bluuebunny/crossref_metadata_2025 huggingface dataset:
Raindrop size distributions and vertical air motions as inferred from zenith‐pointing Doppler radar with the RONSARD system
Authors: Hauser D. , Amayenc P.
Published: April 2008

This paper deals with results of raindrop size distributions and vertical air motions aloft within a cold front rainband, as inferred from vertical incidence Doppler spectra gathered with one radar of the RONSARD system. These results are obtained by using a new method recently proposed by the authors, assuming exponentially shaped raindrop size distributions. Comparison of the radar results with in situ measurements of raindrop size distributions aboard an aircraft indicates that the method provides satisfying estimates of the precipitating water content and of the median volume diameter of raindrops, though the values of the raindrop size distribution parameters N0 or Λ, deduced from the two techniques, are found to be significantly different. It is also found that the proposed method produces values of the deduced parameters with high time and space variability. Owing to this variability, the rainfall bulk parameters are found accurately connected only by means of three‐parameter relationships. These characteristics are similar to those already depicted within another stratiform precipitation by using the same approach.
Frequency scaling propagation parameters using dual‐polarization radar results
Authors: Holt A. R. , McGuinness R. , Evans B. G.
Published: April 2008

Raindrop size distributions derived from dual‐polarization radar data have been used to model attenuation in the rain region of slant paths. Attenuation ratios for 14.455/11.786 GHz, 17.5/11.786 GHz, and 30.0/20.0 GHz have been derived and have been shown to be sensitive to both type of rain and assumed water temperature. An error analysis has shown the dual‐polarization (copolar linear) technique to give a good prediction of attenuation ratios for all but very light rain.
Site diversity advantage as a function of spacing and satellite elevation angle, derived from dual‐polarization radar data
Authors: Goddard John W. F. , Cherry Stephen M.
Published: April 2008

A description is given of the application of dual‐polarization radar (linear, copolar transmit and receive) to estimate the site diversity advantage achieved by two hypothetical satellite receiving stations when attenuation by rain is present on the earth‐satellite path. The site diversity advantage is computed at 11.6 GHz for satellite elevation angles of 10°, 20°, and 30°, and for numerous spacings between the two receivers. The data collection scheme is described, and the limitations of the statistics discussed. A description is given of the computation, from the radar data, of the total path attenuation caused by rain cells and the problems of applying these techniques are discussed.
HF ray tracing at high latitudes using measured meridional electron density distributions
Authors: Villain J. P. , Greenwald R. A. , Vickrey J. F.
Published: April 2008

In many physical cases, small‐ and intermediate‐scale (1–100 m) electron density irregularities in the ionospheric E and F regions are elongated in the direction of the earth's magnetic field. As a result of this elongation or field alignment, radar studies directed toward the analysis of backscattered signals from ionospheric irregularities require that the radar wave vector propagate perpendicular to the magnetic field in the irregularity region. At low and middle latitudes, orthogonality may be achieved at any radar operating frequency, but at higher latitudes this geometry becomes increasingly difficult. Above 60° invariant latitude it is almost impossible to achieve normality with straight line propagation. In order to observe the wealth of ionospheric irregularities in the auroral zone and polar cap E and F regions, one must operate radars at HF frequencies where refraction affects wave propagation and aids one in achieving the orthogonality condition. However, refraction from large‐scale (1–100 km) ionospheric irregularity structure also increases the complexity in accurately determining the location of the scattering volume. In this paper we use a modified version of an existing HF ray tracing program to determine with high precision the HF propagation paths in a realistic ionosphere. In this analysis the ionosphere is simulated by a two‐dimensional array of electron density as a function of geomagnetic latitude and altitude. This analysis is then applied to real measurements obtained from a meridian scan of the Chatanika incoherent scatter radar, and we show how this electron density distribution affects propagation from a simulated HF radar located at Anchorage, Alaska. The variations of the propagation conditions are also studied as one moves the density pattern in the direction of the Anchorage radar.
High‐resolution multipath measurements of the meteor scatter channel
Authors: Weitzen Jay A. , Grossi Mario D. , Birkemeier William P.
Published: April 2008

An experimental method for determining the multipath spread of the meteor scatter communications channel is discussed. The technique measures the spreading of equalization pulses contained in commercial television signals received via meteor scatter. Resolution of spread to within 0.5 μs is possible with this technique.
Radiation of ion acoustic waves from a solid spherical probe in a warm isotropic plasma
Authors: Malingre M.
Published: April 2008

The behavior of a solid spherical probe immersed in a warm isotropic collisionless plasma, biased to plasma potential, and excited from an alternating current source, is investigated for frequencies below the ion plasma frequency. A hydrodynamic description of the plasma is used, together with absorptive boundary conditions for the electrons and ions at the probe surface. Expressions are derived for the alternating potential in the plasma, as a function of frequency, of radial distance, of probe radius, and of the plasma parameters. Much of this potential is due to ion acoustic waves, the amplitude of which depends strongly on the ratio Rp of the probe radius to the Debye length. As soon as Rp exceeds the ratio ƒ/ƒpe of the excitation frequency to the electron plasma frequency, the amplitude is reduced much below its value for Rp = 0 corresponding to a point source. It is shown in particular that under some conditions, for values of Rp over a certain limited range, this reduction is independent of Rp. The smaller the ratio of the excitation frequency to the ion plasma frequency, the larger is this range.
The mobile SOUSY Doppler radar: Technical design and first results
Authors: Czechowsky P. , Schmidt G. , Rüster R.
Published: April 2008

