculate the detection statistics for our signal. We find that the probability density function of the detection statistics is determined by one parameter: the optimal signal-to-noise ratio. We study the signal-to-noise ratio by means of the Monte Carlo method for all long-arm interferometers that are currently under construction. We show how our analysis can be extended to perform a joint search for periodic signals by a network of detectors and we perform a Monte Carlo study of the signal-to-noise ratio for a network of detectors.

A new measure for cosmic shear

Abstract: Cosmic shear, i.e. the distortion of images of high-redshift galaxies through the tidal gravitational field of the large-scale matter distribution in the Universe, offers the opportunity to measure the power spectrum of the cosmic density fluctuations without any reference to the relation of dark matter to luminous tracers. We consider here a new statistical measure for cosmic shear, the aperture mass Map(θ), which is defined as a spatially filtered projected density field and which can be measured directly from the image distortions of high-redshift galaxies. By selecting an appropriate spatial filter function, the dispersion of the aperture mass is a convolution of the power spectrum of the projected density field with a narrow kernel, so that 〈M2ap(θ)〉 provides a well-localized estimate of the power spectrum at wavenumbers s ∼ 5/θ. We calculate 〈M2ap〉 for various cosmological models, using the fully non-linear power spectrum of the cosmic density fluctuations. The non-linear evolution yields a significant increase of 〈M2ap〉 relative to the linear growth on scales below ∼ 0°.5. The third-order moment of Map can be used to define a skewness, which is a measure of the non-Gaussianity of the density field. We present the first calculation of the skewness of cosmic shear in the framework of the quasi-linear theory of structure growth. We show that it yields a sensitive measure of the cosmological model; in particular, it is independent of the normalization of the power spectrum. Several practical estimates for 〈M2ap〉 are constructed and their dispersions calculated. On scales below a few arcminutes, the intrinsic ellipticity distribution of galaxies is the dominant source of noise, whereas on larger scales the cosmic variance becomes the most important contribution. We show that measurements of Map in two adjacent apertures are virtually uncorrelated, which implies that an image with side-length L can yield [L/(2θ)]2 mutually independent estimates for Map. We show that one square degree of a high-quality image is sufficient to detect the cosmic shear with the Map-statistic on scales below ∼ 10 arcmin, and to estimate its amplitude with an accuracy of ∼ 30 per cent on scales below ∼ 5 arcmin.

Ultrasonic Band Gap in a Periodic Two-Dimensional Composite.

Abstract: The first experimentally observed ultrasonic full band gap in periodic bidimensional composites for the longitudinal wave mode is described in this Letter. The structure consists of an aluminum alloy plate with a square periodic arrangement of cylindrical holes filled with mercury. No propagation wave exists at the frequency range between 1000--1120 kHz irrespective of the measurement direction. The experiment was performed by means of an ultrasonic transmission technique, and a measurement of the position dependence of the acoustic amplitude was also performed.

Bipolar electrostatic structures in the shock transition region: Evidence of electron phase space holes

Abstract: We present observations of intense, bipolar, electrostatic structures in the transition region of the terrestrial bow shock from the Wind spacecraft. The electric field signatures are on the order of a tenth of a millisecond in duration and greater than 100 mV/m in amplitude. The measured electric field is generally larger on the smaller dipole antenna, indicating a small spatial size. We compare the potential on the two dipole antennas with a model of antenna response to a Gaussian potential profile. This result agrees with a spatial scale determined by convection and gives a characteristic scale size of 2–7 λd. We interpret the observations as small scale convecting unipolar potential structures, consistent with simulations of electron phase space holes and discuss the results in the context of electron thermalization at strong collisionless shocks.

XFROG — A New Method for Amplitude and Phase Characterization of Weak Ultrashort Pulses

Abstract: We present a new method to characterize the amplitude and phase of weak ultrashort pulses. Our method is based on the spetrally resolved crosscorrelation signal of a weak test pulse with a fully characterized intensive reference pulse and requires no spectral overlap between the signal and the reference. To retrieve the amplitude and phase of the test pulse, we use an iterative Fourier transform algorithm with generalized projections.

