1,615 research outputs found
Ozone Response to Aircraft Emissions: Sensitivity Studies with Two-dimensional Models
Our first intercomparison/assessment of the effects of a proposed high-speed civil transport (HSCT) fleet on the stratosphere is presented. These model calculations should be considered more as sensitivity studies, primarily designed to serve the following purposes: (1) to allow for intercomparison of model predictions; (2) to focus on the range of fleet operations and engine specifications giving minimal environmental impact; and (3) to provide the basis for future assessment studies. The basic scenarios were chosen to be as realistic as possible, using the information available on anticipated developments in technology. They are not to be interpreted as a commitment or goal for environmental acceptability
A first-principles comparison of the electronic properties of MgC_{y}Ni_{3} and ZnC_{y}Ni_{3} alloys
First-principles, density-functional-based electronic structure calculations
are employed to study the changes in the electronic properties of ZnC_{y}Ni_{3}
and MgC_{y}Ni_{3} using the Korringa-Kohn-Rostoker coherent-potential
approximation method in the atomic sphere approximation (KKR-ASA CPA). As a
function of decreasing C at%, we find a steady decrease in the lattice constant
and bulk modulus in either alloys. However, the pressure derivative of the bulk
modulus displays an opposite trend. Following the Debye model, which relates
the pressure derivative of the bulk modulus with the average phonon frequency
of the crystal, it can thus be argued that ZnCNi_{3} and its disordered alloys
posses a different phonon spectra in comparison to its MgCNi_{3} counterparts.
This is further justified by the marked similarity we find in the electronic
structure properties such as the variation in the density of states and the
Hopfield parameters calculated for these alloys. The effects on the equation of
state parameters and the density of states at the Fermi energy, for partial
replacement of Mg by Zn are also discussed.Comment: 19 pages, 15 figure
Ozone Depletion from Nearby Supernovae
Estimates made in the 1970's indicated that a supernova occurring within tens
of parsecs of Earth could have significant effects on the ozone layer. Since
that time, improved tools for detailed modeling of atmospheric chemistry have
been developed to calculate ozone depletion, and advances have been made in
theoretical modeling of supernovae and of the resultant gamma-ray spectra. In
addition, one now has better knowledge of the occurrence rate of supernovae in
the galaxy, and of the spatial distribution of progenitors to core-collapse
supernovae. We report here the results of two-dimensional atmospheric model
calculations that take as input the spectral energy distribution of a
supernova, adopting various distances from Earth and various latitude impact
angles. In separate simulations we calculate the ozone depletion due to both
gamma-rays and cosmic rays. We find that for the combined ozone depletion
roughly to double the ``biologically active'' UV flux received at the surface
of the Earth, the supernova must occur at <8 pc. Based on the latest data, the
time-averaged galactic rate of core-collapse supernovae occurring within 8 pc
is ~1.5/Gyr. In comparing our calculated ozone depletions with those of
previous studies, we find them to be significantly less severe than found by
Ruderman (1974), and consistent with Whitten et al. (1976). In summary, given
the amplitude of the effect, the rate of nearby supernovae, and the ~Gyr time
scale for multicellular organisms on Earth, this particular pathway for mass
extinctions may be less important than previously thought.Comment: 24 pages, 4 Postscript figures, to appear in The Astrophysical
Journal, 2003 March 10, vol. 58
The atmospheric effects of stratospheric aircraft: A current consensus
In the early 1970's, a fleet of supersonic aircraft flying in the lower stratosphere was proposed. A large fleet was never built for economic, political, and environmental reasons. Technological improvements may make it economically feasible to develop supersonic aircraft for current markets. Some key results of earlier scientific programs designed to assess the impact of aircraft emissions on stratospheric ozone are reviewed, and factors that must be considered to assess the environmental impact of aircraft exhaust are discussed. These include the amount of nitrogen oxides injected in the stratosphere, horizontal transport, and stratosphere/troposphere assessment models are presented. Areas in which improvements in scientific understanding and model representation must be made to reduce the uncertainty in model calculations are identified
Reusable Agena study. Volume 2: Technical
The application of the existing Agena vehicle as a reusable upper stage for the space shuttle is discussed. The primary objective of the study is to define those changes to the Agena required for it to function in the reusable mode in the 100 percent capture of the NASA-DOD mission model. This 100 percent capture is achieved without use of kick motors or stages by simply increasing the Agena propellant load by using optional strap-on-tanks. The required shuttle support equipment, launch and flight operations techniques, development program, and cost package are also defined
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Ecological Consequences of a Millennium of Introduced Dogs on Madagascar
