Showing papers on "Amplitude published in 2000"
TL;DR: In this article, the transverse vortex-induced vibrations of an elastically mounted rigid cylinder in a fluid flow were studied for the first time in free vibrations, and the existence of more than one mode transition for low (m*ζ) and high (m *δ) combined mass-damping parameters was analyzed.
Abstract: In this paper, we study the transverse vortex-induced vibrations of an elastically mounted rigid cylinder in a fluid flow. We use simultaneous force, displacement and vorticity measurements (using DPIV) for the first time in free vibrations. There exist two distinct types of response in such systems, depending on whether one has a high or low combined mass–damping parameter (m*ζ). In the classical high-(m*ζ) case, an ‘initial’ and ‘lower’ amplitude branch are separated by a discontinuous mode transition, whereas in the case of low (m*ζ), a further higher-amplitude ‘upper’ branch of response appears, and there exist two mode transitions.To understand the existence of more than one mode transition for low (m*ζ), we employ two distinct formulations of the equation of motion, one of which uses the ‘total force’, while the other uses the ‘vortex force’, which is related only to the dynamics of vorticity. The first mode transition involves a jump in ‘vortex phase’ (between vortex force and displacement), ϕvortex, at which point the frequency of oscillation (f) passes through the natural frequency of the system in the fluid, f ∼ fNwater. This transition is associated with a jump between 2S [harr ] 2P vortex wake modes, and a corresponding switch in vortex shedding timing. Across the second mode transition, there is a jump in ‘total phase’, phis;total , at which point f ∼ fNvacuum. In this case, there is no jump in ϕvortex, since both branches are associated with the 2P mode, and there is therefore no switch in timing of shedding, contrary to previous assumptions. Interestingly, for the high-(m*ζ) case, the vibration frequency jumps across both fNwater and fNvacuum, corresponding to the simultaneous jumps in ϕvortex and ϕtotal. This causes a switch in the timing of shedding, coincident with the ‘total phase’ jump, in agreement with previous assumptions.For large mass ratios, m* = O(100), the vibration frequency for synchronization lies close to the natural frequency (f* = f/fN ≈ 1.0), but as mass is reduced to m* = O(1), f* can reach remarkably large values. We deduce an expression for the frequency of the lower-branch vibration, as follows:formula herewhich agrees very well with a wide set of experimental data. This frequency equation uncovers the existence of a critical mass ratio, where the frequency f* becomes large: m*crit = 0.54. When m* < m*crit, the lower branch can never be reached and it ceases to exist. The upper-branch large-amplitude vibrations persist for all velocities, no matter how high, and the frequency increases indefinitely with flow velocity. Experiments at m* < m*crit show that the upper-branch vibrations continue to the limits (in flow speed) of our facility.
775 citations
TL;DR: In this article, the authors study galaxy clustering using halo models, where gravitational clustering is described in terms of dark matter halos, and find that the galaxy power spectrum shows a power-law behavior even though the dark matter does not, and the galaxy higher-order correlations show smaller amplitudes at small scales than their dark matter counterparts.
Abstract: We study galaxy clustering using halo models, where gravitational clustering is described in terms of dark matter halos. At small scales, clustering statistics are dominated by halo density profiles, whereas at large scales, correlations are the result of combining non-linear perturbation theory with halo biasing. Galaxies are assumed to follow the dark matter profiles, and galaxy formation efficiency is given by the number of galaxies as a function of halo mass. This approach leads to generic predictions: the galaxy power spectrum shows a power-law behavior even though the dark matter does not, and the galaxy higher-order correlations show smaller amplitudes at small scales than their dark matter counterparts, as observed in galaxy catalogs. We find that requiring to fit both the second and third order moments of the APM galaxies provides a strong constraint on galaxy formation models. The data at large scales require that galaxy formation be relatively efficient at small masses, m =10^10 Msun/h, whereas data at smaller scales require that the number of galaxies in a halo scale as the mass to the 0.8th power in the high-mass limit. These constraints are independent of those derived from the luminosity function or Tully-Fisher relation. We also predict the power spectrum, bispectrum, and higher-order moments of the mass density field. Although halo models agree well with measurements of the mass power spectrum and the higher order Sp parameters in N-body simulations, the model assumption that halos are spherical leads to disagreement in the configuration dependence of the bispectrum at small scales. We stress the importance of finite volume effects in higher-order statistics and show how they can be estimated in this approach.
506 citations
Patent•
06 Jan 2000TL;DR: In this article, an intelligent, rule-based processor provides a pulse indicator designating the occurrence of each pulse in a pulse oximeter-derived photo-plethysmograph waveform.
Abstract: An intelligent, rule-based processor provides a pulse indicator designating the occurrence of each pulse in a pulse oximeter-derived photo-plethysmograph waveform. When there is relatively no distortion corrupting the plethysmograph signal, the processor analyzes the shape of the pulses in the waveform to determine where in the waveform to generate the pulse indication. When distortion is present, looser waveform criteria are used to determine if pulses are present. If pulses are present, the pulse indication is based upon an averaged pulse rate. If no pulses are present, no indication occurs. The pulse indicator provides a trigger and amplitude output. The trigger output is used to initiate an audible tone “beep” or a visual pulse indication on a display, such as a vertical spike on a horizontal trace or a corresponding indication on a bar display. The amplitude output is used to indicate data integrity and corresponding confidence in the computed values of saturation and pulse rate. The amplitude output can vary a characteristic of the pulse indicator, such as beep volume or frequency or the height of the visual display spike.
