14,139 research outputs found
Mesospheric scatter and its microstructure
The difference in character between mesospheric returns from about 70 and about 80 km was noted. The 69-km echo is characterized by a single return with about .1 Hz width, while the 82.5 km return extends over more than 3 Hz bandwidth; this difference is also perceptible, but to a lesser degree, on the fading curves. The conclusion seems inescapable that internal random velocities of a few m/s are present within the scattering volume for the 82.5-km echo. The most likely source for these rather large velocities is convective instability arising from deformations of the temperature profile by breaking gravity waves. The distinction between the two types of scatter at these altitudes probably accounts for the behavior with frequency of the nightime fading period at low and very low frequencies. The fading period of D-region reflections at night was constant at about 7 min from 16 to 43 kHz, but that at frequencies of 70 kHz and above, the fading period decreased in such a way as to indicate the presence of irregularities smaller than about 1 km in size. This suggests that frequencies of 48 kHz and above, the fading period decreased in such a way as to indicate the presence of irregularities smaller than about 1 km in size. This suggests that frequencies of 48 kHz and below were reflected primarily from the region below 80 km where the narrow spectral irregularities dominate
Pion double charge exchange on 4He
The doubly differential cross sections for the He
reaction were calculated using both a two-nucleon sequential single charge
exchange model and an intranuclear cascade code. Final state interactions
between the two final protons which were the initial neutrons were included in
both methods. At incident pion energies of 240 and 270 MeV the low-energy peak
observed experimentally in the energy spectrum of the final pions can be
understood only if the contribution of pion production is included. The
calculated cross sections are compared with data.Comment: 25 pages, 9 figure
An intermediate state between the kagome-ice and the fully polarized state in DyTiO
DyTiO is at present the cleanest example of a spin-ice material.
Previous theoretical and experimental work on the first-order transition
between the kagome-ice and the fully polarized state has been taken as a
validation for the dipolar spin-ice model. Here we investigate in further depth
this phase transition using ac-susceptibility and dc-magnetization, and compare
this results with Monte-Carlo simulations and previous magnetization and
specific heat measurements. We find signatures of an intermediate state between
the kagome-ice and full polarization. This signatures are absent in current
theoretical models used to describe spin-ice materials.Comment: 7 pages, 4 figure
Stable Kalman filters for processing clock measurement data
Kalman filters have been used for some time to process clock measurement data. Due to instabilities in the standard Kalman filter algorithms, the results have been unreliable and difficult to obtain. During the past several years, stable forms of the Kalman filter have been developed, implemented, and used in many diverse applications. These algorithms, while algebraically equivalent to the standard Kalman filter, exhibit excellent numerical properties. Two of these stable algorithms, the Upper triangular-Diagonal (UD) filter and the Square Root Information Filter (SRIF), have been implemented to replace the standard Kalman filter used to process data from the Deep Space Network (DSN) hydrogen maser clocks. The data are time offsets between the clocks in the DSN, the timescale at the National Institute of Standards and Technology (NIST), and two geographically intermediate clocks. The measurements are made by using the GPS navigation satellites in mutual view between clocks. The filter programs allow the user to easily modify the clock models, the GPS satellite dependent biases, and the random noise levels in order to compare different modeling assumptions. The results of this study show the usefulness of such software for processing clock data. The UD filter is indeed a stable, efficient, and flexible method for obtaining optimal estimates of clock offsets, offset rates, and drift rates. A brief overview of the UD filter is also given
An efficient frequency-independent numerical method for computing the far-field pattern induced by polygonal obstacles
For problems of time-harmonic scattering by rational polygonal obstacles,
embedding formulae express the far-field pattern induced by any incident plane
wave in terms of the far-field patterns for a relatively small
(frequency-independent) set of canonical incident angles. Although these
remarkable formulae are exact in theory, here we demonstrate that: (i) they are
highly sensitive to numerical errors in practice, and; (ii) direct calculation
of the coefficients in these formulae may be impossible for particular sets of
canonical incident angles, even in exact arithmetic. Only by overcoming these
practical issues can embedding formulae provide a highly efficient approach to
computing the far-field pattern induced by a large number of incident angles.
Here we propose solutions for problems (i) and (ii), backed up by theory and
numerical experiments. Problem (i) is solved using techniques from
computational complex analysis: we reformulate the embedding formula as a
complex contour integral and prove that this is much less sensitive to
numerical errors. In practice, this contour integral can be efficiently
evaluated by residue calculus. Problem (ii) is addressed using techniques from
numerical linear algebra: we oversample, considering more canonical incident
angles than are necessary, thus expanding the space of valid coefficients
vectors. The coefficients vectors can then be selected using either a least
squares approach or column subset selection
Muon-spin rotation measurements of the vortex state in SrRuO: type-1.5 superconductivity, vortex clustering and a crossover from a triangular to a square vortex lattice
Muon-spin rotation has been used to probe vortex state in SrRuO. At
moderate fields and temperatures a lattice of triangular symmetry is observed,
crossing over to a lattice of square symmetry with increasing field and
temperature. At lower fields it is found that there are large regions of the
sample that are completely free from vortices which grow in volume as the
temperature falls. Importantly this is accompanied by {\it increasing} vortex
density and increasing disorder within the vortex-cluster containing regions.
