Showing papers on "Amplitude published in 2004"
TL;DR: In this article, a simplified parameterization is proposed based on a representative amplitude, pulse period, and number of significant pulses in the velocity-time history to estimate the peak ground velocity and period of the velocity pulse (Tv) of available forward-directivity motions.
575 citations
TL;DR: In this paper, the authors introduce a family of approximate (post-Newtonian) capture waveforms, given in nearly analytic form, for use in advancing LISA studies until more accurate versions are available.
Abstract: Captures of stellar-mass compact objects (COs) by massive $(\ensuremath{\sim}{10}^{6}{M}_{\ensuremath{\bigodot}})$ black holes (MBHs) are potentially an important source for LISA, the proposed space-based gravitational-wave (GW) detector. The orbits of the inspiraling COs are highly complicated; they can remain rather eccentric up until the final plunge, and display extreme versions of relativistic perihelion precession and Lense-Thirring precession of the orbital plane. The amplitudes of the strongest GW signals are expected to be roughly an order of magnitude smaller than LISA's instrumental noise, but in principle (i.e., with sufficient computing power) the GW signals can be disentangled from the noise by matched filtering. The associated template waveforms are not yet in hand, but theorists will very likely be able to provide them before LISA launches. Here we introduce a family of approximate (post-Newtonian) capture waveforms, given in (nearly) analytic form, for use in advancing LISA studies until more accurate versions are available. Our model waveforms include most of the key qualitative features of true waveforms, and cover the full space of capture-event parameters (including orbital eccentricity and the MBH's spin). Here we use our approximate waveforms to (i) estimate the relative contributions of different harmonics (of the orbital frequency) to the total signal-to-noise ratio, and (ii) estimate the accuracy with which LISA will be able to extract the physical parameters of the capture event from the measured waveform. For a typical source (a ${10M}_{\ensuremath{\bigodot}}$ CO captured by a ${10}^{6}{M}_{\ensuremath{\bigodot}}$ MBH at a signal-to-noise ratio of 30), we find that LISA can determine the MBH and CO masses to within a fractional error of $\ensuremath{\sim}{10}^{\ensuremath{-}4},$ measure ${S/M}^{2}$ (where S and M are the MBH's mass and spin) to within $\ensuremath{\sim}{10}^{\ensuremath{-}4},$ and determine the location to the source on the sky to within $\ensuremath{\sim}{10}^{\ensuremath{-}3}$ stradians.
479 citations
TL;DR: In this article, the SDSS Ly-alpha forest P_F(k,z) measurement is used to determine the linear theory power spectrum, which is based on fully hydrodynamic simulations, extended using hydro-PM simulations.
Abstract: We analyze the SDSS Ly-alpha forest P_F(k,z) measurement to determine the linear theory power spectrum. Our analysis is based on fully hydrodynamic simulations, extended using hydro-PM simulations. We account for the effect of absorbers with damping wings, which leads to an increase in the slope of the linear power spectrum. We break the degeneracy between the mean level of absorption and the linear power spectrum without significant use of external constraints. We infer linear theory power spectrum amplitude Delta^2_L(k_p=0.009s/km,z_p=3.0)=0.452_{-0.057-0.116}^{+0.069+0.141} and slope n_eff=-2.321_{-0.047-0.102}^{+0.055+0.131} (possible systematic errors are included through nuisance parameters in the fit - a factor >~5 smaller errors would be obtained on both parameters if we ignored modeling uncertainties). The errors are correlated and not perfectly Gaussian, so we provide a chi^2 table to accurately describe the results. The result corresponds to sigma_8=0.85, n=0.94, for a LCDM model with Omega_m=0.3, Omega_b=0.04, and h=0.7, but is most useful in a combined fit with the CMB. The inferred curvature of the linear power spectrum and the evolution of its amplitude and slope with redshift are consistent with expectations for LCDM models, with the evolution of the slope, in particular, being tightly constrained. We use this information to constrain systematic contamination, e.g., fluctuations in the UV background. This paper should serve as a starting point for more work to refine the analysis, including technical improvements such as increasing the size and number of the hydrodynamic simulations, and improvements in the treatment of the various forms of feedback from galaxies and quasars.
