scispace - formally typeset
Search or ask a question

Showing papers on "Amplitude published in 2014"


Journal ArticleDOI
TL;DR: In this paper, a methodology of compensating for attenuation (∼1∕Q) effects in reverse-time migration (Q-RTM) was proposed. But, the attenuation and distorted dispersion of seismic waves caused by attenuation always degrades the resolution of migrated images.
Abstract: Reduced amplitude and distorted dispersion of seismic waves caused by attenuation, especially strong attenuation, always degrades the resolution of migrated images. To improve image resolution, we evaluated a methodology of compensating for attenuation (∼1∕Q) effects in reverse-time migration (Q-RTM). The Q-RTM approach worked by mitigating the amplitude attenuation and phase dispersion effects in source and receiver wavefields. Source and receiver wavefields were extrapolated using a previously published time-domain viscoacoustic wave equation that offered separated amplitude attenuation and phase dispersion operators. In our Q-RTM implementation, therefore, attenuation- and dispersion-compensated operators were constructed by reversing the sign of attenuation operator and leaving the sign of dispersion operator unchanged, respectively. Further, we designed a low-pass filter for attenuation and dispersion operators to stabilize the compensating procedure. Finally, we tested the Q-RTM approach on a simple layer model and the more realistic BP gas chimney model. Numerical results demonstrated that the Q-RTM approach produced higher resolution images with improved amplitude and phase compared to the noncompensated RTM, particularly beneath high-attenuation zones.

243 citations


Journal ArticleDOI
TL;DR: The superpixel method reduces the errors when compared to the state of the art Lee holography method for these test fields by 50% and 18%, with a comparable light efficiency of around 5%.
Abstract: We present a superpixel method for full spatial phase and amplitude control of a light beam using a digital micromirror device (DMD) combined with a spatial filter. We combine square regions of nearby micromirrors into superpixels by low pass filtering in a Fourier plane of the DMD. At each superpixel we are able to independently modulate the phase and the amplitude of light, while retaining a high resolution and the very high speed of a DMD. The method achieves a measured fidelity F = 0.98 for a target field with fully independent phase and amplitude at a resolution of 8 × 8 pixels per diffraction limited spot. For the LG10 orbital angular momentum mode the calculated fidelity is F = 0.99993, using 768 × 768 DMD pixels. The superpixel method reduces the errors when compared to the state of the art Lee holography method for these test fields by 50% and 18%, with a comparable light efficiency of around 5%. Our control software is publicly available.

212 citations


Journal ArticleDOI
Thomas Bauer1, Sergej Orlov1, Ulf Peschel1, Peter Banzer1, Gerd Leuchs1 
TL;DR: In this paper, an easily implementable reconstruction scheme is demonstrated for determining the full vectorial amplitude and relative phase distributions of highly confined electromagnetic fields with subwavelength resolution from a single-scan measurement.
Abstract: An easily implementable reconstruction scheme is demonstrated for determining the full vectorial amplitude and relative phase distributions of highly confined electromagnetic fields with subwavelength resolution from a single-scan measurement. This scheme will help improve microscopy and nanoscopy techniques.

210 citations


Journal ArticleDOI
TL;DR: In this paper, a generalization of Luescher's relation between the finite-volume spectrum and scattering amplitudes to the case of three particles is presented, where the couplings are arbitrary aside from a Z2 symmetry that removes vertices with an odd number of particles.
Abstract: We present a generalization of Luescher's relation between the finite-volume spectrum and scattering amplitudes to the case of three particles. We consider a relativistic scalar field theory in which the couplings are arbitrary aside from a Z2 symmetry that removes vertices with an odd number of particles. The theory is assumed to have two-particle phase shifts that are bounded by \pi/2 in the regime of elastic scattering. We determine the spectrum of the finite-volume theory from the poles in the odd-particle-number finite-volume correlator, which we analyze to all orders in perturbation theory. We show that it depends on the infinite-volume two-to-two K-matrix as well as a nonstandard infinite-volume three-to-three K-matrix. A key feature of our result is the need to subtract physical singularities in the three-to-three amplitude and thus deal with a divergence-free quantity. This allows our initial, formal result to be truncated to a finite dimensional determinant equation. At present, the relation of the three-to-three K-matrix to the corresponding scattering amplitude is not known, although previous results in the non-relativistic limit suggest that such a relation exists.

