Radar Cross Section of Orbital Debris Objects

This discussion is concerned with the radar-data analysis and usage involved in the building of model orbital debris (OD) populations in the near-Earth environment, focusing on radar cross section (RCS). While varying with radar wavelength, physical dimension, material composition, overall shape and structure, the RCS of an irregular object is also strongly dependent on its spatial orientation. The historical records of observed RCSs for cataloged OD objects in the Space Surveillance Network are usually distributed over an RCS range, forming respective characteristic patterns. The National Aeronautics and Space Administration (NASA) Size Estimation Model provides an empirical probability-density function of RCS as a function of effective diameter (or characteristic length), which makes it feasible to predict possible RCS distributions for a given model OD population and to link data with model from a statistical perspective. The discussion also includes application of the widely used method of moments (MoM) and the Generalized Multi-particle Mie-solution (GMM) in the prediction of the RCS of arbitrarily shaped objects. Theoretical calculation results for an aluminum cube are compared with corresponding experimental measurements

A quantum volume hologram

We propose a new scheme for parallel spatially multimode quantum memory for light. The scheme is based on counter-propagating quantum signal wave and strong classical reference wave, like in a classical volume hologram, and therefore can be called a quantum volume hologram. The medium for the hologram consists of a spatially extended ensemble of atoms placed in a magnetic field. The write-in and read-out of this quantum hologram is as simple as that of its classical counterpart and consists of a single pass illumination. In addition we show that the present scheme for a quantum hologram is less sensitive to diffraction and therefore is capable of achieving higher density of storage of spatial modes as compared to previous proposals. A quantum hologram capable of storing entangled images can become an important ingredient in quantum information processing and quantum imaging.Comment: 8 pages, 2 figure

Influence of viral genes on the cell-to-cell spread of RNA silencing

The turnip crinkle virus-based vector TCV–GFPDCP had been devised previously to study cell-to-cell and long-distance spread of virus-induced RNA silencing. TCV–GFPDCP, which had been constructed by replacing the coat protein (CP) gene with a green fluorescent protein (GFP) coding sequence, was able to induce RNA silencing in single epidermal cells, from which RNA silencing spread from cell-to-cell. Using this unique local silencing assay together with mutagenesis analysis, two TCV genes, p8 and p9, which were involved in the intercellular spread of virus-induced RNA silencing, were identified. TCV–GFPDCP and its p8- or p9-mutated derivatives, TCVmp8–GFPDCP and TCVmp9–GFPDCP, replicated efficiently but were restricted to single Nicotiana benthamiana epidermal cells. TCV–GFPDCP, TCVmp8–GFPDCP, or TCVmp9–GFPDCP was able to initiate RNA silencing that targeted and degraded recombinant viral RNAs in inoculated leaves of the GFP-expressing N. benthamiana line 16c. However, cell-to-cell spread of silencing to form silencing foci was triggered only by TCV–GFPDCP. Non-replicating TCVmp88–GFPDCP and TCVmp28mp88–GFPDCP with dysfunctional replicase genes, and single-stranded gfp RNA did not induce RNA silencing. Transient expression of the TCV p9 protein could effectively complement TCVmp9–GFPDCP to facilitate intercellular spread of silencing. These data suggest that the plant cellular trafficking machinery could hijack functional viral proteins to permit cell-to-cell movement of RNA silencing

Extracting Synonymous Gene and Protein Terms From Biological Literature

Genes and proteins are often associated with multiple names. More names are added as new functional or structural information is discovered. Because authors can use any one of the known names for a gene or protein, information retrieval and extraction would benefit from identifying the gene and protein terms that are synonyms of the same substance

Salt Marsh Elevation Limit Determined After Subsidence From Hydrologic Change And Hydrocarbon Extraction