Multiscale pulse dynamics in communication systems with strong dispersion management.

Abstract: The evolution of an optical pulse in a strongly dispersion-managed fiber-optic communication system is studied. The pulse is decomposed into a fast phase and a slowly evolving amplitude. The fast phase is calculated exactly, and a nonlocal equation for the evolution of the amplitude is derived. In the limit of weak dispersion management the equation reduces to the nonlinear Schrodinger equation. A class of stationary solutions of this equation is obtained; they represent pulses with a Gaussian-like core and exponentially decaying oscillatory tails, and they agree with direct numerical solutions of the full system.

Coherence resonance in a Hodgkin-Huxley neuron

Abstract: We study the nonlinear response of the Hodgkin-Huxley model without external periodic signal to the noisy synaptic current near the saddle-node bifurcation of limit cycles. The coherence of the system, estimated from the interspike interval histogram and from the power spectra of membrane potentials and spike trains, is maximal at a certain noise intensity, so that the coherence resonance occurs. The mechanism of this phenomenon is found to be different from previously studied models of coherence resonance and explained in terms of rigid excitations of periodic oscillations, and the combined effect of amplitude and phase fluctuations.

Waveform inversion of very-long-period impulsive signals associated with magmatic injection beneath Kilauea volcano, Hawaii

Abstract: We use data from broadband seismometers deployed around the summit of Kilauea Volcano to quantify the mechanism associated with a transient in the flow of magma feeding the east rift eruption of the volcano. The transient is marked by rapid inflation of the Kilauea summit peaking at 22 μrad 4.5 hours after the event onset, followed by slow deflation over a period of 3 days. Superimposed on the summit inflation is a series of sawtooth displacement pulses, each characterized by a sudden drop in amplitude lasting 5–10 s followed by an exponential recovery lasting 1–3 min. The sawtooth waveforms display almost identical shapes, suggesting a process involving the repeated activation of a fixed source. The particle motion associated with each sawtooth is almost linear, and its major swing shows compressional motion at all stations. Analyses of semblance and particle motion are consistent with a point source located 1 km beneath the northeast edge of the Halemaumau pit crater. To estimate the source mechanism, we apply a moment tensor inversion to the waveform data, assuming a point source embedded in a homogeneous half-space with compressional and shear wave velocities representative of the average medium properties at shallow depth under Kilauea. Synthetic waveforms are constructed by a superposition of impulse responses for six moment tensor components and three single force components. The origin times of individual impulses are distributed along the time axis at appropriately small, equal intervals, and their amplitudes are determined by least squares. In this inversion, the source time functions of the six tensor and three force components are determined simultaneously. We confirm the accuracy of the inversion method through a series of numerical tests. The results from the inversion show that the waveform data are well explained by a pulsating transport mechanism operating on a subhorizontal crack linking the summit reservoir to the east rift of Kilauea. The crack acts like a buffer in which a batch of fluid (magma and/or gas) accumulates over a period of 1–3 min before being rapidly injected into a larger reservoir (possibly the east rift) over a timescale of 5–10 s. The seismic moment and volume change associated with a typical batch of fluid are approximately 1014 N m and 3000 m3, respectively. Our results also point to the existence of a single force component with amplitude of 109 N, which may be explained as the drag force generated by the flow of viscous magma through a narrow constriction in the flow path. The total volume of magma associated with the 4.5-hour-long activation of the pulsating source is roughly 500,000 m3 in good agreement with the integrated volume flow rate of magma estimated near the eruptive site.

Amplification using amplitude reconstruction of amplitude and/or angle modulated carrier

Abstract: The present invention provides high power linear amplification of an amplitude and/or phase modulated signal using multiple saturated (or if desired, unsaturated) amplifiers (4) driven by an appropriate set of switched (5), and/or phase modulated (6) constant amplitude signals derived from the input signal. The present invention combines three amplitude reconstruction techniques and implements the amplitude reconstruction modulator (11) digitally.