432 citations
TL;DR: The authors develop a method to decompose a laser altimeter return waveform into a series of components assuming that the position of each component within the waveform can be used to calculate the mean elevation of a specific reflecting surface within the laser footprint.
Abstract: The authors develop a method to decompose a laser altimeter return waveform into a series of components assuming that the position of each component within the waveform can be used to calculate the mean elevation of a specific reflecting surface within the laser footprint. For simplicity, they assume each component is Gaussian in nature. They estimate the number of Gaussian components from the number of inflection points of a smoothed copy of the laser waveform and obtain initial estimates of the Gaussian half-widths and positions from the positions of its consecutive inflection points. Initial amplitude estimates are obtained using a nonnegative least-squares method (LSM). To reduce the likelihood of fitting the background noise within the waveform and to minimize the number of Gaussians needed in the approximation, we rank the "importance" of each Gaussian in the decomposition using its initial half-width and amplitude estimates. The initial parameter estimates of all Gaussians ranked "important" are optimized using the Levenburg-Marquardt method. If the sum of the Gaussians does not approximate the return waveform to a prescribed accuracy, then additional Gaussians can be included in the optimization procedure or initial parameters can be recalculated. The Gaussian decomposition method is demonstrated on data collected by the airborne laser vegetation imaging sensor (LVIS) in October 1997 over the Sequoia National Forest, California.
355 citations
TL;DR: These equations are used along with the T-matrix method to provide benchmark results for homogeneous, dielectric, rotationally symmetric particles.
Abstract: General equations are derived for computing the amplitude matrix for a nonspherical particle in an arbitrary orientation and for arbitrary illumination and scattering directions with respect to the laboratory reference frame, provided that the scattering problem can be solved with respect to the particle reference frame. These equations are used along with the T-matrix method to provide benchmark results for homogeneous, dielectric, rotationally symmetric particles. The computer code is publicly available on the World-Wide Web at http://www.giss.nasa.gov/~crmim.
314 citations
TL;DR: The cerebral magnetic field of the auditory steady-state response (SSR) to sinusoidal amplitude-modulated (SAM) tones was recorded in healthy humans and waveforms of underlying cortical source activity were calculated at multiples of the modulation frequency to improve the signal-to-noise ratio (SNR).
Abstract: The cerebral magnetic field of the auditory steady-state response (SSR) to sinusoidal amplitude-modulated (SAM) tones was recorded in healthy humans. The waveforms of underlying cortical source activity were calculated at multiples of the modulation frequency using the method of source space projection, which improved the signal-to-noise ratio (SNR) by a factor of 2 to 4. Since the complex amplitudes of the cortical source activity were independent of the sensor position in relation to the subject’s head, a comparison of the results across experimental sessions was possible. The effect of modulation frequency on the amplitude and phase of the SSR was investigated at 30 different values between 10 and 98 Hz. At modulation frequencies between 10 and 20 Hz the SNR of harmonics near 40 Hz were predominant over the fundamental SSR. Above 30 Hz the SSR showed an almost sinusoidal waveform with an amplitude maximum at 40 Hz. The amplitude decreased with increasing modulation frequency but was significantly different from the magnetoencephalographic (MEG) background activity up to 98 Hz. Phase response at the fundamental and first harmonic decreased monotonically with increasing modulation frequency. The group delay (apparent latency) showed peaks of 72 ms at 20 Hz, 48 ms at 40 Hz, and 26 ms at 80 Hz. The effects of stimulus intensity, modulation depth, and carrier frequency on amplitude and phase of the SSR were also investigated. The SSR amplitude decreased linearly when stimulus intensity or the modulation depth were decreased in logarithmic steps. SSR amplitude decreased by a factor of 3 when carrier frequency increased from 250 to 4000 Hz. From the phase characteristics, time delays were found in the range of 0 to 6 ms for stimulus intensity, modulation depth, and carrier frequency, which were maximal at low frequencies, low intensities, or maximal modulation depth.
296 citations
TL;DR: In this paper, an analytical dispersion relation is derived which shows that, in toroidal plasmas, zonal flows can be spontaneously excited via modulations in the radial envelope of a single-n coherent drift wave, with n the toroidal mode number.
Abstract: An analytical dispersion relation is derived which shows that, in toroidal plasmas, zonal flows can be spontaneously excited via modulations in the radial envelope of a single-n coherent drift wave, with n the toroidal mode number. Predicted instability features are verified by three-dimensional global gyrokinetic simulations of the ion-temperature-gradient mode. Nonlinear equations for mode amplitudes demonstrate saturation of the linearly unstable pump wave and nonlinear oscillations of the drift-wave intensity and zonal flows, with a parameter-dependent period doubling route to chaos.
