10,737 research outputs found
Development and application of a non-Gaussian atmospheric turbulence model for use in flight simulators
A method is described for generating time histories which model the frequency content and certain non-Gaussian probability characteristics of atmospheric turbulence including the large gusts and patchy nature of turbulence. Methods for time histories using either analog or digital computation are described. A STOL airplane was programmed into a 6-degree-of-freedom flight simulator, and turbulence time histories from several atmospheric turbulence models were introduced. The pilots' reactions are described
Universal error corrections for finite semiconductor resistivity in cross-kelvin resistor test structures
The Cross-Kelvin Resistor test structure is commonly used for the extraction of the specific contact resistance of ohmic contacts. Analysis using this structure are generally based on a two-dimensional model that assumes zero voltage drop in the semiconductor layer in the direction normal to the plane of the contact. This paper uses a three-dimensional (3-D) analysis to show the magnitude of the errors introduced by this assumption, and illustrates the conditions under which a 3-D analysis should be used. This paper presents for the first time 3-D universal error correction curves that account for the vertical voltage drop due to the finite depth of the semiconductor laye
Finite element modeling of misalignment in interconnect vias
Electrical resistance and hence heat generation in semiconductor chips are becoming more significant issues particularily as generations of silicon devices continue to have smaller features. The resistance of interconnect vias is a significant source of heat generation because of the increasing number of these on chips and increases in via resistance due to reduced size. Finite element modeling of voltage drops and current flow through interconnect vias gives information to aid in designing geometry and materials used in forming vias. It can also be used for modeling the thermal distribution in a via and hence the contribution by vias to heating a chip. In this paper we examine the effect of misalignment of the via between the two metal layers M1 and M2 with regard to the interconnect via resistance. The effect of the interface specific contact resistance is examined in particular. Significant misalignment can be tolerated without increasing the via resistance. The heat generation due to electrical current flow in the via materials and interfaces is modelled using the samefinite element mesh and software. The output of the electrical analysis is used as the heat generation input for the therml analysis
Fe K emission and absorption features in XMM-Newton spectra of Mkn 766 - evidence for reprocessing in flare ejecta
We report on the analysis of a long XMM-Newton EPIC observation in 2001 May
of the Narrow Line Seyfert 1 galaxy Mkn 766. The 3-11 keV spectrum exhibits a
moderately steep power law continuum, with a broad emission line at ~6.7 keV,
probably blended with a narrow line at ~6.4 keV, and a broad absorption trough
above ~8.7 keV. We identify both broad spectral features with reprocessing in
He-like Fe. An earlier XMM-Newton observation of Mkn 766 in 2000 May, when the
source was a factor ~2 fainter, shows a similar broad emission line, but with a
slightly flatter power law and absorption at a lower energy. In neither
observation do we find a requirement for the previously reported broad 'red
wing' to the line and hence of reflection from the innermost accretion disc.
More detailed examination of the longer XMM-Newton observation reveals evidence
for rapid spectral variability in the Fe K band, apparently linked with the
occurrence of X-ray 'flares'. A reduction in the emission line strength and
increased high energy absorption during the X-ray flaring suggests that these
transient effects are due to highly ionised ejecta associated with the flares.
Simple scaling from the flare avalanche model proposed for the luminous QSO PDS
456 (Reeves etal. 2002) confirms the feasibility of coherent flaring being the
cause of the strong peaks seen in the X-ray light curve of \mkn.Comment: 9 pages, 11 figures, submitted to MNRA
Wheat rust epidemics
Stem rust of wheat is probably the most damaging plant disease in the world. The rust fungus attacks stems, leaves and heads, producing characteristic red-brown pustules which affect the plant\u27s ability to fill out the grain. Yield loss is directly related to the amount of rust in the crop and the length of time the crop is infected.
In Western Australia it is not uncommon to find rust on crops in mid-October, but usually there is negligible loss of yield. In fact this pattern is quite normal throughout the world: In general, rust arrives too late to cause an epidemic. Why is this
Extreme X-ray variability in the luminous quasar PDS 456
We present evidence from Beppo-SAX and XMM-Newton of extreme X-ray
variability in the high luminosity radio-quiet quasar PDS 456, the most
luminous known AGN at z<0.3. Repeated X-ray flaring is found in PDS 456, over
the duration of the 340 ksec long Beppo-SAX observation. The X-ray flux doubles
in just 30 ksec, whilst the total energy output of the flaring events is as
high as 10^51 erg. Under the assumption of isotropic emission at the Eddington
limit, this implies that the size of the X-ray emitting region in PDS 456 is
less than 3 Schwarzschild radii, for a 10^9 solar mass black hole. From the
rates of change of luminosity observed during the X-ray flares, we calculate
lower limits for the radiative efficiency between 0.06 and 0.41, implying that
accretion onto a Kerr black hole is likely in PDS 456. We suggest that the
rapid variability is from X-ray flares produced through magnetic reconnection
above the disc and calculate that the energetics and timescale of the flares
are plausible if the quasar is accreting near to the maximum Eddington rate. A
similar mechanism may account for the extreme rapid X-ray variability observed
in many Narrow Line Seyfert 1s. In the case of PDS 456, we show that the X-ray
flaring could be reproduced through a self-induced cascade of about 1000
individual flares over a timescale of the order 1 day.Comment: 5 pages. Accepted for publication in MNRAS Letter
Comet and Meteorite Traditions of Aboriginal Australians
Of the hundreds of distinct Aboriginal cultures of Australia, many have oral
traditions rich in descriptions and explanations of comets, meteors,
meteorites, airbursts, impact events, and impact craters. These views generally
attribute these phenomena to spirits, death, and bad omens. There are also many
traditions that describe the formation of meteorite craters as well as impact
events that are not known to Western science.Comment: Encyclopaedia of the History of Science, Technology, and Medicine in
Non-Western Cultures, 2014. Edited by Helaine Selin. Springer Netherland
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