bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparisons of in situ measurements of cirrus cloud ice water content
Authors: Davis Sean M. , Avallone Linnea M. , Weinstock Elliot M. , Twohy Cynthia H. , Smith Jessica B. , Kok Gregory L.
Published: May 2007

Aircraft‐based in situ measurements of cirrus cloud ice water content (IWC) are important for cloud microphysical/radiative modeling and satellite validation studies, yet the measurements have proven challenging due, in part, to the large dynamic range of IWC values present in cirrus clouds. To date, three instruments designed for the measurement of IWC have been flown aboard the NASA WB‐57F research aircraft: the University of Colorado closed‐path laser hygrometer, the Droplet Measurement Technologies Cloud Spectrometer and Impactor, and the Harvard University Lyman‐α total water photofragment‐fluorescence hygrometer. This paper compares IWC measurements from these three instruments taken during the Midlatitude Cirrus Experiment (MidCiX) in 2004. At larger values of IWC (IWC > 10 mg m−3), the three instruments agree, on average, to within 20%, which is of the order of their estimated instrumental uncertainties. At smaller IWC values (<10 mg m−3), the agreement is worse, in part due to increasing instrument uncertainties. These results have implications for measurements in thin and subvisual cirrus. Particle sampling and evaporation, instrument background levels, and hysteresis are not found to be significant contributors to discrepancies among the measurements. For remote sensing validation studies where IWC data are vertically integrated to obtain ice water path, the agreement between the instruments is better than 20% for thick cirrus (τ > 1), implying that IWC measurements in thicker clouds are of sufficient accuracy for validation studies.
In situ measurements of the thermal properties of a northern peatland: Implications for peatland temperature models
Authors: Kettridge N. , Baird A.
Published: May 2007

Carbon uptake/release in peat soils (the latter especially in the form of methane) is strongly dependent on soil temperature. However, previous peatland temperature models have not represented soil temperatures accurately due to either errors in their parameterization or errors in the representation of heat transfer processes. Dual probe heat pulse sensors (DPHPS) and triple‐probe heat pulse sensors (TPHPS) were used to provide detailed in situ measurements of the soil thermal properties of Sphagnum peat and the advective liquid heat flux resulting from percolating rainfall. The difficulties of applying the DPHPS approach within poorly decomposed Sphagnum peat are first considered. Our results suggest that measurement‐induced advective vapor and liquid heat fluxes occurred when using the DPHPS. The analytical equations normally used to calculate the thermal diffusivity and volumetric heat capacity (C) from DPHPS measurements, therefore, cannot be applied within unsaturated Sphagnum peat. However, a numerical approach enabled the soil thermal properties to be estimated when an instrument‐induced advective vapor flux was significant. When an instrument‐induced advective liquid heat flux was significant, the relevant DPHPS data were identified and rejected. The measurements of the soil thermal properties were used to (1) develop an empirical model of the vertical variations in C through the unsaturated zone of poorly decomposed Sphagnum peat and (2) to develop an empirical relationship between the thermal conductivity (k) and C. The TPHPS measurements showed that the advective heat flux due to percolating rainfall occurred for a maximum of 24 hours after rainfall events. Its influence on soil temperatures may therefore be negligible.
Numerical simulation of polarimetric radar pulse echoes from the lunar regolith layer with scatter inhomogeneity and rough interfaces
Authors: Jin Ya‐Qiu , Xu Feng , Fa Wenzhe
Published: May 2007

To explore the potential utilities of lower‐frequency (L band) radar pulse penetration on Moon exploration, a theoretical model of stratified lunar regolith media and numerical simulation of polarimetric radar pulse echoes are developed. The lunar regolith layer consists of the low lossy regolith layer with randomly rough top and bottom interfaces, and a layer of random stone scatterers (spatially oriented oblate spheroids are assumed) is embedded and overlays the underlying rock media. The time domain Mueller matrix solution derived from vector radiative transfer formulations contains seven scattering mechanisms of the stratified media: surface scattering from the rough top and bottom interfaces, volumetric scattering from random stone scatterers, and their multi‐interactions. Temporal characteristics and structure of the polarimetric echo profile as functional dependence on model parameters such as the layer thickness and the content of FeO + TiO2, are numerically simulated and well display an image of regolith structures. Polarimetric pulse echoes might reveal rich information on the lunar regolith layer depth and other structure properties and might demonstrate a potential new way to explore the Moon surface in the future.
A field assessment of the value of steady shape hydraulic tomography for characterization of aquifer heterogeneities
Authors: Bohling Geoffrey C. , Butler James J. , Zhan Xiaoyong , Knoll Michael D.
Published: May 2007