278 citations
TL;DR: The observation of so-called ‘localized clusters’—periodic antiphase oscillations in one part of the medium, while the remainder appears uniform—in the Belousov–Zhabotinsky reaction–diffusion system with photochemical global feedback is reported.
Abstract: Oscillatory clusters are sets of domains in which nearly all elements in a given domain oscillate with the same amplitude and phase1,2,3,4. They play an important role in understanding coupled neuron systems5,6,7,8. In the simplest case, a system consists of two clusters that oscillate in antiphase and can each occupy multiple fixed spatial domains. Examples of cluster behaviour in extended chemical systems are rare, but have been shown to resemble standing waves9,10,11,12,13, except that they lack a characteristic wavelength. Here we report the observation of so-called ‘localized clusters’—periodic antiphase oscillations in one part of the medium, while the remainder appears uniform—in the Belousov–Zhabotinsky reaction–diffusion system with photochemical global feedback. We also observe standing clusters with fixed spatial domains that oscillate periodically in time and occupy the entire medium, and irregular clusters with no periodicity in either space or time, with standing clusters transforming into irregular clusters and then into localized clusters as the strength of the global negative feedback is gradually increased. By incorporating the effects of global feedback into a model of the reaction, we are able to simulate successfully the experimental data.
254 citations
TL;DR: The Mathematica package FeynArts as discussed by the authors is used for the generation and visualization of Feynman diagrams and amplitudes, with three levels, user-defined model files, and support for supersymmetric models.
Abstract: This paper describes the Mathematica package FeynArts used for the generation and visualization of Feynman diagrams and amplitudes. The main features of version 3 are: generation of diagrams at three levels, user-definable model files, support for supersymmetric models, and publication-quality Feynman diagrams in PostScript or LaTeX.
252 citations
TL;DR: The superior detection ability facilitates the collection of a training set under lower SNR than that of the methods which employ simple amplitude thresholding, so that the statistical characteristics of the input vectors can be better represented in the neural-network classifier.
Abstract: Reports a result on neural spike sorting under conditions where the signal-to-noise ratio is very low. The use of nonlinear energy operator enables the detection of an action potential, even when the SNR is so poor that a typical amplitude thresholding method cannot be applied. The superior detection ability facilitates the collection of a training set under lower SNR than that of the methods which employ simple amplitude thresholding. Thus, the statistical characteristics of the input vectors can be better represented in the neural-network classifier The trained neural-network classifiers yield the correct classification ratio higher than 90% when the SNR is as low as 1.2 (0.8 dB) when applied to data obtained from extracellular recording from Aplysia abdominal ganglia using a semiconductor microelectrode array.
252 citations
TL;DR: In this article, a Boussinesq-type model is derived for a horizontal bottom, and is based explicitly on a fourth-order polynomial representation of the vertical dependence of the velocity potential.
Abstract: A Boussinesq-type model is derived which is accurate to O(kh) 4 and which retains the full representation of the fluid kinematics in nonlinear surface boundary condition terms, by not assuming weak nonlinearity. The model is derived for a horizontal bottom, and is based explicitly on a fourth-order polynomial representation of the vertical dependence of the velocity potential. In order to achieve a (4,4) Pade representation of the dispersion relationship, a new dependent variable is defined as a weighted average of the velocity potential at two distinct water depths. The representation of internal kinematics is greatly improved over existing O(kh) 2 approximations, especially in the intermediate to deep water range. The model equations are first examined for their ability to represent weakly nonlinear wave evolution in intermediate depth. Using a Stokes-like expansion in powers of wave amplitude over water depth, we examine the bound second harmonics in a random sea as well as nonlinear dispersion and stability effects in the nonlinear Schrodinger equation for a narrow-banded sea state. We then examine numerical properties of solitary wave solutions in shallow water, and compare model performance to the full solution of Tanaka (1986) as well as the level 1, 2 and 3 solutions of Shields & Webster (1988).
TL;DR: In this article, a new Monte Carlo stellar wind radiative-transfer code is presented, which employs a three-dimensional opacity grid, and fully treats polarization and multiple scattering, and the line profiles computed match those computed using established radiative transfer codes.
