bluuebunny/crossref_metadata_2025 huggingface dataset:
An off‐line, numerically efficient initialization scheme in an oceanic general circulation model for El Niño–Southern Oscillation prediction
Authors: Tang Youmin , Kleeman Richard , Moore Andrew M. , Vialard Jérôme , Weaver Anthony
Published: May 2004

In this study a simplified initialization scheme, which is “off‐line,” is proposed and applied to an oceanic general circulation model (OGCM) for El Niño–Southern Oscillation (ENSO) prediction. The initialization scheme is based on the National Centers for Environmental Prediction ocean reanalysis and a two‐dimensional variational (2D‐Var) assimilation algorithm. It focuses on two basic issues in data assimilation: observed data and computational cost. Compared with a traditional assimilation system, this simplified scheme avoids model forward integration and the complications of acquiring and processing raw in situ temperature observations. The off‐line scheme only requires around 1/20 of the computational expense of a traditional algorithm. Two hybrid coupled models, an OGCM coupled to a statistical atmosphere, and the same ocean model coupled to a dynamical atmosphere, were used to examine the initialization scheme. A large ensemble of prediction experiments during the period from 1981 to 1998 shows that relative to just a wind forced initialization the off‐line scheme leads to a significant improvement in predictive skills of Niño3 sea surface temperature anomaly (SSTA) for all lead times. The prediction skills obtained by the scheme is as high as that attained by a more traditional “on‐line” assimilation scheme.
Drifter‐derived maps of lateral diffusivity in the Pacific and Atlantic Oceans in relation to surface circulation patterns
Authors: Zhurbas Victor , Oh Im Sang
Published: May 2004

Maps of apparent lateral diffusivity K derived from surface drifters in the Pacific and Atlantic Oceans are analyzed together with mean surface circulation patterns. A remarkable feature of the diffusivity map in the Atlantic was proven to be trans‐ocean belts of enhanced K in the thirties latitudes. It was shown that the North Atlantic belt is caused by the meandering and eddy‐producing Azores Current, while its counterpart in the South Atlantic results from northwestward and westward drift of warm eddies produced by the Agulhas Retroflection. A puzzling feature of the diffusivity map in the Pacific was a zonally elongated tongue of high K at the center of a subtropical gyre in the Southern Hemisphere beginning from the western boundary. A similar tongue observed at the center of a subtropical gyre of the North Pacific is known to result from baroclinic instability in the seasonal North Subtropical Countercurrent (NSTCC). Both tongues were characterized by a strong and identical seasonal cycle of the eddy kinetic energy (EKE), which persuaded us to suggest an identity for the formation mechanisms that would imply the existence of southern counterpart of NSTCC: the would‐be South Subtropical Countercurrent (SSTCC). A seasonal‐latitudinal dependence of the mean zonal current and EKE as well as maps of the mean zonal velocity were derived from drifters that provided some evidence in favor of the existence of this SSTCC.
Model study on production and transport of lightning‐produced NOx in a EULINOX supercell storm
Authors: Fehr Thorsten , Höller Hartmut , Huntrieser Heidi
Published: May 2004

The production of nitrogen oxides (NOx) by lightning has been investigated with a three‐dimensional cloud model. For the study of the NOx transport, the lightning NOx emissions are represented by a Lagrangian particle model instead of volume emissions. The advantages of this approach are (1) the transport of NOx inside the thunderstorm can be analyzed in great detail, (2) the NOx emissions can be represented as a line source instead of a volume emission, and (3) the comparison with high‐resolution aircraft measurements can be conducted. This approach is applied to the 21 July 1998 supercell storm observed during the European Lightning Nitrogen Oxides project (EULINOX). Both, parameterized and observed cloud‐to‐ground and intracloud flash frequencies are used in the simulation and two different lightning NOx production rates are tested. The analysis shows that the parameterization using instantaneous cloud properties can approximately reproduce the lightning activity during the early mature phase of the intensifying storm but cannot be used for the complete life cycle. The comparison of the model results with aircraft NOx measurements indicate that the NOx production rate for intracloud lightning is equivalent or higher than that for cloud‐to‐ground flashes. Experimentally deduced values for the cloud‐to‐ground lightning NOx production of 4.9 kg(N), or equivalent 2.1 × 1026 molecules NO, and a ratio between the intracloud and cloud‐to‐ground NOx production of 1.4 are confirmed by the model leading to an intracloud contribution to the total NOx production of up to 93%. For the applied production parameters 50% to 80% of the total lightning NOx is transported into the anvil, leading to a very pronounced “C”‐shape profile. The estimated total NOx production by lightning during the lifetime of the storm is 22 t(N).
Seasonal and spatial distributions and dry deposition fluxes of atmospheric total and labile iron over the tropical and subtropical North Atlantic Ocean
Authors: Chen Ying , Siefert Ronald L.
Published: May 2004