209 citations


Journal ArticleDOI
TL;DR: The results of a finely resolved large-eddy simulation (LES) of a spatially developing zero-pressure-gradient turbulent boundary layer up to a Reynolds number of Re θ = 8300 are presented in this article.

172 citations


Journal ArticleDOI
TL;DR: In this paper, the joint estimation of a phase shift and the amplitude of phase diffusion at the quantum limit is investigated, and a trade-off bound on the statistical variances for the estimation of phase and phase diffusion, as well as optimum measurement schemes are derived.
Abstract: Phase estimation, at the heart of many quantum metrology and communication schemes, can be strongly affected by noise, whose amplitude may not be known, or might be subject to drift. Here we investigate the joint estimation of a phase shift and the amplitude of phase diffusion at the quantum limit. For several relevant instances, this multiparameter estimation problem can be effectively reshaped as a two-dimensional Hilbert space model, encompassing the description of an interferometer phase probed with relevant quantum states--split single-photons, coherent states or N00N states. For these cases, we obtain a trade-off bound on the statistical variances for the joint estimation of phase and phase diffusion, as well as optimum measurement schemes. We use this bound to quantify the effectiveness of an actual experimental set-up for joint parameter estimation for polarimetry. We conclude by discussing the form of the trade-off relations for more general states and measurements.

166 citations


Journal ArticleDOI
TL;DR: Li et al. as discussed by the authors investigated the effect of following currents on vegetation-induced wave attenuation and found that following currents can either increase or decrease wave dissipation depending on the velocity ratio, which explains the seeming inconsistency in previous studies.

162 citations


Journal ArticleDOI
TL;DR: It is shown that the amplitude and phase information from a two-dimensional complex field can be synthesized from a phase-only optical element with micrometric resolution from a 4-f optical system.
Abstract: We show that the amplitude and phase information from a two-dimensional complex field can be synthesized from a phase-only optical element with micrometric resolution. The principle of the method is based on the combination of two spatially sampled phase elements by using a low-pass filter at the Fourier plane of a 4-f optical system. The proposed encoding technique was theoretically demonstrated, as well as experimentally validated with the help of a phase-only spatial light modulator for phase encoding, a conventional CMOS camera to measure the amplitude of the complex field, and a Shack-Hartmann wavefront sensor to determine its phase.

151 citations


Journal ArticleDOI
TL;DR: In this paper, in situ measurements of ocean surface wave spectra evolution in the Antarctic marginal ice zone are described and attenuation rates of the spectral components of the wavefield are calculated.
Abstract: In situ measurements of ocean surface wave spectra evolution in the Antarctic marginal ice zone are described. Analysis of the measurements shows significant wave heights and peak periods do not vary appreciably in approximately the first 80km of the ice-covered ocean. Beyond this region, significant wave heights attenuate and peak periods increase. It is shown that attenuation rates are insensitive to amplitudes for long-period waves but increase with increasing amplitude above some critical amplitude for short-period waves. Attenuation rates of the spectral components of the wavefield are calculated. It is shown that attenuation rates decrease with increasing wave period. Further, for long-period waves the decrease is shown to be proportional to the inverse of the period squared. This relationship can be used to efficiently implement wave attenuation through the marginal ice zone in ocean-scale wave models.