Abstract: ABSTRA C T A new Monte Carlo stellar wind radiative-transfer code is presented. The code employs a three-dimensional opacity grid, and fully treats polarization and multiple scattering. Either Mie or Rayleigh scattering phase matrices may be used, and the line-transfer is treated by means of the Solobolev approximation. Variance reduction techniques are employed to increase computational efficiency. The results of several tests of the code are reported. It is confirmed that no continuum polarization is produced in the spherically symmetric wind case, and that the line profiles computed match those computed using established radiative-transfer codes. The continuum polarization produced by a latitudinally structured low-density wind is found to be in good agreement with that predicted by the single-scattering analytical treatment of Fox, while in the higher density regime the polarizations are consistent with the multiple-scattering code given by Hillier. Two illustrative applications of the code are described, using the wind parameters of z Puppis [O4 I(n)f] as the base model. In the first the effect on the line profile of a corotating spiral density enhancement is examined. It is found that the spiral gives line profile variations on the order of 5 per cent, and that it produces an S-wave-like pattern as a function of rotational phase. It is noted that the accelerations described by the spiral wave may mimic those produced by tangentially accelerating wind clumps. The variable polarization produced by the spiral is found to have an amplitude of 0.1 per cent, with two maxima per rotational period in phase with the line emission modulation. The second application investigates the profiles and polarization produced in a clumped wind. Although the parameters of the discrete wind clumps are necessarily arbitrary, it is found that a clumpedwind model reproduces the level of spectroscopic variability found by Eversberg et al. It is shown that the wind emission ‘bumps’ produced in the synthetic spectra often arise from the superposition in velocity space of flux from several spatially discrete wind blobs. Although the two example models may, in combination, reproduce the observed spectroscopic variability of OB supergiants, it appears that the predicted polarimetric variability of these models is too weak to explain the polarimetric observations of OB stars by Hayes and Lupie & Nordsieck. Finally, a new line polarization effect is described, resulting from line absorption of continuum photons in a rotating wind. The effect has a striking resemblance to the observations of z Puppis presented by Harries & Howarth (1996), and it is possible that the Ha polarization structure observed arises from this effect, rather than by line dilution of a continuum polarization.
TL;DR: In this paper, a unified description of weak hole equilibria in collisionless plasmas is given, relying on the potential method rather than on the Bernstein, Greene, Kruskal method and associated with electron and ion holes, respectively.
Abstract: A unified description of weak hole equilibria in collisionless plasmas is given. Two approaches, relying on the potential method rather than on the Bernstein, Greene, Kruskal method and associated with electron and ion holes, respectively, are shown to be equivalent. A traveling wave solution is thereby uniquely characterized by the nonlinear dispersion relation and the “classical” potential V(φ), which determine the phase velocity and the spectral decomposition of the wave structure, respectively. A new energy expression for a hole carrying plasma is found. It is dominated by a trapped particle contribution occurring one order earlier in the expansion scheme than the leading term in conventional schemes based on a truncation of Vlasov’s equation. Linear wave theory— reconsidered by taking the infinitesimal amplitude limit—is found to be deficient, as well. Neither Landau nor van Kampen modes and their general superpositions can adequately describe these trapped particle modes due to an incorrect treatmen...
TL;DR: In this paper, an asymptotic formulation of the inverse problem for flow reveals that the inversion may be partitioned into two complementary subproblems: the arrival time associated with the peak slope of the transient curve is directly related to reservoir properties.
Abstract: An asymptotic formulation of the inverse problem for flow reveals that the inversion may be partitioned into two complementary subproblems. In the first problem the arrival time associated with the peak slope of the transient curve is directly related to reservoir properties. The second inverse problem is similar to current methods for interpreting flow data; the transient head amplitudes are related to reservoir storage and conductivity. The first subproblem, the arrival time inversion, involves much less computation than does amplitude matching. Furthermore, it appears to be more robust with respect to the starting model. Therefore the solution to the arrival time inversion provides a starting model for amplitude matching. The methodology is particularly suited to the analysis of observations from well tests. We apply the approach to observations from two interference tests conducted at the Borehole Test Facility in Oklahoma. Using the transient pressure measurements, we image a shallow conductive fracture. The existence and location of the fracture has been verified by both geophysical and borehole data. In particular, core from a slant well contains an open, vertical fracture which coincides with our conductive feature.
TL;DR: Differences between simulated and experimental contrast suggest that the ER‐fMRI amplitude is greater than that predicted by a linear shift‐invariant system.
Abstract: Event-related functional magnetic resonance imaging (ERfMRI) involves the mapping of averaged hemodynamic changes resulting from repeated, brief (<3 sec) brain activation episodes. In this paper, two issues regarding constant-interstimulus interval ER-fMRI were addressed. First, the optimal interstimulus interval (ISI), given a stimulus duration (SD), was determined. Second, the statistical power of ER-fMRI relative to that of a blocked-design paradigm was determined. Experimentally, it was found that with a 2-sec SD, the optimal ISI is 12 to 14 sec. Theoretically, the optimal repetition interval (Topt 5 ISI 1 SD) is 12 to 14 sec for stimuli of 2 sec or less. For longer stimuli, Topt is 8 1 2 z SD. At the optimal ISI for SD 5 2 sec, the experimentally determined functional contrast of ER-fMRI was only 235% lower than that of blocked-design fMRI. Simulations that assumed a linear system demonstrated an event-related functional contrast that was 265% lower than that of the blocked design. These differences between simulated and experimental contrast suggest that the ER-fMRI amplitude is greater than that predicted by a linear shift-invariant system. Magn Reson Med 43:540 ‐548, 2000. © 2000 Wiley-Liss, Inc.
TL;DR: In this article, the authors used the Fast Fourier Transform (FFT) to analyze the vegetation phenology using only the amplitude and phase of the most important periodic components, which is a powerful way to monitor various dynamic parameters of the vegetation in Southern Africa.