Hydraulic tomography is a promising approach for obtaining information on variations in hydraulic conductivity on the scale of relevance for contaminant transport investigations. This approach involves performing a series of pumping tests in a format similar to tomography. We present a field‐scale assessment of hydraulic tomography in a porous aquifer, with an emphasis on the steady shape analysis methodology. The hydraulic conductivity (K) estimates from steady shape and transient analyses of the tomographic data compare well with those from a tracer test and direct‐push permeameter tests, providing a field validation of the method. Zonations based on equal‐thickness layers and cross‐hole radar surveys are used to regularize the inverse problem. The results indicate that the radar surveys provide some useful information regarding the geometry of the K field. The steady shape analysis provides results similar to the transient analysis at a fraction of the computational burden. This study clearly demonstrates the advantages of hydraulic tomography over conventional pumping tests, which provide only large‐scale averages, and small‐scale hydraulic tests (e.g., slug tests), which cannot assess strata connectivity and may fail to sample the most important pathways or barriers to flow.
Spatial and temporal variation of dissolved organic carbon export from gauged and ungauged watersheds of Dee Valley, Scotland: Effect of land cover and C:N
Authors: Aitkenhead‐Peterson J. A. , Smart R. P. , Aitkenhead M. J. , Cresser M. S. , McDowell W. H.
Published: May 2007

Dissolved organic carbon (DOC) concentration and export were quantified in 56 streams within the Dee Valley, Scotland. Percentage land cover was calculated for each of the watersheds. For a subset of 21 watersheds, organic and mineral soil layers were collected for carbon and nitrogen analysis. Peat cover derived from land cover data was a good predictor of annual DOC export for the selected 21 upland watersheds but not for the set of 56 watersheds. Using DOC values and mean watershed C:N ratio, we produced an empirical predictive model for estimation of annual DOC export for the 21 watersheds as well as a model of weekly fluxes in two of the larger watersheds to meet the needs of watershed managers. Annual exports for the 56 streams in the Dee Valley ranged from 1700 to 10,500 kg km−2 yr−1. Land cover was not related to annual DOC export for the 56 streams, but by season, arable land and grassland were both correlated to spring DOC export (R = −0.39 arable land and R = −0.33 grassland p<0.01). Summertime DOC export was correlated to heather, montane, and conifer land cover (R = −0.029 heather, R = −0.39 montane, and R = 0.33 conifer, p<0.05). Autumn and winter DOC export was best described by peat (R = 0.29 and 0.39, respectively, p<0.05). In the subset of 21 watersheds with soil information, watershed soil organic matter C:N ratio and peat had a higher correlation with annual DOC concentrations and export than any other land cover class. Source of peat distribution data may impact the relationship between DOC and peat cover, and this issue is examined. Modeled weekly DOC load compared well with observed DOC load in a time series evaluation.
Flow resistance equations for gravel‐ and boulder‐bed streams
Authors: Ferguson Rob
Published: May 2007

Alternative general forms are considered for equations to predict mean velocity over the full range of relative submergence experienced in gravel‐ and boulder‐bed streams. A partial unification is suggested for some previous semiempirical models and physical concepts. Two new equations are proposed: a nondimensional hydraulic geometry equation with different parameters for deep and shallow flows, and a variable‐power resistance equation that is asymptotic to roughness‐layer formulations for shallow flows and to the Manning‐Strickler approximation of the logarithmic friction law for deep flows. Predictions by existing and new equations using D84 as roughness scale are compared to a compilation of measured velocities in natural streams at relative submergences from 0.1 to over 30. The variable‐power equation performs as well as the best existing approach, which is a logarithmic law with roughness multiplier. For predicting how a known or assumed discharge is partitioned between depth and velocity, a nondimensional hydraulic geometry approach outperforms equations using relative submergence. Factor‐of‐two prediction errors occur with all approaches because of sensitivity to operational definitions of depth, velocity, and slope, the inadequacy of using a single grain‐size length scale, and the complexity of flow physics in steep shallow streams.
Numerical studies on soil moisture distributions in heterogeneous catchments
Authors: Settin T. , Botter G. , Rodriguez‐Iturbe I. , Rinaldo A.
Published: May 2007

The paper deals with numerical studies of basin‐scale dynamics of soil moisture in arbitrarily heterogeneous conditions (i.e., in presence of heterogeneity of climate, soil, vegetation and land use). Its relevance stems from comparative analysis of the probabilistic structure of spatially averaged soil moisture fields with the corresponding exact solutions of the underlying simplified stochastic point processes. The probabilistic structure of coarse‐grained soil moisture fields is largely controlled by temporal fluctuations of intermittent rainfall fields. Averaged properties are also affected by heterogeneous soil and vegetation features and by the spatial scale of aggregation. Here, we employ results from extended Montecarlo simulations of a continuous model of the hydrologic response that proved suitable to describe observed events. The comparison of numerically derived soil moisture probability density functions with exact simplified solutions suggests, somewhat unexpectedly, that the analytical model can reasonably describe the large‐scale behavior of spatially‐averaged hydrologic fluxes through physically meaningful, basin‐scale soil and vegetation parameters. The application of a seasonally variable, stochastic climate model shows pronounced daily fluctuations in the relationship between water losses and soil moisture, related to the underlying climatic fluctuations. The resulting spatially averaged soil moisture probability density functions in heterogeneous catchments, however, do not show appreciable differences with respect to the ones obtained assuming constant mean climate conditions. We thus conclude that effective basin‐scale states, which average highly heterogeneous (spatial/temporal) properties allowing exact probabilistic descriptions, indeed exist, with implications for large scale estimates of soil‐atmosphere interactions.
Intercomparison of four different Southern Hemisphere sea level pressure datasets
Authors: Jones P. D. , Lister D. H.
Published: May 2007

