bluuebunny/crossref_metadata_2025 huggingface dataset:
The large‐scale energy budget of the Arctic
Authors: Serreze Mark C. , Barrett Andrew P. , Slater Andrew G. , Steele Michael , Zhang Jinlun , Trenberth Kevin E.
Published: June 2007

This paper synthesizes a variety of atmospheric and oceanic data to examine the large‐scale energy budget of the Arctic. Assessment of the atmospheric budget relies primarily on the ERA‐40 reanalysis. The seasonal cycles of vertically integrated atmospheric energy storage and the convergence of energy transport from ERA‐40, as evaluated for the polar cap (defined by the 70°N latitude circle), in general compare well with realizations from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis over the period 1979–2001. However, shortcomings in top of atmosphere radiation, as compared to satellite data, and the net surface flux, contribute to large energy budget residuals in ERA‐40. The seasonal cycle of atmospheric energy storage is strongly modulated by the net surface flux, which is also the primary driver of seasonal changes in heat storage within the Arctic Ocean. Averaged for an Arctic Ocean domain, the July net surface flux from ERA‐40 of −100 W m−2 (i.e., into the ocean), associated with sea ice melt and oceanic sensible heat gain, exceeds the atmospheric energy transport convergence of 91 W m−2. During winter (for which budget residuals are large), oceanic sensible heat loss and sea ice growth yield an upward surface flux of 50–60 W m−2, complemented with an atmospheric energy convergence of 80–90 W m−2 to provide a net radiation loss to space of 175–180 W m−2.
Ozone variation in the tropical tropopause layer as seen from ozonesonde data
Authors: Takashima H. , Shiotani M.
Published: June 2007

Ozone variations with seasonal and intraseasonal timescales in the tropical tropopause layer (TTL) are investigated using a 5‐year tropical ozonesonde data set from the SHADOZ (Southern Hemisphere Additional Ozonesondes) archive. The longitudinal ozone distribution in the tropical upper troposphere (TUT) shows a zonal wave one structure with maxima around the Atlantic and Africa and minima around the western Pacific throughout the year, while the annual variation shows maxima during northern summer to autumn at most longitudes. We compare the ozone distribution with the vertical temperature structure and found that the lapse rate is gradual (steep) at the ozone‐enhanced (reduced) longitude and season. The east‐west temperature structure and ozone variation in the TUT may be explained by the longitudinal variation of the large‐scale atmospheric responses to the tropical heat source, which could govern both the temperature structure and the vertical transport processes. Ozone variability in the TUT is also large around the Atlantic and Africa and small around the western Pacific. However, the zonal wave one structure is not clear in the temperature variability and in the correlation coefficient between ozone and temperature, which can be related with wave activities around the tropopause. Remarkably large ozone variabilities with good correlation are observed in Africa during summer and in the central Pacific during autumn‐winter. These are associated with large‐scale equatorial waves, but the longitudinal variation of the wave activities does not seem to be an important factor in the zonal wave one structure of ozone.
Climatic controls on frost cracking and implications for the evolution of bedrock landscapes
Authors: Hales T. C. , Roering Joshua J.
Published: June 2007

In mountainous landscapes the role of periglacial processes in producing sediment is poorly defined, despite evidence of abundant talus slopes. Ice growth in rock has long been recognized as an efficient erosion mechanism, but the effects have not been readily applied to landscape evolution in response to tectonic and climatic forcing. Here, we quantify how and where ice‐driven mechanical erosion occurs in cold, bedrock‐dominated landscapes using a simple one‐dimensional numerical heat flow model. In our model, ice grows by water migration to colder regions in shallow rock by the reduction in chemical potential associated with intermolecular forces between ice and mineral surfaces, a process called segregation ice growth. The depth and intensity of frost cracking is primarily dependent on mean annual temperature (MAT), with positive MAT sites characterized by intense cracking in the top meter of the rock mass and a maximum frost penetration of ∼4 m. In contrast, negative MAT areas have less intense cracking that primarily occurs at depths between 50 and 800 cm. We compare the depth and intensity of frost cracking predicted by our model with measures of the intensity of frost processes determined in three studies: The first measured the timing of rockfall in the Canadian Rockies, Niagara Escarpment, and Japanese Alps; the second analyzed scree deposits in the Southern Alps, New Zealand; and the third documented rockfall frequency in Utah. These natural examples show that rockfalls tend to nucleate at elevations that coincide with zones of intense frost cracking predicted by our model. As such, climatic variations associated with interglacial‐glacial cycles may impart a significant influence on the denudation of mountainous landscapes.
Optimized design of gigahertz transverse electromagnetic cells for dosimetric experiments
Authors: Bozzetti Michele , Calò Giovanna , D'Orazio Antonella , De Sario Marco , Mescia Luciano , Petruzzelli Vincenzo , Prudenzano Francesco
Published: June 2007

