13,447 research outputs found
Geomagnetic model investigations for 1980 - 1989: A model for strategic defense initiative particle beam experiments and a study in the effects of data types and observatory bias solutions
Two suites of geomagnetic field models were generated at the request of Los Alamos National Lab. concerning Strategic Defense Initiative (SDI) research. The first is a progression of five models incorporating MAGSAT data and data from a sequence of batches as a priori information. The batch sequence is: post 1979.5 observatory data, post 1980 land survey and selected aeromagnetic and marine survey data, a special White Sands (NM) area survey by Project Magnet with some additional post 1980 marine survey data, and finally DE-2 satellite data. These models are of 13th deg and order in their main field terms, and deg and order 10 in their first derivative temporal terms. The second suite consists of four models based solely upon post 1983.5 observatory and survey data. They are of deg and order 10 in main field and 8 in a first deg Taylor series. A comprehensive error analysis was applied to both series, which accounted for error sources such as the truncated core and crustal fields, and the neglected Sq and low deg crustal fields. Comparison of the power spectrum of the MGST (10/81) model with those of this series show good agreement
A Reciprocity Theorem for Monomer-Dimer Coverings
The problem of counting monomer-dimer coverings of a lattice is a
longstanding problem in statistical mechanics. It has only been exactly solved
for the special case of dimer coverings in two dimensions. In earlier work,
Stanley proved a reciprocity principle governing the number of dimer
coverings of an by rectangular grid (also known as perfect matchings),
where is fixed and is allowed to vary. As reinterpreted by Propp,
Stanley's result concerns the unique way of extending to so
that the resulting bi-infinite sequence, for , satisfies a
linear recurrence relation with constant coefficients. In particular, Stanley
shows that is always an integer satisfying the relation where unless 2(mod 4) and
is odd, in which case . Furthermore, Propp's method is
applicable to higher-dimensional cases. This paper discusses similar
investigations of the numbers , of monomer-dimer coverings, or
equivalently (not necessarily perfect) matchings of an by rectangular
grid. We show that for each fixed there is a unique way of extending
to so that the resulting bi-infinite sequence, for , satisfies a linear recurrence relation with constant coefficients. We
show that , a priori a rational number, is always an integer, using a
generalization of the combinatorial model offered by Propp. Lastly, we give a
new statement of reciprocity in terms of multivariate generating functions from
which Stanley's result follows.Comment: 13 pages, 12 figures, to appear in the proceedings of the Discrete
Models for Complex Systems (DMCS) 2003 conference. (v2 - some minor changes
An initial analysis of the data from the Polar Orbiting Geophysical (POGS) Satellite
The Polar Orbiting Geophysical Satellite (POGS) was launched in 1990 to measure the geomagnetic field. POGS data from selected magnetically quiet days was chosen, quality checked and deleted where thought to be erroneous. A time and position correction was applied. The resulting data was fit to a degree 13 spherical harmonic model. Evaluation of the quality of the data indicates that it is sufficient for definition of the low degree (approximately less than 8) portion of the geomagnetic field. Further correction of the data time and position may improve this quality
Classification and analysis of emission-line galaxies using mean field independent component analysis
We present an analysis of the optical spectra of narrow emission-line
galaxies, based on mean field independent component analysis (MFICA). Samples
of galaxies were drawn from the Sloan Digital Sky Survey (SDSS) and used to
generate compact sets of `continuum' and `emission-line' component spectra.
These components can be linearly combined to reconstruct the observed spectra
of a wider sample of galaxies. Only 10 components - five continuum and five
emission line - are required to produce accurate reconstructions of essentially
all narrow emission-line galaxies; the median absolute deviations of the
reconstructed emission-line fluxes, given the signal-to-noise ratio (S/N) of
the observed spectra, are 1.2-1.8 sigma for the strong lines. After applying
the MFICA components to a large sample of SDSS galaxies we identify the regions
of parameter space that correspond to pure star formation and pure active
galactic nucleus (AGN) emission-line spectra, and produce high S/N
reconstructions of these spectra.