Four different monthly sea level pressure (MSLP) datasets for the Southern Hemisphere (SH, south of 15°S) are intercompared for the 1961–2000 period, contrasting the differences in decadal‐average pressure and differences between correlations for the 1961–80 (pre‐satellite measurement era) and the 1991–2000 (post‐satellite) period. Agreement is very dependent on the locations in the observing network, and this becomes even clearer for the two decades (1961–80) before the satellite era. Away from the station locations it is impossible to say which provides the ‘best’ decadal average MSLP values, due to assumptions made in the dataset derivation. Improved reanalyses will be undertaken in the next few years, but they are only likely to provide the true decadal average values of MSLP for the 1961–80 period if a much higher percentage of observations made for this period are digitized, and provided they are quality controlled, assessed for homogeneity and given higher priority within the reanalysis system.
Infrasonic forerunners: Exceptionally fast acoustic phases
Authors: Evers L. G. , Haak H. W.
Published: May 2007

A vapor cloud explosion occurred at an oil depot near Buncefield in the UK in 2005. Three infrasound arrays in the Netherlands detected various stratospheric phases. Some of these phases appeared with celerities, i.e., horizontal propagation velocity, in the conventional range of 0.28 to 0.31 km/s. Exceptionally fast arrivals, infrasonic forerunners, were identified with celerities of 0.31 to 0.36 km/s. These phases could be explained by head‐wave‐like propagation in a high velocity acoustic channel between 40 and 50 km height, where stratospheric zonal winds reached values of 120 m/s. The manifestation of infrasonic forerunners is validated by modeling with raytracing through actual atmospheric models and determining the celerity, apparent velocity and back azimuth. One phase occurred with a celerity of 0.25 km/s. Hence, we propose a new celerity range for fast stratospheric phases of 0.31 to 0.36 km/s and to lower the limit of the conventional range to 0.25 km/s.
Atmospheric CO2 and climate from 65 to 30 ka B.P.
Authors: Ahn Jinho , Brook Edward J.
Published: May 2007

Using new and existing ice core CO2 data from 65 ∼ 30 ka a new chronology for CO2 is established and synchronized with Greenland ice core records to study how high latitude climate change and the carbon cycle were linked during the last glacial period. Atmospheric CO2 rose several thousand years before abrupt warming in Greenland associated with Dansgaard‐Oeschger events, 8, 12, 14, 17, four large warm events that follow Heinrich events. The CO2 rise terminated at the onset of Greenland warming for each of these events. Atmospheric CO2 is strongly correlated with the Antarctic isotopic temperature proxy with an average time lag of 720 ± 370 yr (mean ± 1σ) during the time interval studied. The new data and chronology should provide a better target for models attempting to explain CO2 variability and abrupt climate change.
Uppermost mantle structure and its relation with seismic activity in the central Tien Shan
Authors: Xu Yi , Li Zhiwei , Roecker Steven W.
Published: May 2007

Using arrival data from GHENGIS and Kyrgyz Network (KNET) stations, we determined the uppermost mantle structure of the central Tien Shan from Pn tomography. Pn velocities in the north‐central Tien Shan are slower than normal for stable lithosphere. They are very slow in the south‐central Tien Shan, where a hot mantle previously has been postulated. The Pn velocity variation correlates well with seismicity. Most earthquakes are concentrated in the north‐central Tien Shan, but almost no earthquakes in the south‐central Tien Shan, even though both geologic and GPS observations show active convergence there. This correlation suggests a genetic relationship between seismicity and the state of lithospheric mantle. Specifically, we propose that the very slow Pn velocity beneath the south‐central Tien Shan is caused by high temperatures produced by heating from mantle upwelling, and that this heat has conducted well up into the crust, inducing a ductile rheology that prevents significant seismicity.
Prospects for river discharge and depth estimation through assimilation of swath‐altimetry into a raster‐based hydrodynamics model
Authors: Andreadis Konstantinos M. , Clark Elizabeth A. , Lettenmaier Dennis P. , Alsdorf Douglas E.
Published: May 2007

Surface water elevation profiles for a reach of the Ohio River were produced by the Jet Propulsion Laboratory Instrument Simulator to represent satellite measurements representative of those that would be observed by a wide swath altimeter being considered jointly by U.S. and European space agencies. The Ensemble Kalman filter with a river hydrodynamics model as its dynamical core was used to assimilate the water elevation synthetic observations, and to estimate river discharge. The filter was able to recover water depth and discharge, reducing the discharge RMSE from 23.2% to 10.0% over an 84‐day simulation period, relative to a simulation without assimilation. An autoregressive error model was instrumental in correcting boundary inflows, and increasing the persistence of error reductions between times of observations. The nominal 8‐day satellite overpass produced discharge relative errors of 10.0%, while 16‐day and 32‐day overpass frequencies resulted in errors of 12.1% and 16.9% respectively.
Do Children and Adults With Language Impairment Recognize Prosodic Cues?
Authors: Fisher Jennifer , Plante Elena , Vance Rebecca , Gerken LouAnn , Glattke Theodore J.
Published: May 2007

Purpose

Prosodic cues are used to clarify sentence structure and meaning. Two studies, one of children with specific language impairment (SLI) and one of adults with a history of learning disabilities, were designed to determine whether individuals with poor language skills recognize prosodic cues on par with their normal-language peers.