The tropical and subtropical region of the North Atlantic Ocean is under the influence of mineral dust plumes transported from North Africa, resulting in high atmospheric iron (Fe) deposition. Atmospheric Fe, as a major Fe source to this open oceanic region, may influence the rate of nitrogen fixation since Fe is a critical nutrient cofactor for the nitrogenase enzyme in diazotrophic microorganisms. Field measurements of Fe concentrations and Fe speciation in aerosols provide crucial information to understanding the biological role of this atmospheric Fe flux to the ocean. In this study, 24‐hour aerosol samples were collected during winter (6 January 2001 to 18 February 2001) and summer (27 June 2001 to 15 August 2001) research cruises over this North Atlantic region. Three labile Fe species, that included ferrous iron (Fe(II)) and reducible ferric iron (Fe(III)) species, were measured in aerosol samples using a sequential aqueous extraction method. Microwave digestion of the aerosol samples followed by inductively coupled plasma‐mass spectrometry was used to quantify total elemental concentrations. A spatial gradient of over nearly 3 orders of magnitude in the total Fe concentrations (from 1.6 ng m−3 at 28.6°N to 1688 ng m−3 at 10.9°N) was observed during the winter, while this gradient was not as strong in the summer. The mean total Fe concentrations were approximately a factor of 2 higher during the winter (mean value of 670 ng m−3 between 5°N and 26°N) than in the summer (mean value of 324 ng m−3 between 6°N and 26°N). The highest percentage of labile Fe to total Fe was observed between 26°N and 30°N in the winter with a mean value of 32%, which corresponded to low concentrations of total Fe. At latitude 0°N to 10°N, where the lowest Fe concentrations were observed in the summer, the labile Fe fraction with a mean of 5.0% was similar to the region from 10°N to 20°N where there were much higher mineral aerosol concentrations. Air mass back trajectories showed that mineral dust transport from North Africa is a significant atmospheric Fe source in this Atlantic region. However, the highest labile Fe to total Fe ratios were observed in air masses that had been over the ocean for greater than 7 days and also corresponded to high ratios of non‐seasalt‐sulfate (NSS‐sulfate) to total Fe and oxalate to total Fe. The correlation with NSS‐sulfate and oxalate suggests that labile Fe concentrations may have been influenced by anthropogenic activities from North America or Europe.
Global simulation of tropospheric ozone using the University of Maryland Chemical Transport Model (UMD‐CTM): 2. Regional transport and chemistry over the central United States using a stretched grid
Authors: Park R. J. , Pickering K. E. , Allen D. J. , Stenchikov G. L. , Fox‐Rabinovitz M. S.
Published: May 2004

We use the stretched‐grid version of the three‐dimensional global University of Maryland Chemical Transport Model (UMD‐CTM) to examine the effects of mesoscale meteorological features such as fronts and deep convection on regional‐scale chemistry and transport. The stretched‐grid model simulation, with a grid configuration featuring a mesoscale resolution region centered over the central United States, was conducted for June 1985, and evaluated through comparisons with a set of aircraft observations of trace gases. We also present results from a uniform‐grid UMD‐CTM simulation with a more conventional 2° × 2.5° horizontal resolution for the same time period to examine how well the stretched‐grid global model simulates mesoscale features. The changes in middle and upper tropospheric CO and O3 due to convection from the model simulations are in good agreement with the range of measurements. The stretched‐grid model shows better agreement with measurements than the uniform‐grid model for the enhancement of trace gases in upper troposphere outflow due to deep convection and for the gradient of trace gas mixing ratios across a cold front. Peak convective enhancement of CO in the upper troposphere is larger in the stretched‐grid model simulation than in the uniform‐grid simulation, indicating a better representation of locally focused deep convective transport of polluted boundary layer air in the former. This type of vertical transport feature must be handled accurately if a model is to be used for intercontinental transport calculations. However, we find that deep convection in both model simulations, although better simulated in the stretched‐grid model, is too widespread and too frequent. We find that net ozone production in the polluted boundary layer is ∼15% less in the fine‐grid region (0.5° resolution) of the stretched‐grid model than in the same region of the 2° × 2.5° model due to less artificial dilution of ozone precursors. The net ozone production in convective outflow plumes is also smaller in the stretched‐grid model than in the uniform‐grid model. We estimate the net flux of ozone from North America in the lowest 7 km to be 10 Gmol d−1 for the month of June using the results from the stretched‐grid simulation. This value includes direct horizontal boundary layer flux, ozone that has been vertically transported from the boundary layer to free troposphere, and ozone that had been produced photochemically in the free troposphere.
Aerosol optical depth in the UVB and visible wavelength range from Brewer spectrophotometer direct irradiance measurements: 1991–2002
Authors: Gröbner J. , Meleti C.
Published: May 2004

