819 research outputs found
Endogenous Quasicycles and Stochastic Coherence in a Closed Endemic Model
We study the role of demographic fluctuations in typical endemics as
exemplified by the stochastic SIRS model. The birth-death master equation of
the model is simulated using exact numerics and analysed within the linear
noise approximation. The endemic fixed point is unstable to internal
demographic noise, and leads to sustained oscillations. This is ensured when
the eigenvalues () of the linearised drift matrix are complex, which
in turn, is possible only if detailed balance is violated. In the oscillatory
state, the phases decorrelate asymptotically, distinguishing such oscillations
from those produced by external periodic forcing. These so-called quasicycles
are of sufficient strength to be detected reliably only when the ratio
is of order unity. The coherence or regularity of
these oscillations show a maximum as a function of population size, an effect
known variously as stochastic coherence or coherence resonance. We find that
stochastic coherence can be simply understood as resulting from a non-monotonic
variation of with population size. Thus, within the
linear noise approximation, stochastic coherence can be predicted from a purely
deterministic analysis. The non-normality of the linearised drift matrix,
associated with the violation of detailed balance, leads to enhanced
fluctuations in the population amplitudes.Comment: 21 pages, 8 figure
Effects of antibiotic-suppelmented media on recovery of enterobacteria
The frequency at which Salmonella typhimurium (ST) and Escherichia coli were recovered from tryptic soy agar (TSA), brilliant green agar (BGA) and MacConkey agar (MAC) alone or supplemented with 2 and 16 Āµg kanamycin and 0.25 and 2 Āµg enrofloxacin was investigated
Direct observation of electron density reconstruction at the metal-insulator transition in NaOsO3
5d transition metal oxides offer new opportunities to test our understanding
of the interplay of correlation effects and spin-orbit interactions in
materials in the absence of a single dominant interaction. The subtle balance
between solid-state interactions can result in new mechanisms that minimize the
interaction energy, and in material properties of potential use for
applications. We focus here on the 5d transition metal oxide NaOsO3, a strong
candidate for the realization of a magnetically driven transition from a
metallic to an insulating state exploiting the so-called Slater mechanism.
Experimental results are derived from non-resonant and resonant x-ray single
crystal diffraction at the Os L-edges. A change in the crystallographic
symmetry does not accompany the metal-insulator transition in the Slater
mechanism and, indeed, we find no evidence of such a change in NaOsO3. An
equally important experimental observation is the emergence of the (300) Bragg
peak in the resonant condition with the onset of magnetic order. The intensity
of this space-group forbidden Bragg peak continuously increases with decreasing
temperature in line with the square of intensity observed for an allowed
magnetic Bragg peak. Our main experimental results, the absence of crystal
symmetry breaking and the emergence of a space-group forbidden Bragg peak with
developing magnetic order, support the use of the Slater mechanism to interpret
the metal-insulator transition in NaOsO3. We successfully describe our
experimental results with simulations of the electronic structure and, also,
with an atomic model based on the established symmetry of the crystal and
magnetic structure.Comment: 6 figure
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Using Ī“13C-CH4 and Ī“D-CH4 to constrain Arctic methane emissions
We present a global methane modelling study assessing the sensitivity of Arctic atmospheric CH4 mole fractions, Ī“13C-CH4 and Ī“D-CH4 to uncertainties in Arctic methane sources. Model simulations include methane tracers tagged by source and isotopic composition and are compared with atmospheric data at four northern high-latitude measurement sites. We find the model's ability to capture the magnitude and phase of observed seasonal cycles of CH4 mixing ratios, Ī“13C-CH4 and Ī“D-CH4 at northern high latitudes is much improved using a later spring kick-off and autumn decline in northern high-latitude wetland emissions than predicted by most process models. Results from our model simulations indicate that recent predictions of large methane emissions from thawing submarine permafrost in the East Siberian Arctic Shelf region could only be reconciled with global-scale atmospheric observations by making large adjustments to high-latitude anthropogenic or wetland emission inventories
Gas hydrates: Entrance to a methane age or climate threat?
Methane hydrates, ice-like compounds in which methane is held in crystalline cages formed by water molecules, are widespread in areas of permafrost such as the Arctic and in sediments on the continental margins. They are a potentially vast fossil fuel energy source but, at the same time, could be destabilized by changing pressure-temperature conditions due to climate change, potentially leading to strong positive carbon-climate feedbacks. To enhance our understanding of both the vulnerability of and the opportunity provided by methane hydrates, it is necessary (i) to conduct basic research that improves the highly uncertain estimates of hydrate occurrences and their response to changing environmental conditions, and (ii) to integrate the agendas of energy security and climate change which can provide an opportunity for methane hydrates -- in particular if combined with carbon capture and storage -- to be used as a 'bridge fuel' between carbon-intensive fossil energies and zero-emission energies. Taken one step further, exploitation of dissociating methane hydrates could even mitigate against escape of methane to the atmosphere. Despite these opportunities, so far, methane hydrates have been largely absent from energy and climate discussions, including global hydrocarbon assessments and the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
Enhanced stochastic oscillations in autocatalytic reactions
We study a simplified scheme of coupled autocatalytic reactions,
previously introduced by Togashi and Kaneko. The role of stochastic
fluctuations is elucidated through the use of the van Kampen system-size
expansion and the results compared with direct stochastic simulations. Regular
temporal oscillations are predicted to occur for the concentration of the
various chemical constituents, with an enhanced amplitude resulting from a
resonance which is induced by the intrinsic graininess of the system. The
associated power spectra are determined and have a different form depending on
the number of chemical constituents, . We make detailed comparisons in the
two cases and . Agreement between the theoretical and numerical
results for the power spectrum are good in both cases. The resulting spectrum
is especially interesting in the system, since it has two peaks, which
the system-size expansion is still able to reproduce accurately.Comment: Submitted to Phys. Rev.
Making sense of policy choices: understanding the roles of value predispositions, mass media, and cognitive processing in public attitudes toward nanotechnology
Using a nationally representative telephone survey of 1,015 adults in the United States, this study examines how value predispositions, communication variables, and perceptions of risks and benefits are associated with public support for federal funding of nanotechnology. Our findings show that highly religious individuals were less supportive of funding of nanotech than less religious individuals, whereas individuals who held a high deference for scientific authority were more supportive of funding of the emerging technology than those low in deference. Mass media use and elaborative processing of scientific news were positively associated with public support for funding, whereas factual scientific knowledge had no significant association with policy choices. The findings suggest that thinking about and reflecting upon scientific news promote better understanding of the scientific world and may provide a more sophisticated cognitive structure for the public to form opinions about nanotech than factual scientific knowledge. Finally, heuristic cues including trust in scientists and perceived risks and benefits of nanotech were found to be associated with public support for nanotech funding. We conclude with policy implications that will be useful for policymakers and science communication practitioners
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