3,046 research outputs found
Winter Conditions Influence Biological Responses of Migrating Hummingbirds
Conserving biological diversity given ongoing environmental changes requires the knowledge of how organisms respond biologically to these changes; however, we rarely have this information. This data deficiency can be addressed with coordinated monitoring programs that provide field data across temporal and spatial scales and with process-based models, which provide a method for predicting how species, in particular migrating species that face different conditions across their range, will respond to climate change. We evaluate whether environmental conditions in the wintering grounds of broad-tailed hummingbirds influence physiological and behavioral attributes of their migration. To quantify winter ground conditions, we used operative temperature as a proxy for physiological constraint, and precipitation and the normalized difference vegetation index (NDVI) as surrogates of resource availability. We measured four biological response variables: molt stage, timing of arrival at stopover sites, body mass, and fat. Consistent with our predictions, we found that birds migrating north were in earlier stages of molt and arrived at stopover sites later when NDVI was low. These results indicate that wintering conditions impact the timing and condition of birds as they migrate north. In addition, our results suggest that biologically informed environmental surrogates provide a valuable tool for predicting how climate variability across years influences the animal populations
Microflow valve control system design
A design synthesis for a microflow control system is presented based on the interrogation of an analytical model, testing, and observation. The key issues relating to controlling a microflow using a variable geometry flow channel are explored through the implementation and testing of open and closed-loop control systems. The reliance of closed-loop systems on accurate flow measurement and the need for an open-loop strategy are covered. A valve and control system capable of accurately controlling flowrates between 0.09 and 400 ml/h and with a range of 900:1 is demonstrated
Macrospin Models of Spin Transfer Dynamics
The current-induced magnetization dynamics of a spin valve are studied using
a macrospin (single domain) approximation and numerical solutions of a
generalized Landau-Lifshitz-Gilbert equation. For the purpose of quantitative
comparison with experiment [Kiselev {\it et al.} Nature {\bf 425}, 380 (2003)],
we calculate the resistance and microwave power as a function of current and
external field including the effects of anisotropies, damping, spin-transfer
torque, thermal fluctuations, spin-pumping, and incomplete absorption of
transverse spin current. While many features of experiment appear in the
simulations, there are two significant discrepancies: the current dependence of
the precession frequency and the presence/absence of a microwave quiet magnetic
phase with a distinct magnetoresistance signature. Comparison is made with
micromagnetic simulations designed to model the same experiment.Comment: 14 pages, 14 figures. Email [email protected] for a
pdf with higher quality figure
The Variation of Sound Intensity with Distance from the Source; An Interesting Case of Deviation from the Inverse Square Law
The phenomena of sound present many interesting features and diffraction is not the least. The writers have been able, by an extension of Rayleigh\u27s theory, to determine the effect in the neighborhood of a rigid sphere produced by a source of sound located on that sphere. One of the results of this theoretical investigation is the establishment of the variation of the intensity of sound with the distance from the center of the sphere. The computations necessary for approximate numerical results are tediously long, and those given herewith are merely those that were readily available in other investigations of the writers
Studies of Eastern Iowa Spiders
The spiders of Eastern Iowa included in this study constitute a part of a general geographical and seasonal survey of the spiders of Iowa. The majority of them have been collected by the authors, although much assistance has been given by others.1 For the first time some spiders have been collected from the woodland floor by the sifting of dead leaves and other debris. Most of them have been found in Linn County, but there are also species from Wapello, Henry, Muscatine, Pottawattamie, Black Hawk, Clayton, Louisa, and Washington counties. Of the ninety-four different spiders here reported, thirty-five species and ten families are represented. Twelve are males, seventy-seven females, and five immature spiders. For the first time, specimens of the family Gnaphosidae have been added to our collection. More species have been found of the orb weavers, the Argiopidae, than of any other family
The first North American records of the synanthropic spider \u3ci\u3eCithaeron praedonius\u3c/i\u3e O. P.-Cambridge (Araneae: Gnaphosoidea: Cithaeronidae), with notes on its biology
Cithaeron praedonius O. P.-Cambridge 1872 (Araneae: Gnaphosoidea: Cithaeronidae) is an Old World species with a distribution from The Gambia, western Africa, and Greece to Malaysia and Australia. In the New World, it was recently found in Brazil, and is now reported for the first time in North America, in the United States. Multiple individuals of both sexes and various life stages, including multiple eggsacs, have been found in a home in Port Richey, Pasco County, Florida. An adult female was found on the outside wall of the house feeding on another spider, suggesting that C. praedonius are no longer contained as a spot introduction in this one house. Observations in captivity indicate that this species may prefer feeding on other spiders. The eggsac and molting nest are described for the first time, and the first records on fecundity are reported
Ideal Spin Filters: Theoretical Study of Electron Transmission Through Ordered and Disordered Interfaces Between Ferromagnetic Metals and Semiconductors
It is predicted that certain atomically ordered interfaces between some
ferromagnetic metals (F) and semiconductors (S) should act as ideal spin
filters that transmit electrons only from the majority spin bands or only from
the minority spin bands of the F to the S at the Fermi energy, even for F with
both majority and minority bands at the Fermi level. Criteria for determining
which combinations of F, S and interface should be ideal spin filters are
formulated. The criteria depend only on the bulk band structures of the S and F
and on the translational symmetries of the S, F and interface. Several examples
of systems that meet these criteria to a high degree of precision are
identified. Disordered interfaces between F and S are also studied and it is
found that intermixing between the S and F can result in interfaces with spin
anti-filtering properties, the transmitted electrons being much less spin
polarized than those in the ferromagnetic metal at the Fermi energy. A patent
application based on this work has been commenced by Simon Fraser University.Comment: RevTeX, 12 pages, 5 figure
Spin Pumping of Current in Non-Uniform Conducting Magnets
Using irreversible thermodynamics we show that current-induced spin transfer
torque within a magnetic domain implies spin pumping of current within that
domain. This has experimental implications for samples both with conducting
leads and that are electrically isolated. These results are obtained by
deriving the dynamical equations for two models of non-uniform conducting
magnets: (1) a generic conducting magnet, with net conduction electron density
n and net magnetization ; and (2) a two-band magnet, with up and down
spins each providing conduction and magnetism. For both models, in regions
where the equilibrium magnetization is non-uniform, voltage gradients can drive
adiabatic and non-adiabatic bulk spin torques. Onsager relations then ensure
that magnetic torques likewise drive adiabatic and non-adiabatic currents --
what we call bulk spin pumping. For a given amount of adiabatic and
non-adiabatic spin torque, the two models yield similar but distinct results
for the bulk spin pumping, thus distinguishing the two models. As in the recent
spin-Berry phase study by Barnes and Maekawa, we find that within a domain wall
the ratio of the effective emf to the magnetic field is approximately given by
, where P is the spin polarization. The adiabatic spin torque
and spin pumping terms are shown to be dissipative in nature.Comment: 13 pages in pdf format; 1 figur
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