Direct irradiance measurements from Brewer spectrophotometer #66 at Ispra, Italy, were reanalyzed for the period 1991–2002. Two calibration methodologies, one adapted to the ultraviolet (UV) and one adapted to the visible wavelength range, have been developed to retrieve the aerosol optical depth (AOD) from these measurements. The measurements in the UV were calibrated in absolute units using laboratory lamp calibrations of the global entrance port of the same instrument, while in the visible wavelength range a zero air mass Langley extrapolation methodology was implemented. The retrieved aerosol optical depths were validated with a colocated CIMEL Sun photometer operating within the Aerosol Robotic Network (AERONET) since 1997. AOD values at Ispra show a strong short‐term variability between 0.05 and 2 at 320 nm as well as a pronounced seasonal variability. The highest monthly mean values of around 0.6 (at 320 nm) are usually found in spring and summer while the lowest values, of about 0.3, are found in the winter months. An aerosol characterization by means of the Ångström power law yields a mean wavelength exponent of 1.6. The influence of Föhn events on the aerosol climatology at Ispra was investigated. The AOD measurements at Ispra show the strong influence of the Mount Pinatubo eruption in June 1991 and the following decay to background levels in late 1997.
Composition and structure of the Martian surface at high southern latitudes from neutron spectroscopy
Authors: Prettyman T. H. , Feldman W. C. , Mellon M. T. , McKinney G. W. , Boynton W. V. , Karunatillake S. , Lawrence D. J. , Maurice S. , Metzger A. E. , Murphy J. R. , Squyres S. W. , Starr R. D. , Tokar R. L.
Published: May 2004

Neutron spectroscopy data acquired by Mars Odyssey are analyzed to determine the abundance and depth of near‐surface water ice as a function of latitude in the southern hemisphere as well as the inventory of CO2 in the south polar residual cap. The surface is modeled as a semi‐infinite, water‐rich permafrost layer covered by desiccated material, which is consistent with theoretical models of ground ice stability. Latitude‐dependent parameters, water abundance and depth, are determined from zonally averaged neutron counting data. Spatial mixing of the output of neutrons from regions within the footprint of the spectrometer is modeled, and asymmetrical features such as the residual cap are included in the analysis. Absorption of thermal neutrons by major elements other than hydrogen is found to have a significant influence on the determination of water abundance. Poleward of −60°, the water‐rich layer contains 60% ± 10% water by weight (70% to 85% by volume) and is covered by less than 15 g/cm2 ± 5 g/cm2 of dry material. The volume fraction of water is generally higher than can be accommodated in the pore space of surface soils, which implies that water vapor diffusion processes alone cannot explain the observations. Alternatives for the formation of the water‐rich layer are discussed. Results of our analysis of the residual‐cap CO2 inventory support conclusions that the atmosphere is not buffered by a larger reservoir of surface CO2 at the poles and that Mars' total CO2 inventory is well represented by the present atmospheric mass.
Multiple scattering effects in ionospheric radio sounding
Authors: Zabotin N. A. , Wright J. W. , Kovalenko E. S.
Published: May 2004

Properties of radio wave reflection from an optically thick, plane monotonic layer of ionospheric plasma with random density irregularities are considered by investigating numerically the influence of multiple scattering on the angular distribution and the integral intensity of the reflected signal. We use an improved solution of the radiative transfer equation in the approximation of small‐angle scattering in invariant ray coordinates (“SASIRC”). The case of midlatitude ionosphere conditions is treated in most detail, but some conclusions about latitudinal dependence are also obtained, for both vertical and slightly oblique sounding cases. Conclusions from earlier versions of the theory showing a strong anomalous‐attenuation effect are confirmed, with adjustments of the quantitative results. We describe a special amplitude calibration procedure for estimation of anomalous attenuation in practical experiments using dynasonde techniques, and discuss the application of our results to an explanation of ionogram spread F.
Shear zones and magma ascent: A model based on a review of the Tertiary magmatism in the Alps
Authors: Rosenberg C. L.
Published: May 2004

The Alpine Oligocene plutons are spatially and temporally associated with the activity of the Periadriatic Fault System (PFS), an orogen‐parallel, crustal‐scale transpressive mylonitic belt. Excellent three‐dimensional exposure, combined with a wealth of structural, seismic, petrological, geochronological, geochemical, and paleomagnetic data collected over the last decades help to constrain the relationships between deformation, ascent, and emplacement of the plutons. Magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the Periadriatic Fault System, which was used as ascent pathway to cover vertical lengths of 20 to 40 km. Therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth. Ascent of the melts is controlled by the mylonitic foliation of the PFS, which forms the only steep anisotropy, continuously traversing the entire Alpine crust. In contrast, the flow direction is not influenced by the specific kinematics of the faults. Final emplacement of the plutons occurred by extrusion from the Periadriatic Fault System into the adjacent country rocks. The transition from ascent to final emplacement is favored by partitioning of transpressive deformation.
Geography of international water conflict and cooperation: Data sets and applications
Authors: Yoffe Shira , Fiske Greg , Giordano Mark , Giordano Meredith , Larson Kelli , Stahl Kerstin , Wolf Aaron T.
Published: May 2004

The Transboundary Freshwater Dispute Database provides a framework for quantitative, global‐scale explorations of the relationship between freshwater resources and international cooperation and conflict. Projects were designed to test common theories linking freshwater resources to cooperation and conflict, in particular within the context of geography and environmental security. The projects, which follow in sequence, consider three main hypotheses on the likelihood and intensity of water resource disputes. To test these hypotheses, a unique set of tools was created that links water‐specific event data with a geographic information system (GIS) that meshes biophysical, political, and socioeconomic data sets at the river basin and other scales. There are three linked data sets: (1) an event data set documenting historical water relations, including a methodology for identifying and classifying events by their intensity of cooperation/conflict; (2) a GIS data set of countries and international basins, both current and historical; and (3) a spatial data set of biophysical, socioeconomic, and political variables, linked to the GIS. This paper describes the hypotheses, the above tools created to test them, and a methodological framework for utilizing the linked event and GIS data sets, providing three projects as examples: (1) indicators of international basins at risk of political tensions, (2) relationships between internal and international hydropolitics in three geographic regions, and (3) hydroclimatological variables and international water relations.
A model of the bifurcated current sheet: 2. Flapping motions
Authors: Sitnov M. I. , Swisdak M. , Drake J. F. , Guzdar P. N. , Rogers B. N.
Published: May 2004