The physical properties of the pure star formation and pure AGN spectra are
investigated by means of a series of photoionization models, exploiting the
faint emission lines that can be measured in the reconstructions. We are able
to recreate the emission line strengths of the most extreme AGN case by
assuming the central engine illuminates a large number of individual clouds
with radial distance and density distributions, f(r) ~ r^gamma and g(n) ~
n^beta, respectively. The best fit is obtained with gamma = -0.75 and beta =
-1.4. From the reconstructed star formation spectra we are able to estimate the
starburst ages. These preliminary investigations serve to demonstrate the
success of the MFICA-based technique in identifying distinct emission sources,
and its potential as a tool for the detailed analysis of the physical
properties of galaxies in large-scale surveys.Comment: MNRAS accepted. 29 pages, 24 figures, 3 table
Locally Optimally-emitting Clouds and the Narrow Emission Lines in Seyfert Galaxies
The narrow emission line spectra of active galactic nuclei are not accurately
described by simple photoionization models of single clouds. Recent Hubble
Space Telescope images of Seyfert 2 galaxies show that these objects are rich
with ionization cones, knots, filaments, and strands of ionized gas. Here we
extend to the narrow line region the ``locally optimally emitting cloud'' (LOC)
model, in which the observed spectra are predominantly determined by powerful
selection effects. We present a large grid of photoionization models covering a
wide range of physical conditions and show the optimal conditions for producing
many of the strongest emission lines. We show that the integrated narrow line
spectrum can be predicted by an integration of an ensemble of clouds, and we
present these results in the form of diagnostic line ratio diagrams making
comparisons with observations. We also predict key diagnostic line ratios as a
function of distance from the ionizing source, and compare these to
observations. The predicted radial dependence of the [O III]/[O II] ratio may
be matched to the observed one in NGC4151, if the narrow line clouds see a more
intense continuum than we see. The LOC scenario when coupled with a simple
Keplerian gravitational velocity field will quite naturally predict the
observed line width versus critical density relationship. The influence of dust
within the ionized portion of the clouds is discussed and we show that the more
neutral gas is likely to be dusty, although a high ionization dust-free region
is most likely present too. This argues for a variety of NLR cloud origins.Comment: 29 pages plus 16 figures, accepted for publication in Ap
Interpreting the Ionization Sequence in AGN Emission-Line Spectra
We investigate the physical cause of the great range in the ionization level
seen in the spectra of narrow lined active galactic nuclei (AGN). Mean field
independent component analysis identifies examples of individual SDSS galaxies
whose spectra are not dominated by emission due to star formation (SF), which
we designate as AGN. We assembled high S/N ratio composite spectra of a
sequence of these AGN defined by the ionization level of their narrow-line
regions (NLR), extending down to very low-ionization cases. We used a local
optimally emitting cloud (LOC) model to fit emission-line ratios in this AGN
sequence. These included the weak lines that can be measured only in the
co-added spectra, providing consistency checks on strong line diagnostics.
After integrating over a wide range of radii and densities our models indicate
that the radial extent of the NLR is the major parameter in determining the
position of high to moderate ionization AGN along our sequence, providing a
physical interpretation for their systematic variation. Higher ionization AGN
contain optimally emitting clouds that are more concentrated towards the
central continuum source than in lower ionization AGN. Our LOC models indicate
that for the objects that lie on our AGN sequence, the ionizing luminosity is
anticorrelated with the NLR ionization level, and hence anticorrelated with the
radial concentration and physical extent of the NLR. A possible interpretation
that deserves further exploration is that the ionization sequence might be an
age sequence where low ionization objects are older and have systematically
cleared out their central regions by radiation pressure. We consider that our
AGN sequence instead represents a mixing curve of SF and AGN spectra, but argue
that while many galaxies do have this type of composite spectra, our AGN
sequence appears to be a special set of objects with negligible SF excitation.Comment: 57 pages; 18 figures, accepted by MNRA
Hydrogen-oxygen proton-exchange membrane fuel cells and electrolyzers