151 citations


Journal ArticleDOI
TL;DR: In this article, the authors show that small amplitude transverse MHD waves can lead in a few periods time to strand-like structure in loops in EUV intensity images, which can be observed for spatial resolutions of a tenth of loop radius.
Abstract: Current analytical and numerical modeling suggest the existence of ubiquitous thin current sheets in the corona that could explain the observed heating requirements. On the other hand, new high resolution observations of the corona indicate that its magnetic field may tend to organize itself in fine strand-like structures of few hundred kilometers widths. The link between small structure in models and the observed widths of strand-like structure several orders of magnitude larger is still not clear. A popular theoretical scenario is the nanoflare model, in which each strand is the product of an ensemble of heating events. Here, we suggest an alternative mechanism for strand generation. Through forward modeling of three-dimensional MHD simulations we show that small amplitude transverse MHD waves can lead in a few periods time to strand-like structure in loops in EUV intensity images. Our model is based on previous numerical work showing that transverse MHD oscillations can lead to Kelvin-Helmholtz instabilities that deform the cross-sectional area of loops. While previous work has focused on large amplitude oscillations, here we show that the instability can occur even for low wave amplitudes for long and thin loops, matching those presently observed in the corona. We show that the vortices generated from the instability are velocity sheared regions with enhanced emissivity hosting current sheets. Strands result as a complex combination of the vortices and the line-of-sight angle, last for timescales of a period, and can be observed for spatial resolutions of a tenth of loop radius.

148 citations


Journal ArticleDOI
TL;DR: In this article, the authors detect fine structure of chorus elements with peak instantaneous amplitudes of a few hundred picotesla but exceptionally reaching up to 3 nT, i.e., more than 1% of the background magnetic field.
Abstract: Whistler mode chorus waves in the outer Van Allen belt can have consequences for acceleration of relativistic electrons through wave-particle interactions. New multicomponent waveform measurements have been collected by the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science's Waves instrument. We detect fine structure of chorus elements with peak instantaneous amplitudes of a few hundred picotesla but exceptionally reaching up to 3 nT, i.e., more than 1% of the background magnetic field. The wave vector direction turns by a few tens of degrees within a single chorus element but also within its subpackets. Our analysis of a significant number of subpackets embedded in rising frequency elements shows that amplitudes of their peaks tend to decrease with frequency. The wave vector is quasi-parallel to the background magnetic field for large-amplitude subpackets, while it turns away from this direction when the amplitudes are weaker.

Journal ArticleDOI
TL;DR: To evaluate the error in T1 estimates using inversion‐recovery‐based T1 mapping due to imperfect inversion and to perform a systematic study of adiabatic inversion pulse designs in order to maximize inversion efficiency for values of transverse relaxation in the myocardium subject to a peak power constraint.
Abstract: Purpose To evaluate the error in T1 estimates using inversion-recovery-based T1 mapping due to imperfect inversion and to perform a systematic study of adiabatic inversion pulse designs in order to maximize inversion efficiency for values of transverse relaxation (T2) in the myocardium subject to a peak power constraint. Methods The inversion factor for hyperbolic secant and tangent/hyperbolic tangent adiabatic full passage waveforms was calculated using Bloch equations. A brute-force search was conducted for design parameters: pulse duration, frequency range, shape parameters, and peak amplitude. A design was selected that maximized the inversion factor over a specified range of amplitude and off-resonance and validated using phantom measurements. Empirical correction for imperfect inversion was performed. Results The tangent/hyperbolic tangent adiabatic pulse was found to outperform hyperbolic secant designs and achieve an inversion factor of 0.96 within ±150 Hz over 25% amplitude range with 14.7 µT peak amplitude. T1 mapping errors of the selected design due to imperfect inversion was ∼4% and could be corrected to <1%. Conclusions Nonideal inversion leads to significant errors in inversion-recovery-based T1 mapping. The inversion efficiency of adiabatic pulses is sensitive to transverse relaxation. The tangent/hyperbolic tangent design achieved the best performance subject to the peak amplitude constraint. Magn Reson Med 71:1428–1434, 2014. © 2013 Wiley Periodicals, Inc.

Journal ArticleDOI
TL;DR: In this article, the response of atmospheric blocks and the wave amplitude of midlatitude jets to changes in the mid-latitude to pole, near-surface temperature difference (ΔT) was studied using an idealized dry general circulation model (GCM) with Held-Suarez forcing.
Abstract: The response of atmospheric blocks and the wave amplitude of midlatitude jets to changes in the midlatitude to pole, near-surface temperature difference (ΔT), is studied using an idealized dry general circulation model (GCM) with Held-Suarez forcing. Decreasing ΔT results in slower zonal winds, a mean state with reduced meridional gradient of the 500 hPa geopotential height (Z500), a smaller variance of Z500 anomalies, and a robust decrease in blocks and meridional amplitude of waves. Neglecting the decrease of variance associated with reduced ΔT would lead to the incorrect expectation that mean states with smaller Z500 gradients produce more blocks and higher wave amplitudes. Our results suggest further investigation of the hypothesis that reduced ΔT due to Arctic Amplification would increase blocking events and wave amplitude, hence leading to more midlatitude extreme weather events.