Abstract: The distribution and phenology of vegetation is largely associated with climate, terrain characteristics and human activity. Satellite data provide the opportunity to monitor continuously the dynamics of vegetation, its changes and its impact on the environment. Monthly normalized difference vegetation index (NDVI) values which had been extracted from NOAA-AVHRR GAC data for Southern Africa (south of the Equator) from 1981 till 1991 were analysed using the Fast Fourier Transform (FFT). The FFT gives a new representation of the time series of images, i.e. a set of images of amplitude and phase (pixelwise) of periodic functions with different frequencies, which allows the analysis of the vegetation phenology using only the amplitude and phase of the most important periodic components. This approach is a powerful way to monitor various dynamic parameters of the vegetation in Southern Africa. The relationship between amplitude values and aridity and vegetation type was studied by correlating the amplitude and...
TL;DR: In this article, a new discrete variable representation (DVR) in generalized vibrational coordinates is proposed together with a new mixed diabatic/adiabatic contraction technique for the treatment of multidimensional vibrational problems up to high vibrational excitations.
Abstract: A new discrete variable representation (DVR) in generalized vibrational coordinates is proposed together with a new mixed diabatic/adiabatic contraction technique for the treatment of multidimensional vibrational problems up to high vibrational excitations. Formally based on the equidistant Chebyshev DVR in the grid index the new formulation is particularly suitable for multidimensional minimum energy paths. The new Z-matrix DVR proposed in this paper encompasses usual valence coordinates as well as nonlinear maps of coordinates on optimal nonequidistant grids. The pointwise numerical calculation of all kinetic energy terms avoids the algebraic derivation of specialized analytical forms of the kinetic energy adding to the flexibility of the method. With efficient truncation schemes the generalized DVR allows for a compact representation of the time-dependent wave-packet dynamics in up to six dimensions. Vibrationally adiabatic approaches to the detailed modelling of multidimensional quantum-dynamics usually are hampered by the typically large number of (avoided) crossings in dense spectra. This problem is particularly severe for discrete variable representations. A solution is provided by the new technique of diabatic rotations leading to a systematic construction of locally diabatic channels. This allows the treatment of very dense spectra where conventional truncation techniques fail. Applying the new approach to the vibrational problem of tetratomic molecules demonstrates its flexibility and efficiency. The examples of formaldehyde, ammonia, and hydrogen peroxide cover the whole range from semirigid (CH2O) to large amplitude inversion (NH3) and torsional tunnelling dynamics (H2O2). In solving the full six-dimensional vibrational eigenvalue problems for CH2O and NH3 the Z-matrix DVR shows at least comparable if not superior numerical efficiency compared with specialized techniques. In the case of H2O2 the technique of diabatic rotations and adiabatic contraction for the first time allows the treatment of the tunneling dynamics significantly above the dissociation threshold up to the fifth OH stretch overtone. The calculated decrease of the tunneling rate by about one order of magnitude agrees well with experimental observations.
TL;DR: The hemodynamic response to stimulation shows a spatial localization and a level of phase synchronization with the motor stimulation that depends on the stimulation period.
Abstract: We have noninvasively studied the motor cortex hemodynamics in human subjects under rest and motor stimulation conditions using a multichannel near-infrared tissue spectrometer. Our instrument measures optical maps of the cerebral cortex at two wavelengths (758 and 830 nm), with an acquisition time of 160 ms per map. We obtained optical maps of oxy- and deoxy-hemoglobin concentration changes in terms of amplitudes of folding average, power spectrum and coherence at the stimulation repetition frequency, and the phase synchronization index. Under periodic motor stimulation conditions, we observed coherence and frequency or phase synchronization of the local hemodynamic changes with stimulation. Our main findings are the following: (1) The amplitude of the hemodynamic response to the motor stimulation is comparable to the amplitude of the fluctuations at rest. (2) The spatial patterns of the oxy- and deoxy-hemoglobin responses to the stimulation are different. (3) The hemodynamic response to stimulation shows a spatial localization and a level of phase synchronization with the motor stimulation that depends on the stimulation period.
TL;DR: This model, which is derived from investigations of the current distribution across single crystals of NbSe2, is based on a competition between the injection of a disordered vortex phase at the sample edges, and the dynamic annealing of this metastable disorder by the transport current.
Abstract: The magnetic flux line lattice in type II superconductors serves as a useful system in which to study condensed matter flow, as its dynamic properties are tunable. Recent studies have shown a number of puzzling phenomena associated with vortex motion, including: low-frequency noise and slow voltage oscillations; a history-dependent dynamic response, and memory of the direction, amplitude duration and frequency of the previously applied current; high vortex mobility for alternating current, but no apparent vortex motion for direct currents; and strong suppression of an a.c. response by small d.c. bias. Taken together, these phenomena are incompatible with current understanding of vortex dynamics. Here we report a generic mechanism that accounts for these observations. Our model, which is derived from investigations of the current distribution across single crystals of NbSe2, is based on a competition between the injection of a disordered vortex phase at the sample edges, and the dynamic annealing of this metastable disorder by the transport current. For an alternating current, only narrow regions near the edges are in the disordered phase, while for d.c. bias, most of the sample is in the disordered phase--preventing vortex motion because of more efficient pinning. The resulting spatial dependence of the disordered vortex system serves as an active memory of the previous history.