Introduced predators currently threaten endemic animals on Madagascar through predation, facilitation of human-led hunts, competition, and disease transmission, but the antiquity and past consequences of these introductions are poorly known. We use directly radiocarbon dated bones of introduced dogs (Canis familiaris) to test whether dogs could have aided human-led hunts of the island’s extinct megafauna. We compare carbon and nitrogen isotope data from the bone collagen of dogs and endemic “fosa” (Cryptoprocta spp.) in central and southwestern Madagascar to test for competition between introduced and endemic predators. The distinct isotopic niches of dogs and fosa suggest that any past antagonistic relationship between these predators did not follow from predation or competition for shared prey. Radiocarbon dates confirm that dogs have been present on Madagascar for over a millennium and suggest that they at least briefly co-occurred with the island’s extinct megafauna, which included giant lemurs, elephant birds, and pygmy hippopotamuses. Today, dogs share a mutualism with pastoralists who also occasionally hunt endemic vertebrates, and similar behavior is reflected in deposits at several Malagasy paleontological sites that contain dog and livestock bones along with butchered bones of extinct megafauna and extant lemurs. Dogs on Madagascar have had a wide range of diets during the past millennium, but relatively high stable carbon isotope values suggest few individuals relied primarily on forest bushmeat. Our newly generated data suggest that dogs were part of a suite of animal introductions beginning over a millennium ago that coincided with widespread landscape transformation and megafaunal extinction.Introduction Materials and Methods - Specimen and Site Selection - 14C and Stable Isotope Analyses - Data Analysis Results - Chronological Data - Stable Isotope Data Discussio
Stochastic Resonance in Ion Channels Characterized by Information Theory
We identify a unifying measure for stochastic resonance (SR) in voltage
dependent ion channels which comprises periodic (conventional), aperiodic and
nonstationary SR. Within a simplest setting, the gating dynamics is governed by
two-state conductance fluctuations, which switch at random time points between
two values. The corresponding continuous time point process is analyzed by
virtue of information theory. In pursuing this goal we evaluate for our
dynamics the tau-information, the mutual information and the rate of
information gain. As a main result we find an analytical formula for the rate
of information gain that solely involves the probability of the two channel
states and their noise averaged rates. For small voltage signals it simplifies
to a handy expression. Our findings are applied to study SR in a potassium
channel. We find that SR occurs only when the closed state is predominantly
dwelled. Upon increasing the probability for the open channel state the
application of an extra dose of noise monotonically deteriorates the rate of
information gain, i.e., no SR behavior occurs.Comment: 10 pages, 2 figures, to appear in Phys. Rev.
The truth about snitches: an archival analysis of informant testimony
Informants are witnesses who often testify in exchange for an incentive (i.e. jailhouse informant, cooperating witness). Despite the widespread use of informants, little is known about the circumstances surrounding their use at trial. This study content-analyzed trials from 22 DNA exoneration cases involving 53 informants. Because these defendants were exonerated, the prosecution informant testimony is demonstrably false. Informant characteristics including motivation for testifying, criminal history, relationship with the defendant and testimony were coded. Most informants were prosecution jailhouse informants; however, there were also defence jailhouse informants and prosecution cooperating witnesses. Regardless of informant type, most denied receiving an incentive, had criminal histories, were friends/acquaintances of the defendant and had testimonial inconsistencies. In closing statements, attorneys relied on informant testimony by either emphasizing or questioning its reliability. The impact of informant testimony on jurors’ decisions is discussed in terms of truth-default theory (TDT), the fundamental attribution error and prosecutorial vouching
The truth about snitches: an archival analysis of informant testimony
Informants are witnesses who often testify in exchange for an incentive (i.e. jailhouse informant, cooperating witness). Despite the widespread use of informants, little is known about the circumstances surrounding their use at trial. This study content-analyzed trials from 22 DNA exoneration cases involving 53 informants. Because these defendants were exonerated, the prosecution informant testimony is demonstrably false. Informant characteristics including motivation for testifying, criminal history, relationship with the defendant and testimony were coded. Most informants were prosecution jailhouse informants; however, there were also defence jailhouse informants and prosecution cooperating witnesses. Regardless of informant type, most denied receiving an incentive, had criminal histories, were friends/acquaintances of the defendant and had testimonial inconsistencies. In closing statements, attorneys relied on informant testimony by either emphasizing or questioning its reliability. The impact of informant testimony on jurors’ decisions is discussed in terms of truth-default theory (TDT), the fundamental attribution error and prosecutorial vouching
Noncollinear magnetic ordering in small Chromium Clusters
We investigate noncollinear effects in antiferromagnetically coupled clusters
using the general, rotationally invariant form of local spin-density theory.
The coupling to the electronic degrees of freedom is treated with relativistic
non-local pseudopotentials and the ionic structure is optimized by Monte-Carlo
techniques. We find that small chromium clusters (N \le 13) strongly favor
noncollinear configurations of their local magnetic moments due to frustration.
This effect is associated with a significantly lower total magnetization of the
noncollinear ground states, ameliorating the disagreement between Stern-Gerlach
measurements and previous collinear calculations for Cr_{12} and Cr_{13}. Our
results further suggest that the trend to noncollinear configurations might be
a feature common to most antiferromagnetic clusters.Comment: 9 pages, RevTeX plus .eps/.ps figure
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