Journal ArticleDOI
TL;DR: The investigated nonlinear dynamics of a periodic chain of bistable elements consisting of masses connected by elastic springs whose constraint arrangement gives rise to a large-deformation snap-through instability produces three different regimes of wave propagation in the periodic medium, depending on the wave amplitude.
Abstract: We investigate the nonlinear dynamics of a periodic chain of bistable elements consisting of masses connected by elastic springs whose constraint arrangement gives rise to a large-deformation snap-through instability. We show that the resulting negative-stiffness effect produces three different regimes of (linear and nonlinear) wave propagation in the periodic medium, depending on the wave amplitude. At small amplitudes, linear elastic waves experience dispersion that is controllable by the geometry and by the level of precompression. At moderate to large amplitudes, solitary waves arise in the weakly and strongly nonlinear regime. For each case, we present closed-form analytical solutions and we confirm our theoretical findings by specific numerical examples. The precompression reveals a class of wave propagation for a partially positive and negative potential. The presented results highlight opportunities in the design of mechanical metamaterials based on negative-stiffness elements, which go beyond current concepts primarily based on linear elastic wave propagation. Our findings shed light on the rich effective dynamics achievable by nonlinear small-scale instabilities in solids and structures.

Journal ArticleDOI
TL;DR: In this article, the authors revisited the production of gravitational waves from unstable domain walls analyzing their spectrum by the use of field theoretic lattice simulations with grid size 10243, which is larger than the previous study.
Abstract: We revisit the production of gravitational waves from unstable domain walls analyzing their spectrum by the use of field theoretic lattice simulations with grid size 10243, which is larger than the previous study. We have recognized that there exists an error in the code used in the previous study, and the correction of the error leads to the suppression of the spectrum of gravitational waves at high frequencies. The peak of the spectrum is located at the scale corresponding to the Hubble radius at the time of the decay of domain walls, and its amplitude is consistent with the naive estimation based on the quadrupole formula. Using the numerical results, the magnitude and the peak frequency of gravitational waves at the present time are estimated. It is shown that for some choices of parameters the signal of gravitational waves is strong enough to be probed in the future gravitational wave experiments.

Journal ArticleDOI
TL;DR: In this article, the authors show that small amplitude transverse MHD waves can lead in a few periods time to strand-like structure in loops in EUV intensity images, which can be observed for spatial resolutions of a tenth of loop radius.
Abstract: Current analytical and numerical modelling suggest the existence of ubiquitous thin current sheets in the corona that could explain the observed heating requirements. On the other hand, new high resolution observations of the corona indicate that its magnetic field may tend to organise itself in fine strand-like structures of few hundred kilometres widths. The link between small structure in models and the observed widths of strand-like structure several orders of magnitude larger is still not clear. A popular theoretical scenario is the nanoflare model, in which each strand is the product of an ensemble of heating events. Here, we suggest an alternative mechanism for strand generation. Through forward modelling of 3D MHD simulations we show that small amplitude transverse MHD waves can lead in a few periods time to strand-like structure in loops in EUV intensity images. Our model is based on previous numerical work showing that transverse MHD oscillations can lead to Kelvin-Helmholtz instabilities that deform the cross-sectional area of loops. While previous work has focused on large amplitude oscillations, here we show that the instability can occur even for low wave amplitudes for long and thin loops, matching those presently observed in the corona. We show that the vortices generated from the instability are velocity sheared regions with enhanced emissivity hosting current sheets. Strands result as a complex combination of the vortices and the line-of-sight angle, last for timescales of a period and can be observed for spatial resolutions of a tenth of loop radius.