Hydrogen-oxygen solid polymer electrolyte (SPE) fuel cells and SPE electrolyzers (products of Hamilton Standard) both use a Proton-Exchange Membrane (PEM) as the sole electrolyte. These solid electrolyte devices have been under continuous development for over 30 years. This experience has resulted in a demonstrated ten-year SPE cell life capability under load conditions. Ultimate life of PEM fuel cells and electrolyzers is primarily related to the chemical stability of the membrane. For perfluorocarbon proton exchange membranes an accurate measure of the membrane stability is the fluoride loss rate. Millions of cell hours have contributed to establishing a relationship between fluoride loss rates and average expected ultimate cell life. This relationship is shown. Several features have been introduced into SPE fuel cells and SPE electrolyzers such that applications requiring greater than or equal to 100,000 hours of life can be considered. Equally important as the ultimate life is the voltage stability of hydrogen-oxygen fuel cells and electrolyzers. Here again the features of SPE fuel cells and SPE electrolyzers have shown a cell voltage stability in the order of 1 microvolt per hour. That level of stability has been demonstrated for tens of thousands of hours in SPE fuel cells at up to 500 amps per square foot (ASF) current density
Hydrogen-oxygen proton-exchange membrane fuel cells and electrolyzers
Hydrogen-oxygen SPE fuel cells and SPE electrolyzers (products of Hamilton Standard) both use a Proton-Exchange Membrane (PEM) as the sole electrolyte. The SPE cells have demonstrated a ten year life capability under load conditions. Ultimate life of PEM fuel cells and electrolyzers is primarily related to the chemical stability of the membrane. For perfluorocarbon proton-exchange membranes an accurate measure of the membrane stability is the fluoride loss rate. Millions of cell hours have contributed to establishing a relationship between fluroride loss rates and average expected ultimate cell life. Several features were introduced into SPE fuel cells and SPE electrolyzers such that applications requiring greater than or equal to 100,000 hours of life can be considered. Equally important as the ultimate life is the voltage stability of hydrogen-oxygen fuel cells and electrolyzers. Here again the features of SPE fuel cells and SPE electrolyzers have shown a cell voltage stability in the order of 1 microvolt per hour. That level of stability were demonstrated for tens of thousands of hours in SPE fuel cells at up to 500 amps per square foot (ASF) current density. The SPE electrolyzers have demonstrated the same at 1000 ASF. Many future extraterrestrial applications for fuel cells require that they be self recharged. To translate the proven SPE cell life and stability into a highly reliable extraterrestrial electrical energy storage system, a simplification of supporting equipment is required. Static phase separation, static fluid transport and static thermal control will be most useful in producting required system reliability. Although some 200,000 SPE fuel cell hours were recorded in earth orbit with static fluid phase separation, no SPE electrolyzer has, as yet, operated in space
Economic and demographic issues related to deployment of the Satellite Power System (SPS)
Growth in energy consumption stimulated interest in exploitation of renewable sources of electric energy. One technology that was proposed is the Satellite Power System (SPS). Before committing the U.S. to such a large program, the Department of Energy and the National Aeronautics and Space Administration are jointly participating in an SPS Concept Development and Evaluation Program. This white paper on industrial and population relocation is part of the FY 78 preliminary evaluation of related socio-economic issues. Results of four preliminary assessment activities are documented
Phaseless VLBI mapping of compact extragalactic radio sources
The problem of phaseless aperture synthesis is of current interest in
phase-unstable VLBI with a small number of elements when either the use of
closure phases is not possible (a two-element interferometer) or their quality
and number are not enough for acceptable image reconstruction by standard
adaptive calibration methods. Therefore, we discuss the problem of unique image
reconstruction only from the spectrum magnitude of a source. We suggest an
efficient method for phaseless VLBI mapping of compact extragalactic radio
sources. This method is based on the reconstruction of the spectrum magnitude
for a source on the entire UV plane from the measured visibility magnitude on a
limited set of points and the reconstruction of the sought-for image of the
source by Fienup's method from the spectrum magnitude reconstructed at the
first stage. We present the results of our mapping of the extragalactic radio
source 2200 +420 using astrometric and geodetic observations on a global VLBI
array. Particular attention is given to studying the capabilities of a
two-element interferometer in connection with the putting into operation of a
Russian-made radio interferometer based on Quasar RT-32 radio telescopes.Comment: 21 pages, 6 figure
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