21,223 research outputs found
Inference of historical population-size changes with allele-frequency data
With up to millions of nearly neutral polymorphisms now being routinely sampled in population-genomic surveys, it is possible to estimate the site-frequency spectrum of such sites with high precision. Each frequency class reflects a mixture of potentially unique demographic histories, which can be revealed using theory for the probability distributions of the starting and ending points of branch segments over all possible coalescence trees. Such distributions are completely independent of past population history, which only influences the segment lengths, providing the basis for estimating average population sizes separating tree-wide coalescence events. The history of population-size change experienced by a sample of polymorphisms can then be dissected in a model-flexible fashion, and extension of this theory allows estimation of the mean and full distribution of long-term effective population sizes and ages of alleles of specific frequencies. Here, we outline the basic theory underlying the conceptual approach, develop and test an efficient statistical procedure for parameter estimation, and apply this to multiple population-genomic datasets for the microcrustacean Daphnia pulex
Magnetic-Island Contraction and Particle Acceleration in Simulated Eruptive Solar Flares
The mechanism that accelerates particles to the energies required to produce
the observed high-energy impulsive emission in solar flares is not well
understood. Drake et al. (2006) proposed a mechanism for accelerating electrons
in contracting magnetic islands formed by kinetic reconnection in multi-layered
current sheets. We apply these ideas to sunward-moving flux ropes (2.5D
magnetic islands) formed during fast reconnection in a simulated eruptive
flare. A simple analytic model is used to calculate the energy gain of
particles orbiting the field lines of the contracting magnetic islands in our
ultrahigh-resolution 2.5D numerical simulation. We find that the estimated
energy gains in a single island range up to a factor of five. This is higher
than that found by Drake et al. for islands in the terrestrial magnetosphere
and at the heliopause, due to strong plasma compression that occurs at the
flare current sheet. In order to increase their energy by two orders of
magnitude and plausibly account for the observed high-energy flare emission,
the electrons must visit multiple contracting islands. This mechanism should
produce sporadic emission because island formation is intermittent. Moreover, a
large number of particles could be accelerated in each
magnetohydrodynamic-scale island, which may explain the inferred rates of
energetic-electron production in flares. We conclude that island contraction in
the flare current sheet is a promising candidate for electron acceleration in
solar eruptions.Comment: Accepted for publication in The Astrophysical Journal (2016
The Helicopter Antenna Radiation Prediction Code (HARP)
The first nine months effort in the development of a user oriented computer code, referred to as the HARP code, for analyzing the radiation from helicopter antennas is described. The HARP code uses modern computer graphics to aid in the description and display of the helicopter geometry. At low frequencies the helicopter is modeled by polygonal plates, and the method of moments is used to compute the desired patterns. At high frequencies the helicopter is modeled by a composite ellipsoid and flat plates, and computations are made using the geometrical theory of diffraction. The HARP code will provide a user friendly interface, employing modern computer graphics, to aid the user to describe the helicopter geometry, select the method of computation, construct the desired high or low frequency model, and display the results
The S2 VLBI Correlator: A Correlator for Space VLBI and Geodetic Signal Processing
We describe the design of a correlator system for ground and space-based
VLBI. The correlator contains unique signal processing functions: flexible LO
frequency switching for bandwidth synthesis; 1 ms dump intervals, multi-rate
digital signal-processing techniques to allow correlation of signals at
different sample rates; and a digital filter for very high resolution
cross-power spectra. It also includes autocorrelation, tone extraction, pulsar
gating, signal-statistics accumulation.Comment: 44 pages, 13 figure
Tidal effects and the Proximity decay of nuclei
We examine the decay of the 3.03 MeV state of Be evaporated from an
excited projectile-like fragment following a peripheral heavy-ion collision.
The relative energy of the daughter particles exhibits a dependence on
the decay angle of the Be, indicative of a tidal effect. Comparison of
the measured tidal effect with a purely Coulomb model suggests the influence of
a measurable nuclear proximity interaction.Comment: 5 pages, 4 figure
Mid-Infrared Ethane Emission on Neptune and Uranus
We report 8- to 13-micron spectral observations of Neptune and Uranus from
the NASA Infrared Telescope Facility spanning more than a decade. The
spectroscopic data indicate a steady increase in Neptune's mean atmospheric
12-micron ethane emission from 1985 to 2003, followed by a slight decrease in
2004. The simplest explanation for the intensity variation is an increase in
stratospheric effective temperature from 155 +/- 3 K in 1985 to 176 +/- 3 K in
2003 (an average rate of 1.2 K/year), and subsequent decrease to 165 +/- 3 K in
2004. We also detected variation of the overall spectral structure of the
ethane band, specifically an apparent absorption structure in the central
portion of the band; this structure arises from coarse spectral sampling
coupled with a non-uniform response function within the detector elements. We
also report a probable direct detection of ethane emission on Uranus. The
deduced peak mole fraction is approximately an order of magnitude higher than
previous upper limits for Uranus. The model fit suggests an effective
temperature of 114 +/- 3 K for the globally-averaged stratosphere of Uranus,
which is consistent with recent measurements indicative of seasonal variation.Comment: Accepted for publication in ApJ. 16 pages, 10 figures, 2 table
Resonant and Non-Resonant Effects in Photon-Technipion Production at Lepton Colliders
Lepton collider experiments can search for light technipions in final states
made striking by the presence of an energetic photon: e+e- \to
\photon\technipion. To date, searches have focused on either production
through anomalous coupling of the technipions to electroweak gauge bosons or on
production through a technivector meson (\technirho, \techniomega) resonance.
This paper creates a combined framework in which both contributions are
included. This will allow stronger and more accurate limits on technipion
production to be set using existing data from LEP or future data from a
higher-energy linear collider. We provide explicit formulas and sample
calculations (analytic and Pythia) in the framework of the Technicolor Straw
Man Model, a model that includes light technihadrons.Comment: 11 pages, including title page, 3 figures; version 2: references
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STS in management education: connecting theory and practice
This paper explores the value of science and technology studies (STS) to management education. The work draws on an ethnographic study of second year management undergraduates studying decision making. The nature and delivery of the decision making module is outlined and the value of STS is demonstrated in terms of both teaching method and module content. Three particular STS contributions are identified and described: the social construction of technological systems; actor network theory; and ontological politics. Affordances and sensibilities are identified for each contribution and a discussion is developed that illustrates how these versions of STS are put to use in management education. It is concluded that STS has a pivotal role to play in critical management (education) and in the process offers opportunities for new forms of managin
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