This paper highlights the design criteria of a gigahertz transverse electromagnetic (GTEM) cell to be used for the evaluation of potential biological effects induced by radio frequency electromagnetic waves on animal cell cultures. In particular, the estimation of the electromagnetic power absorbed by the cell culture allows us to evaluate the scalar specific absorption rate (SAR) of the sample under test. A uniform distribution of the electromagnetic field is assured in the region where the Petri dishes containing the samples under test are placed. The electromagnetic field is simulated by a homemade computer code based on the analytical transverse resonance diffraction method. Moreover, the characterization results of the first GTEM cell prototype are reported. Finally, the SAR of different biological samples placed in the test site of the GTEM cell is assessed by means of the method of moments.
Differential validation of the US‐TEC model
Authors: Araujo‐Pradere E. A. , Fuller‐Rowell T. J. , Spencer P. S. J. , Minter C. F.
Published: June 2007

This paper presents a validation and accuracy assessment of the total electron content (TEC) from US‐TEC, a new product presented by the Space Environment Center over the contiguous United States (CONUS). US‐TEC is a real‐time operational implementation of the MAGIC code and provides TEC maps every 15 min and the line‐of‐sight electron content between any point within the CONUS and all GPS satellites in view. Validation of TEC is difficult since there are no absolute or true values of TEC. All methods of obtaining TEC, for instance, from GPS, ocean surface monitors (TOPEX), and lightning detectors (FORTE), have challenges that limit their accuracy. GPS data have interfrequency biases; TOPEX also has biases, and data are collected only over the oceans; and FORTE can eliminate biases, but because of the lower operating frequency, the signals suffer greater bending on the rays. Because of the difficulty in obtaining an absolute unbiased TEC measurement, a “differential” accuracy estimate has been performed. The method relies on the fact that uninterrupted GPS data along a particular receiver‐satellite link with no cycle slips are very precise. The phase difference (scaled to TEC units) from one epoch to the next can be determined with an accuracy of less than 0.01 TEC units. This fact can be utilized to estimate the uncertainty in the US‐TEC vertical and slant path maps. By integrating through US‐TEC inversion maps at two different times, the difference in the slant TEC can be compared with the direct phase difference in the original RINEX data file for nine receivers not used in the US‐TEC calculations. The results of this study, for the period of April–September 2004, showed an average root mean square error of 2.4 TEC units, which is equivalent to less than 40 cm of signal delay at the GPS L1 frequency. The accuracy estimates from this “differential” method are similar to the results from a companion paper utilizing an “absolute” validation method by comparing with FORTE data.
Validation of the Utah State University Global Assimilation of Ionospheric Measurements (GAIM) model predictions of the maximum usable frequency for a 3000 km circuit
Authors: McNamara Leo F. , Decker Dwight T. , Welsh Judith A. , Cole David G.
Published: June 2007

The accuracy of the Utah State University Global Assimilation of Ionospheric Measurements (GAIM) monthly median values of the maximum usable frequency for an HF communications circuit has been determined for 3000 km circuits centered on Australian ionosonde locations for March and April 2004. The accuracy is discussed in terms of foF2, M(3000)F2, and the product MUF(3000)F2 = foF2 * M(3000)F2. Ground truth is provided by hourly values of foF2 and M(3000)F2 that have been hand‐scaled from ionograms. In general, the accuracy of the GAIM predictions of foF2 exceeds that for M(3000)F2. Given that M(3000)F2 is a function of the height and shape of the F2 subpeak, it follows that these parameters of the GAIM F2 peak need to be improved. The GAIM errors in foF2 are quite large at Learmonth during the day, even though Learmonth is a digital ionospheric sounding systems (DISS) station that provides real‐time data to GAIM. We interpret these errors in terms of limitations of the current GAIM model. Comparisons of the GAIM errors in the monthly median MUF(3000)F2 with those given by the propagation prediction program Advanced Stand Alone Prediction System (ASAPS) show that the GAIM errors are generally somewhat greater than the ASAPS errors, but we consider GAIM's performance to be creditable given that most of the data available for assimilation is GPS total electron content observations. Overall, the daytime GAIM errors are 3 MHz or 10%, while the nighttime errors are 3 MHz or 16%. On average, the GAIM errors are about 3/2 the ASAPS errors during the day and twice the ASAPS errors at night.
Depolarization ratio–effective lidar ratio relation: Theoretical basis for space lidar cloud phase discrimination
Authors: Hu Yongxiang
Published: June 2007

This paper introduces a technique for cloud phase discrimination based on linear polarization measurements made by space‐based lidar. Using CALIPSO Level 2 data products, a 3‐dimensional histogram of the depolarization ratio, δ, and the effective lidar ratio, Sc,eff, is derived for all optically thick clouds measured during July 2006. A second histogram is derived using data from early November, 2006. By analysis of the relationship between δ and Sc,eff, and an examination of their spatial correlations, water clouds, ice clouds with randomly oriented particles and ice clouds with horizontally oriented particles are clearly differentiated. For clouds having the same depolarization ratios, the effective lidar ratios of water clouds are significantly smaller than the effective lidar ratios of ice clouds comprised of randomly oriented particles. The variance of the depolarization ratios within water clouds is significantly larger than it is in ice clouds containing horizontally oriented particles. Depolarization ratios and effective lidar ratios are negatively correlated for water clouds, and positively correlated for ice clouds with horizontally‐oriented particles. For the optically thick ice clouds observed by CALIPSO, approximately half contain some fraction (up to 100%) of horizontally oriented particles.
Effect of the Earth's oblateness on the estimation of global vertical total electron content maps
Authors: Hobiger T. , Kondo T. , Koyama Y. , Ichikawa R. , Weber R.
Published: June 2007