Both effects are expected to result from the strongly temperature-dependent
long-range vortex attractive forces arising from the multi-band chiral-order
superconductivity.Comment: 13 pages, 4 figure
Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation
Among the many additive manufacturing (AM) processes for metallic materials,
selective laser melting (SLM) is arguably the most versatile in terms of its
potential to realize complex geometries along with tailored microstructure.
However, the complexity of the SLM process, and the need for predictive
relation of powder and process parameters to the part properties, demands
further development of computational and experimental methods. This review
addresses the fundamental physical phenomena of SLM, with a special emphasis on
the associated thermal behavior. Simulation and experimental methods are
discussed according to three primary categories. First, macroscopic approaches
aim to answer questions at the component level and consider for example the
determination of residual stresses or dimensional distortion effects prevalent
in SLM. Second, mesoscopic approaches focus on the detection of defects such as
excessive surface roughness, residual porosity or inclusions that occur at the
mesoscopic length scale of individual powder particles. Third, microscopic
approaches investigate the metallurgical microstructure evolution resulting
from the high temperature gradients and extreme heating and cooling rates
induced by the SLM process. Consideration of physical phenomena on all of these
three length scales is mandatory to establish the understanding needed to
realize high part quality in many applications, and to fully exploit the
potential of SLM and related metal AM processes
Struggling and juggling: a comparison of assessment loads in research and teaching-intensive universities
In spite of the rising tide of metrics in UK higher education, there has been scant attention paid to assessment loads, when evidence demonstrates that heavy demands lead to surface learning. Our study seeks to redress the situation by defining assessment loads and comparing them across research-and teaching intensive universities. We clarify the concept of ‘assessment load’ in response to findings about high volumes of summative assessment on modular degrees. We define assessment load across whole undergraduate degrees, according to four measures: the volume of summative assessment; volume of formative assessment; proportion of examinations to coursework; number of different varieties of assessment. All four factors contribute to the weight of an assessment load, and influence students’ approaches to learning. Our research compares programme assessment data from 73 programmes in 14 UK universities, across two institutional categories. Research-intensives have higher summative assessment loads and a greater proportion of examinations; teaching-intensives have higher varieties of assessment. Formative assessment does not differ significantly across both university groups. These findings pose particular challenges for students in different parts of the sector. Our study questions the wisdom that ‘more’ is always better, proposing that lighter assessment loads may make room for ‘slow’ and deep learning
Optical bistability involving planar metamaterial with broken structural symmetry
We report on a bistable light transmission through a planar metamaterial
composed of a metal pattern of weakly asymmetric elements placed on a nonlinear
substrate. Such structure bears the Fano-like sharp resonance response of a
trapped-mode excitation. The feedback required for bistability is provided by
the coupling between the strong antiphased trapped-mode-resonance currents
excited on the metal elements and the intensity of inner field in the nonlinear
substrate.Comment: 4 pages, 4 figure
Symbiont 'bleaching' in planktic foraminifera during the Middle Eocene Climatic Optimum
Many genera of modern planktic foraminifera are adapted to nutrient-poor (oligotrophic) surface waters by hosting photosynthetic symbionts, but it is unknown how they will respond to future changes in ocean temperature and acidity. Here we show that ca. 40 Ma, some fossil photosymbiont-bearing planktic foraminifera were temporarily 'bleached' of their symbionts coincident with transient global warming during the Middle Eocene Climatic Optimum (MECO). At Ocean Drilling Program (ODP) Sites 748 and 1051 (Southern Ocean and mid-latitude North Atlantic, respectively), the typically positive relationship between the size of photosymbiont-bearing planktic foraminifer tests and their carbon isotope ratios (δ13C) was temporarily reduced for ∼100 k.y. during the peak of the MECO. At the same time, the typically photosymbiont-bearing planktic foraminifera Acarinina suffered transient reductions in test size and relative abundance, indicating ecological stress. The coincidence of minimum δ18O values and reduction in test size–δ13C gradients suggests a link between increased sea-surface temperatures and bleaching during the MECO, although changes in pH and nutrient availability may also have played a role. Our findings show that host-photosymbiont interactions are not constant through geological time, with implications for both the evolution of trophic strategies in marine plankton and the reliability of geochemical proxy records generated from symbiont-bearing planktic foraminifera
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