28,566 research outputs found
Poly-essential and general Hyperelastic World (brane) models
This article provides a unified treatment of an extensive category of
non-linear classical field models whereby the universe is represented (perhaps
as a brane in a higher dimensional background) in terms of a structure of a
mathematically convenient type describable as hyperelastic, for which a
complete set of equations of motion is provided just by the energy-momentum
conservation law. Particular cases include those of a perfect fluid in
quintessential backgrounds of various kinds, as well as models of the elastic
solid kind that has been proposed to account for cosmic acceleration. It is
shown how an appropriately generalised Hadamard operator can be used to
construct a symplectic structure that controles the evolution of small
perturbations, and that provides a characteristic equation governing the
propagation of weak discontinuities of diverse (extrinsic and extrinsic) kinds.
The special case of a poly-essential model - the k-essential analogue of an
ordinary polytropic fluid - is examined and shown to be well behaved (like the
fluid) only if the pressure to density ratio is positive.Comment: 16 pages Latex, Contrib. to 10th Peyresq Pysics Meeting, June 2005:
Micro and Macro Structures of Spacetim
Global mapping of iron and titanium oxides in the lunar megaregolith and subsurface
[Abstract]:
This paper reports mapping results obtained by remote sensing analysis of Iron and Titanium oxides in the megaregolith under the lunar Highlands regolith and in the subsurface under the Mare and South Pole Aitken basin regolith. FeO and TiO2 images were mosaicked from data extracted from the 1994 Clementine lunar orbiter mission from 600 N to 600 S, using the Lucey et al. technique (2000). These images then used to study the ejecta blanket for each of 2059 craters analysed using ISIS software (US Geological Survey). Average weight percentage values for each crater ejecta blanket were interpolated to derive underlying global Province Maps for FeO and TiO2. The Moon was divided into five (5) provinces as a balance of the needs of analysis requirements and simplicity. Division of global TiO2 weight percentages in the megaregolith /subsurface five provinces was matching the observed distribution of that at the surface. In contrast, division of lunar FeO into 5 Provinces reveals unexpectedly elevated iron concentrations (3.8 to 6.4%) in some areas of the Highland megaregolith. This Province of elevated iron oxide is termed âHighland IIâ
What motivates African-American charitable giving: findings from a national sample
Given the growing wealth of minority families in America, including that of African-American families, the potential for charitable donations from these households is much greater. The purpose of this secondary analysis is to examine those variables that may influence African-American charitable giving patterns. This study uses the Panel Study of Income Dynamics (PSID) data to analyze the effects of multiple factors on the giving habits of African-Americans. Based on this study\u27s findings, social workers employed as executive directors or fund-raisers in private nonprofit organizations may want to identify and cultivate individual African-American donors directly, instead of relying on United Way and other federated campaigns
Entrainment coefficient and effective mass for conduction neutrons in neutron star crust: II Macroscopic treatment
Phenomena such as pulsar frequency glitches are believed to be attributable
to differential rotation of a current of ``free'' superfluid neutrons at
densities above the ``drip'' threshold in the ionic crust of a neutron star.
Such relative flow is shown to be locally describable by adaption of a
canonical two fluid treatment that emphasizes the role of the momentum
covectors constructed by differentiation of action with respect to the
currents, with allowance for stratification whereby the ionic number current
may be conserved even when the ionic charge number Z is altered by beta
processes. It is demonstrated that the gauge freedom to make different choices
of the chemical basis determining which neutrons are counted as ``free'' does
not affect their ``superfluid'' momentum covector, which must locally have the
form of a gradient (though it does affect the ``normal'' momentum covector
characterising the protons and those neutrons that are considered to be
``confined'' in the nuclei). It is shown how the effect of ``entrainment''
(whereby the momentum directions deviate from those of the currents) is
controlled by the (gauge independent) mobility coefficient K, estimated in
recent microscopical quantum mechanical investigations, which suggest that the
corresponding (gauge dependent) ``effective mass'' m* of the free neutrons can
become very large in some layers. The relation between this treatment of the
crust layers and related work (using different definitions of ``effective
mass'') intended for the deeper core layers is discussed.Comment: 21 pages Latex. Part II of article whose Part I (Simple microscopic
models) is given by nucl-th/0402057. New version extended to include figure
Symplectic structure for elastic and chiral conducting cosmic string models
This article is based on the covariant canonical formalism and corresponding