A generalization of the Harris [1962] model has recently been proposed to explain the multi‐spacecraft Cluster observations of thin bifurcated current sheets. It utilizes features of the ion motion in thin sheets and assumes anisotropy of the ion species. We report the results of 2D PIC simulations based on this model. Simulations confirm its self‐consistency. They also reveal the lower‐hybrid drift instability at the edges of the thinned sheet and a transition to a new quasi‐equilibrium state with a considerable contribution of electrons to the bifurcated current. This can be explained using a modification of the original equilibrium model that assumes a small electron anisotropy. At later time instabilities of both parities are detected inside the sheet. They have much larger wavelengths and end up with the kink‐type flapping motions of the bifurcated sheet as a whole.
Variations in the age of Arctic sea‐ice and summer sea‐ice extent
Authors: Rigor Ignatius G. , Wallace John M.
Published: May 2004

Three of the past six summers have exhibited record low sea‐ice extent on the Arctic Ocean. These minima may have been dynamically induced by changes in the surface winds. Based on results of a simple model that keeps track of the age of ice as it moves about on the Arctic Ocean, we show that the areal coverage of thick multi‐year ice decreased precipitously during 1989–1990 when the Arctic Oscillation was in an extreme “high index” state, and has remained low since that time. Under these conditions, younger, thinner ice anomalies recirculate back to the Alaskan coast more quickly, decreasing the time that new ice has to ridge and thicken before returning for another melt season. During the 2002 and 2003 summers this anomalously younger, thinner ice was advected into Alaskan coastal waters where extensive melting was observed, even though temperatures were locally colder than normal. The age of sea‐ice explains more than half of the variance in summer sea‐ice extent.
Dynamics of Venus' superrotation: The eddy momentum transport processes newly found in a GCM
Authors: Yamamoto Masaru , Takahashi Masaaki
Published: May 2004

A 4‐day superrotation 60‐times faster than the planetary rotation (243 days) is observed in Venus' atmosphere. Although it has been difficult to reproduce the extraordinary phenomenon in GCMs, the superrotation is reproduced by meridional circulation and planetary‐scale waves with phase velocities slower than 50 m s−1 in our improved GCM. Thermally induced waves produce equatorward momentum fluxes in the middle atmosphere, while the planetary‐scale pattern of two different gravity and Rossby waves with the same frequency is found to produce the equatorward momentum flux in the lower atmosphere. These processes play crucial roles in dynamics of the simulated Venus superrotation.
Causes of low frequency North Atlantic SST variability in a coupled GCM
Authors: Wu Zhaohua , Schneider E. K. , Kirtman B. P.
Published: May 2004

The relative roles of stable and unstable coupled feedbacks in inducing the low frequency SST variability in various regions of the North Atlantic are studied using a coupled general circulation model. The interactive ensemble technique is applied to reduce the strength of “weather noise” in the model and isolate the atmospheric feedback to boundary forcing. We find that the low frequency SST variability in the subtropical North Atlantic is mainly induced by stable coupled feedbacks in which the weather noise plays a central role. However, in the Gulf Stream extension area, the SST variability may be attributed to processes internal to the ocean. Although the results are not definitive, there is no compelling evidence that unstable coupled feedbacks are important for low frequency SST variability in the North Atlantic in this model.
Data‐derived forecasting model for relativistic electron intensity at geosynchronous orbit
Authors: Ukhorskiy A. Y. , Sitnov M. I. , Sharma A. S. , Anderson B. J. , Ohtani S. , Lui A. T. Y.
Published: May 2004

In this paper we present a data‐derived model of relativistic electron flux at geosynchronous orbit. The model is driven by multiple solar wind and magnetospheric inputs and combines the deterministic approach of nonlinear dynamics with conditional probability consideration. The model is used for one‐day predictions of daily flux maxima for the years 1995–2000. The deterministic part of the model yields average prediction efficiency of 0.77 and linear correlation coefficient of 0.89. It identifies solar wind velocity and SymH index as most relevant input parameters. The probabilistic part of the model quantifies the risks of deviations from deterministic predictions as a function of average solar wind and magnetosphere conditions.
Sensitivity of the δ18O‐temperature relationship to the distribution of continents
Authors: Brown Josephine , Simmonds Ian
Published: May 2004

Paleoclimate studies commonly interpret the stable water isotope ratio in precipitation as a proxy for temperature based on observed present day spatial and temporal correlations. The temperature‐isotope relationship results both from the greater efficiency of isotopic fractionation at low temperatures and from the preferential condensation of the heavy isotopic species during moisture transport, which tend to coincide for the present day climate. The Melbourne University General Circulation Model is used to simulate an alternative scenario where these two effects are opposed, with idealized continents located in such a way that moisture transport occurs from cooler to warmer latitudes. It is found that the positive correlation between isotopic ratio and temperature breaks down for cases with oceans limited to mid‐ and high latitudes, indicating that the observed present day correlation between isotopic ratio and temperature is not invariant, but depends on the specific distributions of continents and resulting moisture transport.
The gravity recovery and climate experiment: Mission overview and early results
Authors: Tapley B. D. , Bettadpur S. , Watkins M. , Reigber C.
Published: May 2004

