15,058 research outputs found
F-14 modeling study
Preliminary results in the application of a closed loop pilot/simulator model to the analysis of some simulator fidelity issues are discussed in the context of an air to air target tracking task. The closed loop model is described briefly. Then, problem simplifications that are employed to reduce computational costs are discussed. Finally, model results showing sensitivity of performance to various assumptions concerning the simulator and/or the pilot are presented
On the attenuation coefficient of monomode periodic waveguides
It is widely accepted that, on ensemble average, the transmission T of guided
modes decays exponentially with the waveguide length L due to small
imperfections, leading to the important figure of merit defined as the
attenuation-rate coefficient alpha = -/L. In this letter, we evidence
that the exponential-damping law is not valid in general for periodic monomode
waveguides, especially as the group velocity decreases. This result that
contradicts common beliefs and experimental practices aiming at measuring alpha
is supported by a theoretical study of light transport in the limit of very
small imperfections, and by numerical results obtained for two waveguide
geometries that offer contrasted damping behaviours
Whispering Gallery States of Antihydrogen
We study theoretically interference of the long-living quasistationary
quantum states of antihydrogen atoms, localized near a concave material
surface. Such states are an antimatter analog of the whispering gallery states
of neutrons and matter atoms, and similar to the whispering gallery modes of
sound and electro-magnetic waves. Quantum states of antihydrogen are formed by
the combined effect of quantum reflection from van der Waals/Casimir-Polder
(vdW/CP) potential of the surface and the centrifugal potential. We point out a
method for precision studies of quantum reflection of antiatoms from vdW/CP
potential; this method uses interference of the whispering gallery states of
antihydrogen.Comment: 13 pages 7 figure
On the High--Velocity Ejecta of the Type Ia Supernova 1994D
Synthetic spectra generated with the parameterized supernova
synthetic-spectrum code SYNOW are compared to spectra of the Type Ia SN 1994D
that were obtained before the time of maximum brightness. Evidence is found for
the presence of two-component Fe II and Ca II features, forming in high
velocity ( \kms) and lower velocity ( \kms) matter.
Possible interpretations of these spectral splits, and implications for using
early--time spectra of SNe Ia to probe the metallicity of the progenitor white
dwarf and the nature of the nuclear burning front in the outer layers of the
explosion, are discussed.Comment: 15 pages, 3 figures, 3 tables, Astrophysical Journal, in pres
Depositional Environment of Coral–Rudist Associations in the Upper Cretaceous Cardenas Formation (Central Mexico)
In the Cardenas Formation (central Mexico), a 175 m thick sedimentary sequence of Maastrichtian age was analyzed with respect to its palaeontology and sedimentology. A wide variety of lithological and palaeontological features characterize this sequence comprising unfossiliferous and fossil-bearing sand- and siltstones, and diverse rudist and coral–rudist associations in carbonate or mixed carbonate/clastic lithologies. A total of 24 rudist and coral–rudist associations are exposed in the investigated section, which are grouped into 5 limestone units. Radiolitid assemblages, coral–rudist reefs, coral-dominated reefs, and hippuritid-dominated reefs are present. The stacking pattern of these reef intervals indicates a general transgressive trend through the entire section. Smaller-scale facies trends could be distinguished within each limestone unit, comprising deepening-upward sequences, defined by a shoreface–calcareous algae–radiolitid–marl facies transition, and shallowing-upward sequences defined by a hippuritid–actaeonellid–coral/rudist facies transition. This cyclic sedimentation pattern is obscured by an episodic input of clastic sediments derived from the uplifting Sierra Madre Oriental, which in turn triggered either the development or decline of reefs
Simulations of snow distribution and hydrology in a mountain basin
We applied a version of the Regional Hydro‐Ecologic Simulation System (RHESSys) that implements snow redistribution, elevation partitioning, and wind‐driven sublimation to Loch Vale Watershed (LVWS), an alpine‐subalpine Rocky Mountain catchment where snow accumulation and ablation dominate the hydrologic cycle. We compared simulated discharge to measured discharge and the simulated snow distribution to photogrammetrically rectified aerial (remotely sensed) images. Snow redistribution was governed by a topographic similarity index. We subdivided each hillslope into elevation bands that had homogeneous climate extrapolated from observed climate. We created a distributed wind speed field that was used in conjunction with daily measured wind speeds to estimate sublimation. Modeling snow redistribution was critical to estimating the timing and magnitude of discharge. Incorporating elevation partitioning improved estimated timing of discharge but did not improve patterns of snow cover since wind was the dominant controller of areal snow patterns. Simulating wind‐driven sublimation was necessary to predict moisture losses
A model for human controller remnant Final report
Model for predicting human controller remnant due to underlying psychophysical sources in single display control situation
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Elevated fetal steroidogenic activity in autism
Autism affects males more than females, giving rise to the idea that the influence of steroid hormones on early fetal brain development may be one important early biological risk factor. Utilizing the Danish Historic Birth Cohort and Danish Psychiatric Central Register, we identified all amniotic fluid samples of males born between 1993 and 1999 who later received ICD-10 (International Classification of Diseases, 10th Revision) diagnoses of autism, Asperger syndrome or PDD-NOS (pervasive developmental disorder not otherwise specified) (n=128) compared with matched typically developing controls. Concentration levels of Δ4 sex steroids (progesterone, 17α-hydroxy-progesterone, androstenedione and testosterone) and cortisol were measured with liquid chromatography tandem mass spectrometry. All hormones were positively associated with each other and principal component analysis confirmed that one generalized latent steroidogenic factor was driving much of the variation in the data. The autism group showed elevations across all hormones on this latent generalized steroidogenic factor (Cohen's d=0.37, P=0.0009) and this elevation was uniform across ICD-10 diagnostic label. These results provide the first direct evidence of elevated fetal steroidogenic activity in autism. Such elevations may be important as epigenetic fetal programming mechanisms and may interact with other important pathophysiological factors in autism
Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering
The phonon dispersion was measured at room temperature along (0,0,L) in the
tetragonal phase of LaFeAsO using inelastic x-ray scattering. Spin-polarized
first-principles calculations imposing various types of antiferromagnetic order
are in better agreement with the experimental results than nonmagnetic
calculations, although the measurements were made well above the magnetic
ordering temperature, T_N. Splitting observed between two A_{1g} phonon modes
at 22 and 26 meV is only observed in spin-polarized calculations.
Magneto-structural effects similar to those observed in the AFe_2As_2 materials
are confirmed present in LaFeAsO. The presence of Fe-spin is necessary to find
reasonable agreement of the calculations with the measured spectrum well above
T_N. On-site Fe and As force constants show significant softening compared to
nonmagnetic calculations, however an investigation of the real-space force
constants associates the magnetoelastic coupling with a complex renormalization
instead of softening of a specific pairwise force.Comment: 7 pages, 4 figure
Softening of Cu-O bond stretching phonon in tetragonal HgBaCuO
Phonons in nearly optimally doped HgBaCuO were studied by
inelastic X-ray scattering. The dispersion of the low energy modes is well
described by a shell model, while the Cu-O bond stretching mode at high energy
shows strong softening towards the zone boundary, which deviates strongly from
the model. This seems to be common in the hole-doped high-
superconducting cuprates, and, based on this work, not related to a lattice
distortion specific to each material.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Let
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