Global Navigation Satellite Systems like the Global Positioning System and the Russian counterpart GLONASS permit the estimation of ionosphere parameters in the form of vertical total electron content (vTEC) values. Although the GNSS reference systems are rotation‐symmetric ellipsoids, simple approximations of gravitational equipotential surfaces, the modeling of global vTEC maps is carried out in spherical systems, since this simplifies mathematical expressions. Due to the Earth's oblateness this approximation is in slight disagreement to the geometric situation of the observations. Thus it is expected that estimated vTEC values will change when the WGS84 ellipsoid is taken as a reference for all computations. We will discuss both, the direct effect (i.e due to a shift of the ionospheric pierce point) and the indirect effect, caused by a change of the mapping function, on global vTEC values using a single layer model of the ionosphere. We show that the estimated global TEC maps change slightly by up to ±0.4 TECU when refined mathematical models are considered and we discuss how differential code biases are affected by this change.
Estimating submarine discharge of fresh groundwater from a volcanic island using a freshwater budget of the coastal water column
Authors: Lee Jong‐Mi , Kim Guebuem
Published: June 2007

The magnitude of submarine fresh groundwater discharge (SFGD) was estimated using a freshwater budget of the coastal water, which was calculated from salinity anomalies off a volcanic island, Jeju, Korea. We obtained high‐resolution profiles of salinity from the entire coastal area of Jeju Island using accurate CTD sensors. Lower salinity anomalies relative to the pristine offshore seawaters were observed and assumed to be a result of SFGD on the basis of negligible surface runoffs from the island and the relatively high levels of 222Rn in the low‐salinity waters. The SFGD flux during the study period calculated from a freshwater budget of the water column was 2.5−3.8 × 106 m3 d−1, which is close to the long‐term average based on the inland water balance. Our study shows that SFGD can be accurately and easily measured using a freshwater budget of the coastal waters off a highly permeable coastal zone with little surface runoff.
Stress induced elastic anisotropy of the Etnean basalt: Theoretical and laboratory examination
Authors: Becker K. , Shapiro S. A. , Stanchits S. , Dresen G. , Vinciguerra S.
Published: June 2007

A theory, the stress‐sensitivity approach, has been developed, which relates the elastic moduli of anisotropic rocks to the stress tensor and pore pressure for an arbitrary symmetry of the applied load. The theory explains the stress‐induced changes of seismic velocities in terms of stress‐induced changes of the pore space geometry. The stress dependent anisotropy is described in terms of Thomsen's anisotropy parameters ε, γ and δ. To test the theory we analyze the laboratory (high frequency) results of deformation of an isotropically crack damaged dry lava flow basalt from Mt. Etna volcano. The theory states that, under an anisotropic (i.e. axisymmetric triaxial) load and in the case of an initially isotropic rock, (1) the anisotropy parameters are linear functions of the stress exponentials (i. e. exponential functions of principal stress components) and (2) the ratio of these anisotropy parameters as a function of the stress is constant. In order to verify these relationships, the stress exponentials and the anisotropy parameters based on the measured velocities are computed as well as the expected ratio of the Thomsen's parameters. Our experimental results are in very good agreement with the theoretically predicted relations.
Meridional heat transport across the Antarctic Circumpolar Current by the Antarctic Bottom Water overturning cell
Authors: Heywood Karen J. , Stevens David P.
Published: June 2007

The heat transported by the lower limb of the Southern Ocean meridional overturning circulation is commonly held to be negligible in comparison with that transported by eddies higher in the water column. We use output from one of the first global high resolution models to have a reasonably realistic export of Antarctic Bottom Water, the OCCAM one twelfth degree model. The heat fluxed southward by the deep overturning cell using the annual mean field for 1994 at 56S is 0.033 PW, but the 5‐day mean fields give a larger heat flux (0.048 and 0.061 PW depending on calculation method). This is more than 30% of previous estimates of the total heat flux. Eddies and other transients add considerably to the heat flux. These results imply that this component of meridional heat flux may not be negligible as has been supposed.
Variability of mixed layer depth in Kuroshio/Oyashio Extension region: 2005–2006
Authors: Kako Shin'ichiro , Kubota Masahisa
Published: June 2007

In December 2005, the winter monsoon was abnormally strong around Japan. Therefore, it was expected that the cooling effect on the upper layer of the ocean would be very strong. However, the mixed layer depth (MLD) in the Kuroshio/Oyashio Extension region was not as deep as those observed during the last four winters. We investigated the reason for the shallow MLD by using a one‐dimensional turbulent closure model and the temperature and salinity data obtained using Argo floats. We found that summer heating plays an important role in the determination of the maximum MLD. In 2005, the heat storage rate was very high from July to October since summer heating was significantly stronger than those during the other summers. Consequently, a large vertical density gradient existed at the bottom of the mixed layer and prevented the deepening of the mixed layer.
A numerical study of the Schumann resonances in Mars with the FDTD method
Authors: Soriano A. , Navarro E. A. , Morente J. A. , Portí J. A.
Published: June 2007

