835 research outputs found
Clementine Observations of the Zodiacal Light and the Dust Content of the Inner Solar System
Using the Moon to occult the Sun, the Clementine spacecraft used its
navigation cameras to map the inner zodiacal light at optical wavelengths over
elongations of 3-30 degrees from the Sun. This surface brightness map is then
used to infer the spatial distribution of interplanetary dust over heliocentric
distances of about 10 solar radii to the orbit of Venus. We also apply a simple
model that attributes the zodiacal light as being due to three dust populations
having distinct inclination distributions, namely, dust from asteroids and
Jupiter-family comets (JFCs), dust from Halley-type comets, and an isotropic
cloud of dust from Oort Cloud comets. The best-fitting scenario indicates that
asteroids + JFCs are the source of about 45% of the optical dust cross-section
seen in the ecliptic at 1 AU, but that at least 89% of the dust cross-section
enclosed by a 1 AU radius sphere is of a cometary origin. When these results
are extrapolated out to the asteroid belt, we find an upper limit on the mass
of the light-reflecting asteroidal dust that is equivalent to a 12 km asteroid,
and a similar extrapolation of the isotropic dust cloud out to Oort Cloud
distances yields a mass equivalent to a 30 km comet, although the latter mass
is uncertain by orders of magnitude.Comment: To be published in Icaru
All-optical steering of light via spatial Bloch oscillations in a gas of three-level atoms
A standing-wave control field applied to a three-level atomic medium in a
planar hollow-core photonic crystal waveguide creates periodic variations of
linear and nonlinear refractive indexes of the medium. This property can be
used for efficient steering of light. In this work we study, both analytically
and numerically, the dynamics of probe optical beams in such structures. By
properly designing the spatial dependence of the nonlinearity it is possible to
induce long-living Bloch oscillations of spatial gap solitons, thus providing
desirable change in direction of the beam propagation without inducing
appreciable diffraction. Due to the significant enhancement of the
nonlinearity, such self-focusing of the probe beam can be reached at extremely
weak light intensities.Comment: 8 pages, 4 figure
Silicon-on ceramic process: Silicon sheet growth and device development for the large-area silicon sheet task of the low-cost solar array project
The technical feasibility of producing solar-cell-quality sheet silicon to meet the Department of Energy (DOE) 1986 overall price goal of $0.70/watt was investigated. With the silicon-on-ceramic (SOC) approach, a low-cost ceramic substrate is coated with large-grain polycrystalline silicon by unidirectional solidification of molten silicon. This effort was divided into several areas of investigation in order to most efficiently meet the goals of the program. These areas include: (1) dip-coating; (2) continuous coating designated SCIM-coating, and acronym for Silicon Coating by an Inverted Meniscus (SCIM); (3) material characterization; (4) cell fabrication and evaluation; and (5) theoretical analysis. Both coating approaches were successful in producing thin layers of large grain, solar-cell-quality silicon. The dip-coating approach was initially investigated and considerable effort was given to this technique. The SCIM technique was adopted because of its scale-up potential and its capability to produce more conventiently large areas of SOC
Origins of Solar System Dust Beyond Jupiter
The measurements of cosmic interplanetary dust by the instruments on board the Pioneer 10 and 11 spacecraft contain the dynamical signature of dust generated by Edgeworth-Kuiper Belt objects, as well as short period Oort Cloud comets and short period Jupiter family comets. While the dust concentration detected between Jupiter and Saturn is mainly due to the cometary components, the dust outside Saturn's orbit is dominated by grains originating from the Edgeworth-Kuiper Belt. In order to sustain a dust concentration that accounts for the Pioneer measurements, short period external Jupiter family comets, on orbits similar to comet 29P/Schwassmann-Wachmann-1, have to produce of dust grains with sizes between 0.01 and . A sustained production rate of has to be provided by short period Oort cloud comets on 1P/Halley-like orbits. The comets can not, however, account for the dust flux measured outside Saturn's orbit. The measurements there can only be explained by a generation of dust grains in the Edgeworth-Kuiper belt by mutual collisions of the source objects and by impacts of interstellar dust grains onto the objects' surfaces. These processes have to release in total of dust from the Edgeworth Kuiper belt objects in order to account for the amount of dust found by Pioneer beyond Saturn, making the Edgeworth-Kuiper disk the brightest extended feature of the Solar System when observed from afar
The solar maximum satellite capture cell: Impact features and orbital debris and micrometeoritic projectile materials