Explicitly Covariant Light-Front Dynamics and Relativistic Few-Body Systems

Abstract: The wave function of a composite system is defined in relativity on a space-time surface. In the explicitly covariant light-front dynamics, reviewed in the present article, the wave functions are defined on the plane $\omega \cd x=0$, where $\omega$ is an arbitrary four-vector with $\omega^2=0$. The standard non-covariant approach is recovered as a particular case for $\omega = (1,0,0,-1)$. Using the light-front plane is of crucial importance, while the explicit covariance gives strong advantages emphasized through all the review. The properties of the relativistic few-body wave functions are discussed in detail and are illustrated by examples in a solvable model. The three-dimensional graph technique for the calculation of amplitudes in the covariant light-front perturbation theory is presented. The structure of the electromagnetic amplitudes is studied. We investigate the ambiguities which arise in any approximate light-front calculations, and which lead to a non-physical dependence of the electromagnetic amplitude on the orientation of the light-front plane. The elastic and transition form factors free from these ambiguities are found for spin 0, 1/2 and 1 systems. The formalism is applied to the calculation of the relativistic wave functions of two-nucleon systems (deuteron, scattering state), with particular attention to the role of their new components in the deuteron elastic and electrodisintegration form factors and to their connection with meson exchange currents. Straigthforward applications to the pion and nucleon form factors and the $\rho-\pi$ transition are also made.

Experimental study of interfacial solitary waves

Abstract: A small-scale experiment was conducted (in a 3 m long flume) to study interfacial long-waves in a two-immiscible-fluid system (water and petrol were used). Experiments and nonlinear theories are compared in terms of wave profiles, phase velocity and mainly frequency--amplitude relationships. As expected, the KdV solitary waves match the experiments for small-amplitude waves for all layer thickness ratios. The characteristics of 'large'-amplitude waves (that is when the crest is close to the critical level - approximately located at mid-depth) asymptotically tend to be predicted by a 'KdV-mKdV' equation containing both quadratic and cubic nonlinear terms. In addition a numerical solution of the complete Euler equations, based on Fourier series expansions, is devised to describe solitary waves of intermediate amplitude. In all cases, solitary interfacial waves in this numerical theory tally with the experimental data. When the layer thicknesses are almost equal (ratio of lower layer to total depth equal to 0.4 or 0.63) both the KdV-mKdV and the numerical solutions match the experimental points.

Inversion of surface NMR data

Abstract: The main advantage of the surface nuclear magnetic resonance (NMR) method compared to other geophysical methods in the field of groundwater investigation is the ability to measure an NMR signal directly from the water molecules An NMR signal stimulated by an alternating current pulse through an antenna at the surface, confirms the existence of water in the subsurface with a high degree of reliability The NMR signal amplitude depends on the pulse parameter (the product of the pulse amplitude and its duration), bulk water volume, and water depth Measurements are performed while varying the pulse parameter, and subsequent data processing reveals the number of water‐saturated layers, and data concerning their depth, thickness, and water content One of the major problems in the practical application of the NMR method is the very weak signal (<3000 nV): hence the problem of signal to noise ratio (S/N) S/N can be improved by stacking the signal, but measurement time is increased We have developed an algori

Time-resolved studies of stick-slip friction in sheared granular layers

Abstract: Sensitive and fast force measurements are performed on sheared granular layers undergoing stick-slip motion, along with simultaneous optical imaging. A full study has been done for spherical glass particles with a 20% size distribution. Stick-slip motion due to repetitive fluidization of the granular layer occurs for low driving velocities. Between major slip events, slight creep occurs that is highly variable from one event to the next. The effects of varying the stiffness k of the driving system and the driving velocity V are studied in detail. The stick-slip motion is almost periodic for spherical particles over a wide range of parameters, whereas it becomes irregular when k is large and V is relatively small. At larger V, the motion becomes smoother and is affected by the inertia of the upper plate bounding the layer. Measurements of the period and amplitude of the relative motion are presented as a function of V. At a critical value ${V}_{c}$ a transition to continuous sliding motion occurs. The transition is discontinuous for k not too large, and large fluctuations occur in the neighborhood of the transition. The time dependence of the instantaneous velocity of the upper plate and the frictional force produced by the granular layer are determined within individual slipping events. The frictional force is found to be a multivalued function of the instantaneous velocity during slip, with pronounced hysteresis and a sudden drop just prior to resticking. Measurements of vertical displacement reveal a very small dilation of the material (about one-tenth of the mean particle size in a layer 20 particles deep) associated with each slip event; the dilation reaches its maximum amplitude close to the time of maximum acceleration. Finally, optical imaging reveals that localized microscopic rearrangements precede (and follow) each macroscopic slip event; their number is highly variable and the accumulation of these local displacements is associated with macroscopic creep. The behavior of smooth particles is contrasted qualitatively with that of rough particles.