Natural electromagnetic waves generated near the surface by electrostatic discharges in dust storms (dust devils) or by geological activity could be trapped in the resonant cavity formed by the surface and lower ionosphere of Mars, as it occurs on Earth giving rise to Schumann resonances. The finite difference time‐domain technique (FDTD) is applied to model the atmosphere of Mars in order to determine Schumann resonant frequencies, and natural electromagnetic fields at the extremely low frequency range (ELF). A numerical tool is provided to analyze the electrical conductivity profile of the Martian atmosphere, with the aim of obtaining Schumann resonance frequencies and their dependence on various ionization sources and solar activity. The effect of local enhancement of conductivity profile at several altitudes is also studied to understand its consequences on Schumann resonances. A criterion to fix the upper limit of the numerical model is established from these results.
Interpretation of the near‐IR spectra of the Kuiper Belt Object (136472) 2005 FY9
Authors: Eluszkiewicz Janusz , Cady‐Pereira Karen , Brown Michael E. , Stansberry John A.
Published: June 2007

Visible and near‐IR observations of the Kuiper Belt Object (136472) 2005 FY9 have indicated the presence of unusually long (1 cm or more) optical path lengths in a layer of methane ice. Using microphysical and radiative transfer modeling, we show that even at the frigid temperatures in the outer reaches of the solar system, a slab of low‐porosity methane ice can indeed form by pressureless sintering of micron‐sized grains, and it can qualitatively reproduce the salient features of the measured spectra. A good semiquantitative match with the near‐IR spectra can be obtained with a realistic slab model, provided the spectra are scaled to a visible albedo of 0.6, at the low end of the values currently estimated from Spitzer thermal measurements. Consistent with previous modeling studies, matching spectra scaled to higher albedos requires the incorporation of strong backscattering effects. The albedo may become better constrained through an iterative application of the slab model to the analysis of the thermal measurements from Spitzer and the visible/near‐IR reflectance spectra. The slab interpretation offers two falsifiable predictions: (1) Absence of an opposition surge, which is commonly attributed to the fluffiness of the optical surface. This prediction is best testable with a spacecraft, as Earth‐based observations at true opposition will not be possible until early next century. (2) Unlikelihood of the simultaneous occurrence of very long spectroscopic path lengths in both methane and nitrogen ice on the surface of any Kuiper Belt Object, as the more volatile nitrogen would hinder densification in methane ice.
A high affinity T cell receptor?
Authors: ALBERTI S
Published: June 2007

Summar

T cell receptors (TCR) have a low affinity for the MHC and the presented peptides (MHCpep). The low affinity of the TCR is crucial in T cell recognition and activation. Nevertheless, I propose that the TCR is perfectly capable of specifically binding MHCpep with high affinity. This hypothesis is supported by several data. Among them, the frequency of negatively selected immature T cells and of high affinity alloreactive T cells. Structural and functional analysis indicates that the TCR binding regions are quite similar to those of immunoglobulins. This implies similar binding strategies and suggests that there are no structural constraints on TCR affinity. The possible existence of high affinity TCR is relevant for current views on immune recognition, alloreactivity and peptide antagonism. In particular, it supports a model of immune recognition as unselected higher affinity binding to newly encountered peptides. The actual production of specific affinity TCR may also prove crucial for a soluble T cell receptor‐based immunotherapy and for the co‐crystallization of TCR with MHC and peptides.
Fast and Scriptable Molecular Graphics in Web Browsers without Java3D
Authors: Willighagen Egon , Howard Miguel
Published: June 2007

Jmol is a free, open source molecule viewer for chemistry and biochemistry. It is cross-platform, running on Windows, Mac OS X, and Linux/Unix systems. The software consists of three parts: the JmolApplet is a web browser applet that can be integrated into web pages; the Jmol application is a standalone Java application that runs on the desktop; and the JmolViewer is a development tool kit that can be integrated into other Java applications.
The Effect of S. Pneumoniae Bacteremia on Cerebral Blood Flow Autoregulation in Rats
Authors: Pedersen Michael , Brandt Christian T , Knudsen Gitte M , Østergaard Christian , Skinhøj Peter , Skovsted Ian C , Frimodt-Møller Niels , Møller Kirsten
Published: June 2007