The GRACE mission is designed to track changes in the Earth's gravity field for a period of five years. Launched in March 2002, the two GRACE satellites have collected nearly two years of data. A span of data available during the Commissioning Phase was used to obtain initial gravity models. The gravity models developed with this data are more than an order of magnitude better at the long and mid wavelengths than previous models. The error estimates indicate a 2‐cm accuracy uniformly over the land and ocean regions, a consequence of the highly accurate, global and homogenous nature of the GRACE data. These early results are a strong affirmation of the GRACE mission concept.
Crater degradation in the Martian highlands: Morphometric analysis of the Sinus Sabaeus region and simulation modeling suggest fluvial processes
Authors: Forsberg‐Taylor Nancy K. , Howard Alan D. , Craddock Robert A.
Published: May 2004

Results from simulation modeling of crater degradation by fluvial and eolian processes are compared with size‐frequency and depth of infilling statistics for the heavily cratered Sinus Sabaeus quadrangle of Mars. The fractional degree of infilling of craters greater than 10 km in diameter in this region is bimodal, with a small population of post‐Noachian craters with little infilling, whereas most Noachian craters are strongly infilled. This pattern is most consistent with fluvial erosion of craters, because modeling indicates that craters fill rapidly at first, but the rate of infilling diminishes through time as crater wall heights diminish and the area of deposition on the crater floor increases. Simulated rates of crater infilling by eolian processes are more constant, which would be expected to produce craters equally distributed in degree of infilling, which is not observed. The small slope of the size‐frequency distribution in the 10–30 km size range is also consistent with the more rapid fluvial erosion of smaller craters. The analysis also suggests that rates of crater production and of crater degradation were in rough balance during the Noachian epoch in the 10–30 km size range.
Dynamic hydrologic and geochemical response in a perennial karst spring
Authors: Winston W. E. , Criss R. E.
Published: May 2004

Storms induce rapid variations in the discharge, specific conductivity, and temperature of a perennial karst spring in eastern Missouri that are followed by gradual return to normal conditions. These dynamics reflect the varying relative proportions of “base flow” and “event water” components that have different δ18O signatures, solute concentrations, flow paths, and transport timescales, which combine with other transport impedances to govern the temporal behavior of water quality parameters. A new Darcian model accurately reproduces the hydrograph and its separated components, defines the time constants that govern their physical and geochemical responses, and affords a quantitative method to investigate these linked behaviors. Analysis of 58 storm events reveals an average pulse time constant of 0.4 ± 0.2 days that is much shorter than the ∼2 year residence time of water in the aquifer derived from long‐term δ18O variations. For individual pulses this short time constant for total flow approximates that of the base flow component, but the time constant for the event water component is even shorter. The same model also approximates other storm‐induced variations and indicates they are all triggered at the same time but respond according to different time constants of 1.6 ± 0.2 days for oxygen isotopes, 1.6 ± 0.9 days for temperature, and 3.4 ± 1.0 days for specific conductivity and major ion concentrations. The time constant for discharge decreases somewhat with greater peak flows, while the geochemical time constants increase.
Retracting fronts induce spatiotemporal intermittency
Authors: Coullet Pierre , Kramer Lorenz
Published: May 2004

The intermittent route to spatiotemporal complexity is analyzed in simple models which display a subcritical bifurcation without hysteresis. A new type of spatiotemporal complex behavior is found, induced by fronts which “clean” the perturbations around an unstable state. The mechanism which generates these “retracting fronts” through nonlinear dispersion is analyzed in the frame of the complex Ginzburg–Landau equation. For sufficiently strong nonlinear dispersion the effects also occur for a supercritical bifurcation.
The structure of synchronization sets for noninvertible systems
Authors: Josić Krešimir , Sander Evelyn
Published: May 2004

Unidirectionally coupled systems (x,y)⟼(f(x),g(x,y)) occur naturally, and are used as tractable models of networks with complex interactions. We analyze the structure and bifurcations of attractors in the case the driving system is not invertible, and the response system is dissipative. We discuss both cases in which the driving system is a map, and a strongly dissipative flow. Although this problem was originally motivated by examples of nonlinear synchrony, we show that the ideas presented can be used more generally to study the structure of attractors, and examine interactions between coupled systems.
Breaking a secure communication scheme based on the phase synchronization of chaotic systems
Authors: Álvarez G. , Montoya F. , Pastor G. , Romera M.
Published: May 2004

A security analysis of a recently proposed secure communication scheme based on the phase synchronization of chaotic systems is presented. It is shown that the system parameters directly determine the cipher text waveform, hence it can be readily broken by system parameter estimation from the cipher text signal.
Dynamics and bistability in a reduced model of the lac operon
Authors: Yildirim Necmettin , Santillán Moisés , Horike Daisuke , Mackey Michael C.
Published: May 2004