On the basis of the experience obtained with the stationary SOUSY radar in the Harz mountains, a mobile VHF Doppler system was developed during the last two years. The electronic part is installed in a 20‐foot container and was tested for the first time during a joint experiment at the Arecibo Observatory (Puerto Rico) using a special log‐periodic aerial to illuminate the 300‐m dish. In 1982 this system was extended by designing a mobile phased antenna array with 576 Yagi elements. The grouping of the single Yagis, the feeding network, the phase shifters, the power splitters, and the transmit receive switch are described. A summary of results obtained during the first two campaigns is presented as well as a survey of future programs demonstrating the flexibility of this mobile system.
Modulated beam injection from the space shuttle during magnetic conjunctions of STS 3 with the DE 1 satellite
Authors: Inan U. S. , Pon M. , Banks P. M. , Williamson P. R. , Raitt W. J. , Shawhan S. D.
Published: April 2008

An electron beam emitted from the Office of Space Sciences 1 pallet on STS 3 was pulsed with specially designed very low frequency (VLF) formats in an attempt to generate whistler mode waves. Modulated operations of the beam emitted by a fast pulse electron generator (FPEG) were initiated during times of magnetic conjunctions between STS 3 and the high‐altitude DE 1 satellite equipped with broadband VLF receivers. Coordinated FPEG/VLF modulation and DE 1 wideband data acquisition were achieved in 12 different cases. No evidence of any waves generated by FPEG were detected on the DE 1 analog wideband data. However, it is shown that in all of the cases, either the STS 3 attitude was such that the emitted electrons struck the main body of the vehicle, or it was not possible for whistler mode waves to propagate from the STS 3 location up to the vicinity of the DE 1 satellite.
Auroral kilometric radiation/aurora correlation
Authors: Benson Robert F. , Akasofu Syun‐Ichi
Published: April 2008

Auroral kilometric radiation (AKR) observations from the ISIS 1 topside sounder receiver were compared with visual auroral observations from the network of Alaskan all‐sky camera stations. The goal was to relate AKR source region encounters to specific auroral forms on the same magnetic field line. Thirty‐eight simultaneous data sets were identified and analyzed. In general, intense AKR was associated with bright auroral arcs, and conditions of weak or no AKR corresponded to times when either no aurora or only a faint arc or weak diffuse aurora was observed. Five cases, when both intense AKR and bright visual aurora were present, were analyzed in detail. Complete electron density Ne contours, from the satellite altitude down to the F region ionization peak, were obtained along N‐S traversals of the AKR source region. In addition, the ISIS 1 orbital tracks were projected down the magnetic field lines to the auroral altitude and compared to auroral features on a map derived from the all‐sky camera images. Density cavities (regions where Ne < 100 cm−3) were encountered on each of these passes. In one case the cavity extended in altitude down to just below 2000 km. The latitudinal extent of these cavities ranged from 3° to almost 30°. A variety of situations relating AKR to the visual aurora were identified, and both spatial (within about 100 km at the satellite altitude) and temporal (within about l min) correlations were identified. In addition to confirming that a density cavity is a necessary but not sufficient condition for the generation of AKR, the present results indicate that even the combination of a density cavity on an auroral‐arc field line is not always sufficient for AKR generation. Thus additional constraints must be satisfied in order to generate intense AKR from density cavities. These constraints most likely involve the characteristics of the energetic electron distribution function and possibly also latitudinal Ne gradients.
Scattering from two‐scale surfaces for large distances
Authors: Yordanov Oleg I.
Published: April 2008

The scattering of high‐frequency scalar waves by a reflecting surface containing two‐scale, two‐dimensional cylindrical random irregularities is considered. All possible specular contributions, as well as the effects of large‐scale shadowing, are accounted for. Expressions for the average field and the average intensity are derived and evaluated numerically. The results obtained differ qualitatively from those known in the literature and agree with the latest experimental data.
Multifrequency study of ionospheric scintillation at Ascension Island
Authors: Franke S. J. , Liu C. H. , Fang D. J.
Published: April 2008

Multifrequency amplitude scintillation data at L and C bands from the equatorial region at Ascension Island are used to study the spectral index n for the scintillation index S4. The average frequency dependence of the scintillation index (S4) is found to be f−1.68. It is shown that the spectral index tends to be larger for events which occur after 2030 local time than for events which occur before 2030. This may indicate temporal evolution of the irregularity spectrum. Model studies based on a two component power law irregularity spectrum are applied to relate changes in n to spectral evolution and to show that the observations are consistent with recent in situ measurements. Possible geophysical implications are also discussed.
Multistation study of nighttime scintillations in low latitudes: Evidence of control by equatorial F region irregularities
Authors: Somayajulu Y. V. , Garg S. C. , Dabas R. S. , Singh Lakha , Tyagi T. R. , Lokanadham B. , Ramakrishna S. , Navneeth G.
Published: April 2008