In the present study, we studied the effect of bacteremia on cerebral blood flow (CBF) autoregulation in a rat model of pneumococcal bacteremia and meningitis. Anesthetized rats were divided into five groups (A to E) and inoculated with pneumococci intravenously and normal saline intracisternally (group A, N = 10); saline intravenously and pneumococci intracisternally (group B, N = 10); pneumococci intravenously and pneumococci intracisternally (group C, N = 5); saline intravenously, antipneumococcal antibody intravenously (to prevent bacteremia), and pneumococci intracisternally (group D, N = 10); or saline intravenously and saline intracisternally (group E, N = 10), respectively. Positive cultures occurred in the blood for all rats in groups A, B, and C, and in the cerebrospinal fluid for all rats in groups D and E. Twenty-four hours after inoculation, CBF was measured with laser-Doppler ultrasound during incremental reductions in cerebral perfusion pressure (CPP) by controlled hemorrhage. Autoregulation was preserved in all rats without meningitis (groups A and E) and was lost in 24 of 25 meningitis rats (groups B, C, and D) ( P <0.01). In group A, the lower limit was higher than that of group E ( P <0.05). The slope of the CBF/CPP regression line differed between the meningitis groups ( P < 0.001), being steeper for group B than groups C and D, with no difference between these two groups. The results suggest that pneumococcal bacteremia in rats triggers cerebral vasodilation, which right shifts the lower limit of, but does not entirely abolish, CBF autoregulation in the absence of meningitis, and which may further aggravate the vasoparalysis induced by concomitant pneumococcal meningitis.
Dynamic interaction of point vortices and a two-dimensional cylinder
Authors: Borisov Alexey V. , Mamaev Ivan S. , Ramodanov Sergey M.
Published: June 2007

In this paper we consider the system of an arbitrary two-dimensional cylinder interacting with point vortices in a perfect fluid. We present the equations of motion and discuss their integrability. Simulations show that the system of an elliptic cylinder (with nonzero eccentricity) and a single point vortex already exhibits chaotic features and the equations of motion are nonintegrable. We suggest a Hamiltonian form of the equations. The problem we study here, namely, the equations of motion, the Hamiltonian structure for the interacting system of a cylinder of arbitrary cross-section shape, with zero circulation around it, and N vortices, has been addressed by Shashikanth [Regular Chaotic Dyn. 10, 1 (2005)]. We slightly generalize the work by Shashikanth by allowing for nonzero circulation around the cylinder and offer a different approach than that by Shashikanth by using classical complex variable theory.
Transparent boundary conditions as dissipative subgrid closures for the spectral representation of scalar advection by shear flows
Authors: Oliver Marcel , Bühler Oliver
Published: June 2007

We consider the evolution of a passive scalar in a shear flow in its representation as a system of lattice differential equations in wave number space. When the velocity field has small support, the interaction in wave number space is local and can be studied in terms of dispersive linear lattice waves. We close the restriction of the system to a finite set of wave numbers by implementing transparent boundary conditions for lattice waves. This closure is studied numerically in terms of energy dissipation rate and energy spectrum, both for a time-independent velocity field and for a time-dependent synthetic velocity field whose Fourier coefficients follow independent Ornstein-Uhlenbeck stochastic processes.
Fabrication of polymer photonic crystal superprism structures using polydimethylsiloxane soft molds
Authors: Wang Li , Jiang Wei , Chen Xiaonan , Gu Lanlan , Chen Jiaqi , Chen Ray T.
Published: June 2007

We presented a soft lithography technique of fabricating polymer photonic crystal superprism structures using elastomeric polydimethylsiloxane templates. Dense two-dimensional photonic crystal superprism structures with feature sizes of 150–500nm and aspect ratios of up to 1.25 were replicated. Large field size and easy fabrication are two major advantages when compared with other imprint technology. Atomic force microscopy images showed that the molded structures had high fidelity to the masters. Less than 3% reduction of the depth in the molded structures was achieved with respect to the master. The increase of the surface roughness from the master to the molded structures is minimal. The issue of pattern collapse during pattern transfer of submicron structures was analyzed against the pattern dimensions and aspect ratios; and the experimental results were found in agreement with a prior theory. We also experimentally demonstrated the superprism effect in two-dimensional photonic crystal structure at near-infrared wavelength. The propagation beam changed 39° in the photonic crystal with respect to the input wavelength varying from 1546to1572nm. Such an effective, low cost, and high throughput soft lithography technique could find wide use in making photonic crystal based nanostructures.
Simulation and laboratory validation of magnetic nozzle effects for the high power helicon thruster
Authors: Winglee R. , Ziemba T. , Giersch L. , Prager J. , Carscadden J. , Roberson B. R.
Published: June 2007

The efficiency of a plasma thruster can be improved if the plasma stream can be highly focused, so that there is maximum conversion of thermal energy to the directed energy. Such focusing can be potentially achieved through the use of magnetic nozzles, but this introduces the potential problem of detachment of plasma from the magnetic field lines tied to the nozzles. Simulations and laboratory testing are used to investigate these processes for the high power helicon (HPH) thruster, which has the capacity of producing a dense (1018−1020m−3) energetic (tens of eV) plasma stream which can be both supersonic and super-Alfvénic within a few antenna wavelengths. In its standard configuration, the plasma plume generated by this device has a large opening angle, due to relatively high thermal velocity and rapid divergence of the magnetic field. With the addition of a magnetic nozzle system, the plasma can be directed/collimated close to the pole of the nozzle system causing an increase in the axial velocity of the plasma, as well as an increase in the Alfvén Mach number. As such the magnetic field of the nozzle is insufficient to pull the plasma back to the spacecraft, i.e., plasma attachment is not a problem for the system. Laboratory results show that the specific impulse (Isp) of the system can be increased by ∼30% by the addition of the nozzle due to the conversion of thermal energy into directed energy in association with a highly collimated profile. An interesting feature of the system is that self-collimation of the beam is expected to occur during continuous operation through plasma currents induced downstream from the magnetic nozzle. These currents lead to magnetic fields that have a smaller divergence than the original vacuum magnetic field so that the following plasma will be more collimated than the proceeding plasma. This self-focusing can lead to beam propagation over extended distances.
Stability of piecewise affine systems with application to chaos stabilization
Authors: Li Chuandong , Chen Guanrong , Liao Xiaofeng
Published: June 2007