TL;DR: This work proposes to use correlations of the type discussed by EPR in their original 1935 paradox for continuous variables to transmit with high efficiency messages in such a way that the receiver and sender may later determine whether eavesdropping has occurred.
Abstract: Correlations of the type discussed by EPR in their original 1935 paradox for continuous variables exist for the quadrature phase amplitudes of two spatially separated fields. These correlations were first experimentally reported in 1992. We propose to use such EPR beams in quantum cryptography, to transmit with high efficiency messages in such a way that the receiver and sender may later determine whether eavesdropping has occurred. The merit of the new proposal is in the possibility of transmitting a reasonably secure yet predetermined key. This would allow relay of a cryptographic key over long distances in the presence of lossy channels.
TL;DR: In this article, the authors presented the two-loop pure gauge contribution to the gluon-gluon scattering amplitude with maximal helicity violation, which does not rely directly on Feynman diagrams, but instead uses its analytic properties 4-2{epsilon} dimensions.
Abstract: The authors present the two-loop pure gauge contribution to the gluon-gluon scattering amplitude with maximal helicity violation. The construction of the amplitude does not rely directly on Feynman diagrams, but instead uses its analytic properties 4--2{epsilon} dimensions. The authors evaluate the loop integrals appearing in the amplitude through order({epsilon}{sup 0})in terms of polylogarithms.
TL;DR: In this paper, a quantitative relationship between pulse amplitude and pulse width was found, with the smaller amplitude peaks tend to be wider, with pulse width following a power law with an index of about -2.8.
Abstract: Many cosmological models of gamma-ray bursts (GRBs) envision the energy source to be a cataclysmic stellar event leading to a relativistically expanding fireball. Particles are thought to be accelerated at shocks and produce nonthermal radiation. The highly variable temporal structure observed in most GRBs has significantly constrained models. By using different methods of statistical analysis in the time domain, we find that the width of the large-amplitude pulses in GRB time histories remains remarkably constant throughout the classic GRB phase. This is also true for small-amplitude pulses. However, small and large pulses do not have the same pulse width within a single time history. We find a quantitative relationship between pulse amplitude and pulse width: the smaller amplitude peaks tend to be wider, with the pulse width following a power law with an index of about -2.8. Internal shocks simulated by randomly selecting the Lorentz factor and energy per shell are consistent with a power-law relationship. This is strong quantitative evidence that GRBs are indeed caused by internal shocks. The dependency of the width-versus-intensity relationship on the maximum Lorentz factor provides a way to estimate that elusive parameter. Our observed power-law index indicates that Γmax is 103. We also interpret the narrowness of the pulse width distribution as indicating that the emission that occurs when one shell overtakes another is produced over a small range of distances from the central site.
TL;DR: In this article, the number of independent parameter space points for which corrections must be applied to the data stream in a coherent search is computed, and the sensitivity of a search for continuous gravitational-wave signals is computationally bound when data analysis proceeds at a similar rate to data acquisition.
Abstract: The detection of quasi-periodic sources of gravitational waves requires the accumulation of signal to noise over long observation times. This represents the most difficult data analysis problem facing experimenters with detectors such as those at LIGO. If not removed, Earth-motion induced Doppler modulations and intrinsic variations of the gravitational-wave frequency make the signals impossible to detect. These effects can be corrected (removed) using a parametrized model for the frequency evolution. In a previous paper, we introduced such a model and computed the number of independent parameter space points for which corrections must be applied to the data stream in a coherent search. Since this number increases with the observation time, the sensitivity of a search for continuous gravitational-wave signals is computationally bound when data analysis proceeds at a similar rate to data acquisition. In this paper, we extend the formalism developed by Brady et al. [Phys. Rev. D 57, 2101 (1998)], and we compute the number of independent corrections ${N}_{p}(\ensuremath{\Delta}T,N)$ required for incoherent search strategies. These strategies rely on the method of stacked power spectra---a demodulated time series is divided into N segments of length $\ensuremath{\Delta}T,$ each segment is Fourier transformed, a power spectrum is computed, and the N spectra are summed up. This method is incoherent; phase information is lost from segment to segment. Nevertheless, power from a signal with fixed frequency (in the corrected time series) is accumulated in a single frequency bin, and amplitude signal to noise accumulates as $\ensuremath{\sim}{N}^{1/4}$ (assuming the segment length $\ensuremath{\Delta}T$ is held fixed). For fixed available computing power, there are optimal values for N and $\ensuremath{\Delta}T$ which maximize the sensitivity of a search in which data analysis takes a total time $N\ensuremath{\Delta}T.