VHF nighttime scintillations, recorded during a high solar activity period at a meridian chain of stations covering a magnetic latitude belt of 3°–21°N (420 km subionospheric points) are analyzed to investigate the influence of equatorial spread F irregularities on the occurrence of scintillation at latitudes away from the equator. Observations show that saturated amplitude scintillations start abruptly about one and a half hours after ground sunset and their onset is almost simultaneous at stations whose subionospheric points are within 12°N latitude of the magnetic equator, but is delayed at a station whose subionospheric point is at 21°N magnetic latitude by 15 min to 4 hours. In addition, the occurrence of postsunset scintillations at all the stations is found to be conditional on their prior occurrence at the equatorial station. If no postsunset scintillation activity is seen at the equatorial station, no scintillations are seen at other stations also. The occurrence of scintillations is explained as caused by rising plasma bubbles and associated irregularities over the magnetic equator and the subsequent mapping of these irregularities down the magnetic field lines to the F region of higher latitudes through some instantaneous mechanism; and hence an equatorial control is established on the generation of postsunset scintillation‐producing irregularities in the entire low‐latitude belt.
Pulse distortion after propagating through an ionospheric bubble
Authors: Tucker M. R. , Yeh K. C.
Published: April 2008

An ionospheric bubble is first modeled based on in situ data measured on board a satellite. The effect such a bubble may have on radio signals propagating through it is next simulated by solving numerically a parabolic equation. Considerable pulse distortion is found. When the computational technique is applied to a frozen but moving bubble, these results imply that the pulse excess arrival time, the pulse width, and its skewness will fluctuate with time, leading to the pulse jitter problem often observed experimentally.
Scintillation results from ionospheric modification experiment
Authors: Johnson Allen L. , Hocutt Anne M.
Published: April 2008

During September 1982 a group of U.S., West German, and Brazilian organizations evaluated the trigger mechanism of equatorial ionospheric bubbles in an ionospheric modification experiment in Brazil. The experiment involved the release of two barium chemical packages at an unstable point in the nighttime F layer. At this point, they would trigger a bubble of low ion density to rise into the middle and upper F regions. Following the release of the barium, an aircraft equipped with a UHF satellite receiver was flown in the shadow of the bubble. It mapped the growth of the resulting irregularities from their initial appearance until they grew to a width of 100 km or more. The experiment demonstrated that a bubble could be triggered by proper placement of the barium ions and that the resultant irregularities would have the appearance of naturally triggered equatorial ionospheric irregularities.
Two‐frequency microwave resonance measurements from an aircraft: A quantitative estimate of the directional ocean surface spectrum
Authors: Johnson J. W. , Weissman D. E.
Published: April 2008

Measurements of the relative strength of ocean surface wave spectral components have been made from an aircraft using the two‐frequency microwave resonance technique. A coherent Ku band radar was used to study the Bragg type resonance matching (at the difference frequency Δf) to the surface wave components. The spatial spectrum of the surface reflectivity modulation was then computed as the value of Δf was varied over a matching range of approximately 15 m to 150 m in surface wavelength. This paper contains experimental results from flights conducted during the 1979 MARSEN project and the 1980 ARSLOE project, plus a theoretical formuation of the relationship between the radar measurement and the directional surface elevation spectrum. The objective here is to evaluate the two‐frequency resonance technique in terms of the strength of the radar response and the effects of surface illumination geometry, and to demonstrate the inversion of the radar data to absolute directional surface elevation spectrum. An independent measurement from the aircraft of the modulation transfer function required in this inversion and the resulting estimate of the surface spectrum are presented. Finally, a comparison is drawn between the Langley two‐frequency results and an estimate of the spectrum produced by the Wallops Flight Center surface contour radar.
Thermal emission of nonspherical particles
Authors: Tsang Leung
Published: April 2008

The thermal emission of nonspherical particles is studied by applying the fluctuation dissipation theorem to calculate the Stokes' vector source terms in radiative transfer theory. Generally, all the four Stokes' parameters in the source term are nonzero and are proportional to the absorption coefficient in the backward direction. The measurements of active and passive remote sensing are related by showing that within the framework of radiative transfer theory, the emissivity of vertically and horizontally polarized waves can be expressed in terms of the bistatic scattering coefficients in the usual manner. In addition, the emissions of the third and fourth Stokes' parameters are also expressed in terms of bistatic scattering coefficients.
Analysis of internally reflected and diffracted fields in electromagnetic backscattering by dielectric spheroids
Authors: Chaudhuri Sujeet K. , Sleator Frederick B. , Boerner Wolfgang‐M.
Published: April 2008