This paper addresses the stability issue for a class of piecewise affine (PWA) systems, where the state spaces are assumed to be dividable into a certain number of hypercuboid subspaces. By constructing appropriate piecewise continuous Lyapunov functions, several numerically tractable stability criteria are developed for four subclasses of such PWA systems, which allow to recast the switching control problem for the PWA systems as a convex optimization problem. Moreover, the proposed method is applied to switching controller design for (globally and locally) stabilizing the unstable equilibrium points of PWA chaotic systems. Numerical simulations on the chaotic Chua’s circuit are presented to verify the theoretical results.
Vibrational characteristics of single-walled carbon-nanotube: Time and frequency domain analysis
Authors: Mitra Mira , Gopalakrishnan S.
Published: June 2007

In this paper, the vibrational characteristics of higher modes of single-walled carbon-nanotube (SWNT) modeled as the continuum axisymmetric cylinder are studied in both time and frequency domains. The modeling of SWNT for the high frequency dynamic analysis is done using the wavelet based spectral element method and this numerical technique involves the Daubechies scaling function approximation in time and one spatial (axial) dimension. This model is capable of capturing the coupled longitudinal-radial vibration arising due to the finiteness of SWNT. Here, first the phonon dispersion relation is obtained and validated with the atomistic and other continuum model simulations available. The effects of dimensional parameters on higher radial, longitudinal, and coupled radial-longitudinal vibrational modes are studied. Dependence of the higher mode frequencies on these parameters are much different from that of the first mode frequencies. Further time domain responses for broadband excitations are simulated and effects of the radius and thickness of the SWNT are studied. The prominent influence of the above geometrical parameters are observed in the time domain results.
Computing Fukui functions without differentiating with respect to electron number. I. Fundamentals
Authors: Ayers Paul W. , De Proft Frank , Borgoo Alex , Geerlings Paul
Published: June 2007

By using perturbations in the molecular external potential, the authors deduce the Fukui function from the change in Kohn-Sham orbital energies, avoiding the troublesome differentiation of the density with respect to electron number. Though this paper focuses on the Fukui function, the same general technique can be used to compute the functional derivative of any observable with respect to the external potential. In this paper, the method is used to compute the Fukui function for the beryllium atom and the formaldehyde molecule. The follow-up paper (part II) addresses the problem of computing condensed reactivity indicators.
Computing Fukui functions without differentiating with respect to electron number. II. Calculation of condensed molecular Fukui functions
Authors: Sablon Nick , De Proft Frank , Ayers Paul W. , Geerlings Paul
Published: June 2007

The Fukui function is a frequently used DFT concept in the description of a system’s regioselective preferences to undergo electrophilic, nucleophilic, or radical attacks. Until now, this function has usually been evaluated using finite difference approximations. The first paper in this series proposed a method for obtaining the Fukui function by a direct calculation of the functional derivative of the chemical potential with respect to the external potential. This paper extends the method to condensed Fukui functions and applies it to an extensive testing set of molecules. Results are promising, which demonstrates the usefulness of the new formalism.
Anharmonicity and tunneling effects in revisited vibrational O(1s) photoelectron spectrum of water gas phase
Authors: Carniato Stéphane
Published: June 2007

The authors have revisited the description of the core-hole ionization dynamics of the oxygen atom in water by re-exploiting the high-resolution, vibrationally resolved, XPS photoelectron spectrum of gas phase at the O(1s) edge. The agreement between theory and experiments is mainly controlled by (i) the description of the tunneling behavior near the barrier top (linear H–O–H conformation) of wave functions with high vibrational quanta, and (ii) the relative displacement of the potential-energy minimum of the O(1s) final state with respect to the ground state one. Accurate change in bond angle between the neutral and core-ionized states is essential to account for the Franck-Condon factors. The O(1s) photoelectron spectrum of water is well reproduced by the molecular ab initio calculations based on density functional theory and Franck-Condon factors calculations in a double-well (2×W) simulation of the bending motion.
Dewetting dynamics in miscible polymer-polymer thin film mixtures
Authors: Besancon Brian M. , Green Peter F.
Published: June 2007