It is known that the lac operon regulatory pathway is capable of showing bistable behavior. This is an important complex feature, arising from the nonlinearity of the involved mechanisms, which is essential to understand the dynamic behavior of this molecular regulatory system. To find which of the mechanisms involved in the regulation of the lac operon is the origin of bistability, we take a previously published model which accounts for the dynamics of mRNA, lactose, allolactose, permease and β-galactosidase involvement and simplify it by ignoring permease dynamics (assuming a constant permease concentration). To test the behavior of the reduced model, three existing sets of data on β-galactosidase levels as a function of time are simulated and we obtain a reasonable agreement between the data and the model predictions. The steady states of the reduced model were numerically and analytically analyzed and it was shown that it may indeed display bistability, depending on the extracellular lactose concentration and growth rate.
Modeling of heat explosion with convection
Authors: Belk Michaël , Volpert Vitaly
Published: May 2004

The work is devoted to numerical simulations of the interaction of heat explosion with natural convection. The model consists of the heat equation with a nonlinear source term describing heat production due to an exothermic chemical reaction coupled with the Navier–Stokes equations under the Boussinesq approximation. We show how complex regimes appear through successive bifurcations leading from a stable stationary temperature distribution without convection to a stationary symmetric convective solution, stationary asymmetric convection, periodic in time oscillations, and finally aperiodic oscillations. A simplified model problem is suggested. It describes the main features of solutions of the complete problem.
Hydrogen transfer in excited pyrrole–ammonia clusters
Authors: David O. , Dedonder-Lardeux C. , Jouvet C. , Kang H. , Martrenchard S. , Ebata T. , Sobolewski A. L.
Published: May 2004

The excited state hydrogen atom transfer reaction (ESHT) has been studied in pyrrole–ammonia clusters [PyH–(NH3)n+hν→Py•+•NH4(NH3)n−1]. The reaction is clearly evidenced through two-color R2P1 experiments using delayed ionization and presents a threshold around 235 nm (5.3 eV). The cluster dynamics has also been explored by picosecond time scale experiments. The clusters decay in the 10–30 ps range with lifetimes increasing with the cluster size. The appearance times for the reaction products are similar to the decay times of the parent clusters. Evaporation processes are also observed in competition with the reaction, and the cluster lifetime after evaporation is estimated to be around 10 ns. The kinetic energy of the reaction products is fairly large and the energy distribution seems quasi mono kinetic. These experimental results rule out the hypothesis that the reaction proceeds through a direct N–H bond rupture but rather imply the existence of a fairly long-lived intermediate state. Calculations performed at the CASSCF/CASMP2 level confirm the experimental observations, and provide some hints regarding the reaction mechanism.
Nonsingular G2 stiff fluid cosmologies
Authors: Fernández-Jambrina L. , González-Romero L. M.
Published: May 2004

In this paper we analyze Abelian diagonal orthogonally transitive space–times with spacelike orbits for which the matter content is a stiff perfect fluid. The Einstein equations are cast in a suitable form for determining their geodesic completeness. A sufficient condition on the metric of these space–times is obtained, that is fairly easy to check and to implement in exact solutions. These results confirm that nonsingular space–times are abundant among stiff fluid cosmologies.
Perturbations of ground states in weakly interacting quantum spin systems
Authors: Yarotsky D. A.
Published: May 2004

We consider a general weak bounded finite range perturbation of a general free quantum spin Hamiltonian on a lattice. We prove that if the free Hamiltonian has a nondegenerate ground state and a spectral gap, then the perturbation also has a ground state, and estimate the localization of the spectrum in the corresponding ground state representation.
A neoclassical model for toroidal rotation and the radial electric field in the edge pedestal
Authors: Stacey W. M.
Published: May 2004

A model for the calculation of toroidal rotation velocities and the radial electric field in the edge pedestal of tokamaks is described. The model is based on particle and momentum balance and the use of the neoclassical gyroviscous expression for the toroidal viscous force. Predicted toroidal rotation velocities in the edge pedestal are found to agree with measured values to within about a factor of 2 or less, for a range of DIII-D [Luxon, Nucl. Fusion 42, 614 (2002)] edge pedestal conditions.
Comparison of toroidal rotation velocities of different impurity ions in the DIII-D tokamak
Authors: Baylor L. R. , Burrell K. H. , Groebner R. J. , Houlberg W. A. , Ernst D. P. , Murakami M. , Wade M. R.
Published: May 2004

Measurements of the relative toroidal rotation velocities of low Z impurity ions have been made in DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] plasmas that have a strong core pressure gradient and reduced anomalous transport. They show that the differences in toroidal rotation velocities agree with predictions of neoclassical transport theory. The toroidal rotation velocities are measured via charge exchange recombination (CER) spectroscopy of the impurity species. Taking into account the non-negligible effect of energy dependent charge exchange cross sections, quantitative agreement is found between the measured and predicted difference in toroidal rotation velocities, including the strong Z dependence. The predicted difference between the measured impurity rotation velocity and the main ion rotation velocity can be substantial and therefore care should be taken in using quantitative results from impurity rotation measurements and applying them to the bulk plasma.
Ultrafast InGaAs/InGaAlAs multiple-quantum-well electro-absorption modulator for wavelength conversion at high bit rates
Authors: El Dahdah N. , Aubin G. , Harmand J.-C. , Ramdane A. , Shen A. , Devaux F. , Garreau A. , Benkelfat B.-E.
Published: May 2004