For a prolate spheroid of arbitrary size various components of the scattering mechanism responsible for the backscattered energy (which is of interest to monostatic radar systems) are analyzed. In the case of a spherical structure, due to the symmetry, the conditions for the existence of glory rays and internal surface waves are straightforward. It is shown that for the prolate spheroid with arbitrary direction of incidence, these conditions are more complex, and it is advantageous to resort to a numerical method of ray tracing. The solution yielded by this numerical method, along with some geometrical arguments, shows that for most of the cases of arbitrary scatterers, the single bounce glory ray will not exist and therefore will not contribute to the backscattered energy. For the axial case of incidence, conditions for the existence of internal surface waves have also been developed. Finally, some results for the approximate backscattered signal incorporating the above components, are presented.
Asymptotic evaluation of high‐frequency fields near a caustic: An introduction to Maslov's method
Authors: Ziolkowski Richard W. , Deschamps Georges A.
Published: April 2008

It is well known that the geometrical optics approximation, generally valid for high‐frequency fields, fails in the vicinity of a caustic. A systematic procedure of V. P. Maslov that remedies this situation will be reviewed in this paper. Maslov's method makes use of a representation of the geometrical optics field in the phase space M = X × K, where a point m = (x, k) is a pair of a position vector x ε X and a wave vector k ε K. A Lagrangian submanifold of M, Λ, that lies in the dispersion surface and is a union of the phase space trajectories selected by the initial conditions is constructed. It can be considered as a global representation of the phase. The phase space amplitudes (half densities) satisfy transport equations defined along those trajectories in Λ. Since trajectories in M never form a caustic, a globally defined amplitude can be established on Λ. The field on X is related to the resultant field on Λ by the “canonical operator,” an operator introduced by Maslov. It generates an integral form of the solution near a caustic that can be evaluated analytically, numerically, or with uniform asymptotic techniques. Away from the caustic it recovers the geometrical optics field. Alternatively, the phase space field can be projected on a hybrid space Y where some of the space coordinates have been replaced by the corresponding wave vector components. For any caustic point in X, one such hybrid space Y where this projection does not encounter a caustic exists. A geometrical optics field results in Y that is related to the original in X by an asymptotic Fourier transform. The solution in X near a caustic can be represented as the Fourier transform to X of that hybrid space geometrical optics solution. These techniques are illustrated with two simple but revealing problems: continuation of the field through a fold caustic in a linear layer medium and through a caustic with a cusp point in a homogeneous medium.
A comparison of radio wave and in situ observations of tropospheric turbulence and wind velocity
Authors: Herben M. H. A. J. , Kohsiek W.
Published: April 2008

The importance of radio wave scintillations introduced by tropospheric turbulence for both telecommunication and meteorological research is indicated. Amplitude, phase, and phase difference measurements of scintillation were performed at 30 GHz on an 8.2‐km line‐of‐sight path. In situ measurements of temperature and humidity fluctuations and wind velocity and direction were done about midway on the propagation path. The measuring systems are described, and theory is presented to derive the refractive‐index structure parameter C2n and wind velocity v from radio wave scintillations and the in situ measurements. Formulas are given to scale C2n and v with height. Finally, results of the experiments are presented and discussed.
Numerical analysis of polarization‐maintaining optical fibers with circular pits in a circular core
Authors: Hosono Toshio , Hinata Takashi , Yoshikawa Hiroshi
Published: April 2008

This paper proposes new optical fibers with angularly nonuniform index distributions featured by one circular pit in the core, two circular pits in the core, and noncoaxial W‐type distribution. The field problems are analyzed by the improved point‐matching method. Waveguide parameters of single circular‐pit fibers, double circular‐pit fibers, and noncoaxial W‐type fibers that provide the largest modal birefringence are investigated. It is found that (1) the maximum modal birefringence Bmax for single circular‐pit fibers and noncoaxial W‐type fibers is much larger than those for double circular pit fibers, (2) noncoaxial W‐type fibers exhibit larger modal birefringence than single circular‐pit fibers in the range Δn < 1.8% (Δn is relative index difference), and (3) the value Bmax of 1.7 × 10−4 has been attained for Δn = 0.42%. These results are quite reliable because (1) the relative error of the modal birefringence computed by our method is less than 10−4, and (2) this new method never produces ghost solutions.
Inversion theory for almost periodic media
Authors: Jaggard D. L. , Jordan A. K.
Published: April 2008

The inversion problem for almost periodic media is considered and formulated. By use of a priori knowledge and the characteristics of the scattering data, two distinct inversion methods are found for determining almost periodic profiles from the scattering data. This is useful and important since almost periodic functions provide a convenient basis for expanding the characteristic functions of a variety of periodic, quasi‐periodic, aperiodic, or random media. In addition, rational function reflection data lead naturally to generalized almost periodic profiles. These concepts are illustrated by several numerical examples.
Prospects for three‐dimensional projective and tomographic imaging radar networks
Authors: Farhat N. H. , Werner C. L. , Chu T. H.
Published: April 2008