The physical properties of impact features observed in the Solar Max main electronics box (MEB) thermal blanket generally suggest an origin by hypervelocity impact. The chemistry of micrometeorite material suggests that a wide variety of projectile materials have survived impact with retention of varying degrees of pristinity. Impact features that contain only spacecraft paint particles are on average smaller than impact features caused by micrometeorite impacts. In case both types of materials co-occur, it is belevied that the impact feature, generally a penetration hole, was caused by a micrometeorite projectile. The typically smaller paint particles were able to penetrate though the hole in the first layer and deposit in the spray pattern on the second layer. It is suggested that paint particles have arrived with a wide range of velocities relative to the Solar Max satellite. Orbiting paint particles are an important fraction of materials in the near-Earth environment. In general, the data from the Solar Max studies are a good calibration for the design of capture cells to be flown in space and on board Space Station. The data also suggest that development of multiple layer capture cells in which the projectile may retain a large degree of pristinity is a feasible goal
The Geography of Scientific Productivity: Scaling in U.S. Computer Science
Here we extract the geographical addresses of authors in the Citeseer
database of computer science papers. We show that the productivity of research
centres in the United States follows a power-law regime, apart from the most
productive centres for which we do not have enough data to reach definite
conclusions. To investigate the spatial distribution of computer science
research centres in the United States, we compute the two-point correlation
function of the spatial point process and show that the observed power-laws do
not disappear even when we change the physical representation from geographical
space to cartogram space. Our work suggests that the effect of physical
location poses a challenge to ongoing efforts to develop realistic models of
scientific productivity. We propose that the introduction of a fine scale
geography may lead to more sophisticated indicators of scientific output.Comment: 6 pages, 3 figures; minor change
Unusual Flaring Activity in the Blazar PKS 1424-418 during 2008-2011
Context. Blazars are a subset of active galactic nuclei (AGN) with jets that
are oriented along our line of sight. Variability and spectral energy
distribution (SED) studies are crucial tools for understanding the physical
processes responsible for observed AGN emission.
Aims. We report peculiar behaviour in the bright gamma-ray blazar PKS
1424-418 and use its strong variability to reveal information about the
particle acceleration and interactions in the jet. Methods. Correlation
analysis of the extensive optical coverage by the ATOM telescope and nearly
continuous gamma-ray coverage by the Fermi Large Area Telescope is combined
with broadband, time-dependent modeling of the SED incorporating supplemental
information from radio and X-ray observations of this blazar.
Results. We analyse in detail four bright phases at optical-GeV energies.
These flares of PKS 1424-418 show high correlation between these energy ranges,
with the exception of one large optical flare that coincides with relatively
low gamma-ray activity. Although the optical/gamma-ray behaviour of PKS
1424-418 shows variety, the multiwavelength modeling indicates that these
differences can largely be explained by changes in the flux and energy spectrum
of the electrons in the jet that are radiating. We find that for all flares the
SED is adequately represented by a leptonic model that includes inverse Compton
emission from external radiation fields with similar parameters.
Conclusions. Detailed studies of individual blazars like PKS 1424-418 during
periods of enhanced activity in different wavebands are helping us identify
underlying patterns in the physical parameters in this class of AGN.Comment: accepted for publication in A&
Structure of the Edgeworth-Kuiper Belt (EKB) Dust Disk and Implications for Extrasolar Planet(s) epsilon Eridani
Numerical simulations of the orbital evolution of dust particles from Edgeworth-Kuiper Belt (EKB) objects show that the three giant planets, Neptune, Jupiter, and Saturn impose distinct and dramatic signatures on the overall distribution of EKB dust particles. The features are very similar to those observed in the dust disk around the nearby star Eridani. Numerical simulations of dust particles in the epsilon Eridani system show that planetary perturbations may be responsible for the observed feature
Dip coating process: Silicon sheet growth development for the large-area silicon sheet task of the low-cost silicon solar array project
The technical and economic feasibility of producing solar cell quality sheet silicon by dip-coating one surface of carbonized ceramic substrates with a thin layer of large grain polycrystalline silicon was investigated. The dip-coating methods studied were directed toward a minimum cost process with the ultimate objective of producing solar cells with a conversion efficiency of 10% or greater. The technique shows excellent promise for low cost, labor-saving, scale-up potentialities and would provide an end product of sheet silicon with a rigid and strong supportive backing. An experimental dip-coating facility was designed and constructed, several substrates were successfully dip-coated with areas as large as 25 sq cm and thicknesses of 12 micron to 250 micron. There appears to be no serious limitation on the area of a substrate that could be coated. Of the various substrate materials dip-coated, mullite appears to best satisfy the requirement of the program. An inexpensive process was developed for producing mullite in the desired geometry
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