761 research outputs found
Using an ontology for interoperability and browsing of museum, library and archive information
Ontologies play an important part in the development of the future âsemantic webâ; the CIDOC conceptual reference model (CRM) is an ontology aimed at the cultural heritage domain. This paper describes a Concept Browser, developed for the EU/IST-funded SCULPTEUR project (semantic and content-based multimedia exploitation for European benefit environment (programme IST-2001-no. 35372); May 2002 to May 2005), which is able to access different museum information systems through a common ontology, the CRM. The development of this Concept Browser has required mappings from the legacy museum database systems to the CRM. The crucial process of creating the mappings is described, using the C2RMF catalogue (EROS) and library databases as a case study
Predicted Properties of Microhollow Cathode Discharges in Xenon
A fluid model has been developed and used to help clarify the physical mechanisms occurring in microhollow cathode discharges (MHCD). Calculated current-voltage (I-V) characteristics and gas temperatures in xenon at 100 Torr are presented. Consistent with previous experimental results in similar conditions, we find a voltage maximum in the I-V characteristic. We show that this structure reflects a transition between a low-current, abnormal discharge localized inside the cylindrical hollow cathode to a higher-current, normal glow discharge sustained by electron emission from the outer surface of the cathode. This transition, due to the geometry of the device, is a factor contributing to the well-known stability of MHCDs
Respiration and Heart Rate at the Surface between Dives in Northern Elephant Seals
All underwater activities of diving mammals are constrained by the need for surface gas exchange. Our aim was to measure respiratory rate (fb) and heart rate (fh) at the surface between dives in free-ranging northern elephant seals Mirounga angustirostris. We recorded fb and fh acoustically in six translocated juveniles, 1.8-2. 4 years old, and three migrating adult males from the rookery at Ano Nuevo, California, USA. To each seal, we attached a diving instrument to record the diving pattern, a satellite tag to track movements and location, a digital audio tape recorder or acoustic datalogger with an external hydrophone to record the sounds of respiration and fh at the surface, and a VHF transmitter to facilitate recovery. During surface intervals averaging 2.2+/â0.4 min, adult males breathed a mean of 32.7+/â5.4 times at a rate of 15. 3+/â1.8 breaths min(â)(1) (means +/â s.d., N=57). Mean fh at the surface was 84+/â3 beats min(â)(1). The fb of juveniles was 26 % faster than that of adult males, averaging 19.2+/â2.2 breaths min(â)(1) for a mean total of 41.2+/â5.0 breaths during surface intervals lasting 2.6+/â0.31 min. Mean fh at the surface was 106+/â3 beats min(â)(1). fb and fh did not change significantly over the course of surface intervals. Surface fb and fh were not clearly associated with levels of exertion, such as rapid horizontal transit or apparent foraging, or with measures of immediately previous or subsequent diving performance, such as diving duration, diving depth or swimming speed. Together, surface respiration rate and the duration of the preceding dive were significant predictors of surface interval duration. This implies that elephant seals minimize surface time spent loading oxygen depending on rates of oxygen uptake and previous depletion of stores
Modeling of the plasma jet of a stationary plasma thruster
We have developed a two-dimensional hybrid fluid â particle-in-cell Monte Carlo collisions (PIC- MCC) model to study the plume of a stationary plasma thruster. The model is based on a fluid description of the electrons (the electron density follows a Boltzmann distribution) and a particle description of the ion and neutral transport. Collisions between heavy species are taken into account with a Monte Carlo method. The electric field is obtained from Poisson's equation or from the quasineutrality assumption. We first show that the results from the PIC-MCC model are close to the results of a more time-consuming direct simulation Monte Carlo approach. We then compare the model predictions of the plume density and ion energy distribution with experimental measurements. Finally, we present a brief discussion on the assumptions of the model and on its ability to give reliable predictions on important issues such as the flux of ions backscattered to the satellite. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70320/2/JAPIAU-91-12-9521-1.pd
Sinking versus suspended particle size distributions in the North Pacific Subtropical Gyre
The particle size distribution (PSD) is a fundamental property that influences all aspects of phytoplankton ecology. In particular, the size (e.g., diameter d [ÎŒm]) and sinking speed w (m/day) of individual particles are inextricable, but much remains unknown about how d and w are related quantitatively for bulk particulate matter. There is significant interest in inferring sinking mass fluxes from PSDs, but doing so requires knowing how both mass and w scale with d . To this end, using both laser diffraction and imaging, we characterized for the first time both sinking and suspended PSDs in the oligotrophic North Pacific subtropical gyre. Comparing these PSDs via a power law parameterization indicates an approximately linear w âtoâd scaling, suggesting particles are more fractalâlike than sphereâlike in this respect. This result is robust across multiple instruments, depths, and sediment trap deployments and is made comparatively precise by a high degree of replication