Results of a study demonstrating the feasibility of broadband and speckle‐free projective imaging of a complex shaped scattering object in the 6‐17 GHz range with centimeter resolution are presented. We show how angular, spectral, and polarization diversities can be combined in the data acquisition process, with a unique target‐derived reference technique to access the three‐dimensional Fourier space of the scatterer cost‐effectively. Image retrieval algorithms, based on the projection slice theorem and knowledge of object symmetry, are utilized in obtaining images of a model aircraft with near optical resolution. The implications for high‐resolution three‐dimensional tomographic imaging radar networks are discussed.
Electromagnetic imaging based on the characteristic properties of the Rayleigh coefficient
Authors: Chaudhuri S. K.
Published: April 2008

In this paper, characteristic parameters of low‐frequency scattered signal are analyzed for their dependence on the object's geometry, and specifically the Rayleigh coefficient is identified as a suitable quantity for the imaging and identification applications involving volume and size estimations. In order to recover the Rayleigh coefficient from the backscattered signals, an inverse scattering model is presented here, which is based on the time domain concepts of electromagnetic theory. Using the first five moment condition integrals, the returned pulse response is processed to recover the Rayleigh coefficient, and the next higher order nonzero coefficient of the power series expansion in k (wave number) of the object backscattering response. The Rayleigh coefficients thus recovered are related to the volume, elongation factor (eccentricity), and orientation of an equivalent spheroid by use of a nonlinear optimization algorithm. Numerical simulation of the scheme of this paper has been tested on different scatterers. The results of these simulations are presented.
Applications of Riemann‐Hilbert problem techniques to electromagnetic coupling through apertures
Authors: Ziolkowski Richard W. , Johnson William A. , Casey Kendall F.
Published: April 2008

The generalized dual series approach, as it is applied to the mixed boundary value problems that characterize the coupling of electromagnetic energy through apertures, is described. This approach is based upon techniques borrowed from the analysis of the Riemann‐Hilbert problem of complex variable theory. It allows one to obtain essentially analytical solutions to families of canonical coupling problems in separable geometries. The generalized dual series approach is illustrated with the example of the coupling of an H‐polarized plane wave to a perfectly conducting infinite cylinder with an infinite axial slot enclosing a concentric impedance cylinder. This problem encompasses the coupling to an empty cylinder as well as to a cylinder enclosing a conducting wire or cable. The solution explicitly exhibits the correct behavior near the edge of the aperture, and it can handle small to large ratios of cylinder radius to wavelength without additional special considerations. The angle of incidence is arbitrary. Results are shown for a normal incidence case and for a case in which the direction of incidence coincides with the edge of the aperture. A brief comparison with related moment method results is given.
The middle and upper atmosphere radar: First results using a partial system
Authors: Kato Susumu , Ogawa Toru , Tsuda Toshitaka , Sato Toru , Kimura Iwane , Fukao Shoichiro
Published: April 2008

The middle and upper atmosphere radar of Radio Atmospheric Science Center, Kyoto University, is now partly in operation. The facility will be complete in 1985. The active array antenna system of the radar makes it possible to steer the radar beam in an interpulse period, i.e., 400 μs. The antenna system consists of independently operative subgroups, each having 19 Yagis. We expect various sophisticated experiments by the system. A preliminary observation was successful to elucidate atmospheric motions during typhoon 5 which approached the radar site in August, 1983.
Simplified theory of first‐ and second‐order scattering of HF radio waves from the sea
Authors: Robson R. E.
Published: April 2008

A simplified, physically intuitive model of diffuse scattering of radio waves from a rough ocean surface is used to derive first‐ and second‐order cross sections essentially in agreement with the standard expressions.
Comparison of winds, waves, and turbulence as observed by airborne lidar, ground‐based radars, and instrumented tower
Authors: Eilts M. D. , Doviak R. J. , Sundara‐Rajan A.
Published: April 2008

On June 29, 1981, two ground‐based Doppler radars, an airborne Doppler lidar, a tall (444 m) instrumented tower, and a rawinsonde collected wind data in the planetary boundary layer (PBL) in central Oklahoma. This allowed, for the first time, intercomparison of wind fields synthesized from airborne Doppler lidar data with those from dual Doppler radar data. The vertical profile of wind in the PBL measured by the radars compared favorably with the profiles measured by the tower and rawinsonde while the one obtained from lidar data differed from the other three by as much as 3 m/s in wind speed and 38° in direction. The time dependence of differences in wind estimates from radar and lidar suggested that these discrepancies could be attributed to a Schuler resonance in the aircraft's inertial navigation system which caused an erroneous component of the aircraft velocity vector to be subtracted from the lidar radial velocities, thus creating errors in the synthesized wind speed and direction. The vertical profiles of horizontal velocity fluctuations detected by the different sensing systems compared well. Also, spectra from the different sensing systems compared well in both magnitude and shape, suggesting that the lidar and radar detected similar structure in the PBL. A peak at the 4‐km wavelength, evident in spectra from all sensing systems, is proposed to be caused by horizontally symmetrical cells having a wavelength 4 times the height of the PBL in accord with theory.
The fine structure of elevated refractive layers: Implications for over‐the‐horizon propagation and radar sounding systems
Authors: Gossard E. E. , Neff W. D. , Zamora R. J. , Gaynor J. E.
Published: April 2008

