367 research outputs found
The rate of colonization by macro-invertebrates on artificial substrate samplers
The influence of exposure time upon macro-invertebrate colonization on modified Hester-Dendy substrate samplers was investigated over a 60-day period. The duration of exposure affected the number of individuals, taxa and community diversity. The numbers of individuals colonizing the samplers reached a maximum after 39 days and then began to decrease, due to the emergence of adult insects. Coefficients of variation for the four replicate samples retrieved each sampling day fluctuated extensively throughout the study. No tendencies toward increasing or decreasing coefficients of variation were noted with increasing time of sampler exposure. The number of taxa colonizing the samplers increased throughout the study period. The community diversity index was calculated for each sampling day and this function tended to increase throughout the same period. This supports the hypothesis that an exposure period of 6 weeks, as recommended by the United States Environmental Protection Agency, may not always provide adequate opportunity for a truly representative community of macro-invertebrates to colonize multiplate samplers. Many of the taxa were collected in quite substantial proportions after periods of absence or extreme sparseness. This is attributed to the growth of periphyton and the collection of other materials that created food and new habitats suitable for the colonization of new taxa. Investigation of the relationship between ‘equitability’ and length of exposure revealed that equitability did not vary like diversity with increased time of exposure.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72073/1/j.1365-2427.1979.tb01522.x.pd
Stabilities of nanohydrated thymine radical cations: insights from multiphoton ionization experiments and ab initio calculations
Multi-photon ionization experiments have been carried out on thymine-water clusters in the gas phase. Metastable H2O loss from T+(H2O)n was observed at n ≥ 3 only. Ab initio quantum-chemical calculations of a large range of optimized T+(H2O)n conformers have been performed up to n = 4, enabling binding energies of water to be derived. These decrease smoothly with n, consistent with the general trend of increasing metastable H2O loss in the experimental data. The lowest-energy conformers of T+(H2O)3 and T+(H2O)4 feature intermolecular bonding via charge-dipole interactions, in contrast with the purely hydrogen-bonded neutrals. We found no evidence for a closed hydration shell at n = 4, also contrasting with studies of neutral clusters
On primordial trispectrum from exchanging scalar modes in general multiple field inflationary models
We make an complementary investigation of the primordial trispectrum from
exchanging intermediate scalar modes in multi-field inflation models with
generalized kinetic terms. Together with the calculation of irreducible
contributions to the primordial trispectrum in Ref.[103], we give the full
leading-order primordial trispectrum in generalized multi-field models.Comment: 15 pages, 1 figure; v2 references adde
Linear temperature dependence of conductivity in the "insulating" regime of dilute two-dimensional holes in GaAs
The conductivity of extremely high mobility dilute two-dimensional holes in
GaAs changes linearly with temperature in the insulating side of the
metal-insulator transition. Hopping conduction, characterized by an
exponentially decreasing conductivity with decreasing temperature, is not
observed when the conductivity is smaller than . We suggest that
strong interactions in a regime close to the Wigner crystallization must be
playing a role in the unusual transport.Comment: 3 pages, 2 figure
Constraining Cut-off Physics in the Cosmic Microwave Background
We investigate the ability to constrain oscillatory features in the
primordial power spectrum using current and future cosmic microwave background
observations. In particular, we study the observability of an oscillation
arising from imprints of physics at the cut-off energy scale. We perform a
likelihood analysis on the WMAP data set, and find that the current data set
constrains the amplitude of the oscillations to be less than 0.77 at 2-sigma,
consistent with a power spectrum without oscillations. In addition, we
investigate the fundamental limitations in the measurement of oscillation
parameters by studying the constraints from a cosmic variance limited
experiment. We find that such an experiment is capable of constraining the
amplitude of such oscillations to be below 0.005, implying that reasonable
models with cut-off energy scales Lambda>200 H_infl are unobservable through
the microwave background.Comment: 16 pages, 7 figures; PRD accepted versio
Review of AdS/CFT Integrability, Chapter V.2: Dual Superconformal Symmetry
Scattering amplitudes in planar N=4 super Yang-Mills theory reveal a
remarkable symmetry structure. In addition to the superconformal symmetry of
the Lagrangian of the theory, the planar amplitudes exhibit a dual
superconformal symmetry. The presence of this additional symmetry imposes
strong restrictions on the amplitudes and is connected to a duality relating
scattering amplitudes to Wilson loops defined on polygonal light-like contours.
The combination of the superconformal and dual superconformal symmetries gives
rise to a Yangian, an algebraic structure which is known to be related to the
appearance of integrability in other regimes of the theory. We discuss two dual
formulations of the symmetry and address the classification of its invariants.Comment: 22 pages, see also overview article arXiv:1012.3982, v2: references
to other chapters updated, v3 added references, typos fixe
Are Solar Active Regions with Major Flares More Fractal, Multifractal, or Turbulent than Others?
Multiple recent investigations of solar magnetic field measurements have
raised claims that the scale-free (fractal) or multiscale (multifractal)
parameters inferred from the studied magnetograms may help assess the eruptive
potential of solar active regions, or may even help predict major flaring
activity stemming from these regions. We investigate these claims here, by
testing three widely used scale-free and multiscale parameters, namely, the
fractal dimension, the multifractal structure function and its inertial-range
exponent, and the turbulent power spectrum and its power-law index, on a
comprehensive data set of 370 timeseries of active-region magnetograms (17,733
magnetograms in total) observed by SOHO's Michelson Doppler Imager (MDI) over
the entire Solar Cycle 23. We find that both flaring and non-flaring active
regions exhibit significant fractality, multifractality, and non-Kolmogorov
turbulence but none of the three tested parameters manages to distinguish
active regions with major flares from flare-quiet ones. We also find that the
multiscale parameters, but not the scale-free fractal dimension, depend
sensitively on the spatial resolution and perhaps the observational
characteristics of the studied magnetograms. Extending previous works, we
attribute the flare-forecasting inability of fractal and multifractal
parameters to i) a widespread multiscale complexity caused by a possible
underlying self-organization in turbulent solar magnetic structures, flaring
and non-flaring alike, and ii) a lack of correlation between the fractal
properties of the photosphere and overlying layers, where solar eruptions
occur. However useful for understanding solar magnetism, therefore, scale-free
and multiscale measures may not be optimal tools for active-region
characterization in terms of eruptive ability or, ultimately,for major
solar-flare prediction.Comment: 25 pages, 7 figures, 2 tables, Solar Phys., in pres
Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics
We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions
The mammalian gene function resource: The International Knockout Mouse Consortium
In 2007, the International Knockout Mouse Consortium (IKMC) made the ambitious promise to generate mutations in virtually every protein-coding gene of the mouse genome in a concerted worldwide action. Now, 5 years later, the IKMC members have developed highthroughput gene trapping and, in particular, gene-targeting pipelines and generated more than 17,400 mutant murine embryonic stem (ES) cell clones and more than 1,700 mutant mouse strains, most of them conditional. A common IKMC web portal (www.knockoutmouse.org) has been established, allowing easy access to this unparalleled biological resource. The IKMC materials considerably enhance functional gene annotation of the mammalian genome and will have a major impact on future biomedical research
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