Measurement of the Amplitude and Phase of a Sculpted Rydberg Wave Packet

Abstract: We have completely determined the amplitude and phase of a quantum wave packet. The wave packet was shaped using a programmable optical pulse. Phase information comes from analysis of covariant fluctuations due to interference between the wave packet and a well-characterized reference.

Nonlinear viscoelastic behavior of sedimentary rocks, Part I: Effect of frequency and strain amplitude

Abstract: Sedimentary rocks display nonlinear elastic behavior. This nonlinearity is a strong function of frequency, strain amplitude, and the properties of the saturating fluid. Experimental observations and potential mechanisms that cause these nonlinearities are presented in this and a companion paper. Young’s moduli and Poisson’s ratios obtained from ultrasonic laboratory measurements (50 kHz, 100 kHz, 180kHz and 1 MHz), low‐frequency measurements (1–2000 Hz) and static measurements (0.001–0.05 Hz) show significant differences under identical stress conditions. A comparison of the laboratory‐measured quantities with log‐derived moduli measured at 20 kHz indicates that Eultrasonic>Elog>Elowfreq>Estatic. This shows clearly that a wide variety of sandstones demonstrate frequency‐dependent elastic behavior (viscoelastic behavior) over a range of frequencies. Differences between static (low‐frequency, high‐strain amplitude) velocities and ultrasonic velocities can be explained partially by differences in frequency a...

Time-Domain Analysis of Large-Amplitude Vertical Ship Motions and Wave Loads

Abstract: The vertical motions and wave induced loads on ships with forward speed are studied in the time domain, considering non-linear effects associated with large amolitude motions and hull flare shape. The method is based on a strip theory, using singularities distributed on the cross sections which satisfy the linear free surface condition. The solution is obtained in the time domain using convolution to account for the memory effects related to the free surface oscillations. In this way the linear radiation forces are represented in terms of impulse response functions, infinite frequency added masses and radiation restoring coefficients. The diffraction forces associated with incident wave scattering are linear. The hydrostatic and Froude-Krylov forces are evaluated over the instantaneous wetted surface of the hull to account for the large amplitude motions and hull flare. The radiation contribution for wave loads is also obtained in the time domain usine convolution to account for the memory effects related to the free surface oscillations. Results of motions and wave loads for the S175 container ship are presented and analyzed. The results from the present method are compared with linear results.

Beam modes beyond the paraxial approximation: A scalar treatment

Abstract: Beam modes are considered by a high-aperture scalar theory based on the complex source-point method. A different form for the beam mode that avoids nonphysical singularities in intensity is introduced. The amplitude in the focal region is explored for the lowest-order mode. Conditions for oscillation of a high-aperture resonator, such as a microcavity, are derived. The amplitude in the focal region is explored for higher-order modes in systems with cylindrical symmetry. These modes are expressed in terms of Jacobi polynomials. @S1050-2947~98!01103-2#

Bounds on the Compactness of Neutron Stars from Brightness Oscillations during X-Ray Bursts

Abstract: The discovery of high-amplitude brightness oscillations at the spin frequency or its first overtone in six neutron stars in low-mass X-ray binaries during type I X-ray bursts provides a powerful new way to constrain the compactness of these stars and hence to constrain the equation of state of the dense matter in all neutron stars. Here we report general relativistic calculations of the maximum fractional rms amplitudes that can be observed during bursts, as a function of stellar compactness. We compute the dependence of the oscillation amplitude on the compactness of the star, on the angular dependence of the emission from the surface, on the rotational velocity at the stellar surface, and on whether there are one or two emitting spots. We show that color oscillations caused by the spectral variation with the angle of emission, the rotation of the star, and the limited bandwidth of the detector all tend to increase the observed amplitude of the oscillation. Nevertheless, if two spots are emitting, as appears to be the case in 4U 1636-536 and KS 1731-26, very restrictive bounds on the compactness of the star can be derived.