Radar and acoustic sounder data are compared with in situ turbulence and meteorological data collected with fast response sensors on a carriage traversing a 300‐m tower. The carriage data reveal a fine structure of sharp laminae within elevated inversions that is associated with laminated echoes in the sounder records. The zones of sharp gradient contain narrow regions of enhanced structure parameters of temperature and humidity that presumably account for the radar and acoustic returns from those zones. In these cases the temperature and humidity profiles are remarkably close (negative) replicas of each other, greatly simplifying the interpretation of radio refractive index from temperature data. A model of specular reflection is compared with a Bragg backscatter model, and it is concluded that specular contributions from the observed gradients can exceed the Bragg return even for back‐scatter if radar wavelengths greater than a few meters are used. For bistatic configurations the specular contributions should commonly exceed the tropospheric scatter contribution when elevated stable layers are present.
Comparisons of Audio and Audiovisual Measures of Stuttering Frequency and Severity in Preschool-Age Children
Authors: Rousseau Isabelle , Onslow Mark , Packman Ann , Jones Mark
Published: April 2008

Purpose

To determine whether measures of stuttering frequency and measures of overall stuttering severity in preschoolers differ when made from audio-only recordings compared with audiovisual recordings.

Method

Four blinded speech-language pathologists who had extensive experience with preschoolers who stutter measured stuttering frequency and rated overall severity from audio-only and audiovisual recordings of 36 preschool children who were stuttering. Stuttering frequency (percentage of syllables stuttered [%SS]) was based on counts of perceptually unambiguous stutterings, made in real time, and overall severity was measured using a 9-point rating scale.

Results

Stuttering frequency was statistically significantly lower by around 20% when made from audio-only recordings. This was found to be directly attributable to differences in the counts of stuttered syllables, rather than to differences in the total numbers of syllables spoken. No significant differences were found between recording modalities for the ratings of overall severity. Correlations between %SS scores in the 2 modalities and severity rating scores in the 2 modalities were high, indicating that observers agreed on data trends across speech samples.

Conclusions

Measures of %SS made from audio-only recordings may underestimate stuttering frequency in preschoolers. Although audio-only %SS measures may underestimate stuttering frequency at the start of a clinical trial to a clinically significant extent, posttreatment scores at or below 1.0%SS are likely to underestimate by 0.2%SS or less, which is clinically insignificant.
Additivity of noise propagation in a protein cascade
Authors: Zheng Xiudeng , Tao Yi
Published: April 2008

Stochastic fluctuations in a protein synthetic cascade are investigated using standard Ω-expansion technique. For the steady-state sensitivity, we show the conditions that result in the ultrasensitive “all-or-none” behavior, and for the noise propagation, we show clearly that (i) for any one given protein species in this cascade, the contributions of fluctuations in upstream proteins to its noise should be additive; and (ii) the output noise levels can vary as a function of the input concentrations and cascade length. Our results provide a possible theoretical explanation for the previous experimental studies.
Critical dynamics of ballistic and Brownian particles in a heterogeneous environment
Authors: Höfling Felix , Munk Tobias , Frey Erwin , Franosch Thomas
Published: April 2008

The dynamic properties of a classical tracer particle in a random, disordered medium are investigated close to the localization transition. For Lorentz models obeying Newtonian and diffusive motion at the microscale, we have performed large-scale computer simulations, demonstrating that universality holds at long times in the immediate vicinity of the transition. The scaling function describing the crossover from anomalous transport to diffusive motion is found to vary extremely slowly and spans at least five decades in time. To extract the scaling function, one has to allow for the leading universal corrections to scaling. Our findings suggest that apparent power laws with varying exponents generically occur and dominate experimentally accessible time windows as soon as the heterogeneities cover a decade in length scale. We extract the divergent length scales, quantify the spatial heterogeneities in terms of the non-Gaussian parameter, and corroborate our results by a thorough finite-size analysis.
Low-field magnetoresistance in nanostructured Sr2FeMoO6∕CeO2 composites
Authors: Li Xianghu , Sun Yuping , Zhu Xuebin , Ang Ran , Lu Wenjian , Song Wenhai , Dai Jianming
Published: April 2008

Nanosized (Sr2FeMoO6)1−x∕(CeO2)x composites were prepared by Sr2FeMoO6 (SFMO) obtained by sol-gel and nanosized CeO2 powders. The magnetoresistance (MR) of composites is explored as a function of the molybdate/insulator composition and magnetic field. The enhancement of the low-field magnetoresistance is observed with increasing CeO2. The MR ratio at 10K with H=5kOe is 7.9% and 10.2% for the x=0.05 and 0.3 samples, which are 1.1 times and 1.7 times as large as that for pure SFMO, respectively. The enhanced MR was attributed to the spin-dependent tunneling at the interface of grain boundaries.
Pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy of metastable He2: Ionization potential and rovibrational structure of He2+
Authors: Raunhardt Matthias , Schäfer Martin , Vanhaecke Nicolas , Merkt Frédéric
Published: April 2008

