413,227 research outputs found

    Finite size effects in metallic superlattice systems

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    Clean metallic superlattice systems composed of alternating layers of superconducting and normal materials are considered, particularly aspects of the proximity effect as it affects the critical temperature. A simple model is used to address the question of when a finite--sized system theoretically approximates well a true infinite superlattice. The methods used in the analysis afford some tests of the approximation used that the pair amplitude of the Cooper pairs is constant over a superconducting region. We also use these methods to construct a model of a single superconducting layer which intends to incorporate a more realistic form of the pair amplitude than a simple constant.Comment: 16 ReVTeX pages + 12 PostScript figures encoded with uufile

    The dielectric properties of soil-water mixtures at microwave frequencies

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    Recent measurements on the dielectric constants of soil-water mixtures show the existence of two frequency regions in which the dielectric behavior of these mixtures was quite different. At the frequencies of 1.4 GHz to 5 GHz, there were strong evidences that the variations of the dielectric (epsilon) with water content (W) depended on soil type. While the real part of epsilon for sandy soils rose rapidly with the increase in W, epsilon for the high-clay content soils rose only slowly with W. As a consequence, epsilon was generally higher for the sandy soils than for the high-clay content soils at a given W. On the other hand, most of the measurements at frequencies 1 GHz indicated the increase of epsilon with W independent of soil types. At a given W, epsilon' (sandy soil) approximately equals epsilon (high-clay content soil) within the precision of the measurements. These observational features can be satisfactorily interpreted in terms of a simple dielectric relaxation model, with an appropriate choice of the mean relaxation frequency f(m) and the range of the activation energy (beta). It was found that smaller f(m) and larger beta were required for the high-clay content soils than the sandy soils in order to be consistent with the measured data

    An Oort cloud origin of the Halley-type comets

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    The origin of the Halley-type comets (HTCs) is one of the last mysteries of the dynamical evolution of the Solar System. Prior investigation into their origin has focused on two source regions: the Oort cloud and the Scattered Disc. From the former it has been difficult to reproduce the non-isotropic, prograde skew in the inclination distribution of the observed HTCs without invoking a multi-component Oort cloud model and specific fading of the comets. The Scattered Disc origin fares better but suffers from needing an order of magnitude more mass than is currently advocated by theory and observations. Here we revisit the Oort cloud origin and include cometary fading. Our observational sample stems from the JPL catalogue. We only keep comets discovered and observed after 1950 but place no a priori restriction on the maximum perihelion distance of observational completeness. We then numerically evolve half a million comets from the Oort cloud through the realm of the giant planets and keep track of their number of perihelion passages with perihelion distance q<2.5AU, below which the activity is supposed to increase considerably. We can simultaneously fit the HTC inclination and semi-major axis distribution very well with a power law fading function of the form m^-k, where m is the number of perihelion passages with q<2.5 AU and k is the fading index. We match both the inclination and semi-major axis distributions when k~1 and the maximum imposed perihelion distance of the observed sample is q~1.8AU. The value of k is higher than the one obtained for the Long-Period Comets (LPCs), with k~0.7. This increase in k is most likely the result of cometary surface processes. We argue the HTC sample is now most likely complete for q<1.8AU. We calculate that the steady-state number of active HTCs with diameter D>2.3km and q<1.8AU is of the order of 100.Comment: Accepted for publication in Astronomy and Astrophysic

    The coronal transport of the flare associated scatter free electrons

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    A total of 11 scatter-free electron events from McMath plage region 8905 were observed by IMP 4 during the period from 29 July to 3 August 1967. The transit times and duration of these electron events are examined in detail. The duration of 170- to 1000-keV electrons shows a strong dependence upon the heliolongitude of the associated flare. Typical values of the duration full width at half maximum vary from 4 min at 50 deg W to 12 min at 10 deg and 90 deg W heliolongitudes. In addition, the difference in times at which the maximum intensities of the 22- to 45-keV and 170- to 1000-keV electrons occur is observed to change from 18 min near 50 deg W to 13 min at 10 deg and 90 deg W heliolongitudes. An idealized two-dimensional diffusion model for the transport of these electrons in the solar corona is proposed. The 22- to 45-keV electrons escape promptly from flare site to the feet of the interplanetary magnetic field lines, whereas 170- to 1000-keV electrons suffer slight scattering. The diffusion coefficient for 170- to 1000-keV electrons is estimated to be 3 X 10 to the 18th power sq cm/sec. Finally, a possible relation between the scatter-free and the classical diffusive-type electron events is discussed

    The SDSS Galaxy Angular Two-Point Correlation Function

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    We present the galaxy two-point angular correlation function for galaxies selected from the seventh data release of the Sloan Digital Sky Survey. The galaxy sample was selected with rr-band apparent magnitudes between 17 and 21; and we measure the correlation function for the full sample as well as for the four magnitude ranges: 17-18, 18-19, 19-20, and 20-21. We update the flag criteria to select a clean galaxy catalog and detail specific tests that we perform to characterize systematic effects, including the effects of seeing, Galactic extinction, and the overall survey uniformity. Notably, we find that optimally we can use observed regions with seeing < 1\farcs5, and rr-band extinction < 0.13 magnitudes, smaller than previously published results. Furthermore, we confirm that the uniformity of the SDSS photometry is minimally affected by the stripe geometry. We find that, overall, the two-point angular correlation function can be described by a power law, ω(θ)=Aωθ(1γ)\omega(\theta) = A_\omega \theta^{(1-\gamma)} with γ1.72\gamma \simeq 1.72, over the range 0\fdg005--10\degr. We also find similar relationships for the four magnitude subsamples, but the amplitude within the same angular interval for the four subsamples is found to decrease with fainter magnitudes, in agreement with previous results. We find that the systematic signals are well below the galaxy angular correlation function for angles less than approximately 5\degr, which limits the modeling of galaxy angular correlations on larger scales. Finally, we present our custom, highly parallelized two-point correlation code that we used in this analysis.Comment: 22 pages, 17 figures, accepted by MNRA

    An empirical model for the complex dielectric permittivity of soils as a function of water content

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    The recent measurements on the dielectric properties of soils shows that the variation of dielectric constant with moisture content depends on soil types. The observed dielectric constant increases only slowly with moisture content up to a transition point. Beyond the transition it increases rapidly with moisture content. The moisture value of transition region was found to be higher for high clay content soils than for sandy soils. Many mixing formulas were compared with, and were found incompatible with, the measured dielectric variations of soil-water mixtures. A simple empirical model was proposed to describe the dielectric behavior of ths soil-water mixtures. The relationship between transition moisture and wilting point provides a means of estimating soil dielectric properties on the basis of texture information

    The Passive Microwave Remote Sensing of Soil Moisture: the Effect of Tilled Row Structure

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    The tilled rowstructure is known to be one of the important factors affecting the observations of the microwave emission from a natural surface. Measurements of this effect were carried out with both I and X band radiometers mounted on a mobile truck on a bare 40 m x 45 m row tilled field. The soil moisture content during the measurements ranged from approximately 10 percent to approximately 30 percent by dry weight. The results of these measurements showed that the variations of the antenna temperatures with incident angle theta changed with the azimuthal angle a measured from the row direction. A numerical calculation based on a composite surface roughness was made and found to predict the observed features within the model's limit of accuracy. It was concluded that the difference between the horizontally and vertically polarized temperatures was due to the change in the local angle of field emission within the antenna field of view caused by the large scale row structure
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