342 citations
TL;DR: In this paper, the authors used the LUQAS sample, a set of 27 high-resolution and high signal-to-noise ratio quasi-stellar object (QSO) absorption spectra at a median redshift of z = 2.25, together with a large suite of highresolution large box-size hydrodynamical simulations, to estimate the linear dark matter power spectrum on scales 0.003 < k < 0.03 s km -1.
Abstract: We use the LUQAS sample, a set of 27 high-resolution and high signal-to-noise ratio quasi-stellar object (QSO) absorption spectra at a median redshift of z = 2.25, and the data from Croft et al. at a median redshift of z = 2.72, together with a large suite of high-resolution large box-size hydrodynamical simulations, to estimate the linear dark matter power spectrum on scales 0.003 < k < 0.03 s km -1 . Our reanalysis of the Croft et al. data agrees well with their results if we assume the same mean optical depth and gas temperature-density relation. The inferred linear dark matter power spectrum at z = 2.72 also agrees with that inferred from LUQAS at lower redshift if we assume that the increase of the amplitude is due to gravitational growth between these redshifts. We further argue that the smaller mean optical depth measured from high-resolution spectra is more accurate than the larger value obtained from low-resolution spectra by Press et al. which Croft et al. used. For the smaller optical depth we obtain a 20 per cent higher value for the rms fluctuation amplitude of the matter density. By combining the amplitude of the matter power spectrum inferred from the Lya forest with the amplitude on large scales inferred from measurements of the CMB we obtain constraints on the primordial spectral index n and the normalization σ 8 . For values of the mean optical depth favoured by high-resolution spectra, the inferred linear power spectrum is consistent with a ACDM model with a scale-free (n = 1) primordial power spectrum.
322 citations
TL;DR: Using long photons of different frequencies emitted from an atom-cavity system, a quantum beat with a visibility close to 100% is observed in the correlation between the photodetections at the output ports of the beam splitter.
Abstract: The interference of two single photons impinging on a beam splitter is measured in a time-resolved manner. Using long photons of different frequencies emitted from an atom-cavity system, a quantum beat with a visibility close to 100% is observed in the correlation between the photodetections at the output ports of the beam splitter. The time dependence of the beat amplitude reflects the coherence properties of the photons. Most remarkably, simultaneous photodetections are never observed, so that a temporal filter allows one to obtain perfect two-photon coalescence even for nonperfect photons.
291 citations
TL;DR: In this paper, the Lomb-Scargle statistic was used to estimate the false alarm probability associated with a Keplerian fit, and the effect of eccentricity on detectability was investigated.
Abstract: Radial velocity surveys are beginning to reach the time baselines required to detect Jupiter analogs, as well as sub-Saturn mass planets in close orbits. Therefore it is important to understand the sensitivity of these surveys at long periods an d low amplitudes. In this paper, I derive analytic expressions for the detectability of plane ts at both short and long periods, for circular and eccentric orbits. In the long period regime, th e scaling of the detection threshold with period depends on the desired detection efficiency. The 99% velocity threshold scales as K / P 2 / a 3 , whereas the 50% velocity threshold scales as K / P / a 3/2 . I suggest an extension of the Lomb-Scargle statistic to Keplerian orbits, and describe how to estimate the false alarm probability associated with a Keplerian fit. I us e this Keplerian periodogram to investigate the effect of eccentricity on detectability. A t short periods, detectability is reduced for eccentric orbits, mainly due to the sparse sampling of th e periastron passage, whereas long period orbits are easier to detect on average if they are eccentric because of the steep velocity gradients near periastron. Fitting Keplerian orb its allows the lost sensitivity at short orbital periods to be recovered for e . 0.6, however there remain significant selection effects against eccentric orbits for e & 0.6, and the small number of highly eccentric planets discovered so far may reflect this. Finally, I present a Bayes ian approach to the periodogram which gives a simple derivation of the probability distribu tions of noise powers, clarifies why the periodogram is an appropriate way to search for long period signals, and emphasises the equivalence of periodogram and least squares techniques.
272 citations
TL;DR: In this article, the authors investigated transverse oscillations in coronal loops of a post-flare loop arcade and deduced oscillation signatures such as displacement amplitude, period, phase and damping time.