A supersonic beam of metastable He* atoms and He2*aΣu+3 molecules has been generated using a pulsed discharge at the exit of a pulsed valve prior to the gas expansion into vacuum. Pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the He2+ X+Σu+2 (v+=0−2)←He2*aΣu+3 (v″=0−2) transitions and photoionization spectra of He2* in the vicinity of the lowest ionization thresholds have been recorded. The energy level structures of He2+4 X+Σu+2 (v+⩽2,N+⩽23) and He2+3 X+Σu+2 (v+=0,N+⩽11) have been determined, and an accurate set of molecular constants for all isotopomers of He2+ has been derived in a global analysis of all spectroscopic data reported to date on the low vibrational levels of He2+. The analysis of the photoionization spectrum by multichannel quantum defect theory has provided a set of parameters describing the threshold photoionization dynamics.
Ambipolar acceleration of ions in a magnetic nozzle
Authors: Arefiev Alexey V. , Breizman Boris N.
Published: April 2008

This paper describes a magnetic nozzle with a magnetic mirror configuration that transforms a collisionless subsonic plasma flow into a supersonic jet expanding into the vacuum. The nozzle converts electron thermal energy into the ion kinetic energy via an ambipolar electric field. The ambipolar potential in the expanding plume involves a time-dependent rarefaction wave. Travelling through the rarefaction wave, electrons lose some kinetic energy and can become trapped downstream from the mirror throat. This work presents a rigorous adiabatic description of the trapped electron population. It examines the impact of the adiabatic cooling of the trapped electrons on the ambipolar potential and the ensuing ion acceleration. The problem is formulated for an arbitrary incoming electron distribution and then a “water-bag” electron distribution is used to obtain a closed-form analytical solution.
Quantum-classical Liouville dynamics of proton and deuteron transfer rates in a solvated hydrogen-bonded complex
Authors: Hanna Gabriel , Kapral Raymond
Published: April 2008

Proton and deuteron transfer reactions in a hydrogen-bonded complex dissolved in a polar solution are studied using quantum-classical Liouville dynamics. Reactive-flux correlation functions that involve quantum-classical Liouville dynamics for species operators and quantum equilibrium sampling are used to calculate the rate constants. Adiabatic and nonadiabatic reaction rates are computed, compared, and analyzed. Large variations of the kinetic isotope effect (KIE) for this reaction have been observed in the literature, which depend on the nature of the approximate calculation used to estimate the proton and deuteron transfer rates. Our estimate of the KIE lies at the low end of the range of previously observed values, suggesting a rather small KIE for this reaction.
Precise nanosecond time resolved infrared radiometry measurements of laser induced silicon phase change and melting front propagation
Authors: Martan J. , Semmar N. , Cibulka O.
Published: April 2008

Nanosecond laser melting of silicon was investigated by infrared (IR) radiometry and time resolved reflectivity (TRR) methods. IR radiometry revealed interesting signal evolutions and their qualitative changes during different stages of the melting process. The observed signals represent melting front propagation, liquid phase overheating, and melt duration. Comparison of IR and TRR methods is shown. The physical origin of the IR signal shapes is discussed in detail.
Thermally enhanced interdiffusion in Mo∕Si multilayers
Authors: Nedelcu I. , van de Kruijs R. W. E. , Yakshin A. E. , Bijkerk F.
Published: April 2008

The formation and development of Mo-Si interfaces in Mo∕Si multilayers upon thermal annealing, including a transition to h-MoSi2, have been investigated using high resolution transmission electron microscopy, x-ray reflectivity, and x-ray diffraction measurements. The silicide layers naturally formed at Mo-Si interfaces, i.e., just upon and after the deposition, are amorphous and have different thicknesses for as-deposited samples, with the Mo-on-Si interlayer being the largest. In addition, silicide growth at Mo-Si interfaces during annealing before the phase transformation predominantly takes place at the Mo-on-Si interface and a MoSi2 interface layer is formed. Diffusion continues until a thick MoSi2 layer is formed at the interface, at which point the interface crystallizes and diffusion speeds up, finally resulting in an abrupt intermixing and phase transition of the entire interface to h-MoSi2. This model predicts an onset of the phase transition which does not depend primarily on the annealing temperature but on a threshold thickness of the MoSi2 interface before crystallization takes place. This crystallization threshold is shown to exist not only in the Mo∕Si system, but also occurs for Mo∕Si multilayers where the natural interfaces are replaced by diffusion barriers.
Restricted domain growth and polarization reversal kinetics in ferroelectric polymer thin films
Authors: Gysel Roman , Stolichnov Igor , Tagantsev Alexander K. , Setter Nava , Mokrý Pavel
Published: April 2008