Journal ArticleDOI
TL;DR: In this article, a new estimator of the polarization amplitude from a single measurement of its normally distributed Stokes components is proposed, based on the properties of the Rice distribution and dubbed "MAS" (Modified ASymptotic), which meets several desirable criteria: its values lie in the whole positive region; its distribution is continuous; it transforms smoothly with the signal-to-noise ratio (SNR) from a Rayleigh-like shape to a Gaussian one; it is unbiased and reaches its components variance as soon as the SNR exceeds 2; and it
Abstract: We propose a novel estimator of the polarization amplitude from a single measurement of its normally distributed $(Q,U)$ Stokes components. Based on the properties of the Rice distribution and dubbed "MAS" (Modified ASymptotic), it meets several desirable criteria:(i) its values lie in the whole positive region; (ii) its distribution is continuous; (iii) it transforms smoothly with the signal-to-noise ratio (SNR) from a Rayleigh-like shape to a Gaussian one ; (iv) it is unbiased and reaches its components variance as soon as the SNR exceeds 2; (v) it is analytic and can therefore be used on large data-sets. We also revisit the construction of its associated confidence intervals, and show how the Feldman-Cousins prescription efficiently solves the issue of classical intervals lying entirely in the unphysical negative domain. Such intervals can be used to identify statistically significant polarized regions and conversely build masks for polarization data. We then consider the case of a general $[Q,U]$ covariance matrix and perform a generalization of the estimator that preserves its asymptotic properties. We show that its bias does not depend on the true polarization angle, and provide an analytic estimate of its variance. The estimator value, together with its variance, provide a powerful point-estimate of the true polarization amplitude that follows an unbiased Gaussian distribution for an SNR as low as 2. These results can be applied to the much more general case of transforming any normally distributed random variable from Cartesian to polar coordinates.

Journal ArticleDOI
TL;DR: A simple self-consistent model is presented that provides a clear description of the saturation mechanism and quantitatively predicts the saturated amplitude and flow fields of the Bénard-von Kármán vortex shedding instability.
Abstract: The Benard–von Karman vortex shedding instability in the wake of a cylinder is perhaps the best known example of a supercritical Hopf bifurcation in fluid dynamics. However, a simplified physical description that accurately accounts for the saturation amplitude of the instability is still missing. Here, we present a simple self-consistent model that provides a clear description of the saturation mechanism and quantitatively predicts the saturated amplitude and flow fields. The model is formally constructed by a set of coupled equations governing the mean flow together with its most unstable eigenmode with finite size. The saturation amplitude is determined by requiring the mean flow to be neutrally stable. Without requiring any input from numerical or experimental data, the resolution of the model provides a good prediction of the amplitude and frequency of the vortex shedding as well as the spatial structure of the mean flow and the Reynolds stress.

Journal ArticleDOI
TL;DR: In this article, a 3D seismic model of the western United States crust derived from a joint version of Rayleigh-wave phase velocity and ellipticity measurements using periods from 8 to 100 s was presented.
Abstract: We present a new 3-D seismic model of the western United States crust derived from a joint inversion of Rayleigh-wave phase velocity and ellipticity measurements using periods from 8 to 100 s. Improved constraints on upper-crustal structure result from use of short-period Rayleigh-wave ellipticity, or Rayleigh-wave H/V (horizontal to vertical) amplitude ratios, measurements determined using multicomponent ambient noise cross-correlations. To retain the amplitude ratio information between vertical and horizontal components, for each station, we perform daily noise pre-processing (temporal normalization and spectrum whitening) simultaneously for all three components. For each station pair, amplitude measurements between cross-correlations of different components (radial–radial, radial–vertical, vertical–radial and vertical–vertical) are then used to determine the Rayleigh-wave H/V ratios at the two station locations. We use all EarthScope/USArray Tranportable Array data available between 2007 January and 2011 June to determine the Rayleigh-wave H/V ratios and their uncertainties at all station locations and construct new Rayleigh-wave H/V ratio maps in the western United States between periods of 8 and 24 s. Combined with previous longer period earthquake Rayleigh-wave H/V ratio measurements and Rayleigh-wave phase velocity measurements from both ambient noise and earthquakes, we invert for a new 3-D crustal and upper-mantle model in the western United States. Correlation between the inverted model and known geological features at all depths suggests good resolution in five crustal layers. Use of short-period Rayleigh-wave H/V ratio measurements based on noise cross-correlation enables resolution of distinct near surface features such as the Columbia River Basalt flows, which overlie a thick sedimentary basin.