Method

Participants were asked to determine whether low-pass filtered sentences matched unfiltered target sentences. Filtered sentences either matched the target sentence exactly or differed on between 1 and 3 parameters that affected the prosodic profile of the sentences.

Results

Children with SLI were significantly poorer than their normal peers in determining whether low-pass filtered sentences matched or were different from unfiltered target sentences. The children’s performance, measured in terms of response accuracy, deteriorated as the similarities between filtered and unfiltered sentences increased. Adults revealed a pattern of differential reaction time to sentence pairs that reflected their relative degree of similarity. There was no difference in performance accuracy for adults with a history of language/learning disabilities compared with their peers.

Conclusion

Given that prosodic cues are known to assist language processing, the weak prosodic skills of preschool children with SLI may limit the amount of benefit that these children derive from the presence of prosodic cues in spoken language. That the adult sample did not show a similar weakness in this skill may reflect developmental differences, sampling differences, or a combination of both.
Streak oscillations of finite length: Disturbance evolution and growth
Authors: Lundell Fredrik
Published: May 2007

Secondary instabilities of low-velocity streaks are studied in a plane channel flow. The secondary instability is forced by a sinusoidal signal consisting of N periods, where N ranges from 0.5 to 30. The disturbance distribution of the secondary instability is found to be similar to previous measurements on streaks forced by infinitely long wave trains. The amplitude growth of the localized secondary instabilities is close to exponential for N>4. However, for N<4 it is found that the exponential growth factor varies considerably both in the streamwise direction and with N with high values at upstream positions. It is proposed that these effects are due to transient growth of the localized secondary instabilities.
Development of low-frequency streaks in Blasius boundary layer
Authors: Boiko A. V. , Chun H. H.
Published: May 2007

A response of flat plate boundary layer to a single free-stream axial vortex of limited spanwise extent with periodically modulated strength is investigated. The response is dominated by a streamwise velocity perturbation (streak), whose characteristics are in agreement with data of previous experiments performed with natural streaks developed under the effect of free-stream turbulence. This confirms that the experimental approach used is suitable for an accurate modeling of the low-frequency natural streaks in controlled conditions. A quantitative comparison of the observed streak characteristics with available theoretical approaches is performed.
Dependence on the initial conditions of scalar mixing in the turbulent wake of a circular cylinder
Authors: Beaulac S. , Mydlarski L.
Published: May 2007

The effect of the initial conditions on scalar mixing in the turbulent wake of a circular cylinder is studied. The scalar under consideration is temperature and it is injected by means of an array of parallel, heated, fine wires (a mandoline). The initial length scale of the velocity field is varied (relative to that of the scalar field) by changing the downstream location of the mandoline. The initial length scale of the scalar field is modified by changing the number of mandoline wires and their spacing. In accordance with previous results in homogeneous, isotropic, grid turbulence, increasing the initial length scale of the velocity field was found to (i) increase the decay rate of scalar variance (m), (ii) decrease the integral scale of the scalar field (lθ), and (iii) increase the mechanical to thermal time scale ratio r. Varying the mandoline wire spacing had little effect on m, lθ, or r. Increasing the width of the mandoline decreased the scalar variance decay rate. Doing so, however, had negligible effect on the structure of the scalar field (i.e., lθ or r). The independence of the scalar variance decay rate and the structure of the scalar field—the opposite of what is observed in grid turbulence—is attributed to the inhomogeneous nature of the flow.
Some specific features of chaotization of the pulsating barotropic flow over elliptic and axisymmetric sea-mounts
Authors: Izrailsky Yu. G. , Kozlov V. F. , Koshel K. V.
Published: May 2007

A comparison of the chaotic advection in the barotropic inviscid unidirectional pulsating flow over a sea-mount of axisymmetric and elliptic form is presented. The process of passive markers’ transport from vortical region into the flow-through region is studied. In particular the evolution of the corresponding Poincaré sections with the change of frequency and amplitude of oscillations is presented. An approach to study the mechanism and parameters of chaotic advection in the open systems with finite lifetime trajectories is presented, which is based on the study of the distribution of time necessary to carry markers into the flow-through region. The optimal for chaotic mixing frequency of external excitation is calculated. The results show that in the case of elliptic sea-mount chaotic transport of markers can be more efficient than in the axisymmetric case, and the corresponding phase portraits have significantly different structures.
Dynamics of a reactive falling film at large Péclet numbers. I. Long-wave approximation
Authors: Trevelyan Philip M. J. , Kalliadasis Serafim
Published: May 2007