A combination of microscopic and macroscopic methods enables an insight into polarization reversal in the copolymer of vinylidene fluoride and trifluoroethylene. Piezoforce microscopy together with switching charge measurements suggests that the polarization reversal is impeded by the restricted geometry of the domain growth. This effect limits the applicability of the conventional switching model of Kolmogorov-Avrami to the first stage of the polarization reversal. The interface-adjacent passive layer is found to impact the switching properties considerably resulting in a retardation of the polarization reversal. Furthermore, an extraordinary dielectric constant increase is observed in the films with a passive layer due to an additional domain wall contribution.
Influence of the radio frequency ponderomotive force on anomalous impurity transport in tokamaks
Authors: Nordman H. , Singh R. , Fülöp T. , Eriksson L.-G. , Dumont R. , Anderson J. , Kaw P. , Strand P. , Tokar M. , Weiland J.
Published: April 2008

Trace impurity transport in tokamaks is studied using an electrostatic, collisionless fluid model for ion-temperature-gradient and trapped-electron mode driven turbulence in the presence of radio frequency (rf) fields, and the results are compared with neoclassical predictions. It is shown that the inward impurity convective velocity (pinch) that is usually obtained can be reduced by the rf fields, in particular close to the wave resonance location where the rf ponderomotive force may be significant. However, the impurity diffusivity and convective velocity are usually similarly affected by the ponderomotive force, and hence the steady-state impurity density peaking factor −∇nz∕nz is only moderately affected by the rf fields.
Asymptotic regimes of ridge and rift formation in a thin viscous sheet model
Authors: Perazzo Carlos Alberto , Gratton Julio
Published: April 2008

We numerically and theoretically investigate the evolution of the ridges and rifts produced by the convergent and divergent motions of two substrates over which an initially uniform layer of a Newtonian liquid rests. We put particular emphasis on the various asymptotic self-similar and quasi-self-similar regimes that occur in these processes. During the growth of a ridge, two self-similar stages occur; the first takes place in the initial linear phase, and the second is obtained for a large time. Initially, the width and the height of the ridge increase as t1∕2. For a very large time, the width grows as t3∕4, while the height increases as t1∕4. On the other hand, in the process of formation of a rift, there are three self-similar asymptotics. The initial linear phase is similar to that for ridges. The second stage corresponds to the separation of the current in two parts, leaving a dry region in between. Last, for a very large t, each of the two parts in which the current has separated approaches the self-similar viscous dam break solution.
Effective passivation of porous silicon optical devices by thermal carbonization
Authors: Torres-Costa V. , Martín-Palma R. J. , Martínez-Duart J. M. , Salonen J. , Lehto V-P.
Published: April 2008

Nanostructured porous silicon (PS) optical filters have been proposed for their use in biological and chemical sensing applications. PS, however, presents a reactive surface that must be adequately passivated in order to achieve the required chemical stability mandatory for sensing applications. In the present work, thermal carbonization (TC) by acetylene decomposition is shown to provide effective passivation of a PS internal surface. Thermally carbonized PS optical filters are stable even in strong oxidizing environments such as absolute ethanol. Moreover, it is shown that the TC process, as opposed to more commonly used oxidation treatments, has only minor effects on the optical properties of PS. Thus, the optical performance of PS interference filters is preserved after the carbonization process. In addition, the hydrophilicity of the PS device surface can be adjusted by setting the appropriate TC process temperature. These results show that it is possible to produce low-cost, reusable, chemically stable PS optical devices for sensing applications.
Inclusion of heat transfer computations for particle laden flows
Authors: Feng Zhi-Gang , Michaelides Efstathios E.
Published: April 2008

A newly developed direct numerical simulation method has been used to study the dynamics of nonisothermal cylindrical particles in particulate flows. The momentum and energy transfer equations are solved to compute the effects of heat transfer in the sedimentation of particles. Among the effects examined is the drag force on nonisothermal particles, which we found strongly depends on the Reynolds and Grashof numbers. It was observed that heat advection between hotter particles and fluid causes the drag coefficient of particles to significantly increase at relatively low Reynolds numbers. For Grashof number of 100, the drag enhancement effect diminishes when the Reynolds number exceeds 50. On the contrary, heat advection with colder particles reduces the drag coefficient for low and medium Reynolds number (Re<50) for Grashof number of −100. We used this numerical method to study the problem of a pair of hot particles settling in a container at different Grashof numbers. In isothermal cases, such a pair of particles would undergo the well-known drafting-kissing-tumbling (DKT) motion. However, it was observed that the buoyancy currents induced by the hotter particles reverse the DKT motion of the particles or suppress it altogether. Finally, the sedimentation of a circular cluster of 172 particles in an enclosure at two different Grashof numbers was studied and the main features of the results are presented.
Effect of bubble deformability in turbulent bubbly upflow in a vertical channel
Authors: Lu Jiacai , Tryggvason Gretar
Published: April 2008

As bubbles rising in a vertical channel with upflow become bigger, it is well known that the void fraction distribution changes in a fundamental way, from a wall peak for small bubbles to a maximum void fraction at the channel center for larger bubbles. Here, we use direct numerical simulations of buoyant bubbles in a turbulent flow to show that it is not the size of the bubbles that matters, but their deformability.