$ We estimate that the optimal sensitivity of an all-sky search that uses incoherent stacks is a factor of $2\ensuremath{-}4$ better than achieved using coherent Fourier transforms, assuming the same available computing power; incoherent methods are computationally efficient at exploring large parameter spaces. We also consider a two-stage hierarchical search in which candidate events from a search using short data segments are followed up in a search using longer data segments. This hierarchical strategy yields a further $20\ensuremath{-}60%$ improvement in sensitivity in all-sky (or directed) searches for old (\ensuremath{\geqslant}1000 yr) slow (\ensuremath{\leqslant}200 Hz) pulsars, and for young (\ensuremath{\geqslant}40 yr) fast (\ensuremath{\leqslant}1000 Hz) pulsars. Assuming enhanced LIGO detectors (LIGO-II) and ${10}^{12}$ flops of effective computing power, we examine the sensitivity to sources in three specialized classes. A limited area search for pulsars in the Galactic core would detect objects with gravitational ellipticities of $\ensuremath{\epsilon}\ensuremath{\gtrsim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ at 200 Hz; such limits provide information about the strength of the crust in neutron stars. Gravitational waves emitted by unstable r-modes of newborn neutron stars would be detected out to distances of $\ensuremath{\sim}8$ Mpc, if the r-modes saturate at a dimensionless amplitude of order unity and an optical supernova provides the position of the source on the sky. In searches targeting low-mass x-ray binary systems (in which accretion-driven spin up is balanced by gravitational-wave spin down), it is important to use information from electromagnetic observations to determine the orbital parameters as accurately as possible. An estimate of the difficulty of these searches suggests that objects with x-ray fluxes exceeding $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8} {\mathrm{erg}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2}{\mathrm{s}}^{\ensuremath{-}1}$ would be detected using the enhanced interferometers in their broadband configuration. This puts Sco X-1 on the verge of detectability in a broadband search; the amplitude signal to noise would be increased by a factor of order $\ensuremath{\sim}5\ensuremath{-}10$ by operating the interferometer in a signal-recycled, narrow-band configuration. Further work is needed to determine the optimal search strategy when limited information is available about the frequency evolution of a source in a targeted search.
TL;DR: The amplitude estimation techniques discussed in this paper do not model the observation noise, and yet, they are all asymptotically statistically efficient and provide a computationally simple and statistically accurate solution to the problem of system identification.
Abstract: This paper considers the problem of amplitude estimation of sinusoidal signals from observations corrupted by colored noise. A relatively large number of amplitude estimators, which encompass least squares (LS) and weighted least squares (WLS) methods, are described. Additionally, filterbank approaches, which are widely used for spectral analysis, are extended to amplitude estimation; more exactly, we consider the matched-filterbank (MAFI) approach and show that by appropriately designing the prefilters, the MAFI approach to amplitude estimation includes the WLS approach. The amplitude estimation techniques discussed in this paper do not model the observation noise, and yet, they are all asymptotically statistically efficient. It is, however, their different finite-sample properties that are of particular interest to this study. Numerical examples are provided to illustrate the differences among the various amplitude estimators. Although amplitude estimation applications are numerous, we focus herein on the problem of system identification using sinusoidal probing signals for which we provide a computationally simple and statistically accurate solution.
TL;DR: In this paper, the authors focused on the problems related to the sensitivity of temporal and spatial changes of the climate in high-altitude regions, particularly in the Qinghai-Tibetan (Q-T) Plateau.
Abstract: On the basis of ice core and meteorological data from the Qinghai-Tibetan (Q-T) Plateau, this article focuses on the discussion of the problems related to the sensitivity of temporal and spatial changes of the climate in high-altitude regions, particularly in the Q-T Plateau. The features of abrupt climatic changes of the past 100 ka, 2 000 a and recent years indicate that the amplitude of these changes in the Q-T Plateau was obviously larger than that in low-altitude regions. The scope of temperature change above 6 000 m in the Q-T Plateau between glacial and interglacial stages could reach over 10°C, but only about 4°C in low-elevation regions close to sea level. During the last 2 000 a, the amplitude of temperature changes at Guliya (over 6 000 m a.s.l.) in the Q-T Plateau reached 7°C, in comparison with 2°C in eastern China at low altitude. In the present age, apparent differences of climatic warming have been observed in the Q-T Plateau, indicating that the warming in high-elevation regions is much higher than that in low-elevation regions. The temperature in over 3 500 m regions of the Q-T Plateau have been increasing at a rate of 0.25×101/a in recent 30 years, but almost no change has taken place in the regions below 500 m. Thus, we concluded that high-altitude regions are more sensitive to climatic changes than the low-altitude regions.
Journal Article•
TL;DR: In this article, a two-loop pure gauge contribution to the gluon-gluon scattering amplitude with maximal helicity violation was presented. But this contribution does not rely directly on Feynman diagrams, but instead uses its analytic properties in $4-2 ϵπσon$ dimensions.
Abstract: We present the two-loop pure gauge contribution to the gluon-gluon scattering amplitude with maximal helicity violation. Our construction of the amplitude does not rely directly on Feynman diagrams, but instead uses its analytic properties in $4-2\epsilon$ dimensions. We evaluate the loop integrals appearing in the amplitude through ${\cal O}(\epsilon^0)$ in terms of polylogarithms.
TL;DR: In this paper, a multichannel analysis is introduced to constrain seismic anisotropy from the shear wave splitting of SKS and SKKS, which utilizes simultaneously a set of records coming from different azimuths.