We study the dynamics of a vertically falling film in the presence of a first-order (exothermic or endothermic) chemical reaction. We extend the work by Trevelyan et al. [Phys. Fluids 14, 2402 (2002)] on the same problem to large heat/mass transport Péclet numbers and so we take into account the convective terms of the heat/mass transport equations. Our analysis is based on a long-wave expansion of the equations of motion and associated boundary conditions. Previous results by Trevelyan et al. are reviewed and compared with present results. Particular emphasis is given on permanent-form traveling solitary waves. We show that the inclusion of the heat/mass transport convective effects can have a dramatic effect on the evolution of the interface and in fact can make the solitary waves dispersive. The size of dispersion depends on the size of the Prandtl and Schmidt numbers while its sign can change from positive to negative leading to negative-hump solitary waves. We show that for large dispersion and for a sufficiently large region of Reynolds numbers, the liquid layer can be excited in the form of nondissipative solitary pulses which close to criticality assume the form of Korteweg–de Vries solitons.
Dynamics of a reactive falling film at large Péclet numbers. II. Nonlinear waves far from criticality: Integral-boundary-layer approximation
Authors: Trevelyan Philip M. J. , Kalliadasis Serafim
Published: May 2007

We consider the dynamics of a reactive falling film far from criticality. Our analysis is based on the integral-boundary-layer (IBL) approximation of the equations of motion, energy and concentration, and associated free-surface boundary conditions. We develop a hierarchy of IBL models starting from a simplified Shkadov approach to large IBL systems based on high-order Galerkin projections. We show that these high-order models correct the deficiencies of Shkadov’s approach and predict correctly all relevant quantities including the critical Reynolds number. We also numerically construct nonlinear solutions of the solitary wave type for a simplified Shkadov approximation and we show that unlike the long-wave theory in Paper I which leads to branch multiplicity and limit points as well as points where the solitary wave solution branches terminate, the IBL model predicts the existence of solitary waves for all Reynolds numbers.
Wakes behind towed and self-propelled bodies: Asymptotic theory
Authors: Afanasyev Y. D.
Published: May 2007

Solutions for the (steady or unsteady) wake flows induced by a localized single force or a force doublet in a uniform stream are obtained in Oseen approximation for two-dimensional (planar) as well as for three-dimensional (axisymmetric) flows. These solutions are compared with the steady solutions obtained previously by other authors in boundary layer approximation. The straightforward approach to the general problem of the flow induced by any distribution of localized forces which was developed here, can be used to obtain the vorticity and stream function distributions for the flows generated by forcing of more complicated spatial symmetry in both two and three dimensions.
Single-photon source characterization with twin infrared-sensitive superconducting single-photon detectors
Authors: Hadfield Robert H. , Stevens Martin J. , Mirin Richard P. , Nam Sae Woo
Published: May 2007

We report on the high fidelity characterization, via spontaneous emission lifetime and g(2)(τ) measurements, of a cavity-coupled quantum dot single-photon source at 902nm using a pair of nanowire-based superconducting single-photon detectors (SSPDs). We analyze the suitability of the twin SSPD scheme reported here for the characterization of single-photon sources at telecommunications wavelengths (1310 and 1550nm).
Hyperlabyrinth chaos: From chaotic walks to spatiotemporal chaos
Authors: Chlouverakis Konstantinos E. , Sprott J. C.
Published: May 2007

In this paper we examine a very simple and elegant example of high-dimensional chaos in a coupled array of flows in ring architecture that is cyclically symmetric and can also be viewed as an N-dimensional spatially infinite labyrinth (a “hyperlabyrinth”). The scaling laws of the largest Lyapunov exponent, the Kaplan–Yorke dimension, and the metric entropy are investigated in the high-dimensional limit (3<N≤101) together with its routes to chaos. It is shown that by tuning the single bifurcation parameter b that governs the dissipation and the number of coupled systems N, the attractor dimension can span the entire range of 0 to N including Hamiltonian (conservative) hyperchaos in the limit of b=0 and, furthermore, spatiotemporal chaotic behavior. Finally, stability analysis reveals interesting and important changes in the dynamics, whether N is even or odd.
Complex network study of Asian Go players
Authors: Xu Xinping , Hu Junhui , Liu Feng
Published: May 2007

Tournaments of the game of Go can be represented as a directed network in which the vertices are players and a directed link is pointing from the winner to the loser for each game. In this article, we present some interesting results for the network of Asian Go players, which is composed of 756 Go players and 9473 tournaments. It is found that the topological structure of this network displays the small-world property and a significant rich-club phenomenon where high-degree nodes are tightly interconnected. In addition, we consider the weighted version of the network, and find the weights obey power-law distributions, while the strengths follow stretched exponential distributions. The time evolution of the network structure is studied and the corresponding results discussed. The analysis of this work provides a deeper understanding for the competition network structure of Asian Go players.
Micropropulsion in space via dust-plasma thruster
Authors: Avinash K. , Zank G. P.
Published: May 2007

A new engine for propulsion in space, i.e., the dust-plasma thruster, is proposed. The scheme uses plasma thermal energy to charge externally injected sub-micrometer-sized particles and simultaneously create electric fields in the plasma that accelerates them. Particles are subsequently charge stripped and exhausted to produce electrically neutral thrust obviating the need of a charge neutralizer. For reasonable plasma and particle parameters, thrust and specific impulse over a broad range may be produced. The dependence of thrust on particle size, number of injected particles, and plasma temperature density allows for a better thruster precision. The scheme is shown to have modest power requirements. It may be realized in a simple design where there are no high voltage grids or electrodes, charge neutralizer, valves, pressurized gases, etc., and can operate in space or vacuum. A layout for the possible configuration is described.
A simple Lorenz circuit and its radio frequency implementation
Authors: Blakely Jonathan N. , Eskridge Michael B. , Corron Ned J.
Published: May 2007

