3,908 research outputs found
Statistical properties of high performance cesium standards
The intermediate term frequency stability of a group of new high-performance cesium beam tubes at the U.S. Naval Observatory were analyzed from two viewpoints: (1) by comparison of the high-performance standards to the MEAN(USNO) time scale and (2) by intercomparisons among the standards themselves. For sampling times up to 5 days, the frequency stability of the high-performance units shows significant improvement over older commercial cesium beam standards
Greenstone belts: Their boundaries, surrounding rock terrains and interrelationships
Greenstone belts are an important part of the fragmented record of crustal evolution, representing samples of the magmatic activity that formed much of the Earth's crust. Most belts developed rapidly, in less than 100 Ma, leaving large gaps in the geological record. Surrounding terrains provide information on the context of greenstone belts. The effects of tectonic setting, structural geometry and evolution, associated plutonic activity and sedimentation are discussed
Vegetation Effects on Fish Distribution in Impounded Salt Marshes
We compared the density and biomass of resident fish in vegetated and
unvegetated flooded habitats of impounded salt marshes in the northern Indian
River Lagoon (IRL) Estuary of east-central Florida. A 1-m2 throw trap was used
to sample fish in randomly located, paired sample plots (n = 198 pairs) over 5 seasons
in 7 impoundments. We collected a total of 15 fish taxa, and 88% of the fishes
we identified from the samples belonged to three species: Cyprinodon variegatus
(Sheepshead Minnow), Gambusia holbrooki (Eastern Mosquitofish), and Poecilia
latipinna (Sailfin Molly). Vegetated habitat usually had higher density and biomass
of fish. Mean fish density (and 95% confidence interval) for vegetated and unvegetated
sites were 8.2 (6.7–9.9) and 2.0 (1.6–2.4) individuals m-2, respectively; mean
biomass (and 95% confidence interval) for vegetated and unvegetated sites were
3.0 (2.5–3.7) and 1.1 (0.9–1.4) g m-2, respectively. We confirmed previous findings
that impounded salt marshes of the northern IRL Estuary produce a high standing
stock of resident fishes. Seasonal patterns of abundance were consistent with fish
moving between vegetated and unvegetated habitat as water levels changed in the
estuary. Differences in density, mean size, and species composition of resident
fishes between vegetated and unvegetated habitats have important implications for
movement of biomass and nutrients out of salt marsh by piscivores (e.g., wading
birds and fishes) via a trophic relay
Scale-Dependent Habitat Selection of Nesting Great Egrets and Snowy Egrets
Foraging habitat selection of nesting Great Egrets (
Ardea alba
) and Snowy Egrets (
Egretta thula
) was
investigated within an estuary with extensive impounded salt marsh habitat. Using a geographic information system,
available habitat was partitioned into concentric bands at five, ten, and 15 km radius from nesting colonies to assess
the relative effects of habitat composition and distance on habitat selection. Snowy Egrets were more likely than
Great Egrets to depart colonies and travel to foraging sites in groups, but both species usually arrived at sites that
were occupied by other wading birds. Mean flight distances were 6.2 km (SE = 0.4, N = 28, range 1.8-10.7 km) for
Great Egrets and 4.7 km (SE = 0.48, N = 31, range 0.7-12.5 km) for Snowy Egrets. At the broadest spatial scale both
species used impounded (mostly salt marsh) and estuarine edge habitat more than expected based on availability
while avoiding unimpounded (mostly fresh water wetland) habitat. At more local scales habitat use matched availability.
Interpretation of habitat preference differed with the types of habitat that were included and the maximum
distance that habitat was considered available. These results illustrate that caution is needed when interpreting the
results of habitat preference studies when individuals are constrained in their choice of habitats, such as for central
place foragers
The Square Root Depth Wave Equations
We introduce a set of coupled equations for multilayer water waves that
removes the ill-posedness of the multilayer Green-Naghdi (MGN) equations in the
presence of shear. The new well-posed equations are Hamiltonian and in the
absence of imposed background shear they retain the same travelling wave
solutions as MGN. We call the new model the Square Root Depth equations, from
the modified form of their kinetic energy of vertical motion. Our numerical
results show how the Square Root Depth equations model the effects of
multilayer wave propagation and interaction, with and without shear.Comment: 10 pages, 5 figure
Tunable Whispering Gallery Mode Resonators for Cavity Quantum Electrodynamics
We theoretically study the properties of highly prolate shaped dielectric
microresonators. Such resonators sustain whispering gallery modes that exhibit
two spatially well separated regions with enhanced field strength. The field
per photon on the resonator surface is significantly higher than e.g. for
equatorial whispering gallery modes in microsphere resonators with a comparable
mode volume. At the same time, the frequency spacing of these modes is much
more favorable, so that a tuning range of several free spectral ranges should
be attainable. We discuss the possible application of such resonators for
cavity quantum electrodynamics experiments with neutral atoms and reveal
distinct advantages with respect to existing concepts.Comment: 4 pages, 3 figure
Damped Lyman alpha systems and disk galaxies: number density, column density distribution and gas density
We present a comparison between the observed properties of damped Lyman alpha
systems (DLAs) and the predictions of simple models for the evolution of
present day disk galaxies, including both low and high surface brightness
galaxies. We focus in particular on the number density, column density
distribution and gas density of DLAs, which have now been measured in
relatively large samples of absorbers. From the comparison we estimate the
contribution of present day disk galaxies to the population of DLAs, and how it
varies with redshift. Based on the differences between the models and the
observations, we also speculate on the nature of the fraction of DLAs which
apparently do not arise in disk galaxies.Comment: 11 pages, 10 figures, accepted in MNRA
Sewing sound quantum flesh onto classical bones
Semiclassical transformation theory implies an integral representation for
stationary-state wave functions in terms of angle-action variables
(). It is a particular solution of Schr\"{o}dinger's time-independent
equation when terms of order and higher are omitted, but the
pre-exponential factor in the integrand of this integral
representation does not possess the correct dependence on . The origin of
the problem is identified: the standard unitarity condition invoked in
semiclassical transformation theory does not fix adequately in a
factor which is a function of the action written in terms of and
. A prescription for an improved choice of this factor, based on
succesfully reproducing the leading behaviour of wave functions in the vicinity
of potential minima, is outlined. Exact evaluation of the modified integral
representation via the Residue Theorem is possible. It yields wave functions
which are not, in general, orthogonal. However, closed-form results obtained
after Gram-Schmidt orthogonalization bear a striking resemblance to the exact
analytical expressions for the stationary-state wave functions of the various
potential models considered (namely, a P\"{o}schl-Teller oscillator and the
Morse oscillator).Comment: RevTeX4, 6 page
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