20,996 research outputs found
Antenna induced range smearing in MST radars
There is considerable interest in developing stratosphere troposphere (ST) and mesosphere stratosphere troposphere (MST) radars for higher resolution to study small-scale turbulent structures and waves. At present most ST and MST radars have resolutions of 150 meters or larger, and are not able to distinguish the thin (40 - 100 m) turbulent layers that are known to occur in the troposphere and stratosphere, and possibly in the mesosphere. However the antenna beam width and sidelobe level become important considerations for radars with superior height resolution. The objective of this paper is to point out that for radars with range resolutions of about 150 meters or less, there may be significant range smearing of the signals from mesospheric altitudes due to the finite beam width of the radar antenna. At both stratospheric and mesospheric heights the antenna sidelobe level for lear equally spaced phased arrays may also produce range aliased signals. To illustrate this effect the range smearing functions for two vertically directed antennas have been calculated, (1) an array of 32 coaxial-collinear strings each with 48 elements that simulates the vertical beam of the Poker Flat, Glaska, MST radar; and (2) a similar, but smaller, array of 16 coaxial-collinear strings each with 24 elements
Origin and Detection of Microstructural Clustering in Fluids with Spatial-Range Competitive Interactions
Fluids with competing short-range attractions and long-range repulsions mimic
dispersions of charge-stabilized colloids that can display equilibrium
structures with intermediate range order (IRO), including particle clusters.
Using simulations and analytical theory, we demonstrate how to detect cluster
formation in such systems from the static structure factor and elucidate links
to macrophase separation in purely attractive reference fluids. We find that
clusters emerge when the thermal correlation length encoded in the IRO peak of
the structure factor exceeds the characteristic lengthscale of interparticle
repulsions. We also identify qualitative differences between the dynamics of
systems that form amorphous versus micro-crystalline clusters.Comment: 6 pages, 5 figure
Selective MR imaging approach for evaluation of patients with Horner's syndrome.
Journal ArticlePURPOSE: To assess the usefulness of MR in the evaluation of patients with Horner's syndrome. PATIENTS AND METHODS: We prospectively performed MR imaging in 33 patients with Horner's syndrome (13 preganglionic and 20 postganglionic) using a protocol specifically designed for pre- and postganglionic varieties of this syndrome. Assignment of patients to pre- or postganglionic categories was performed on the basis of pharmacologic testing. RESULTS: Abnormalities in one-half of the patients with preganglionic Horner's syndrome included lateral medullary infarct, spinal cord/root disease, apical lung tumor, and paravertebral metastatic mass. Three of 20 patients with postganglionic Horner's syndrome had carotid artery dissection. CONCLUSION: Routine scanning of patients who have postganglionic Horner's syndrome with cluster headaches was not helpful in our small serie
Thin Animals
Lattice animals provide a discretized model for the theta transition
displayed by branched polymers in solvent. Exact graph enumeration studies have
given some indications that the phase diagram of such lattice animals may
contain two collapsed phases as well as an extended phase. This has not been
confirmed by studies using other means. We use the exact correspondence between
the q --> 1 limit of an extended Potts model and lattice animals to investigate
the phase diagram of lattice animals on phi-cubed random graphs of arbitrary
topology (``thin'' random graphs). We find that only a two phase structure
exists -- there is no sign of a second collapsed phase.
The random graph model is solved in the thermodynamic limit by saddle point
methods. We observe that the ratio of these saddle point equations give
precisely the fixed points of the recursion relations that appear in the
solution of the model on the Bethe lattice by Henkel and Seno. This explains
the equality of non-universal quantities such as the critical lines for the
Bethe lattice and random graph ensembles.Comment: Latex, 10 pages plus 6 ps/eps figure
Wealth Condensation in Pareto Macro-Economies
We discuss a Pareto macro-economy (a) in a closed system with fixed total
wealth and (b) in an open system with average mean wealth and compare our
results to a similar analysis in a super-open system (c) with unbounded wealth.
Wealth condensation takes place in the social phase for closed and open
economies, while it occurs in the liberal phase for super-open economies. In
the first two cases, the condensation is related to a mechanism known from the
balls-in-boxes model, while in the last case to the non-integrable tails of the
Pareto distribution. For a closed macro-economy in the social phase, we point
to the emergence of a ``corruption'' phenomenon: a sizeable fraction of the
total wealth is always amassed by a single individual.Comment: 4 pages, 1 figur
Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR
In the iron pnictide superconductors, theoretical calculations have
consistently shown enhancements of the static magnetic susceptibility at both
the stripe-type antiferromagnetic (AFM) and in-plane ferromagnetic (FM)
wavevectors. However, the possible existence of FM fluctuations has not yet
been examined from a microscopic point of view. Here, using As NMR data,
we provide clear evidence for the existence of FM spin correlations in both the
hole- and electron-doped BaFeAs families of iron-pnictide
superconductors. These FM fluctuations appear to compete with superconductivity
and are thus a crucial ingredient to understanding the variability of and the shape of the superconducting dome in these and other iron-pnictide
families.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Let
Electrochemical deposition of silver crystals aboard Skylab 4
Silver crystals were grown aboard Skylab 4 by an electro-chemical reaction and subsequently returned to earth for comparison with crystals grown at 1- and 5-g. Both the Skylab and earth-grown crystals show a variety of structures. Certain tendencies in structure dependency on gravity level, however, can be discerned. In addition, downward growing dendrite streamers; upward growing chunky crystal streamers; growth along an air/liquid interface; and ribbon, film, and fiber crystal habits were observed in experiments conducted on the ground with solutions of varying concentrations. It was also observed that the crystal structures of space and ground electro-deposited silver crystals were very similar to the structures of germanium selenide and germanium telluride crystals grown in space and on the ground by a vapor transport technique. Consideration of the data leads to the conclusions that: (1) the rate of electrochemical displacement of silver ions from a 5 percent aqueous solution by copper is predominantly diffussion controlled in space and kinetically controlled in 1- and higher-g because of augmentation of mass transport by convection; (2) downward and upward crystal streamers are the result of gravity-driven convection, the flow patterns of which can be delineated. Lateral growths along an air/liquid interface are the result of surface-tension-driven convection, the pattern of which also can be delineated; (3) electrolysis in space or low-g environments can produce either dendritic crystals with more perfect microcrystalline structures or massive, single crystals with fewer defects than those grown on ground or at higher g-levels. Ribbons or films of space-grown silicon crystals would find a ready market for electronic substrate and photocell applications. Space-grown dendritic, metal crystals present the possibility of unique catalysts. Large perfect crystals of various materials are desired for a number of electronic and optical applications; and (4) vapor transport growth of germanium selenide and germanium telluride is affected by convection mechanisms similar to the mechanisms hypothesized for the electrochemical deposition of silver crystals. Evidence and considerations leading to the preceding summaries and conclusions are presented. The implications of the findings and conclusions for technological applications are discussed, and recommendations for further experiments are presented
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