1,028,799 research outputs found
Causality-based criteria for a negative refractive index must be used with care
Using the principle of causality as expressed in the Kramers-Kronig
relations, we derive a generalized criterion for a negative refractive index
that admits imperfect transparency at an observation frequency . It
also allows us to relate the global properties of the loss (i.e. its frequency
response) to its local behaviour at . However, causality-based criteria
rely the on the group velocity, not the Poynting vector. Since the two are not
equivalent, we provide some simple examples to compare the two criteria.Comment: slightly longer version of published PR
Properties of extra-planar HI clouds in the outer part of the Milky Way
There is mounting evidence for an extra-planar gas layer around the Milky Way
disk, similar to the anomalous HI gas detected in a few other galaxies. As much
as 10% of the gas may be in this phase. We analyze HI clouds located in the
disk-halo interface outside the solar circle to probe the properties of the
extra-planar HI gas, which is following Galactic rotation. We use the
Leiden/Argentine/Bonn (LAB) 21-cm line survey to search for HI clouds which
take part in the rotation of the Galactic plane, but are located above the disk
layer. Selected regions are mapped with the Effelsberg 100-m telescope. Two of
the HI halo clouds are studied in detail for their small scale structure using
the Westerbork Synthesis Radio Telescope (WSRT). Data from the 100m telescope
allow for the parameterization of 25 distinct HI halo clouds at Galactocentric
radii 10 kpc <R<15 kpc and heights 1 kpc <z<5 kpc. The clouds have a median
temperature of 620 K, column densities of NH~10E19 cm^-2, and most of them are
surrounded by an extended envelope of warmer HI gas. Interferometer
observations for two selected regions resolve the HI clouds into several
arc-minute sized cores. These cores show narrow line widths (FWHM ~3 km/s),
they have volume densities of n > 1.3 cm^-3, masses up to 24 M_{sol}, and are
on average in pressure equilibrium with the surrounding envelopes. Pressures
and densities fall within the expectations from theoretical phase diagrams (P
vs ). The HI cores tend to be unstable if one assumes a thermally
bistable medium, but are in better agreement with models that predict thermal
fragmentation driven by a turbulent flow.Comment: 9 pages, 5 figures, 3 tables, Accepted for publication in A&
Apollo-Soyuz pamphlet no. 3: Sun, stars, in between
The structure of the sun and its surface temperature and brightness are discussed as background for explaining the ASTP joint experiment to photograph the solar corona from Soyuz while the Apollo spacecraft created an artificial eclipse by blocking out the sun. Stellar spectra, stellar evolution, and the Milky Way galaxy are explored in relation to the MA-083 experiment to survey the sky for extreme ultraviolet sources and background radiation. Interstellar gas and the spectrum of helium are discussed in relation to the MA-088 experiment designed to detect interstellar helium entering the solar system and to measure its density and motion
Apollo-Soyuz pamphlet no. 5: The earth from orbit
Astronaut training in the recognition of various geological features from space is described as well as the cameras, lenses and film used in experiment MA-136 to measure their effectiveness in photographing earth structural features from orbit. Aerosols that affect climate and weather are discussed in relation to experiment Ma-007 which relied on infrared observations of the setting or rising sun, as seen from Apollo, to measure the amount of dust and droplets in the lower 150 km of earth's atmosphere. The line spectra of atomic oxygen and nitrogen and their densities at 22 km above the earth's surface are examined along with experiment MA-059 which measured ultraviolet absorption at that altitude
Assessing Aquatic Insect Flight Behavior with Sticky Traps
A suspended sticky trap was designed to analyze flight behavior of aquatic insects, including both direction and vertical distribution of flight. Specifications of trap construction are detailed. Possible applications were explored and preliminary results from the study of a small Indiana stream demonstrated primary upstream migration of females, and indicated vertical partitioning of flight activity at the species level
Apparatus for reducing aerodynamic noise in a wind tunnel
An apparatus is described for reducing the background noise produced by the porous walls of the test section of a wind tunnel. A finely meshed screen member is placed over the perforations in the test section walls. The mesh wire screen attached to the interior wall provides a smoother surface for the air stream to flow against reducing the vorticies produced by the edges of the perforations in the test section walls
Vibration limiting of rotors by feedback control
Experimental findings of a three mass rotor with four channels of feedback control are reported. The channels are independently controllable with force being proportional to the velocity and/or instantaneous displacement from equilibrium of the shaft at the noncontacting probe locations (arranged in the vertical and horizontal attitudes near the support bearings). The findings suggest that automatic feedback control of rotors is feasible for limiting certain vibration levels. Control of one end of a rotor does afford some predictable vibration limiting of the rotor at the other end
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