Maximum likelihood track-before-detect with fluctuating target amplitude

Abstract: An important problem in target tracking is the detection and tracking of targets in very low signal-to-noise ratio (SNR) environments. In the past, several approaches have been used, including maximum likelihood. The major novelty of this work is the incorporation of a model for fluctuating target amplitude into the maximum likelihood approach for tracking of constant velocity targets. Coupled with a realistic sensor model, this allows the exploitation of signal correlation between resolution cells in the same frame, and also from one frame to the next. The fluctuating amplitude model is a first order model to reflect the inter-frame correlation. The amplitude estimates are obtained using a Kalman filter, from which the likelihood function is derived. A numerical maximization technique avoids problems previously encountered in "velocity filtering" approaches due to mismatch between assumed and actual target velocity, at the cost of additional computation. The Cramer-Rao lower bound (CRLB) is derived for a constant, known amplitude case. Estimation errors are close to this CRLB even when the amplitude is unknown. Results show track detection performance for unknown signal amplitude is nearly the same as that obtained when the correct signal model is used.

Investigation of Tuned Liquid Dampers under Large Amplitude Excitation

Abstract: The behavior of tuned liquid dampers (TLD) was investigated through laboratory experiments and numerical modeling. Large amplitude excitation is the primary focus, as previous research was limited to small amplitude motion. Time histories of the base shear force and water-surface variations were measured by precisely controlled shaking table tests. The results are compared with a numerical model. The random-choice numerical method was used to solve the fully nonlinear shallow-water wave equations. The results suggest that the model captures the underlying physical phenomenon adequately, including wave breaking, for most of the frequency range of interest and over a wide range of amplitude excitation. It was found that the response frequency of tuned liquid dampers increases as excitation amplitude increases, and the TLD behaves as a hardening spring system. To achieve the most robust system, the design frequency for the damper, if it is computed by the linearized water-wave theory, should be set at the value lower than that of the structure response frequency; hence, the actual nonlinear frequency of the damper matches the structural response. It was found that, even if the damper frequency had been mistuned slightly, the TLD always performed favorably; we observed no adverse effect in the wide range of experimental parameters tested in this study.

Gravity-Modes in ZZ Ceti Stars: I.Quasiadiabatic Analysis of Overstability

Abstract: We analyze the stability of g-modes in variable white dwarfs with hydrogen envelopes. In these stars, the radiative layer contributes to mode damping because its opacity decreases upon compression and the amplitude of the Lagrangian pressure perturbation increases outward. The overlying convective envelope is the seat of mode excitation because it acts as an insulating blanket with respect to the perturbed flux that enters it from below. A crucial point is that the convective motions respond to the instantaneous pulsational state. Driving exceeds damping by as much as a factor of two provided $\omega\tau_c\geq 1$, where $\omega$ is the radian frequency of the mode and $\tau_c\approx 4\tau_{th}$ with $\tau_{th}$ being the thermal time constant evaluated at the base of the convective envelope. As a white dwarf cools, its convection zone deepens, and modes of lower frequency become overstable. However, the deeper convection zone impedes the passage of flux perturbations from the base of the convection zone to the photosphere. Thus the photometric variation of a mode with constant velocity amplitude decreases. These factors account for the observed trend that longer period modes are found in cooler DAVs. The linear growth time, ranging from hours for the longest period observed modes ($P\approx 20$ minutes) to thousands of years for those of shortest period ($P\approx 2 $ minutes), probably sets the time-scale for variations of mode amplitude and phase. This is consistent with observations showing that longer period modes are more variable than shorter period ones. Our investigation confirms many results obtained by Brickhill in his pioneering studies of ZZ Cetis.