Journal ArticleDOI
TL;DR: In this article, the authors explore the impact of modified gravity on $B$-modes, identifying two main separate effects: lensing and propagation of tensor modes, and obtain that the gravitational wave speed is compatible with the speed of light and constrained to within about 10%.
Abstract: We explore the impact of modified gravity on $B$-modes, identifying two main separate effects: lensing and propagation of tensor modes. The location of the inflationary peak of the $BB$ spectrum depends on the speed of gravitational waves; the amplitude of the lensing contribution depends on the anisotropic stress. We single out these effects using the quasistatic regime and considering models for which the background and the growth of matter perturbations are standard. Using available data we obtain that the gravitational wave speed is compatible with the speed of light and constrained to within about 10%.

Journal ArticleDOI
TL;DR: In this paper, the amplitude of a Bose-Einstein condensate (BEC) wave was estimated using relativistic quantum metrology techniques, and the optimal precision bound on the estimation of the wave's amplitude was provided.
Abstract: We show that gravitational waves create phonons in a Bose–Einstein condensate (BEC). A traveling spacetime distortion produces particle creation resonances that correspond to the dynamical Casimir effect in a BEC phononic field contained in a cavity-type trap. We propose to use this effect to detect gravitational waves. The amplitude of the wave can be estimated applying recently developed relativistic quantum metrology techniques. We provide the optimal precision bound on the estimation of the waveʼs amplitude. Finally, we show that the parameter regime required to detect gravitational waves with this technique could be, in principle, within experimental reach in a medium-term timescale.

Journal ArticleDOI
TL;DR: In this paper, the authors determined the number of distinct observables that can be derived from pulse profile modeling and showed that using only bolometric pulse profiles is insufficient for breaking the degeneracy between inferred neutron-star radius and mass.
Abstract: Modeling the amplitudes and shapes of the X-ray pulsations observed from hot, rotating neutron stars provides a direct method for measuring neutron-star properties. This technique constitutes an important part of the science case for the forthcoming NICER and proposed LOFT X-ray missions. In this paper, we determine the number of distinct observables that can be derived from pulse profile modeling and show that using only bolometric pulse profiles is insufficient for breaking the degeneracy between inferred neutron-star radius and mass. However, we also show that for moderately spinning (300-800 Hz) neutron stars, analysis of pulse profiles in two different energy bands provides additional constraints that allow a unique determination of the neutron-star properties. Using the fractional amplitudes of the fundamental and the second harmonic of the pulse profile in addition to the amplitude and phase difference of the spectral color oscillations, we quantify the signal-to-noise ratio necessary to achieve a specified measurement precision for neutron star radius. We find that accumulating 106 counts in a pulse profile is sufficient to achieve a 5% uncertainty in the neutron star radius, which is the level of accuracy required to determine the equation of state of neutron-star matter. Finally, we formally derive the background limits that can be tolerated in the measurements of the various pulsation amplitudes as a function of the system parameters.

Journal ArticleDOI
TL;DR: Estimates of the anomalous collision frequency based on experimental measurements indicate that the ion acoustic turbulence collision frequency can exceed the classical rate at high discharge current densities by nearly two orders of magnitude.
Abstract: The temporal fluctuations in the near plume of a 100-A ${\mathrm{LaB}}_{6}$ hollow cathode are experimentally investigated. A probe array is employed to measure the amplitude and dispersion of axial modes in the plume, and these properties are examined parametrically as a function of cathode operating conditions. The onset of ion acoustic turbulence is observed at high current and is characterized by a power spectrum that exhibits a cutoff at low frequency and an inverse dependence on frequency at high values. The amplitude of the turbulence is found to decrease with flow rate but to depend nonmonotonically on discharge current. Estimates of the anomalous collision frequency based on experimental measurements indicate that the ion acoustic turbulence collision frequency can exceed the classical rate at high discharge current densities by nearly two orders of magnitude.