Abstract: TRACE observations from 15 April 2001 of transverse oscillations in coronal loops of a post-flare loop arcade are investigated. They are considered to be standing fast kink oscillations. Oscillation signatures such as displacement amplitude, period, phase and damping time are deduced from 9 loops as a function of distance along the loop length. Multiple oscillation modes are found with different amplitude profile along the loop length, suggesting the presence of a second harmonic. The damping times are consistent with the hypothesis of phase mixing and resonant absorption, although there is a clear bias towards longer damping times compared with previous studies. The coronal magnetic field strength and coronal shear viscosity in the loop arcade are derived.
267 citations
TL;DR: In this article, a large suite of high-resolution large box-size hydro-dynamical simulations is used to estimate the linear dark matter power spectrum on scales 0.003 s/km < k < 0.03 s/ km.
Abstract: We use the LUQAS sample (Kim et al. 2004), a set of 27 high-resolution and high signal-to-noise QSO absorption spectra at a median redshift of z=2.25, and the data from Croft et al. (2002) at a median redshift of z=2.72, together with a large suite of high-resolution large box-size hydro-dynamical simulations, to estimate the linear dark matter power spectrum on scales 0.003 s/km < k <0.03 s/km. Our re-analysis of the Croft et al. data agrees well with their results if we assume the same mean optical depth and gas temperature-density relation. The inferred linear dark matter power spectrum at z=2.72 also agrees with that inferred from LUQAS at lower redshift if we assume that the increase of the amplitude is due to gravitational growth between these redshifts. We further argue that the smaller mean optical depth measured from high-resolution spectra is more accurate than the larger value obtained from low-resolution spectra by Press et al. (1993) which Croft et al. used. For the smaller optical depth we obtain a ~ 20% higher value for the rms fluctuation amplitude of the matter density. By combining the amplitude of the matter power spectrum inferred from the Lyman-alpha forest with the amplitude on large scales inferred from measurements of the CMB we obtain constraints on the primordial spectral index n and the normalisation sigma_8. For values of the mean optical depth favoured by high-resolution spectra, the inferred linear power spectrum is consistent with a LambdaCDM model with a scale-free (n=1) primordial power spectrum.
254 citations
University of Edinburgh1, Durham University2, Queen's University3, Liverpool John Moores University4, University of New South Wales5, Rutherford Appleton Laboratory6, University of Oxford7, Australian National University8, University of Cambridge9, California Institute of Technology10, Johns Hopkins University11, Australia Telescope National Facility12, University College London13, University of Leeds14, University of Nottingham15, ETH Zurich16
TL;DR: In this paper, a decomposition of the 2dF Galaxy Redshift Survey (2dFGRS) galaxy overdensity field into an orthonormal basis of spherical harmonics and spherical Bessel functions is presented.
Abstract: We present the result of a decomposition of the 2dF Galaxy Redshift Survey (2dFGRS) galaxy overdensity field into an orthonormal basis of spherical harmonics and spherical Bessel functions. Galaxies are expected to directly follow the bulk motion of the density field on large scales, so the absolute amplitude of the observed large-scale redshift-space distortions caused by this motion is expected to be independent of galaxy properties. By splitting the overdensity field into radial and angular components, we linearly model the observed distortion and obtain the cosmological constraint Omega(m)(0.6)sigma(8)=0.46+/-0.06. The amplitude of the linear redshift-space distortions relative to the galaxy overdensity field is dependent on galaxy properties and, for L-*, galaxies at redshift z=0, we measure beta(L-*, 0)=0.58+/-0.08, and the amplitude of the overdensity fluctuations b(L-*, 0)sigma(8)=0.79+/-0.03, marginalizing over the power spectrum shape parameters. Assuming a fixed power spectrum shape consistent with the full Fourier analysis produces very similar parameter constraints.
229 citations
TL;DR: In this article, a database of 1700 digital seismograms from 186 earthquakes of magnitude mN 2.5-5.6 that occurred in southeastern Canada and the northeastern United States from 1990 to 2003 was compiled.