Thin polystyrene films supported by oxidized silicon (SiOx∕Si) substrates may be unstable or metastable, depending on the film thickness, h, and can ultimately dewet the substrate when heated above their glass transition. In the metastable regime, holes nucleate throughout the film and subsequently grow due to capillary driving forces. Recent studies have shown that the addition of a second component, such as a copolymer or miscible polymer, can suppress the dewetting process and stabilize the film. We examined the hole growth dynamics and the hole morphology in thin film mixtures composed of polystyrene and tetramethyl bisphenol-A polycarbonate (TMPC) supported by SiOx∕Si substrates. The hole growth velocity decreased with increasing TMPC content beyond that expected from changes in the bulk viscosity. The authors show that the suppression of the dewetting velocity is primarily due to reductions in the capillary driving force for dewetting and to increased friction at the substrate-polymer interface. The viscosity, as determined from the hole growth dynamics, decreases with decreasing film thickness, and is connected to a depression of the glass transition of the film.
Ab initio calculation of proton-coupled electron transfer rates using the external-potential representation: A ubiquinol complex in solution
Authors: Yamamoto Takeshi , Kato Shigeki
Published: June 2007

In quantum-mechanical/molecular-mechanical (QM/MM) treatment of chemical reactions in condensed phases, one solves the electronic Schrödinger equation for the solute (or an active site) under the electrostatic field from the environment. This Schrödinger equation depends parametrically on the solute nuclear coordinates R and the external electrostatic potential V. This fact suggests that one may use R and V as natural collective coordinates for describing the entire system, where V plays the role of collective solvent variables. In this paper such an (R,V) representation of the QM/MM canonical ensemble is described, with particular focus on how to treat charge transfer processes in this representation. As an example, the above method is applied to the proton-coupled electron transfer of a ubiquinol analog with phenoxyl radical in acetonitrile solvent. Ab initio free-energy surfaces are calculated as functions of R and V using the reference interaction site model self-consistent field method, the equilibrium points and the minimum free-energy crossing point are located in the (R,V) space, and then the kinetic isotope effects (KIEs) are evaluated approximately. The results suggest that a stiffer proton potential at the transition state may be responsible for unusual KIEs observed experimentally for related systems.
Adaptive dynamical networks via neighborhood information: Synchronization and pinning control
Authors: Lu Wenlian
Published: June 2007

In this paper, we introduce a model of an adaptive dynamical network by integrating the complex network model and adaptive technique. In this model, the adaptive updating laws for each vertex in the network depend only on the state information of its neighborhood, besides itself and external controllers. This suggests that an adaptive technique be added to a complex network without breaking its intrinsic existing network topology. The core of adaptive dynamical networks is to design suitable adaptive updating laws to attain certain aims. Here, we propose two series of adaptive laws to synchronize and pin a complex network, respectively. Based on the Lyapunov function method, we can prove that under several mild conditions, with the adaptive technique, a connected network topology is sufficient to synchronize or stabilize any chaotic dynamics of the uncoupled system. This implies that these adaptive updating laws actually enhance synchronizability and stabilizability, respectively. We find out that even though these adaptive methods can succeed for all networks with connectivity, the underlying network topology can affect the convergent rate and the terminal average coupling and pinning strength. In addition, this influence can be measured by the smallest nonzero eigenvalue of the corresponding Laplacian. Moreover, we provide a detailed study of the influence of the prior parameters in this adaptive laws and present several numerical examples to verify our theoretical results and further discussion.
Nitrogen diffusion enhancement in a ferrous alloy by deuterium isotopic effect
Authors: Figueroa C. A. , Czerwiec T. , Driemeier C. , Baumvol I. J. R. , Weber S.
Published: June 2007

Studies of nitrogen implantation in an iron alloy using photoemission electron spectroscopy, sputtered neutral mass spectrometry, and elastic recoil detection analysis, reveal an enhancement of nitrogen diffusion when deuterium replaces hydrogen in the gas. Compared to hydrogen, deuterium reduces NOx species on the surface (geometric barrier), increasing the nitrogen activity at the surface and consequently nitrogen diffusion into the solid solution.
Anharmonic effects in IR, Raman, and Raman optical activity spectra of alanine and proline zwitterions
Authors: Daněček Petr , Kapitán Josef , Baumruk Vladimír , Bednárová Lucie , Kopecký Vladimír , Bouř Petr
Published: June 2007

The difference spectroscopy of the Raman optical activity (ROA) provides extended information about molecular structure. However, interpretation of the spectra is based on complex and often inaccurate simulations. Previously, the authors attempted to make the calculations more robust by including the solvent and exploring the role of molecular flexibility for alanine and proline zwitterions. In the current study, they analyze the IR, Raman, and ROA spectra of these molecules with the emphasis on the force field modeling. Vibrational harmonic frequencies obtained with 25 ab initio methods are compared to experimental band positions. The role of anharmonic terms in the potential and intensity tensors is also systematically explored using the vibrational self-consistent field, vibrational configuration interaction (VCI), and degeneracy-corrected perturbation calculations. The harmonic approach appeared satisfactory for most of the lower-wavelength (200–1800cm−1) vibrations. Modern generalized gradient approximation and hybrid density functionals, such as the common B3LYP method, provided a very good statistical agreement with the experiment. Although the inclusion of the anharmonic corrections still did not lead to complete agreement between the simulations and the experiment, occasional enhancements were achieved across the entire region of wave numbers. Not only the transitional frequencies of the C–H stretching modes were significantly improved but also Raman and ROA spectral profiles including N–H and C–H lower-frequency bending modes were more realistic after application of the VCI correction. A limited Boltzmann averaging for the lowest-frequency modes that could not be included directly in the anharmonic calculus provided a realistic inhomogeneous band broadening. The anharmonic parts of the intensity tensors (second dipole and polarizability derivatives) were found less important for the entire spectral profiles than the force field anharmonicities (third and fourth energy derivatives), except for a few weak combination bands which were dominated by the anharmonic tensor contributions.
Surface wave suppression in antenna systems using magnetic metamaterial
Authors: Wu Bae-Ian , Chen Hongsheng , Kong Jin Au , Grzegorczyk Tomasz M.
Published: June 2007