Journal ArticleDOI
TL;DR: In this paper, the authors identify electromagnetic field variations from the Van Allen Probes which have the properties of Doppler shifted kinetic scale Alfvenic field line resonances during injections of energetic plasmas into the inner magnetosphere.
Abstract: We identify electromagnetic field variations from the Van Allen Probes which have the properties of Doppler shifted kinetic scale Alfvenic field line resonances. These variations are observed during injections of energetic plasmas into the inner magnetosphere. These waves have scale sizes perpendicular to the magnetic field which are determined to be of the order of an ion gyro-radius (ρi) and less. Cross-spectral analysis of the electric and magnetic fields reveals phase transitions at frequencies correlated with enhancements and depressions in the ratio of the electric and magnetic fields. Modeling shows that these observations are consistent with the excitation of field-line resonances over a broad range of wave numbers perpendicular to the magnetic field (k⊥) extending to k⊥ρi ≫ 1. The amplitude of these waves is such that E/Bo ≳ Ωi/k⊥ (E, Bo, and Ωi are the wave amplitude, background field strength, and ion gyro-frequency, respectively) leading to ion demagnetization and acceleration for multiple transitions through the wave potential.

Journal Article
TL;DR: In this article, experimental gradient-based optimization is used to maximize the propulsive efficiency of a heaving and pitching flexible panel, where the Strouhal number is within an optimal range that varies weakly with amplitude and boundary conditions.
Abstract: Experimental gradient-based optimization is used to maximize the propulsive efficiency of a heaving and pitching flexible panel. Optimum and near-optimum conditions are studied via direct force measurements and particle image velocimetry (PIV). The net thrust and power scale predictably with the frequency and amplitude of the leading edge, but the efficiency shows a complex multimodal response. Optimum pitch and heave motions are found to produce nearly twice the efficiencies of optimum heave-only motions. Efficiency is globally optimized when (i) the Strouhal number is within an optimal range that varies weakly with amplitude and boundary conditions; (ii) the panel is actuated at a resonant frequency of the fluid–panel system; (iii) heave amplitude is tuned such that trailing-edge amplitude is maximized while the flow along the body remains attached; and (iv) the maximum pitch angle and phase lag are chosen so that the effective angle of attack is minimized. The multi-dimensionality and multi-modality of the efficiency response demonstrate that experimental optimization is well-suited for the design of flexible underwater propulsors.

Journal ArticleDOI
TL;DR: In this paper, a sample of 68 massive stars in our galaxy whose projected rotational velocity, effective temperature, and gravity are available from high-precision spectroscopic measurements were used to estimate the nitrogen abundance.
Abstract: We have composed a sample of 68 massive stars in our galaxy whose projected rotational velocity, effective temperature, and gravity are available from high-precision spectroscopic measurements. The additional seven observed variables considered here are their surface nitrogen abundance, rotational frequency, magnetic field strength, and the amplitude and frequency of their dominant acoustic and gravity modes of oscillation. A multiple linear regression to estimate the nitrogen abundance combined with principal component analysis, after addressing the incomplete and truncated nature of the data, reveals that the effective temperature and the frequency of the dominant acoustic oscillation mode are the only two significant predictors for the nitrogen abundance, while the projected rotational velocity and the rotational frequency have no predictive power. The dominant gravity mode and the magnetic field strength are correlated with the effective temperature but have no predictive power for the nitrogen abundance. Our findings are completely based on observations and their proper statistical treatment and call for a new strategy in evaluating the outcome of stellar evolution computations.