Abstract: A database of 1700 digital seismograms from 186 earthquakes of mag- nitude mN 2.5-5.6 that occurred in southeastern Canada and the northeastern United States from 1990 to 2003 was compiled. Maximum-likelihood regression analysis of the database was performed to determine a model for the attenuation of Fourier spectral amplitudes for the shear window, for the vertical and horizontal component of motion, for frequencies from 0.2 to 20 Hz. Fourier amplitudes follow a hinged trilinear attenuation model. Fourier spectral amplitudes decay as R 1.3 (where R is hypocentral distance) within 70 km of the source. There is a transition zone from 70 to 140 km as the direct waves are joined by strong postcritical reflections, where the attenuation is described as R 0.2 ; spectral amplitudes actually increase with distance in this range for low frequencies. Beyond 140 km, the attenuation is well described by R 0.5 , corresponding to geometric spreading in two dimensions. The associated model for the regional quality factor for frequencies greater than 1 Hz can be ex- pressed as Q 893f 032 . Q can be better modeled over a wider frequency range (0.2- 20 Hz) by a polynomial expression: log Q 3.052 0.393 log f 0.945 (log f ) 2 0.327 (log f ) 3 . The polynomial expression accommodates the observation that Q values are at a minimum (about 1000) near 1 Hz and rise at both lower and higher frequencies. Correction factors for the spectral amplitude model that describe the effects of focal depth on the amplitudes and their attenuation are developed using the subset of events with known focal depth. The attenuation model is similar to that determined from an earlier study with more limited data (Atkinson and Mereu, 1992), but the enlarged database indicates more rapid near-source amplitude decay and higher Q. The attenuation model is used to play back attenuation effects to determine the apparent source spectrum for each earthquake in the database and hence determine moment magnitude (M) and Brune stress drop. The events have moment magnitude in the range from 2.5 to 5. Stress drop increases with moment magnitude for events of M 4.3, then appears to attain a relatively constant level in the range from 100 to 200 bars for the larger events, as previously noted in Atkinson (1993b). The results of this study provide a useful framework for improving regional ground-motion relations in eastern North America. They further our understanding of attenuation in the region through analysis of an enlarged ground-motion database. In particular, the inclusion of the three-component broadband data gathered over the last decade allows extension of attenuation models to both horizontal and vertical components over a broad frequency range (0.2-20 Hz).
229 citations
TL;DR: The hysteresis acts as “solid” friction on the contour oscillations, and gives rise to a stick-slip regime at intermediate amplitude, which is interpreted as the decrease of the resonance frequencies at larger vibration amplitudes.
Abstract: We study the effects of vertical vibrations on non-wetting large water sessile drops flattened by gravity. The solid substrate is characterized by a finite contact angle hysteresis (10-15 degrees). By varying the frequency and the amplitude of the vertical displacement, we observe two types of oscillations. At low amplitude, the contact line remains pinned and the drop presents eigen modes at different resonance frequencies. At higher amplitude, the contact line moves: it remains circular but its radius oscillates at the excitation frequency. The transition between these two regimes arises when the variations of contact angle exceed the contact angle hysteresis. We interpret different features of these oscillations, such as the decrease of the resonance frequencies at larger vibration amplitudes. The hysteresis acts as “solid” friction on the contour oscillations, and gives rise to a stick-slip regime at intermediate amplitude.
TL;DR: In this paper, a comparison of laboratory-modeling results with a diffusive-viscous-theory model showed that low (<5) values of the quality factor Q can explain the observations of frequency dependence.
Abstract: There is a complex relationship between seismic attributes, including the frequency dependence of reflections and fluid saturation in a reservoir. Observations in both laboratory and field data indicate that reflections from a fluid-saturated layer have an increased amplitude and delayed traveltime at low frequencies, when compared with reflections from a gas-saturated layer. Comparison of laboratory-modeling results with a diffusive-viscous-theory model show that low (<5) values of the quality factor Q can explain the observations of frequency dependence. At the field scale, conventional processing of time-lapse VSP data found minimal changes in seismic response of a gas-storage reservoir when the reservoir fluid changed from gas to water. Lowfrequency analysis found significant seismic-reflectionattribute variation in the range of 15‐50 Hz. The field observations agree with effects seen in laboratory data and predicted by the diffusive-viscous theory. One explanation is that very low values of Q are the result of internal diffusive losses caused by fluid flow. This explanation needs further theoretical investigation. The frequencydependent amplitude and phase-reflection properties presented in this paper can be used for detecting and monitoring fluid-saturated layers.
