31,147 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
Transient radiation and conduction in a slotted slab and a hollow cylinder
Transient radiation and conduction in slotted slab and hollow cylinde
Use of a transcendental approximation in transient conduction analysis
Variation method applied to transient thermal conductivity in semiinfinite slab with nonlinear boundary conditio
Use of a transcendental approximation in transient conduction analysis
Boundary condition solution and transcendental approximation for transient heat conduction proble
Evaluating semi-automatic annotation of domestic energy consumption as a memory aid
Frequent feedback about energy consumption can help conservation, one of the current global challenges. Such feedback is most helpful if users can relate it to their own day-to-day activities. In earlier work we showed that manual annotation of domestic energy consumption logs aids users to make such connection and discover patterns they were not aware of. In this poster we report how we augmented manual annotation with machine learning classification techniques. We propose the design of a lab study to evaluate the system, extending methods used to evaluate context aware memory aids, and we present the results of a pilot with 5 participants
Novel Fiber Design for Wideband Conversion and Amplification in Multimode Fibers
We propose an operating principle to achieve broadband and highly tunable
mode conversion and amplification exploiting inter-modal four wave mixing in a
multimode fiber. A bandwidth of 30 nanometers is demonstrated by properly
designing a simple step-index silica fiber.Comment: Ecoc conference 201
Tungsten thermal neutron dosimeter
Tungsten-185 activity, which is produced by neutron activation of tungsten-184, determines thermal neutron flux. Radiochemical separation methods and counting techniques for irradiated tungsten provide accurate determination of the radiation exposure
Improvements in estimating proportions of objects from multispectral data
Methods for estimating proportions of objects and materials imaged within the instantaneous field of view of a multispectral sensor were developed further. Improvements in the basic proportion estimation algorithm were devised as well as improved alien object detection procedures. Also, a simplified signature set analysis scheme was introduced for determining the adequacy of signature set geometry for satisfactory proportion estimation. Averaging procedures used in conjunction with the mixtures algorithm were examined theoretically and applied to artificially generated multispectral data. A computationally simpler estimator was considered and found unsatisfactory. Experiments conducted to find a suitable procedure for setting the alien object threshold yielded little definitive result. Mixtures procedures were used on a limited amount of ERTS data to estimate wheat proportion in selected areas. Results were unsatisfactory, partly because of the ill-conditioned nature of the pure signature set
Climate change at the ecosystem scale: a 50-year record in New Hampshire
Observing the full range of climate change impacts at the local scale is difficult. Predicted rates of change are often small relative to interannual variability, and few locations have sufficiently comprehensive long-term records of environmental variables to enable researchers to observe the fine-scale patterns that may be important to understanding the influence of climate change on biological systems at the taxon, community, and ecosystem levels. We examined a 50-year meteorological and hydrological record from the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, an intensively monitored Long-Term Ecological Research site. Of the examined climate metrics, trends in temperature were the most significant (ranging from 0.7 to 1.3 °C increase over 40–50 year records at 4 temperature stations), while analysis of precipitation and hydrologic data yielded mixed results. Regional records show generally similar trends over the same time period, though longer-term (70–102 year) trends are less dramatic. Taken together, the results from HBEF and the regional records indicate that the climate has warmed detectably over 50 years, with important consequences for hydrological processes. Understanding effects on ecosystems will require a diversity of metrics and concurrent ecological observations at a range of sites, as well as a recognition that ecosystems have existed in a directionally changing climate for decades, and are not necessarily in equilibrium with the current climate
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