We report on InGaAs/InGaAlAs electro-absorption modulators for ultrafast all-optical signal processing. The structure design is based on the use of small conduction and valence band offsets. Switching windows as short as 5 ps for a 2 V applied reverse bias have been obtained by cross-absorption modulation.
Wiebel instability of microwave gas discharge in strong linear and circular pulsed fields
Authors: Shokri B. , Ghorbanalilu M.
Published: May 2004

Being much weaker than the atomic fields, the gas breakdown produced by high-power pulsed microwave fields is investigated in the nonrelativistic case. The distribution function of the electrons produced by the interaction with intense linearly and circularly polarized microwave fields is obtained and it is shown that it is in a nonequilibrium state and anisotropic. The discharge mechanism for the gas atoms is governed by electron-impact avalanche ionization. By analyzing the instability of the system and by finding its growth rate, it is shown that the instability which is governed by the anisotropic property of the distribution function is Wiebel instability.
Modeling of stimulated Brillouin scattering near the critical-density surface in the plasmas of direct-drive inertial confinement fusion targets
Authors: Maximov A. V. , Myatt J. , Seka W. , Short R. W. , Craxton R. S.
Published: May 2004

The nonlinear propagation of laser beams, smoothed by spatial and temporal bandwidth, near the critical density surface of direct-drive inertial confinement fusion targets has been modeled. The interplay between filamentation and forward and backward stimulated Brillouin scattering (SBS) is described in the presence of light reflected from the critical density surface and high absorption of light near the critical density. The spectrum of backscattered light develops a red shift due to SBS, which can be seeded by the reflection of light from the critical surface. The intensity of backscattered light decreases moderately as the bandwidth of smoothing by spectral dispersion is increased.
A full radius gyrokinetic stability analysis for large aspect ratio finite-β tokamaks
Authors: Ganesh R. , Angelino P. , Vaclavik J. , Villard L.
Published: May 2004

Linear, fully gyrokinetic, full radius (global), large aspect ratio studies of Alfvén-ion temperature gradient mode (AITG) or kinetic ballooning modes or beta-induced Alfvén eigenmodes considering only “passing” species is presented. Effects hitherto completely neglected in a full radius approach such as B∥-fluctuations and the ones which have been treated partly [Phys. Plasmas 10, 1424 (2003)] such as Shafranov shifts are included. To this end, an existing code EM-GLOGYSTO has been upgraded to incorporate these effects. Among others, the most interesting results include: (i) For relatively large positive magnetic shear ŝ [1.25<ŝ<4.25, ŝ=d ln qs/d ln ρ, where qs(ρ) is safety factor and ρ minor radius], B∥ fluctuations have a benign effect on AITG growth rates and for positive but small shear (0.0<ŝ<2.7), B∥ fluctuations are too weak to play any crucial role. (ii) In the later case, inclusion of Shafranov shift leads to the following: (a) Growth rates without Shafranov shift effects are in general larger than those including Shafranov shift; (b) nonmonotonous dependence of growth rates and frequencies on β=2μ0NTi/B02; (c) the presence of multiple eigenmodes with competing growth rates for same values of β; (d) no sign of complete stabilization with increasing β. Finally eigenmode structures [φ,A∥,Aθ] with and without Shafranov shift are reported. The growth rates and frequencies thus obtained may serve as estimates of transport coefficients and for future bench marking of the (then) global electromagnetic, gyrokinetic, time evolution codes (particle-in-cell or otherwise).
Exact solutions for axisymmetric ideal magnetized plasma steady state with incompressible poloidal flow
Authors: Khater A. H. , Moawad S. M.
Published: May 2004

The steady state equations of an axisymmetric ideal magnetized plasma with incompressible poloidal flow and toroidal magnetic field are investigated. The state is governed by a second order elliptic partial differential equation of the stream function. New exact analytic solutions to this equation associated with cusp-mirror-like, divertor-tokamak-like, and loop-like configurations are obtained. The latter solutions have not appeared in literature as far as we know.
Growth of a ring ripple on a Gaussian beam in a plasma
Authors: Sodha M. S. , Sharma Ashutosh , Prakash Gyan , Verma M. P.
Published: May 2004

The growth of a ring ripple, riding on an intense Gaussian laser beam, through plasma has been studied. The amplitude ratio p of the ripple and the beam and the dimensionless width ρ1 of the ripple are chosen as significant coordinates. It is observed that the positive quadrant of the p, ρ1 space can be divided in three distinct regions corresponding to steady divergence, oscillatory divergence and oscillatory convergence of the ripple. The variation of ripple width with distance of propagation has been obtained for typical points in the three regions. Collisions, ponderomotive force, and relativistic dependence of mass on quiver velocity have been considered as the mechanisms, which introduce nonlinearity.
Collective behavior of ion Bernstein waves in a multi-ion-species plasma
Authors: Toida Mieko , Suzuki Takamasa , Ohsawa Yukiharu
Published: May 2004