TL;DR: In this paper, a fully non-linear finite difference model was developed based on inviscid flow equations for liquid sloshing induced by harmonic base excitations, which is valid for any water depth except for small depth when viscous effects would become important.
TL;DR: The effect of noise on images obtained using quantitative phase‐amplitude microscopy is explored and an approach that allows good‐quality quantitative phase retrieval to be obtained even for very noisy data is proposed.
Abstract: We explore the effect of noise on images obtained using quantitative phase-amplitude microscopy - a new microscopy technique based on the determination of phase from the intensity evolution of propagating radiation. We compare the predictions with experimental results and also propose an approach that allows good-quality quantitative phase retrieval to be obtained even for very noisy data.
TL;DR: In this paper, a small-amplitude Tollmien-Schlichting wave (TS wave) was introduced into the boundary layer without (case I) and with (case II) spanwise forcing of steady 3D disturbances.
Abstract: When a laminar boundary layer separates be- cause of an adverse streamwise pressure gradient, the flow is subject to increased instability with respect to small- amplitude disturbances. Laminar-turbulent transition oc- curs under a rapid three-dimensional (3D) development within the separated shear layer. When the following tur- bulent boundary layer reattaches, a laminar separation bubble is formed. To allow controlled measurements, a small-amplitude Tollmien-Schlichting wave (TS wave) was introduced into the boundary layer without (case I) and with (case II) spanwise forcing of steady 3D disturbances. Combined application of laser-Doppler anemometry (LDA) and particle image velocimetry (PIV) demonstrates the suitability of both measurement techniques to capture the development of unsteady, periodic phenomena. The transition mechanism occurring in the flow field under consideration is discussed, and results obtained by con- trolled measurements are compared to direct numerical simulations (DNS) and predictions from linear stability theory (LST). Flow visualizations and stereoscopic PIV measurements give better insight into the 3D breakdown of the separated shear layer.
TL;DR: In this article, a linear superposition of two macroscopically distinguishable optical coherent states can be generated using a single photon source and simple all-optical operations, which does not need photon number resolving measurements nor Kerr-type nonlinear interactions.
Abstract: It is shown that a linear superposition of two macroscopically distinguishable optical coherent states can be generated using a single photon source and simple all-optical operations. Weak squeezing on a single photon, beam mixing with an auxiliary coherent state, and photon detecting with imperfect threshold detectors are enough to generate a coherent state superposition in a free propagating optical field with a large coherent amplitude (alpha>2) and high fidelity (F>0.99). In contrast to all previous schemes to generate such a state, our scheme does not need photon number resolving measurements nor Kerr-type nonlinear interactions. Furthermore, it is robust to detection inefficiency and exhibits some resilience to photon production inefficiency.
TL;DR: In this article, the authors report results from their $z\simeq 5.5$--7 galaxy search in the HST Ultra Deep Field (UDF) using the 400-orbit of ACS data.
Abstract: We report results from our $z\simeq 5.5$--7 galaxy search in the HST Ultra Deep Field (UDF). Using the 400-orbit of ACS data, we found 108 plausible $5.5\leq z\leq 6.5$ (or $z\simeq 6$ for short) candidates to $m_{AB}(z_{850})=30.0$ mag. The contamination to the sample, either due to image artifacts or known types of astronomical objects, is likely negligible. The inferred surface densities of $z\simeq 6$ galaxies are consistent with our earlier predictions from $m_{AB}(z_{850})=26.5$ to 28.5 mag. After correcting for detection incompleteness, the counts of $z\simeq 6$ candidates to $m_{AB}(z_{850})=29.2$ mag suggests that the faint-end slope of the galaxy luminosity function (LF) at this redshift is likely between $\alpha=-1.8$ and -1.9, which is sufficient to account for the entire Lyman photon budget necessary to complete the reionization of the universe at $z\simeq 6$. We also searched for $z\simeq 6.5$--7 candidates using the UDF NICMOS data, and have found four candidates to $J_{110}=27.2$ mag. However, the infrared colors of three candidates cannot be easily explained by galaxies in this redshift range. We tentatively derive an upper limit to the cumulative surface density of galaxies at $z\simeq 7$ of 0.36 per arcmin$^2$ to $J_{110}=26.6$ mag, which suggest a noticeable drop in the LF amplitude from $z\simeq 6$ to $z\simeq 7$.