In the presence of normal substrates, it is well known that there exists strong transverse magnetic (TM) surface wave coupling between antenna ports on a shared ground plane. Through the analysis of surface wave propagation, such coupling is proposed to be substantially decreased via the use of magnetic metamaterials. The design and the characterization of the metamaterials are presented, and it is shown that within a certain frequency band, the material behaves as an anisotropic magnetic plasma, suppressing the propagation of a TM surface wave. Experimental measurements further confirm such a suppression, and an isolation improvement of 15 dB between an aperture spaced 9.2 cm apart is observed, in agreement with numerical simulations.
Near-wall passive scalar transport at high Prandtl numbers
Authors: Bergant Robert , Tiselj Iztok
Published: June 2007

Very accurate numerical simulations of a passive scalar field in the turbulent channel and flume flow were performed at friction Reynolds numbers Reτ=150 and Reτ=395 and Prandtl numbers Pr=100, Pr=200. Direct numerical simulation is used for description of the velocity field. The temperature field is described with the LES-like approach with the smallest resolved temperature scales equal to the smallest scales of the velocity field. The consistency of the applied physical modelling and pseudospectral scheme is first tested with comparison of the results with the existing DNS simulations of F. Schwertfirm and M. Manhart [Proceedings of Turbulence, Heat, and Mass Transfer (2006)] at Reτ=180 and Pr=25. The sensitivity of the method to the grid refinement and time step variations is performed with simulations at Reτ=150 and Pr=200. Both tests show that the proposed approach produces very accurate mean temperature profiles, heat transfer coefficients, and other low-order moments of the turbulent thermal field. It is shown that the mean temperature profiles near the wall can be accurately predicted even when the temperature scales between the Batchelor and Kolmogorov scale are not resolved. The key to the success of the proposed approach lies in the fact that the large-scale structures govern the turbulent heat transfer at high Prandtl numbers. Resolved spectra of the temperature fluctuations and the rms temperature fluctuations in the diffusive sublayer and the thermal buffer layer (y+<5) are practically unaffected by the unresolved temperature scales. The contribution of the sub-Kolmogorov thermal scales becomes relevant above the thermal buffer layer (y+>5), where the unresolved temperature scales affect spectra and rms temperature fluctuations, but not the log-law shape of the mean temperature profile and the mean heat transfer coefficient. Further results are obtained at Reτ=150, Pr=100, Pr=500, and Reτ=395, Pr=100, Pr=200. These results are compared with Kader empirical temperature profiles and other available experimental and numerical results. Significant difference in the mean temperature profiles is demonstrated between the profiles calculated at friction Reynolds numbers 150 and 395. Kader correlation is shown to be very accurate at higher Reynolds number but underpredicts temperatures at low Reynolds number.
Slip and accommodation coefficients from rarefaction and roughness in rotating microscale disk flows
Authors: Blanchard Danny , Ligrani Phil
Published: June 2007

Accommodation coefficients are determined from experimental results and analysis based on the Navier-Stokes equations for rotation-induced flows in C-shaped fluid chamber passages formed between a rotating disk and a stationary surface. A first-order boundary condition is used to model the slip flow. The fluid chamber passage height ranges from 6.85to29.2μm to give Knudsen numbers from 0.0025 to 0.031 for air and helium. In all cases, roughness size is large compared to molecular mean free path. The unique method presented for deducing tangential momentum accommodation coefficients gives values with less uncertainty compared to procedures that rely on flows in stationary tubes and channels. When channel height is defined at the tops of the roughness elements, slip velocity magnitudes and associated accommodation coefficients are a result of rarefaction at solid-gas interfaces and shear at the gas-gas interfaces. With this arrangement, tangential accommodation coefficients obtained with this approach decrease, and slip velocity magnitudes increase, at a particular value of Knudsen number, as the level of surface roughness increases. At values of the mean roughness height greater than 500nm, accommodation coefficients then appear to be lower in air flows than in helium flows, when compared for a particular roughness configuration. When channel height is defined midway between the crests and troughs of the roughness elements, nondimensional pressure rise data show little or no dependence on the level of disk surface roughness and working fluid. With this arrangement, slip is largely independent of surface roughness magnitude and mostly due to rarefaction, provided the appropriate channel height is chosen to define the roughness height.