Abstract: A multichannel analysis is introduced to constrain seismic anisotropy from the shear wave splitting of SKS and SKKS. This technique utilizes simultaneously a set of records coming from different azimuths. The splitting intensity of SKS waves, measured by the amplitude of the transverse component, depends on the angle between the back azimuth of the earthquake and the direction of the symmetry axis and on the delay time δt between the two quasi-shear waves. It is shown that the splitting parameters can be determined from the azimuthal dependence of the splitting intensity, which is given by the first right eigenvector of the matrix containing the transverse components of all the records. Alternatively, the splitting intensity can be measured by projecting the transverse components on the radial components derivatives. Experiments on synthetic seismograms demonstrate that both approaches provide robust estimates of the splitting parameters. However, the projection approach gives measurements that are closer to the inputs and with smaller error bars, which suggests that it should be preferred when the signal-to-noise ratio is low. The new technique is applied to analyze shear wave splitting under stations BCAO (Bangui, Central African Republic) and BDFB (Brasilia, Brazil). The analysis of 75 seismograms recorded by station BCAO gives a fast polarization direction in very close agreement with the present-day motion direction of the African plate. Comparisons with the results of previous studies, obtained with other techniques and smaller data sets, suggest that some of the published measurements may be strongly biased. The main limitation of the multichannel analysis is a good azimuthal distribution of the seismicity. Maps of the azimuthal coverage were determined, and it is found that the most favorable regions to study anisotropy with SKS waves are a latitudinal band that goes from South America to Africa, the eastern part of North America, and India.
TL;DR: Experimental aspects of guided wave analysis include phase velocity, group velocity, and attenuation dispersion curves; boundary element model analysis for reflection and transmission factor analysis; use of wave structure for defect detection sensitivity; source influence on the phase velocity spectrum, and the use of angle beam and comb transducer technology.
Abstract: Recent developments in guided wave generation, reception, and mode control show that increased penetration power and sensitivity are possible. A tone burst function generator and appropriate signal processing are generally used. Variable angle beam and comb-type transducers are the key to this effort. Problems in tubing, piping, hidden corrosion detection in aging aircraft, adhesive and diffusion bonding, and ice detection are discussed. Additionally, sample configurations, inspection objectives, and logic are being developed for such sample problems as defect detection and analysis in lap splice joints, tear straps, cracks in a second layer, hidden corrosion in multiple layers, cracks from rivet holes, transverse cracking in a beam, and cracks in landing gear assembly. Theoretical and experimental aspects of guided wave analysis include phase velocity, group velocity, and attenuation dispersion curves; boundary element model analysis for reflection and transmission factor analysis; use of wave structure for defect detection sensitivity; source influence on the phase velocity spectrum, and the use of angle beam and comb transducer technology. Probe design and modeling considerations are being explored. Utilization of in-plane and out-of-plane displacement patterns on the surface and longitudinal power distribution across the structural cross-section are considered for improved sensitivity, penetration power, and resolution in nondestructive evaluation. Methods of controlling the phase velocity spectrum for mode and frequency selection are available. Such features as group velocity change, mode cut-off measurements, mode conversion, amplitude ratios of transmission, and reflection factors of specific mode and frequency as input will be introduced for their ability to be used in flaw and material characterization analysis.
TL;DR: In this article, a real space Green's function theory of solid-state contributions and polarization dependence of the x-ray elastic scattering amplitude is presented, which is implemented in a generalization of the ab initio, self-consistent code, FEFF8.10.
Abstract: We present a real space Green's function theory of solid-state contributions and polarization dependence of the x-ray elastic scattering amplitude. In this approach the calculation separates naturally into contributions from the central (embedded) absorbing atom and multiple-scattering contributions from the environment. Both real and imaginary parts of the anomalous x-ray scattering amplitude are calculated simultaneously in the complex energy plane, without the necessity of a Kramers-Kronig transform. This approach also takes into account final-state effects, including core-hole lifetime, the finite temperature Fermi distribution and Debye-Waller factors, as well as experimental resolution. The approach is implemented in a generalization of the ab initio, self-consistent code, FEFF8.10, which permits applications to a number of x-ray spectroscopies for general, not necessarily periodic systems. The solid-state effect on the fine structure in the anomalous scattering amplitude near an absorption edge is illustrated for Cu metal. Calculations are also presented of the x-ray anomalous cross scattering amplitude ${F}_{\ensuremath{\pi}\ensuremath{\sigma}}$ for Cd metal, and x-ray natural circular dichroism in ${\mathrm{LiIO}}_{3},$ which are both due entirely to solid-state effects.
TL;DR: In this paper, the authors considered the Aharonov-Bohm effect on the noncommutative plane and derived an upper bound for the non-commutativity parameter.
Abstract: The Aharonov-Bohm effect on the noncommutative plane is considered. Developing the path integral formulation of quantum mechanics, we find the propagation amplitude for a particle in a noncommutative space. We show that the corresponding shift in the phase of the particle propagator due to the magnetic field of a thin solenoid receives certain gauge invariant corrections because of the noncommutativity. Evaluating the numerical value for this correction, an upper bound for the noncommutativity parameter is obtained.