TL;DR: In this article, a comparison of ray-theoretical and finite-frequency travel-time tomography for compressional waves is presented, showing that wave front healing cannot be neglected when interpreting long-period seismic waves.
Abstract: SUMMARY This paper presents a comparison of ray-theoretical and finite-frequency traveltime tomography for compressional waves. Our data set consists of 86 405 long-period P and PP‐P traveltimes measured by cross-correlation. The traveltime of a finite-frequency wave is sensitive to anomalies in a hollow banana-shaped region surrounding the unperturbed ray path, with the sensitivity being zero on the ray. Because of the minimax nature of the surface-reflected PP wave, its sensitivity is more complicated. We compute the 3-D traveltime sensitivity efficiently by using the paraxial approximation in conjunction with ray theory and the Born approximation. We compare tomographic models with the same χ 2 fit for both ray theory and finite-frequency analysis. Depending on the depth and size of the anomaly, the amplitudes of the velocity perturbations in the finite-frequency tomographic images are 30‐50 per cent larger than in the corresponding ray-theoretical images, demonstrating that wave front healing cannot be neglected when interpreting long-period seismic waves. The images obtained provide clear evidence that a limited number of hotspots are fed by plumes originating in the lower mantle.
TL;DR: In this paper, the average shape of a large crest is found to be the scaled auto-correlation function, as is shown mathematically by Lindgren [Lindgren G. 1970;41:1870-83] and Boccotti [Boccotti P. 1983;5:134-40].
TL;DR: In this article, the effects of solar flares on the D-region electron densities were investigated using the NOSC Earth-ionosphere waveguide programs (LWPC and ModeFinder) over a wide range of VLF frequencies.
TL;DR: In this paper, the wave amplitude and seasonal variability in the mesosphere and lower thermosphere region of the world were analyzed using the National Center for Atmospheric Research (NCER) time-varying general circulation model.
Abstract: [1] The zonal wave number 1 planetary wave of period near 6.5 days is a robust feature in the mesosphere and lower thermosphere (MLT) region with prominent seasonal variability as revealed by ground based and satellite observations. This wave and its seasonal variability are well reproduced in a recent one model year run of the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) with its lower boundary specified according to the National Centers for Environmental Prediction analysis (year 1993). Wavelet analysis of the model output shows that in the MLT region the wave maximizes before and after the equinoxes and minimizes at solstices. The wave amplitudes at the equinoxes are smaller than the peaks before and after but are still larger than the wave amplitudes at solstices. However, at the lower boundary near 30 km the wave peaks are predominantly between fall and the following spring. By examining the episodes of maximum and minimum wave amplitude and by conducting additional control experiments using the TIME-GCM, the structure of this planetary wave and the factors determining the wave characteristics and seasonal variability are studied in detail. It is found that the wave source, mean wind structure, instability, and the critical layers of the wave can all affect the wave response in the MLT region and can have a strong seasonal dependence. Before and after equinox, the wave follows the waveguide and propagates from the stratosphere to the summer mesosphere/mesopause, where it may amplify due to baroclinic/barotropic instability. Such instability is usually absent from the equinoctial atmosphere, so that there is no wave amplification at equinox. At solstice the wave decays significantly when propagating away from its winter source due to the strong eastward winter stratospheric jet. In the summer side the westward jet is also strong, and the meridional and vertical extension of the critical layer of the wave is large enough to enclose the instability in the summer mesosphere/mesopause at middle to high latitudes. The wave is thus reflected away and prevented from reaching and amplifying at the unstable region. The seasonal variation of the quasi-two-day wave, which has zonal phase speed similar to the 6.5-day wave, is also studied using similar diagnostics. It is further shown that within certain seasonal “window” periods, the variability in the MLT, especially the summer MLT, may closely track the lower atmospheric variability associated with these waves.
TL;DR: In this article, the authors used multoint measurements obtained by the Cluster spacecraft in the premidnight equatorial region outside the plasmasphere at a radial distance of 4 Earth radii (L = 4.0 − 4.2).