Collective behavior of ion Bernstein waves propagating perpendicular to an external magnetic field is studied with attention to the effect of multiple-ion species. In a thermal-equilibrium, multi-ion-species plasma, a great number of Bernstein waves are excited near the harmonics of many different ion cyclotron frequencies. The autocorrelation function of the quasimode consisting of these waves is initially damped and is not recovered to its initial value. This is predicted by the theory and is confirmed by numerical calculations and by particle simulations. It is also demonstrated by particle simulations that a perpendicular macroscopic disturbance is damped in a multi-ion-species plasma. The electric-field energy associated with this disturbance is significantly reduced and is transferred to the ions, indicating that the presence of multiple-ion species affects the energy transport.
Visible quantum cutting through downconversion in Eu3+-doped KGd3F10 and KGd2F7 crystals
Authors: Kodama Nobuhiro , Watanabe Yamato
Published: May 2004

Visible quantum cutting (visible two-photon luminescence) through downconversion is observed for the Gd3+–Eu3+ couple in Eu3+-doped KGd3F10 (KG3F) and KGd2F7 (KG2F). The dependence of the efficiency of cross-relaxation required for visible quantum cutting on the Eu3+-doping concentration and crystal structure is discussed in terms of the probability of energy transfer as a function of the rare-earth interatomic distance. Eu3+-doped KG3F exhibits a higher efficiency in the cross-relaxation step than Eu3+-doped KG2F. The highest efficiency of the cross-relaxation step, 0.65, is obtained for the KG3F crystal doped with 2 at. % Eu3+. It is also suggested from dependence of the lifetime of 5D0 emission of Eu3+ in KG3F on Eu3+ concentration that interactions between rare-earth ions are strong at greater than 2 at. % Eu3+.
Expansion of dust grains in electron depleted two-temperature–ion plasma
Authors: Tribeche Mouloud , Houili Hocine , Zerguini Taha Houssine
Published: May 2004

The nonlinear space–time evolution of an expanding electron depleted dusty plasma into a vacuum region is numerically investigated without imposing the quasineutrality approximation. Based on hydrodynamic equations a Lagrangian algorithm is developed. A small group of dust particles is accelerated ahead of the dominant plasma expansion by a local electric field set up by massless ions rushing into the vacuum region. Dust wave breaking and associated dust bunching is found to take place limiting therefore the time scale of the simulation.
Critical assessment of a two-dimensional hybrid Hall thruster model: Comparisons with experiments
Authors: Bareilles J. , Hagelaar G. J. M. , Garrigues L. , Boniface C. , Boeuf J. P. , Gascon N.
Published: May 2004

A discussion is presented on the results and predictive capabilities of a two-dimensional (2D) hybrid Hall effect thruster (HET) model. It is well known that classical (collision-induced) cross-field electron transport and energy losses are not sufficient to explain the observed HET characteristics. The 2D, quasineutral, hybrid discharge model uses empirical parameters to describe additional, anomalous electron transport and energy loss phenomena. It is shown that, for properly adjusted empirical parameters, the model can qualitatively reproduce the observed thruster behavior over a large range of operating conditions. The ionization and transit-time oscillations predicted by the model are described, and their consequences on the time-averaged thruster properties are discussed. Finally, the influence of the empirical parameters on the model results is shown, especially on quantities that can be measured experimentally.
Near steady-state orbits and injection in free electron lasers with ion-channel guiding
Authors: Mirzanejhad Saeed , Maraghechi Pouya , Maraghechi Behrouz
Published: May 2004

A numerical and an analytical investigation of near steady-state orbits in free electron lasers (FELs) with ion-channel guiding are carried out and two kinds of orbits are found. Injection of electrons is studied numerically using an adiabatically tapered wiggler field and ion-channel density. A considerable reduction in the amplitude of oscillations about steady-state orbits was found. Near steady-state orbits, to second order in wiggler-induced velocity, are found in FELs with axial magnetic field guiding.
Integrated pedestal and core modeling of Joint European Torus (JET) triangularity scan discharges
Authors: Onjun Thawatchai , Kritz Arnold H. , Bateman Glenn , Parail Vassili , Lönnroth Johnny , Huysmans Guido
Published: May 2004

Simulations of four Joint European Torus (JET) [Rebut et al., Nucl. Fusion 25, 1011 (1985)] type I ELMy high confinement mode discharges in a triangularity scan are carried out using the JETTO integrated modeling code [Erba et al., Plasma Phys. Contolled Fusion 39, 261 (1997)] with a predictive core transport model and a pedestal model that includes the effects of edge localized modes (ELMs). The pedestal pressure gradient is limited by the magnetohydrodynamic (MHD) ballooning mode instability, which triggers ELM crashes in these simulations. The validation of the pressure gradient limit used in the simulations is confirmed by a stability analysis carried out using the HELENA and MISHKA codes [Mikhailovskii et al., Plasma Phys. Rep 23, 713 (1997)]. The MHD stability analysis includes infinite-n ideal ballooning, finite-n ballooning, and low-n kink/peeling modes. It is shown that higher triangularity plasmas have easier access to the second stability region, which allows the edge pressure gradients in the higher triangularity discharges to increase to higher levels.