26,207 research outputs found
I. Collisional evolution and reddening of asteroid surfaces: The problem of conflicting timescales and the role of size-dependent effects
Space weathering is the generic term used for processes that modify the
optical properties of surfaces of atmosphereless rocky bodies under exposure to
the space environment. The general agreement about the relevance of the effects
of space weathering on the spectral properties of S-complex asteroids fails
when some basic quantitative estimates are attempted. In particular, there is
severe disagreement regarding the typical timescales for significant spectral
reddening to occur, ranging from 1 Myr to 1 Gyr. Generally speaking, the
spectral reddening of an individual object can be considered as the sum of
three terms, one (which is relevant for statistical analyses) depending on the
exposure of the object to space weathering during its lifetime, a second one
due to the original surface composition, and a third one (a "noise" term) due
to the combination of poorly constrained effects (e.g., structure and texture
of the surface). The surface of an asteroid is usually covered by regolith, and
its presence and properties presumably play a critical role in the weathering
processes. In this paper we discuss the role played by collisional evolution in
affecting the spectral properties of asteroids and refreshing the surfaces due
to the formation of ejecta, and the necessity of a simultaneous modeling of
collisions and weathering processes. We introduce a new idea, based on the
possibility of a sort of saturation of the refreshing process whenever a
massive reaccumulation of the impact ejecta takes place. In this case, a
dependence of the overall reddening on the asteroid size should naturally come
out. We show that this conclusion is indeed supported by available main belt
asteroid spectroscopic data.Comment: Accepted by MNRA
Asymptotic solution of a model for bilayer organic diodes and solar cells
The current voltage characteristics of an organic semiconductor diode made by placing together two materials with dissimilar electron affinities and ionisation potentials is analysed using asymptotic methods. An intricate boundary layer structure is examined. We find that there are three regimes for the total current passing through the diode. For reverse bias and moderate forward bias the dependency of the voltage on the current is similar to the behaviour of conventional inorganic semiconductor diodes predicted by the Shockley equation and are governed by recombination at the interface of the materials. There is then a narrow range of currents where the behaviour undergoes a transition. Finally for large forward bias the behaviour is different with the current being linear in voltage and is primarily controlled by drift of charges in the organic layers. The size of the interfacial recombination rate is critical in determining the small range of current where there is rapid transition between the two main regimes. The extension of the theory to organic solar cells is discussed and the analogous current voltage curves derived in the regime of interest
A reconfigurable optical header recognition system for optical packet routing applications
We demonstrate a reconfigurable all-optical packet processing system. The key device is a code-reconfigurable header decoder based on a fiber Bragg grating. The performance of the system is tested for different packet headers, and error-free operation is confirmed
Integrating multiple representations: fighting asthma
This paper seeks to engage debates about integrating pluralisms regarding multiple forms/representations and how they might function smoothly if they are closely aligned. This paper offers, narrative poetry with an artistic impression aimed at seeing how these might interact with each other. Like poetry, visual images are unique and can evoke particular kinds of emotional and visceral responses. By offering narrative poetry together with an artistic representation it is not meant to de-value the importance of either, but it is aimed at seeing how these arts-based methods and creative analytical practices might unite as a narrative to offer knew ways of ‘knowing’ and ‘seeing
Methods of editing cloud and atmospheric layer affected pixels from satellite data
Subvisible cirrus clouds (SCi) were easily distinguished in mid-infrared (MIR) TIROS-N daytime data from south Texas and northeast Mexico. The MIR (3.55-3.93 micrometer) pixel digital count means of the SCi affected areas were more than 3.5 standard deviations on the cold side of the scene means. (These standard deviations were made free of the effects of unusual instrument error by factoring out the Ch 3 MIR noise on the basis of detailed examination of noisy and noise-free pixels). SCi affected areas in the IR Ch 4 (10.5-11.5 micrometer) appeared cooler than the general scene, but were not as prominent as in Ch 3, being less than 2 standard deviations from the scene mean. Ch 3 and 4 standard deviations and coefficients of variation are not reliable indicators, by themselves, of the presence of SCi because land features can have similar statistical properties
Development of a carbon fibre composite active mirror: Design and testing
Carbon fibre composite technology for lightweight mirrors is gaining
increasing interest in the space- and ground-based astronomical communities for
its low weight, ease of manufacturing, excellent thermal qualities and
robustness. We present here first results of a project to design and produce a
27 cm diameter deformable carbon fibre composite mirror. The aim was to produce
a high surface form accuracy as well as low surface roughness. As part of this
programme, a passive mirror was developed to investigate stability and coating
issues. Results from the manufacturing and polishing process are reported here.
We also present results of a mechanical and thermal finite element analysis, as
well as early experimental findings of the deformable mirror. Possible
applications and future work are discussed.Comment: Accepted by Optical Engineering. Figures 1-7 on
http://www.star.ucl.ac.uk/~sk/OEpaper_files
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