Abstract: We investigate intense whistler-mode chorus emissions which occurred during the geomagnetic storm on 31 March 2001. We use multipoint measurements obtained by the Cluster spacecraft in the premidnight equatorial region outside the plasmasphere at a radial distance of 4 Earth radii (L = 4.0 − 4.2). Observed spatio-temporal variations of the direction of the Poynting flux manifest a consistent pattern: the central position of the chorus source fluctuates at time scales of minutes within 1000–2000 km of the geomagnetic equator. We demonstrate that estimates of the electromagnetic planarity can be used to characterize the extent of the source, obtaining a range of 3000–5000 km. Discrete wave packets of chorus are observed to rise in frequency between 0.13 and 0.5 of the local electron cyclotron frequency, at a rate up to 20 kHz/s, having the maximum peak amplitudes of ∼20 mV/m. We observe a fine structure of subpackets with large amplitudes embedded in the interior of the wave packets. This fine structure has a typical delay of a few milliseconds between the two neighboring maxima of the wave amplitude. Longer delays occur with a decreasing probability density.
TL;DR: In this paper, a degree-8 3D Q model (QRLW8) of the upper mantle, derived from three-component surface waveform data in the period range 60-400 s, is presented.
Abstract: SUMMARY
We present a degree-8 3-D Q model (QRLW8) of the upper mantle, derived from three-component surface waveform data in the period range 60–400 s. The inversion procedure involves two steps. In the first step, 3-D whole-mantle velocity models are derived separately for elastic SH (transverse component) and SV (vertical and longitudinal component) velocity models, using both surface and body waveforms and the non-linear asymptotic coupling theory (NACT) approach. In the second step, the surface waveforms thus aligned in phase are inverted to obtain a 3-D Q model in the depth range 80–670 km. Various stability tests are performed to assess the quality of the resulting Q model and, in particular, to assess possible contamination from focusing effects. We find that the 3-D patterns obtained are stable, but the amplitude of the lateral variations in Q is not well constrained, because large damping is necessary to extract the weak Q signal from data. The model obtained agrees with previous results in that a strong correlation of Q with tectonics is observed in the first 250 km of the upper mantle, with high attenuation under oceanic regions and low attenuation under continental shields. It is gradually replaced by a simpler pattern at larger depth. At the depths below 400 km, the Q distribution is generally dominated by two strong minima, one under the southern Pacific and one under Africa, yielding a strong degree-2 pattern. Most hotspots are located above regions of low Q at this depth. Ridges are shallow features in both velocity and Q models.
TL;DR: It is shown from a consideration of different characteristic waveforms that there is typically an optimal drive amplitude for the most consistent response; internal noise sources dominate for small amplitude driving while deterministic system nonlinearity reduces consistency for large amplitudes.
Abstract: The consistency of a nonlinear system's response to a repeated complex waveform drive signal is an important consideration in classical and quantum systems as diverse as lasers, neuronal networks, and manufacturing plants. We show from a consideration of different characteristic waveforms that there is typically an optimal drive amplitude for the most consistent response; internal noise sources dominate for small amplitude driving while deterministic system nonlinearity reduces consistency for large amplitudes. We test this general concept and its measurement experimentally and numerically on the specific example of a laser system.
TL;DR: In this paper, the authors investigate the hydrodynamic interactions between oscillating flexible cylinders and fluid forces using a discrete vortex method, and compare results obtained using the quasi-steady theory, as proposed by Bearman et al.
TL;DR: In this paper, the googly amplitudes in gauge theory are computed by using the off shell MHV vertices with the newly proposed rules of Cachazo, Svrcek and Witten.
Abstract: The googly amplitudes in gauge theory are computed by using the off shell MHV vertices with the newly proposed rules of Cachazo, Svrcek and Witten. The result is in agreement with the previously well-known results. In particular we also obtain a simple result for the all negative but one positive helicity amplitude when one of the external line is off shell.
TL;DR: In this article, the amplitude and frequency dependence of the noise in superconducting transition-edge sensors with eight different geometries was measured and it was shown that the unexplained noise contribution is inversely correlated with the temperature width of the super-conducting-to-normal transition.
