14,793 research outputs found
Vorticity imbalance and stability in relation to convection
A complete synoptic-scale vorticity budget was related to convection storm development in the eastern two-thirds of the United States. The 3-h sounding interval permitted a study of time changes of the vorticity budget in areas of convective storms. Results of analyses revealed significant changes in values of terms in the vorticity equation at different stages of squall line development. Average budgets for all areas of convection indicate systematic imbalance in the terms in the vorticity equation. This imbalance resulted primarily from sub-grid scale processes. Potential instability in the lower troposphere was analyzed in relation to the development of convective activity. Instability was related to areas of convection; however, instability alone was inadequate for forecast purposes. Combinations of stability and terms in the vorticity equation in the form of indices succeeded in depicting areas of convection better than any one item separately
Development and verification of design methods for ducts in a space nuclear shield
A practical method for computing the effectiveness of a space nuclear shield perforated by small tubing and cavities is reported. Performed calculations use solutions for a two dimensional transport code and evaluate perturbations of that solution using last flight estimates and other kernel integration techniques. In general, perturbations are viewed as a change in source strength of scattered radiation and a change in attenuation properties of the region
Escape path complexity and its context dependency in Pacific blue-eyes (Pseudomugil signifer)
The escape trajectories animals take following a predatory attack appear to
show high degrees of apparent 'randomness' - a property that has been described
as 'protean behaviour'. Here we present a method of quantifying the escape
trajectories of individual animals using a path complexity approach. When fish
(Pseudomugil signifer) were attacked either on their own or in groups, we find
that an individual's path rapidly increases in entropy (our measure of
complexity) following the attack. For individuals on their own, this entropy
remains elevated (indicating a more random path) for a sustained period (10
seconds) after the attack, whilst it falls more quickly for individuals in
groups. The entropy of the path is context dependent. When attacks towards
single fish come from greater distances, a fish's path shows less complexity
compared to attacks that come from short range. This context dependency effect
did not exist, however, when individuals were in groups. Nor did the path
complexity of individuals in groups depend on a fish's local density of
neighbours. We separate out the components of speed and direction changes to
determine which of these components contributes to the overall increase in path
complexity following an attack. We found that both speed and direction measures
contribute similarly to an individual's path's complexity in absolute terms.
Our work highlights the adaptive behavioural tactics that animals use to avoid
predators and also provides a novel method for quantifying the escape
trajectories of animals.Comment: 9 page
Potential solar axion signatures in X-ray observations with the XMM-Newton observatory
The soft X-ray flux produced by solar axions in the Earth's magnetic field is
evaluated in the context of ESA's XMM-Newton observatory. Recent calculations
of the scattering of axion-conversion X-rays suggest that the sunward
magnetosphere could be an observable source of 0.2-10 keV photons. For
XMM-Newton, any conversion X-ray intensity will be seasonally modulated by
virtue of the changing visibility of the sunward magnetic field region. A
simple model of the geomagnetic field is combined with the ephemeris of
XMM-Newton to predict the seasonal variation of the conversion X-ray intensity.
This model is compared with stacked XMM-Newton blank sky datasets from which
point sources have been systematically removed. Remarkably, a seasonally
varying X-ray background signal is observed. The EPIC count rates are in the
ratio of their X-ray grasps, indicating a non-instrumental, external photon
origin, with significances of 11(pn), 4(MOS1) and 5(MOS2) sigma. After
examining the constituent observations spatially, temporally and in terms of
the cosmic X-ray background, we conclude that this variable signal is
consistent with the conversion of solar axions in the Earth's magnetic field.
The spectrum is consistent with a solar axion spectrum dominated by
bremsstrahlung- and Compton-like processes, i.e. axion-electron coupling
dominates over axion-photon coupling and the peak of the axion spectrum is
below 1 keV. A value of 2.2e-22 /GeV is derived for the product of the
axion-photon and axion-electron coupling constants, for an axion mass in the
micro-eV range. Comparisons with limits derived from white dwarf cooling may
not be applicable, as these refer to axions in the 0.01 eV range. Preliminary
results are given of a search for axion-conversion X-ray lines, in particular
the predicted features due to silicon, sulphur and iron in the solar core, and
the 14.4 keV transition line from 57Fe.Comment: Accepted for publication in MNRAS. 67 pages total, including 39
figures, 6 table
Wick's Theorem and a New Perturbation Theory Around the Atomic Limit of Strongly Correlated Electron Systems
A new type of perturbation expansion in the mixing of localized orbitals
with a conduction-electron band in the Anderson model is
presented. It is built on Feynman diagrams obeying standard rules. The local
correlations of the unperturbed system (the atomic limit) are included exactly,
no auxiliary particles are introduced. As a test, an infinite-order ladder-type
resummation is analytically treated in the Kondo regime, recovering the correct
energy scale. An extension to the Anderson-lattice model is obtained via an
effective-site approximation through a cumulant expansion in on the
lattice. Relation to treatments in infinite spatial dimensions are indicated.Comment: selfextracting postscript file containing entire paper (10 pages)
including 3 figures, in case of trouble contact author for LaTeX-source or
hard copies (prep0994
The tidal stripping of satellites
We present an improved analytic calculation for the tidal radius of
satellites and test our results against N-body simulations.
The tidal radius in general depends upon four factors: the potential of the
host galaxy, the potential of the satellite, the orbit of the satellite and
{\it the orbit of the star within the satellite}. We demonstrate that this last
point is critical and suggest using {\it three tidal radii} to cover the range
of orbits of stars within the satellite. In this way we show explicitly that
prograde star orbits will be more easily stripped than radial orbits; while
radial orbits are more easily stripped than retrograde ones. This result has
previously been established by several authors numerically, but can now be
understood analytically. For point mass, power-law (which includes the
isothermal sphere), and a restricted class of split power law potentials our
solution is fully analytic. For more general potentials, we provide an equation
which may be rapidly solved numerically. Over short times (\simlt 1-2 Gyrs
satellite orbit), we find excellent agreement between our analytic and
numerical models. Over longer times, star orbits within the satellite are
transformed by the tidal field of the host galaxy. In a Hubble time, this
causes a convergence of the three limiting tidal radii towards the prograde
stripping radius. Beyond the prograde stripping radius, the velocity dispersion
will be tangentially anisotropic.Comment: 10 pages, 5 figures. Final version accepted for publication in MNRAS.
Some new fully analytic tidal radii have been added for power law density
profiles (including the isothermal sphere) and some split power law
Valence bond spin liquid state in two-dimensional frustrated spin-1/2 Heisenberg antiferromagnets
Fermionic valence bond approach in terms of SU(4) representation is proposed
to describe the frustrated Heisenberg antiferromagnetic (AF)
model on a {\it bipartite} square lattice. A uniform mean field solution
without breaking the translational and rotational symmetries describes a
valence bond spin liquid state, interpolating the two different AF ordered
states in the large and large limits, respectively. This novel
spin liquid state is gapless with the vanishing density of states at the Fermi
nodal points. Moreover, a sharp resonance peak in the dynamic structure factor
is predicted for momenta and in the strongly
frustrated limit , which can be checked by neutron
scattering experiment.Comment: Revtex file, 4 pages, 4 figure
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