A remarkably simple electronic circuit design based on the chaotic Lorenz system is described. The circuit consists of just two active nonlinear elements (high-speed analog multipliers) and a few passive linear elements. Experimental implementations of the circuit exhibit the classic butterfly attractor and the hysteretic transition from steady state to chaos observed in the Lorenz equations. The simplicity of the circuit makes it suitable for radio frequency applications. The power spectrum of the observed oscillations displays a peak frequency as high as 930kHz and significant power beyond 1MHz.
Selective image encryption using a spatiotemporal chaotic system
Authors: Xiang Tao , Wong Kwok-wo , Liao Xiaofeng
Published: May 2007

A universal selective image encryption algorithm, in which the spatiotemporal chaotic system is utilized, is proposed to encrypt gray-level images. In order to resolve the tradeoff between security and performance, the effectiveness of selective encryption is discussed based on simulation results. The scheme is then extended to encrypt RGB color images. Security analyses for both scenarios show that the proposed schemes achieve high security and efficiency.
The K239 2Σg+3 state: Observation and analysis
Authors: Li Dan , Xie Feng , Li Li , Sovkov Vladimir B. , Ivanov Valery S. , Ahmed Ergin , Marjatta Lyyra A. , Huennekens John , Magnier Sylvie
Published: May 2007

The K239 2Σg+3 state has been observed by perturbation-facilitated infrared-infrared double resonance spectroscopy and two-photon excitation. Resolved fluorescence spectra into the aΣu+3 state have been recorded. The observed vibrational levels have been assigned as the v=23–25, 27, 28, 31–33, 38–45, 47, and 53 levels by comparing the observed and calculated spectra of the 2Σg+3→aΣu+3 transitions. Molecular constants have been obtained using a global fitting procedure with a comprehensive set of experimental data. Fine and hyperfine splittings have been resolved in the excitation spectra. Perturbations between the 2Σg+3 and 2Πg3 states were observed. The hyperfine patterns of the 2Σg+3 levels are strongly affected by the perturbation. The perturbation-free and weakly perturbed levels follow the case bβS coupling scheme, while the perturbed levels follow case bβJ coupling. A Fermi contact constant, bF=65±10MHz, has been obtained. Intensity anomalies of rotational lines appeared both in the 2Σg+3∼2Πg3←bΠu3 excitation spectra and in the 2Σg+3∼2Πg3→aΣu+3 resolved fluorescence spectra. These intensity anomalies can be explained in terms of a quantum-mechanical interference effect.
Formation, detection, and stability studies of neutral vanadium sulfide clusters
Authors: He Sheng-Gui , Xie Yan , Guo Yuanqing , Bernstein Elliot R.
Published: May 2007

Neutral vanadium sulfide clusters are generated by the reaction of seeded hydrogen sulfide in a helium carrier gas with laser ablated vanadium metal within a supersonic nozzle. The exiting clusters are expanded into a vacuum in a molecular beam and are ionized by both ultraviolet (UV) and vacuum UV (VUV) laser radiation. The generated ions are detected by a time of flight mass spectrometer. With single photon ionization (SPI) employing VUV (118nm) radiation, sulfur rich clusters (VmSn, n>m+1) and hydrogen containing clusters (VmSnHx, x>0) are observed. With multiphoton ionization (MPI) through nanosecond UV (193nm) radiation, these sulfur rich and hydrogen containing clusters cannot be observed, indicating severe fragmentation generated by MPI and the importance of SPI in determining the neutral vanadium sulfide cluster distribution. With MPI through femtosecond UV (226nm) radiation, a few sulfur rich and hydrogen containing clusters are detected, but most clusters observed by SPI are still undetected even by femtosecond MPI. Density functional theory calculations are applied to optimize energies and structures of the clusters with m=1–3 and n=0–7. The experimental results are well interpreted based on the calculations. The calculated and experimental results for vanadium sulfides are compared with those of vanadium oxides in literature.
Numerical experiments on quantum chaotic billiards
Authors: de Menezes D. D. , Jar e Silva M. , de Aguiar F. M.
Published: May 2007

A recently proposed numerical technique for generation of high-quality unstructured meshes is combined with a finite-element method to solve the Helmholtz equation that describes the quantum mechanics of a particle confined in two-dimensional cavities. Different shapes are treated on equal footing, including Sinai, stadium, annular, threefold symmetric, mushroom, cardioid, triangle, and coupled billiards. The results are shown to be in excellent agreement with available measurements in flat microwave resonator counterparts with nonintegrable geometries.
Homological error correction: Classical and quantum codes
Authors: Bombin H. , Martin-Delgado M. A.
Published: May 2007