Abstract: The noise in superconducting transition-edge sensors (TESs) commonly exceeds simple theoretical predictions. The reason for this discrepancy is presently unexplained. We have measured the amplitude and frequency dependence of the noise in TES sensors with eight different geometries. In addition, we have measured the dependence of the noise on operating resistance, perpendicular magnetic field, and bath temperature. We find that the unexplained noise contribution is inversely correlated with the temperature width of the superconducting-to-normal transition and is reduced by a perpendicular field and in certain geometries. These results suggest paths to improved sensor performance.
TL;DR: In this article, the Ca II K line chromospheric fluxes measured from high-resolution spectra in 35 G dwarf stars of 5 open clusters were used to determine the age-activity-rotation relationship from the young Hyades and Praesepe (0.6 Gyr) to the old M 67 (∼4.5 Gyr).
Abstract: We present Ca II K line chromospheric fluxes measured from high-resolution spectra in 35 G dwarf stars of 5 open clusters to determine the age-activity-rotation relationship from the young Hyades and Praesepe (0.6 Gyr) to the old M 67 (∼4.5 Gyr) through the two intermediate age clusters IC 4651 and NGC 3680 (∼1.7 Gyr). The full amplitude of the activity index within a cluster is slightly above 60 % for all clusters but one, NGC 3680, in which only two stars were observed. As a comparison, the same Solar Ca II index varies by ∼40% during a solar cycle. Four of our clusters (Hyades and Praesepe, IC 4651 and NGC 3680) are pairs of twins as far as age is concerned; the Hyades have the same chromospheric-activity level as Praesepe, at odds with early claims based on X-ray observations. Both stars in NGC 3680 are indistinguishable, as far as chromospheric activity is concerned, from those in the coeval IC 4651. This is a validation of the existence of an age-activity relationship. On the other hand, the two intermediate age clusters have the same activity level as the much older M 67 and the Sun. Our data therefore shows that a dramatic decrease in chromospheric activity takes place in solar stars between the Hyades and the IC 4651 age, of about 1 Gyr. Afterwards, activity remains virtually constant for more than 3 Gyr. We have also measured v sini for all of our stars and the average rotational velocity shows the same trend as the chromospheric-activity index. We briefly investigate the impact of this result on the age determinations of field G dwarfs in the solar neighborhood; the two main conclusions are that a consistent group of "young" stars (i.e. as active as Hyades stars) is present, and that it is virtually impossible to give accurate chromospheric ages for stars older than ∼2 Gyr. The observed abrupt decline in activity explains very well the Vaughan-Preston gap.
TL;DR: In this paper, a numerical procedure for the simulation of non-Gaussian surfaces has been developed, which can simulate surfaces with given skewness and kurtosis and with spectral density or auto-correlation function.
TL;DR: In this article, the authors employ a simple hot spot model to explain the position and amplitude of high-frequency quasi-periodic oscillations in the X-ray flux from accreting black hole binary systems.
Abstract: Observations from the Rossi X-Ray Timing Explorer have shown the existence of high-frequency quasi-periodic oscillations (HFQPOs) in the X-ray flux from accreting black hole binary systems. In at least two systems, these HFQPOs come in pairs with a 2 : 3 frequency commensurability. We employ a simple "hot spot" model to explain the position and amplitude of the HFQPO peaks. Using the exact geodesic equations for the Kerr metric, we calculate the trajectories of massive test particles, which are treated as isotropic, monochromatic emitters in their rest frames. Photons are traced from the accretion disk to a distant observer to produce time- and frequency-dependent images of the orbiting hot spot and background disk. The power spectrum of the X-ray light curve consists of multiple peaks at integral combinations of the black hole coordinate frequencies. In particular, if the radial frequency is one-third of the azimuthal frequency (as is the case near the innermost stable circular orbit), beat frequencies appear in the power spectrum at two-thirds and four-thirds of the fundamental azimuthal orbital frequency, in agreement with observations. In addition, we model the effects of shearing the hot spot in the disk, producing an arc of emission that also follows a geodesic orbit, as well as the effects of nonplanar orbits that experience Lens-Thirring precession around the black hole axis. By varying the arc length, we are able to explain the relative amplitudes of the QPOs at either 2ν or 3ν in observations from XTE J1550-564 and GRO J1655-40. In the context of this model, the observed power spectra allow us to infer values for the black hole mass and angular momentum and also constrain the parameters of the model, such as the hot spot size and luminosity.