14,368 research outputs found
The Construction of Spinor Fields on Manifolds with Smooth Degenerate Metrics
We examine some of the subtleties inherent in formulating a theory of spinors
on a manifold with a smooth degenerate metric. We concentrate on the case where
the metric is singular on a hypersurface that partitions the manifold into
Lorentzian and Euclidean domains. We introduce the notion of a complex spinor
fibration to make precise the meaning of continuity of a spinor field and give
an expression for the components of a local spinor connection that is valid in
the absence of a frame of local orthonormal vectors. These considerations
enable one to construct a Dirac equation for the discussion of the behavior of
spinors in the vicinity of the metric degeneracy. We conclude that the theory
contains more freedom than the spacetime Dirac theory and we discuss some of
the implications of this for the continuity of conserved currents.Comment: 24 pages, LaTeX (RevTeX 3.0, no figures), To appear in J. Math. Phy
Continental land cover classification using meteorological satellite data
The use of the National Oceanic and Atmospheric Administration's advanced very high resolution radiometer satellite data for classifying land cover and monitoring of vegetation dynamics over an extremely large area is demonstrated for the continent of Africa. Data from 17 imaging periods of 21 consecutive days each were composited by a technique sensitive to the in situ green-leaf biomass to provide cloud-free imagery for the whole continent. Virtually cloud-free images were obtainable even for equatorial areas. Seasonal variation in the density and extent of green leaf vegetation corresponded to the patterns of rainfall associated with the inter-tropical convergence zone. Regional variations, such as the 1982 drought in east Africa, were also observed. Integration of the weekly satellite data with respect to time produced a remotely sensed assessment of biological activity based upon density and duration of green-leaf biomass. Two of the 21-day composited data sets were used to produce a general land cover classification. The resultant land cover distributions correspond well to those of existing maps
The Transition State in a Noisy Environment
Transition State Theory overestimates reaction rates in solution because
conventional dividing surfaces between reagents and products are crossed many
times by the same reactive trajectory. We describe a recipe for constructing a
time-dependent dividing surface free of such recrossings in the presence of
noise. The no-recrossing limit of Transition State Theory thus becomes
generally available for the description of reactions in a fluctuating
environment
A computer operated mass spectrometer system
Digital computer system for processing mass spectrometer output dat
Spiral cracks in drying precipitates
We investigate the formation of spiral crack patterns during the desiccation
of thin layers of precipitates in contact with a substrate. This
symmetry-breaking fracturing mode is found to arise naturally not from torsion
forces, but from a propagating stress front induced by the fold-up of the
fragments. We model their formation mechanism using a coarse-grain model for
fragmentation and successfully reproduce the spiral cracks. Fittings of
experimental and simulation data show that the spirals are logarithmic,
corresponding to constant deviation from a circular crack path. Theoretical
aspects of the logarithmic spirals are discussed. In particular we show that
this occurs generally when the crack speed is proportional to the propagating
speed of stress front.Comment: 4 pages, 5 figures, RevTe
Isomorphism between Non-Riemannian gravity and Einstein-Proca-Weyl theories extended to a class of Scalar gravity theories
We extend the recently proved relation between certain models of
Non-Riemannian gravitation and Einstein- Proca-Weyl theories to a class of
Scalar gravity theories. This is used to present a Black-Hole Dilaton solution
with non-Riemannian connection.Comment: 13 pages, tex file, accepted in Class. Quant. Gra
Mammal population densities at a global scale are higher in human‐modified areas
Global landscapes are changing due to human activities with consequences for both biodiversity and ecosystems. For single species, terrestrial mammal population densities have shown mixed responses to human pressure, with both increasing and decreasing densities reported in the literature. How the impacts of human activities on mammal populations translates into altered global density patterns remains unclear. Here we aim to disentangle the effect of human impacts on large‐scale patterns of mammal population densities using a global dataset of 6729 population density estimates for 468 mammal species (representing 59% and 44% of mammalian orders and families). We fitted a mixed effect model to explain the variation in density based on a 1‐degree resolution as a function of the human footprint index (HFI), a global proxy of direct and indirect human disturbances, while accounting for body mass, trophic level and primary productivity (normalized vegetation index; NDVI). We found a significant positive relationship between population density and HFI, where population densities were higher in areas with a higher HFI (e.g. agricultural or suburban areas – no populations were located in very high HFI urban areas) compared to areas with a low HFI (e.g. wilderness areas). We also tested the effect of the individual components of the HFI and still found a consistent positive effect. The relationships remained positive even across populations of the same species, although variability among species was high. Our results indicate shifts in mammal population densities in human modified landscapes, which is due to the combined effect of species filtering, increased resources and a possible reduction in competition and predation. Our study provides further evidence that macroecological patterns are being altered by human activities, where some species will benefit from these activities, while others will be negatively impacted or even extirpated
Mammal population densities at a global scale are higher in human-modified areas
Global landscapes are changing due to human activities with consequences for both biodiversity and ecosystems. For single species, terrestrial mammal population densities have shown mixed responses to human pressure, with both increasing and decreasing densities reported in the literature. How the impacts of human activities on mammal populations translates into altered global density patterns remains unclear. Here we aim to disentangle the effect of human impacts on large-scale patterns of mammal population densities using a global dataset of 6729 population density estimates for 468 mammal species (representing 59% and 44% of mammalian orders and families). We fitted a mixed effect model to explain the variation in density based on a 1-degree resolution as a function of the human footprint index (HFI), a global proxy of direct and indirect human disturbances, while accounting for body mass, trophic level and primary productivity (normalized vegetation index; NDVI). We found a significant positive relationship between population density and HFI, where population densities were higher in areas with a higher HFI (e.g. agricultural or suburban areas – no populations were located in very high HFI urban areas) compared to areas with a low HFI (e.g. wilderness areas). We also tested the effect of the individual components of the HFI and still found a consistent positive effect. The relationships remained positive even across populations of the same species, although variability among species was high. Our results indicate shifts in mammal population densities in human modified landscapes, which is due to the combined effect of species filtering, increased resources and a possible reduction in competition and predation. Our study provides further evidence that macroecological patterns are being altered by human activities, where some species will benefit from these activities, while others will be negatively impacted or even extirpated
The kinetics of surfactant desorption at the air–solution interface
The kinetics of desorption of the anionic surfactant sodium dodecylbenzene sulfonate at the air–solution interface have been studied using neutron reflectivity (NR). The experimental arrangement incorporates a novel flow cell in which the subphase can be exchanged (diluted) using a laminar flow whilst the surface region remains unaltered. The kinetics of the desorption is relatively slow and occurs over many tens of minutes compared with the dilution timescale of approximately 10–30 minutes. A detailed mathematical model, in which the rate of the desorption is determined by transport through a near-surface diffusion layer into a diluted bulk solution below, is developed and provides a good description of the timedependent adsorption data.\ud
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A key parameter of the model is the ratio of the depth of the diffusion layer, Hc , to the depth of the fluid, Hf, and we find that this is related to the reduced Péclet number, Pe*, for the system, via Hc/Hf, = C/Pe* 1/ 2 . Although from a highly idealised experimental arrangement, the results provide an important insight into the ‘rinse mechanism’, which is applicable to a wide variety of domestic and industrial circumstances
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