77,872 research outputs found
Local properties of patterned vegetation: quantifying endogenous and exogenous effects
Dryland ecosystems commonly exhibit periodic bands of vegetation, thought to
form due to competition between individual plants for heterogeneously
distributed water. In this paper, we develop a Fourier method for locally
identifying the pattern wavenumber and orientation, and apply it to aerial
images from a region of vegetation patterning near Fort Stockton, Texas. We
find that the local pattern wavelength and orientation are typically coherent,
but exhibit both rapid and gradual variation driven by changes in hillslope
gradient and orientation, the potential for water accumulation, or soil type.
Endogenous pattern dynamics, when simulated for spatially homogeneous
topographic and vegetation conditions, predict pattern properties that are much
less variable than the orientation and wavelength observed in natural systems.
Our local pattern analysis, combined with ancillary datasets describing soil
and topographic variation, highlights a largely unexplored correlation between
soil depth, pattern coherence, vegetation cover and pattern wavelength. It
also, surprisingly, suggests that downslope accumulation of water may play a
role in changing vegetation pattern properties
Reversal Modes of Simulated Iron Nanopillars in an Obliquely Oriented Field
Stochastic micromagnetic simulations are employed to study switching in
three-dimensional magnetic nanopillars exposed to highly misaligned fields. The
switching appears to proceed through two different decay modes, characterized
by very different average lifetimes and different average values of the
transverse magnetization components.Comment: 3 pages, 4 figure
Analysis of the financial factors governing the profitability of lunar helium-3
Financial factors influencing the profitability of the mining and utilization of lunar helium-3 are examined. The analysis addressed the following questions: (1) which financial factors have the greatest leverage on the profitability of He-3; (2) over what range can these factors be varied to keep the He-3 option profitable; and (3) what ultimate effect could this energy source have on the price of electricity for U.S. consumers. Two complementary methods of analysis were used in the assessment: rate of return on incremental investment required and reduction revenue requirements (total cost to customers) achieved. Some of the factors addressed include energy demand, power generation costs with and without fusion, profitability for D-He(3) fusion, annual capital and operating costs, launch mass and costs, He-3 price, and government funding. Specific conclusions are made with respect to each of the companies considered: utilities, lunar mining company, and integrated energy company
Highly-improved lattice field-strength tensor
We derive an O(a^4)-improved lattice version of the continuum field-strength
tensor. Discretization errors are reduced via the combination of several clover
terms of various sizes, complemented by tadpole improvement. The resulting
improved field-strength tensor is used to construct O(a^4)-improved topological
charge and action operators. We compare the values attained by these operators
as we cool several configurations to self-duality with a previously defined
highly-improved action and assess the relative scale of the remaining
discretization errors.Comment: 22 pages, 7 postscript figure
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