symplectic structure on phase space developed by Witten, Zuckerman and others
in the context of field theory. After recalling the basic principles of this
procedure, we construct the conserved bilinear symplectic current for generic
elastic string models. These models describe current carrying cosmic strings
evolving in an arbitrary curved background spacetime. Particular attention is
paid to the special case of the chiral string for which the worldsheet current
is null. Different formulations of the chiral string action are discussed in
detail, and as a result the integrability property of the chiral string is
clarified.Comment: 18 page
Capabilities and limitations of existing MST radars: Poker Flat
Designed as a prototype system to continuously monitor the atmosphere up to approximately 100 km, the Poker Flat MST radar began operating in 1979 at a relatively low sensitivity. In almost continuous operation since then, the system is steadily increasing in sensitivity to its ultimate design characteristics. Current and final parameters are listed. The advantages of its modular design, which uses 64 transmitting modules distributed through the 200 mx 200 m antenna array include: easy maintenance, beam switching using very low power switching, air cooled transmitting tubes, lower feedline costs, and no moving parts. Continuous, uninterrupted operation ( 4 years) and less man-made interference because of the remote location) are other assets. Most disadvantages are related to its not-yet-finished status, climate, moose excursions, and operating expenses
The NOAA TOGA antenna array
The Aeronomy Laboratory recently installed a 100 x 100 meter array antenna with limited beam steering on Christmas Island as a part of the TOGA (Tropical Ocean and Global Atmosphere) program. The array and the associated beam steering and indicating hardware are described
The Ponape ST radar
In May, 1984, a 50-MHz ST radar was installed on the island of Ponape in the western equatorial Pacific (7 deg N, 158 deg E) by the Aeronomy Laboratory of NOAA. The radar consists of a 100 m x 100 m array with a single, vertically directed, beam and is initially transmitting micro sec. (2.25 km) pulses. The radar is operating continuously, with Doppler spectra being recorded at approximately 1 1/2 minute intervals and sent to Boulder for later analysis. One of the principal goals of the radar is to measure vertical motions in the troposphere and lower stratosphere at a location which is within the intertropical convergence zone during part of the year. First results, during generally fair weather conditions, show detectable echoes up to about 21 km with the tropopause at 17-18 km. Once daily balloon soundings are available locally from a NOAA Weather Service Office on the island, it is planned that this radar will be joined in the coming year by two others with oblique as well as vertical beams on two yet-to-be-selected equatorial islands as part of the TOGA (Tropical Oceans Global Atmosphere) program
Saturated laser fluorescence in turbulent sooting flames at high pressure
The primary objective was to develop a quantitative, single pulse, laser-saturated fluorescence (LSF) technique for measurement of radical species concentrations in practical flames. The species of immediate interest was the hydroxyl radical. Measurements were made in both turbulent premixed diffusion flames at pressures between 1 and 20 atm. Interferences from Mie scattering were assessed by doping with particles or by controlling soot loading through variation of equivalence ratio and fuel type. The efficacy of the LSF method at high pressure was addressed by comparing fluorescence and adsorption measurements in a premixed, laminar flat flame at 1-20 atm. Signal-averaging over many laser shots is sufficient to determine the local concentration of radical species in laminar flames. However, for turbulent flames, single pulse measurements are more appropriate since a statistically significant number of laser pulses is needed to determine the probability function (PDF). PDFs can be analyzed to give true average properties and true local kinetics in turbulent, chemically reactive flows
Energy dynamics in a simulation of LAPD turbulence
Energy dynamics calculations in a 3D fluid simulation of drift wave
turbulence in the linear Large Plasma Device (LAPD) [W. Gekelman et al., Rev.
Sci. Inst. 62, 2875 (1991)] illuminate processes that drive and dissipate the
turbulence. These calculations reveal that a nonlinear instability dominates
the injection of energy into the turbulence by overtaking the linear drift wave
instability that dominates when fluctuations about the equilibrium are small.
The nonlinear instability drives flute-like () density
fluctuations using free energy from the background density gradient. Through
nonlinear axial wavenumber transfer to fluctuations, the
nonlinear instability accesses the adiabatic response, which provides the
requisite energy transfer channel from density to potential fluctuations as
well as the phase shift that causes instability. The turbulence characteristics
in the simulations agree remarkably well with experiment. When the nonlinear
instability is artificially removed from the system through suppressing
modes, the turbulence develops a coherent frequency spectrum
which is inconsistent with experimental data
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