Nonlinear standing waves in an acoustical resonator

Abstract: A one-dimensional model is developed to analyze nonlinear standing waves in an acoustical resonator. The time domain model equation is derived from the fundamental gasdynamics equations for an ideal gas. Attenuation associated with viscosity is included. The resonator is assumed to be of an axisymmetric, but otherwise arbitrary shape. In the model the entire resonator is driven harmonically with an acceleration of constant amplitude. The nonlinear spectral equations are integrated numerically. Results are presented for three geometries: a cylinder, a cone, and a bulb. Theoretical predictions describe the amplitude related resonance frequency shift, hysteresis effects, and waveform distortion. Both resonance hardening and softening behavior are observed and reveal dependence on resonator geometry. Waveform distortion depends on the amplitude of oscillation and the resonator shape. A comparison of measured and calculated wave shapes shows good agreement.

Internal wave reflection and mixing at Fieberling Guyot

Abstract: The structure of internal wave reflection off a sloping bottom on the steep flank of a tall North Pacific Ocean seamount is observed in year-long moored array records to differ substantially from the form predicted by linear theory, although linear theory accounts for several qualitative features of the process. This study documents new features of wave reflection as described below. Wave reflection is detectable as far as 750 m above the bottom. Motions are dominated by a single empirical mode whose phase structure obeys linear internal wave dispersion but whose amplitude decays with a scale comparable to the wavenumber. While the dominant mode has scales appropriate to the reflection of a first baroclinic mode wave incident from the open ocean, its decay from the bottom is such that current vectors in the vertical plane rotate clockwise in time when viewed with shallow water to the right. This flow resembles the lower half of the deepest cell pattern predicted by linear reflection from a uniform slope. At the local internal wave critical frequency, the dominant mode has nearly a vanishing wavenumber rather than the infinite wavenumber predicted by linear reflection. Reflected waves are aligned parallel to the bottom slope measured on wave spatial scales, rather than shorter ones. Wave reflection causes large, frequent density overturns, implying mixing. The rate and strength of these overturns imply a local vertical eddy viscosity of 2–6 × 10−4 m2/s over the bottom few hundred meters. The contribution of bottom-intensified mixing to the open deep ocean is roughly equivalent to that found in situ, although reflection from gentler slopes or at lower latitude may produce greater contribution from internal wave-reflection-induced mixing.

Solitary waves and weak double layers in a two‐electron temperature auroral plasma

Abstract: We show that the main characteristics of ion acoustic solitary waves and weak double layers observed by the Swedish satellite Viking can be well reproduced assuming the presence of two electron components in the auroral plasma. The characteristics of the ion acoustic solitons excited in such a plasma are derived with the help of the Sagdeev potential. The results show that the interactions between the hot and the cold electron component in the presence of a finite ion temperature produce rarefactions of the localized density. Such nonlinear structures exist in a more extended range of plasma parameters than the one previously studied in the small amplitude limit case using the Korteweg-de-Vries equation. We find that the density of the cold population must be always smaller than the hot one, while the hot to cold temperature ratio must be greater than ∼ 10. The characteristics of these structures are quite different from those obtained in the small amplitude limit case and better reproduce the Viking observations in terms of their velocity, width, and amplitude scales.

Particle-core model for transverse dynamics of beam halo

Abstract: The transverse motion of beam halo particles is described by a particle-core model which uses the space-charge field of a continuous cylindrical oscillating beam core in a uniform linear focusing channel to provide the force that drives particles to large amplitudes. The model predicts a maximum amplitude for the resonantly-driven particles as a function of the initial mismatch. We have calculated these amplitude limits and have estimated the growth times for extended-halo formation as a function of both the space-charge tune-depression ratio and a mismatch parameter. We also present formulas for the scaling of the maximum amplitudes as a function of the beam parameters. The model results are compared with multiparticle simulations and we find very good agreement for a variety of initial particle distributions.