We prove several theorems characterizing the existence of homological error correction codes both classically and quantumly. Not every classical code is homological, but we find a family of classical homological codes saturating the Hamming bound. In the quantum case, we show that for nonorientable surfaces it is impossible to construct homological codes based on qudits of dimension D>2, while for orientable surfaces with boundaries it is possible to construct them for arbitrary dimension D. We give a method to obtain planar homological codes based on the construction of quantum codes on compact surfaces without boundaries. We show how the original Shor’s 9-qubit code can be visualized as a homological quantum code. We study the problem of constructing quantum codes with optimal encoding rate. In the particular case of toric codes we construct an optimal family and give an explicit proof of its optimality. For homological quantum codes on surfaces of arbitrary genus we also construct a family of codes asymptotically attaining the maximum possible encoding rate. We provide the tools of homology group theory for graphs embedded on surfaces in a self-contained manner.
Identification of network modules by optimization of ratio association
Authors: Angelini L. , Boccaletti S. , Marinazzo D. , Pellicoro M. , Stramaglia S.
Published: May 2007

We introduce a novel method for identifying the modular structures of a network based on the maximization of an objective function: the ratio association. This cost function arises when the communities detection problem is described in the probabilistic autoencoder frame. An analogy with kernel k-means methods allows us to develop an efficient optimization algorithm, based on the deterministic annealing scheme. The performance of the proposed method is shown on real data sets and on simulated networks.
Frequency selection in globally unstable round jets
Authors: Lesshafft Lutz , Huerre Patrick , Sagaut Pierre
Published: May 2007

The self-sustained formation of synchronized ring vortices in hot subsonic jets is investigated by direct numerical simulation of the axisymmetric equations of motion. The onset of global instability and the global frequency of synchronized oscillations are examined as functions of the ambient-to-jet temperature ratio and the initial jet shear layer thickness. The numerical results are found to follow the predictions from nonlinear global instability theory; global instability sets in as the unperturbed flow is absolutely unstable over a region of finite streamwise extent at the inlet, and the global frequency near the global instability threshold corresponds to the absolute frequency of the inlet profile. In strongly supercritical thin shear layer jets, however, the simulations display global frequencies well above the absolute frequency, in agreement with experimental results. The inner structure of rolled-up vortices in hot jets displays fine layers of positive and negative vorticity that are produced and maintained by the action of the baroclinic torque.
Characterization of planar periodic structure using inverse laser scanning confocal microscopy moiré method and its application in the structure of butterfly wing
Authors: Xie Huimin , Wang Qinghua , Kishimoto Satoshi , Dai Fulong
Published: May 2007

An effective measuring method for characterizing the planar periodic structure on specimen surface, i.e., inverse laser scanning confocal microscopy moiré method, is presented and verified by experiments in this paper. The planar character of the periodic surface structure in a large region of several hundreds of millimeters, as well as the dimension at micro- and nanoscales, can be obtained through seeking the specimen grating equation as well as the virtual strain distribution. Verification experiment is carried out using a 1200lines∕mm holography grating as the specimen. The calculated results are in good coincidence with the actual character, demonstrating the validity, feasibility, and high accuracy of this method. Applying this method into identifying the structure of butterfly wing, gratings composed of a cluster of curves with pitches of a little less than 438nm are found on multiscales of butterfly wing, which provides the possibility for further explaining its attractive iridescence. Furthermore, this method is greatly promising for providing simple, reliable support for bionics, grating fabrication, and maybe the sliding mechanism of crystal lattice.
Fabrication and analysis of one-dimensional defect-induced ultrawide photonic band gaps
Authors: Lu Y. H. , Huang M. D. , Park S. Y. , Kim P. J. , Lee Y. P. , Rhee J. Y. , Hwangbo C. K. , Chen L. Y.
Published: May 2007

An approach named the defect-induced extension of photonic band gaps (PBGs) is proposed, in which a stratified periodic structure is combined with another stratified structure containing defect layers. For comparison, three structures composed of TiO2 and SiO2 alternating layers, perfect, five-defect and combined-photonic crystals (PCs), were fabricated by using an electron-beam evaporation system. The measured PBG width of combined-PCs extended to 370 nm at the normal incidence, nearly three times more than that of perfect ones, 130 nm, but a few ripples are observed within the PBG. In order to elucidate the origin of the ripples, the influence of the refractive-index contrast on the reflectance was analyzed theoretically. When the refractive-index contrast nH∕nL≥1.7, the ripples are eliminated. More importantly, an overdoubled omnidirectional PBG is realized at nH∕nL=2.56, covering the visible region completely. This approach is considered to be based on the light localization and a heterostructure resulting from introducing the impurity band and compensating for it using PCs with a strictly periodic structure, in which the thickness of every layer remains unchanged. These characteristics distinguish it from other methods proposed before, with the fabrication much easier.
Effect of sulfur passivation of silicon (100) on Schottky barrier height: Surface states versus surface dipole
Authors: Ali Muhammad Yusuf , Tao Meng
Published: May 2007