Levee construction aboveground and hydrocarbon removal from belowground in coastal wetlands can create hydrologic changes that increase plant stress through flooding. But the significance of the subsidence they cause individually or in combination is contested. This study untangled them to demonstrate elevational limits of salt marshes by studying dredged and natural waterways in two salt marshes in Louisiana, USA. The areas had a homogenous plant cover before drilling for oil and gas extraction peaked in the 1960s, and now are a mixed network of natural waterways and dredged canals used to drill wells with an average drill date of 1965.8 +/- 2.7 (mu +/- 1 SEM; n = 18) and well depth of 4661.0 m +/- 56.6 (mu +/- 1 SEM; n = 18). Aerial imagery was used to document how canals widened to become 2 to 4 times larger than their original construction width at the high production site and 50% larger at the low production site, whereas increases at the nearby natural channels were much less. Light detection and ranging (LIDAR) measurements at the high production site from 2002 showed that the marsh surface near wells subsided by 34 cm compared to undredged sites. Elevation in marshes at producing and dry wells were equal at the low production site, but high production well locations were even lower than at dry wells. An elevation vs. percent open water curve developed from these data overlapped with an independent analysis of a brackish marsh. A relative subsidence rate between 7.4 to 10.4 mm y(-1) transformed these salt marshes to an open water habitat within a few decades. The local creation of accommodation space through hydrocarbon removal and leveed wetlands is a parsimonious explanation for the spatial and temporal land loss rates on this deltaic coast over the last 80 years of oil and gas exploration. Substantial losses from the accelerating rates of sea level rise are indicated to occur before 2050

Plasmon attenuation and optical conductivity of a two-dimensional electron gas

Journal ArticleIn a ballistic two-dimensional electron gas, the Landau damping does not lead to plasmon attenuation in a broad interval of wave vectors q≤kF . Similarly, it does not contribute to the optical conductivity σ(ω,q) in a wide domain of its arguments, EF>ω>qvF , where EF , kF , and vF are, respectively, the Fermi energy, wave vector, and velocity of the electrons. We identify processes that result in the plasmon attenuation in the absence of Landau damping. These processes are: the excitation of two electron-hole pairs, phonon-assisted excitation of one pair, and a direct plasmon-phonon conversion. We evaluate the corresponding contributions to the plasmon linewidth and to the optical conductivity

Ductility enhancement of high performance cementitious composites and structures

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2002.Includes bibliographical references (p. 280-287).High performance cementitious composites (HP2C) are a new generation of fiber reinforced cementitious composites (FRCC) with substantial improvements in mechanical behavior. The most important development in these HP2C materials may be the nearly elasto-plastic ductile behavior, which allows safe exploitation of the tensile and shear capacity in structural elements. This thesis presents a comprehensive investigation into the ductility enhancement of HP2C structures. Beginning at the micromechanical level, sources of ductility are examined and micro-to-macro relations are derived from homogenization theory and fracture mechanics. These micro-to-macro relations form the basis for a novel 3-D two-phase material model, which captures macroscopically observed behavior. Currently existing models which describe the mechanical behavior of FRCC are often micromechanical in nature. However, this macroscopic approach permits one to model the mechanical behavior of HP2C in a continuous fashion, i.e. through the various states of cracking in HP2C, while capturing - through the two-phase composite structure of the model - the micromechanical sources of energy dissipation in the fiber reinforced composite.(cont.) The 3-D model is implemented in a finite element program to simulate the behavior of two HP2C applications: a flexural girder and a shear girder, which have recently been tested by the FHWA. It is shown how the two-phase model aptly and accurately predicts the structural behavior of HP2C. Next, a sensitivity analysis of the HP2C model parameters elucidates how changes in HP2C mechanical behavior, observed at material level, manifest themselves at the structural level. By setting limits on the permanent composite matrix strain, which accounts for cracking in HP2C, one can set service limits on HP2C structures.Hence, a comprehensive (micromechanical, macroscopic, and structural) method for the assessment of the ductility enhancement of HP2C structures is presented. A significant scientific benefit of this research is the HP2C model which links micromechanical processes to macroscopic behavior and ultimately to structural behavior. This research also provides a design tool, that is the finite element application, which can be used to predict the behavior of HP2C structures and suggest improvements in HP2C structural and material design.by Eugene Chuang.Sc.D

Poisson's ratio in cryocrystals under pressure

We present results of lattice dynamics calculations of Poisson's ratio (PR) for solid hydrogen and rare gas solids (He, Ne, Ar, Kr and Xe) under pressure. Using two complementary approaches - the semi-empirical many-body calculations and the first-principle density-functional theory calculations we found three different types of pressure dependencies of PR. While for solid helium PR monotonically decreases with rising pressure, for Ar, Kr, and Xe it monotonically increases with pressure. For solid hydrogen and Ne the pressure dependencies of PR are non-monotonic displaying rather deep minimums. The role of the intermolecular potentials in this diversity of patterns is discussed.Comment: Fizika Nizkikh Temperatur 41, 571 (2015