62,534 research outputs found
Transverse modulational instability of partially incoherent soliton stripes
Based on the Wigner distribution approach, an analysis of the effect of
partial incoherence on the transverse instability of soliton structures in
nonlinear Kerr media is presented. It is explicitly shown, that for a
Lorentzian incoherence spectrum the partial incoherence gives rise to a damping
which counteracts, and tends to suppress, the transverse instability growth.
However, the general picture is more complicated and it is shown that the
effect of the partial incoherence depends crucially on the form of the
incoherence spectrum. In fact, for spectra with finite rms-width, the partial
incoherence may even increase both the growth rate and the range of unstable,
transverse wave numbers.Comment: 5 pages, submitted to Phys. Rev.
Comparative utility of LANDSAT-1 and Skylab data for coastal wetland mapping and ecological studies
Skylab 190-A photography and LANDSAT-1 analog data have been analyzed to determine coastal wetland mapping potential as a near term substitute for aircraft data and as a long term monitoring tool. The level of detail and accuracy of each was compared. Skylab data provides more accurate classification of wetland types, better delineation of freshwater marshes and more detailed analysis of drainage patterns. LANDSAT-1 analog data is useful for general classification, boundary definition and monitoring of human impact in wetlands
Trapping of Cold Excitons with Laser Light
Optical trapping and manipulation of neutral particles has led to a variety
of experiments from stretching DNA-molecules to trapping and cooling of neutral
atoms. An exciting recent outgrowth of the technique is an experimental
implementation of atom Bose-Einstein condensation. In this paper, we propose
and demonstrate laser induced trapping for a new system--a gas of excitons in
quantum well structures. We report on the trapping of a highly degenerate Bose
gas of excitons in laser induced traps.Comment: 9 pages, 3 figure
Skylab-EREP investigations of wetlands ecology
There are no author-identified significant results in this report
Skylab - EREP investigations of wetlands ecology
There are no author-identified significant results in this report
Untangling the Recombination Line Emission from HII Regions with Multiple Velocity Components
HII regions are the ionized spheres surrounding high-mass stars. They are
ideal targets for tracing Galactic structure because they are predominantly
found in spiral arms and have high luminosities at infrared and radio
wavelengths. In the Green Bank Telescope HII Region Discovery Survey (GBT HRDS)
we found that >30% of first Galactic quadrant HII regions have multiple
hydrogen radio recombination line (RRL) velocities, which makes determining
their Galactic locations and physical properties impossible. Here we make
additional GBT RRL observations to determine the discrete HII region velocity
for all 117 multiple-velocity sources within 18deg. < l < 65deg. The
multiple-velocity sources are concentrated in the zone 22deg. < l < 32deg.,
coinciding with the largest regions of massive star formation, which implies
that the diffuse emission is caused by leaked ionizing photons. We combine our
observations with analyses of the electron temperature, molecular gas, and
carbon recombination lines to determine the source velocities for 103 discrete
H II regions (88% of the sample). With the source velocities known, we resolve
the kinematic distance ambiguity for 47 regions, and thus determine their
heliocentric distances.Comment: 44 pages, 5 figures, 16 pages of tables; Accepted by ApJ
Mapping Atlantic coastal marshlands, Maryland, Georgia, using ERTS-1 imagery
Eastern coastal marshes are the most extensive and productive in the United States. A relatively low cost, moderately accurate method is needed to map these areas for management and protection. Groundbased and low-altitude aircraft methods for mapping are time-consuming and quite expensive. The launch of NASA's Earth Resources Technology Satellite has provided an opportunity to test the feasibility of mapping wetlands using small scale imagery. The test sites selected were in Chesapeake Bay, Maryland, and Ossabaw Island, Georgia. Results of the investigation indicate that the following may be ascertained from ERTS imagery, enlarged to 1:250,000: (1) upper wetland boundary; (2) drainage pattern in the wetland; (3) plant communities; (4) ditching activities associated with agriculture; and (5) lagooning for water-side home development. Conclusions are that ERTS will be an excellent tool for many types of coastal wetland mapping
Universality in Glassy Low-Temperature Physics
We propose a microscopic translationally invariant glass model which exhibits
two level tunneling systems with a broad range of asymmetries and barrier
heights in its glassy phase. Their distribution is qualitatively different from
what is commonly assumed in phenomenological models, in that symmetric
tunneling systems are systematically suppressed. Still, the model exhibits the
usual glassy low-temperature anomalies. Universality is due to the collective
origin of the glassy potential energy landscape. We obtain a simple explanation
also for the mysterious {\em quantitative} universality expressed in the
unusually narrow universal glassy range of values for the internal friction
plateau.Comment: 4 pages, 5 figures, uses RevTeX
Internal and external axial corner flows
The inviscid, internal, and external axial corner flows generated by two intersecting wedges traveling supersonically are obtained by use of a second-order shock-capturing, finite-difference approach. The governing equations are solved iteratively in conical coordinates to yield the complicated wave structure of the internal corner and the simple peripheral shock of the external corner. The numerical results for the internal flows compare favorably with existing experimental data
The Green Bank Telescope H II Region Discovery Survey: IV. Helium and Carbon Recombination Lines
The Green Bank Telescope H II Region Discovery Survey (GBT HRDS) found
hundreds of previously unknown Galactic regions of massive star formation by
detecting hydrogen radio recombination line (RRL) emission from candidate H II
region targets. Since the HRDS nebulae lie at large distances from the Sun,
they are located in previously unprobed zones of the Galactic disk. Here we
derive the properties of helium and carbon RRL emission from HRDS nebulae. Our
target sample is the subset of the HRDS that has visible helium or carbon RRLs.
This criterion gives a total of 84 velocity components (14% of the HRDS) with
helium emission and 52 (9%) with carbon emission. For our highest quality
sources, the average ionic He-4+/H+ abundance ratio by number, , is 0.068
+/- 0.023 (1-sigma). This is the same ratio as that measured for the sample of
previously known Galactic H II regions. Nebulae without detected helium
emission give robust y+ upper limits. There are 5 RRL emission components with
y+ less than 0.04 and another 12 with upper limits below this value. These H II
regions must have either a very low He-4 abundance or contain a significant
amount of neutral helium. The HRDS has 20 nebulae with carbon RRL emission but
no helium emission at its sensitivity level. There is no correlation between
the carbon RRL parameters and the 8 microns mid-infrared morphology of these
nebulae.Comment: Accepted to ApJ. The survey website can be found here:
http://go.nrao.edu/hrd
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