Semimetalic antiferromagnetism in the half-Heusler compound CuMnSb
The half-Heusler compound CuMnSb, the first antiferromagnet (AFM) in the
Mn-based class of Heuslers and half-Heuslers that contains several conventional
and half metallic ferromagnets, shows a peculiar stability of its magnetic
order in high magnetic fields. Density functional based studies reveal an
unusual nature of its unstable (and therefore unseen) paramagnetic state, which
for one electron less (CuMnSn, for example) would be a zero gap semiconductor
(accidentally so) between two sets of very narrow, topologically separate bands
of Mn 3d character. The extremely flat Mn 3d bands result from the environment:
Mn has four tetrahedrally coordinated Cu atoms whose 3d states lie well below
the Fermi level, and the other four tetrahedrally coordinated sites are empty,
leaving chemically isolated Mn 3d states. The AFM phase can be pictured
heuristically as a self-doped CuMnSb compensated semimetal
with heavy mass electrons and light mass holes, with magnetic coupling
proceeding through Kondo and/or antiKondo coupling separately through the two
carrier types. The ratio of the linear specific heat coefficient and the
calculated Fermi level density of states indicates a large mass enhancement
, or larger if a correlated band structure is taken as the
reference
Organochloride pesticides in California sea lions revisited
BACKGROUND: Dichlorodiphenyltrichloroethane (DDT) and polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that have been banned in most countries, but considerable amounts continue to cycle the ecosphere. Top trophic level predators, like sea birds and marine mammals, bioaccumulate these lipophilic compounds, reflecting their presence in the environment. RESULTS: We measured concentrations of tDDT (p,p' - DDT + p,p' - DDD + p,p' - DDE) and PCBs in the blubber of dead California sea lions stranded along the California coast. tDDT and PCB concentrations were 150 ± 257 ug/g lipid weight (mean ± SD) and 44 ± 78 ug/g lipid weight, respectively. There were no differences in tDDT or PCB concentrations between animal categories varying in sex or age. There was a trend towards a decrease in tDDT and PCB concentrations from northern to southern California. The lipid content of the blubber was negatively correlated with levels of tDDT and PCBs. tDDT concentrations were approximately 3 times higher than PCB concentrations. CONCLUSIONS: tDDT levels in the blubber of California sea lions decreased by over one order of magnitude from 1970 to 2000. PCB level changes over time were unclear owing to a paucity of data and analytical differences over the years. Current levels of these pollutants in California sea lions are among the highest among marine mammals and exceed those reported to cause immunotoxicity or endocrine disruption
Organochloride pesticides in California sea lions revisited: Correction
<p/> <p>There has been an error in the publication of this paper <abbrgrp><abbr bid="B1">1</abbr></abbrgrp>; the title should read: 'Organochlorine pollutants in California sea lions revisited'.</p
Polarization Control of the Non-linear Emission on Semiconductor Microcavities
The degree of circular polarization () of the non-linear emission in
semiconductor microcavities is controlled by changing the exciton-cavity
detuning. The polariton relaxation towards \textbf{K} cavity-like
states is governed by final-state stimulated scattering. The helicity of the
emission is selected due to the lifting of the degeneracy of the spin
levels at \textbf{K} . At short times after a pulsed excitation
reaches very large values, either positive or negative, as a result of
stimulated scattering to the spin level of lowest energy ( spin for
positive/negative detuning).Comment: 8 pages, 3 eps figures, RevTeX, Physical Review Letters (accepted
Azimuthal structures and turbulent transport in Penning discharge
Azimuthal structures in cylindrical Penning discharge are studied with 2D3V
radial-azimuthal PIC/MCC model with the axial magnetic field. The discharge is
self-consistently supported by ionization due to the axial injection of
electrons. It is shown that the steady-state discharge can be supported in two
different regimes with different type of observed azimuthal structures. The
transition between the regimes is controlled by the mechanism of the energy
input to the discharge. In the first regime (low energy of the injected
electrons), with the pronounced spoke activity, the power input is
dominated by the energy absorption due to the radial current and
self-consistent electric field. In the other regime (higher energy of the
injected electrons), with prevalent small scale spiral structures, and
the lower values of the anomalous transport, the total energy deposited to the
discharge is lower and is mostly due to the direct input of the kinetic energy
from the axial electron beam. We show that the large (m=1) spoke and small
scale structures occur as a result of Simon-Hoh and lower hybrid instabilities
driven by the electric field, density gradient, and collisions. We show that
the spoke frequency follows the equilibrium ion rotation frequency
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