Journal ArticleDOI
TL;DR: In this article, the authors extracted light curves from the pixel photometric data of the Kepler space telescope for 15 Blazhko RR Lyrae stars and carried out detailed Fourier analysis of the light curves and the amplitude independent O-C diagram.
Abstract: In order to benefit from the 4-year unprecedented precision of the Kepler data, we extracted light curves from the pixel photometric data of the Kepler space telescope for 15 Blazhko RR Lyrae stars. For collecting all the flux from a given target as accurately as possible, we defined tailor-made apertures for each star and quarter. In some cases the aperture finding process yielded sub-optimal result, because some flux have been lost even if the aperture contains all available pixels around the star. This fact stresses the importance of those methods that rely on the whole light curve instead of focusing on the extrema (O-C diagrams and other amplitude independent methods). We carried out detailed Fourier analysis of the light curves and the amplitude independent O-C diagram. We found 12 (80%) multiperiodically modulated stars in our sample. This ratio is much higher than previously found. Resonant coupling between radial modes, a recent theory to explain of the Blazhko effect, allows single, multiperiodic or even chaotic modulations. Among the stars with two modulations we found three stars (V355 Lyr, V366 Lyr and V450 Lyr) where one of the periods dominate in amplitude modulation, but the other period has larger frequency modulation amplitude. The ratio between the primary and secondary modulation periods is almost always very close to ratios of small integer numbers. It may indicate the effect of undiscovered resonances. Furthermore, we detected the excitation of the second radial overtone mode $f_2$ for three stars where this feature was formerly unknown. Our data set comprises the longest continuous, most precise observations of Blazhko RR Lyrae stars ever published. These data which is made publicly available will be unprecedented for years to come.

Journal ArticleDOI
TL;DR: A figure-8 dual-pump passively mode-locked fiber laser to generate square-wave pulse tunable by both width and amplitude is proposed and demonstrated.
Abstract: We have proposed and demonstrated a figure-8 dual-pump passively mode-locked fiber laser to generate square-wave pulse tunable by both width and amplitude. Just by simply adjusting the power of the pumps, both the amplitude and width of the output square-wave pulse can be tuned independently and continuously. One pump is used to tune the output pulsewidth while the other is used to tune amplitude.

Journal ArticleDOI
TL;DR: In this article, the power spectrum of the velocity field of the intracluster medium using only information on the plasma density fluctuations, which can be measured today by Chandra and XMM-Newton observatories, is evaluated.
Abstract: We address the problem of evaluating the power spectrum of the velocity field of the intracluster medium using only information on the plasma density fluctuations, which can be measured today by Chandra and XMM-Newton observatories. We argue that for relaxed clusters there is a linear relation between the rms density and velocity fluctuations across a range of scales, from the largest ones, where motions are dominated by buoyancy, down to small, turbulent scales: , where δρ k /ρ is the spectral amplitude of the density perturbations at wavenumber k, is the mean square component of the velocity field, cs is the sound speed, and η1 is a dimensionless constant of the order of unity. Using cosmological simulations of relaxed galaxy clusters, we calibrate this relation and find η1 1 ± 0.3. We argue that this value is set at large scales by buoyancy physics, while at small scales the density and velocity power spectra are proportional because the former are a passive scalar advected by the latter. This opens an interesting possibility to use gas density power spectra as a proxy for the velocity power spectra in relaxed clusters across a wide range of scales.

Journal ArticleDOI
TL;DR: In this article, the authors quantified wind speed asymmetries in 35 hurricanes in terms of their amplitude and phase, based on aircraft observations from 128 individual flights between 1998 and 2011.
Abstract: Wavenumber-1 wind speed asymmetries in 35 hurricanes are quantified in terms of their amplitude and phase, based on aircraft observations from 128 individual flights between 1998 and 2011. The impacts of motion and 850–200-mb environmental vertical shear are examined separately to estimate the resulting asymmetric structures at the sea surface and standard 700-mb reconnaissance flight level. The surface asymmetry amplitude is on average around 50% smaller than found at flight level, and while the asymmetry amplitude grows in proportion to storm translation speed at the flight level, no significant growth at the surface is observed, contrary to conventional assumption. However, a significant upwind storm-motion-relative phase rotation is found at the surface as translation speed increases, while the flight-level phase remains fairly constant. After removing the estimated impact of storm motion on the asymmetry, a significant residual shear direction-relative asymmetry is found, particularly at the ...