Aluminum and nickel contacts were prepared by evaporation on sulfur-passivated n- and p-type Si(100) substrates. The Schottky diodes were characterized by current-voltage, capacitance-voltage, and activation-energy measurements. Due to the passivation of Si dangling bonds by S, surface states are reduced to a great extent and Schottky barriers formed by Al and Ni on Si(100) substrates show greater sensitivity to their respective work functions. Aluminum, a low work function metal, shows a barrier height of <0.11 eV on S-passivated n-type Si(100) and ∼0.80 eV on S-passivated p-type Si(100), as compared to 0.56 and ∼0.66 eV for nonpassivated n- and p-type Si(100), respectively. Nickel, a high work function metal, shows ∼0.72 and ∼0.51 eV on S-passivated n and p-type Si(100), respectively, as compared to ∼0.61 and ∼0.54 eV on nonpassivated n and p-type Si(100), respectively. Though a surface dipole forms due to the adsorption of S on Si(100), our experimental results indicate that the effect of surface states is the dominant factor in controlling the Schottky barrier height in these metal-Si systems.
Magnetic behavior of manganese-doped ZnSe quantum dots
Authors: Lad Amit D. , Rajesh Ch. , Khan Mahmud , Ali Naushad , Gopalakrishnan I. K. , Kulshreshtha S. K. , Mahamuni Shailaja
Published: May 2007

Magnetic properties of manganese-doped ZnSe quantum dots with the size of approximately 3.6 nm are investigated. The amount of Mn in the ZnSe quantum dots has been varied from 0.10% to 1.33%. The doping level in the quantum dots is much less than that used in the precursor. The co-ordination of Mn in the ZnSe lattice has been determined by electron paramagnetic resonance (EPR). Two different hyperfine couplings 67.3×10−4 and 60.9×10−4 cm−1 observed in the EPR spectrum imply that Mn atoms occupy two distinct sites; one uncoordinated (near the surface) and other having a cubic symmetric environment (nanocrystal core), respectively. Photoluminescence measurements also confirm the incorporation of Mn in ZnSe quantum dots. From the Curie-Weiss behavior of the susceptibility, the effective Mn-Mn antiferromagnetic exchange constant (J1) has been evaluated. The spin-glass behavior is observed in 1.33% Mn-doped ZnSe quantum dots, at low temperature. Magnetic behavior at a low temperature is discussed.
The role of dipolar interactions in magnetic nanoparticles: Ferromagnetic resonance in discontinuous magnetic multilayers
Authors: Schmool D. S. , Rocha R. , Sousa J. B. , Santos J. A. M. , Kakazei G. N. , Garitaonandia J. S. , Lezama L.
Published: May 2007

We have performed detailed ferromagnetic resonance (FMR) studies of [Co80Fe20(t)∕Al2O3(40 Å)]10 (t=7−13 Å) discontinuous multilayers, with measurements taken as a function of the angle of the applied static magnetic field with respect to the sample at room temperature. We outline an approach to FMR in granular systems by using the magnetic dipolar interaction between magnetic nanoparticles. This can be easily incorporated into the free energy of the system and provides a method for evaluating the average interaction strength in particulate media. Angular dependent measurements show how FMR can be used to evaluate interparticle interactions. Our measurements indicate that there is a change of coupling regime, going from superferromagnetic, for thicker samples (with larger average particle sizes and smaller separations), to superparamagnetic in thinner samples.
Interfacial-layer modulation of domain switching current in ferroelectric thin films
Authors: Jiang A. Q. , Lin Y. Y. , Tang T. A.
Published: May 2007

It is found that the voltage drop across a 170-nm-thick Pb(Zr0.4Ti0.6)O3 film keeps constantly at a well-defined coercive voltage during domain switching, irrespective of the applied voltage and frequency, and that the switching current of domains is reversely proportional to the resistance of loading resistors in the circuit. A simple formalism is derived for the speed of polarization reversal short into a few nanoseconds. The maximum speed of domain switching is limited by the time of compensation charge dissipation via loading resistors in the circuit, instead of reverse domain nucleation and growth. However, in most cases, the switching current decays with time and is thus peaked under different applied voltages, as observed in an 87-nm-thick film. This phenomenon is understood from our work due to the presence of interfacial passive layers that modulate switching current transient through the circuit RC-time constant, besides the consideration of a broad coercive-voltage distribution in a genuine thin film.
Photocurrent spectroscopy of optical absorption enhancement in silicon photodiodes via scattering from surface plasmon polaritons in gold nanoparticles
Authors: Lim S. H. , Mar W. , Matheu P. , Derkacs D. , Yu E. T.
Published: May 2007

Experimental characterization and finite-element numerical simulations of the electromagnetic interaction between Au nanoparticles positioned atop a Si pn junction photodiode and incident electromagnetic plane waves have been performed as a function of wavelength. The presence of the Au nanoparticles is found to lead to increased electromagnetic field amplitude within the semiconductor, and consequently increased photocurrent response, over a broad range of wavelengths extending upward from the nanoparticle surface plasmon polariton resonance wavelength. At shorter wavelengths, a reduction in electromagnetic field amplitude and a corresponding decrease in photocurrent response in the semiconductor are observed. Numerical simulations reveal that these different behaviors are a consequence of a shift in the phase of the nanoparticle polarizability near the surface plasmon polariton wavelength, leading to interference effects within the semiconductor that vary strongly with wavelength. These observations have substantial implications for the optimization of device structures in which surface plasmon polariton resonances in metallic nanoparticles are exploited to engineer the performance of semiconductor photodetectors and related devices.