Electromagnetic wave scattering from the sea surface in the presence of wind wave patterns

The study is concerned with electromagnetic wave (EM) scattering by a random sea surface in the presence of coherent wave patterns. The coherent patterns are understood in a broad sense as the existence of certain dynamical coupling between linear Fourier components of the water wave field. We show that the presence of weakly nonlinear wave patterns can significantly change the EM scattering compared to the case of a completely random wave field. Generalizing the Random Phase Approximation (RPA) we suggest a new paradigm for EM scattering by a random sea surface.

The specific analysis carried out in the paper synthesizes the small perturbation method for EM scattering and a weakly nonlinear approach for wind wave dynamics. By investigating, in detail, two examples of a random sea surface composed of either Stokes waves or horse-shoe (‘crescent-shaped’) patterns the mechanism of the pattern effect on scattering is revealed. Each Fourier harmonic of the scattered EM field is found to be a sum of contributions due to different combinations of wave field harmonics. Among these ‘partial scatterings’ there are phase-dependent ones and, therefore, the intensity of the resulting EM harmonic is sensitive to the phase relations between the wind wave harmonics. The effect can be interpreted as interference of partial scatterings due to the co-existence of several phase-related periodic scattering grids. A straightforward generalization of these results enables us to obtain, for a given wind wave field and an incident EM field, an a priori estimate of whether the effects due to the patterns are significant and the commonly used RPA is inapplicable. When the RPA is inapplicable, we suggest its natural generalization by re-defining the statistical ensemble for water surface. First, EM scattering by an ‘elementary’ constituent pattern should be considered. Each such scattering is affected by the interference because the harmonics comprising the pattern are dynamically linked. Then, ensemble averaging, which takes into account the distribution of the pattern parameters (based on the assumption that the phases between the patterns are random), should be carried out. It is shown that, generally, this interference does not vanish for any statistical ensemble due to dynamical coupling between water wave harmonics. The suggested RPA generalization takes into account weak non-Gaussianity of water wave field m contrast to the traditional RPA which ignores it.     

The finer points of lying online: E-mail versus pen and paper.

The authors present 3 experimental studies that build on moral disengagement theory by exploring lying in online environments. Findings indicate that, when e-mail is compared with pen and paper communication media (both of which are equal in terms of media richness, as both are text only), people are more willing to lie when communicating via e-mail than via pen and paper and feel more justified in doing so. The findings were consistent whether the task assured participants that their lie either would or would not be discovered by their counterparts. Implications for theory and practice are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Electing a leader in a ring with link failures

Summary We investigate the message complexity of electing a leader in a ring of asynchronous processors. Our work deviates from the previous works on electing a leader in that we consider the effect of link failures. A link is said to fail if some message sent through it never reaches its destination. We distinguish the case where n is known from the case n unknown. Our main result is a O(n · log n) algorithm for electing a leader on a n-processor ring (the case n is known).     

On the complexity of computation in the presence of link failures: the case of a ring

Summary We investigate the message complexity of distributed computations on rings of asynchronous processors. In such computations, each processor has an initial local value and the task is to compute some predetermined function of all local values. Our work deviates from previous works concerning the complexity of ring computations in that we consider the effect oflink failures. A link is said to fail if some message sent through it never reaches its destination. We show that the message complexity of any function, which is sensitive to all its inputs, is (n logn) whenn, the number of processors, is a-priori known; and is (n ² ) whenn is not known. Interestingly, these tight bounds do not depend on whether the identity of a leader is a-priori known before the computation starts. These results stand in sharp contrast to the situation in asynchronous rings with no link failures, where the message complexity is affected by the a-priori knowledge of a leader but is not affected by the knowledge ofn. Oded Goldreich was born in Tel-Aviv, Israel, on February 4th 1957. Received B.A., M.Sc., and D.Sc. in Computer Science from the Technion — Israel Institute of Technology, Haifa, Israel, in 1980, 1982, and 1983, respectively. He is currently an Associate Professor of Computer Science in the Technion. From 1983 to 1986, he was a postdoctoral fellow in MIT's Laboratory for Computer Science. His research interests include cryptography and related areas, relation between randomness and algorithms, and distributed computation. Luiba Shrira was born in Vilnius, Lithuania. Received B.A., M.Sc., and D.Sc. in Computer Science from the Technion—Israel Institute of Technology, Haifa, Israel in 1977, 1980, and 1985, respectively. from 1986 to 1989 she was a postdoctoral fellow at Laboratory for Computer science at MIT, where she is currently a Research Associate. Her research interests include highly-available and reliable distributed algorithms and systems, persistent object systems, and programming methodology.Part of the work has been done while the first author was in the Laboratory for Computer Science of MIT and the second author was in the Computer Science Department of the Technion. First author was partially supported by a Weizmann Postdoctoral Fellowship, an IBM Postdoctoral Fellowship, and Albert Einstein Research Fund (through Technion's V.P.R. Fund)     

A technique for constructing highly available services

This paper describes a general method for constructing a highly available service for use in a distributed system. It gives a specific implementation of the method and proves the implementation correct. The service consists of replicas that reside at several different locations in a network. It presents its clients with a consistent view of its state, but the view may contain old information. Clients can indicate how recent the information must be. The method can be used in applications satisfying certain semantic constraints. For applications that can use it, the method performs better than other replication techniques.This research was supported in part by the Advanced Research Projects Agency of the Department of Defense, monitored by the Office of Naval Research under Contract No. N00014-83-K.-0125, by the National Science Foundation under Grant No. DCR-8503662, and by the HTI Postdoctoral Fellowship.     

Pyrrole chemical polymerization on textiles: Kinetics and operating conditions

The chemical polymerization of pyrrole was studied with the aim of defining the best operating conditions for obtaining conductive fabrics of synthetic fibers. Ammonium persulfate was chosen as the oxidant, and various dopants were tested. Among these, naphtalene‐2,6‐disulfonic acid disodium salt enabled the lowest surface resistivity to be reached. Hence, several kinetic runs were performed to explain the influence of some operating conditions as oxidant concentration and temperature. A pseudo‐first‐order kinetic equation was derived, and the reaction rates were found to be practically unaffected by the presence of the textile substrate. Pyrrole polymerization was faster than adsorption of the monomer, suggesting the hypothesis of fast polymerization outside the fibers followed by adsorption onto the fiber surface. The surface coating of polypyrrole was evidenced by microscopic observation of the fiber cross sections. ©2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4121–4126, 2006     

What makes the Peregrine soliton so special as a prototype of freak waves?

The formation of breathers as prototypes of freak waves is studied within the framework of the classic ‘focussing’ nonlinear Schrödinger (NLS) equation. The analysis is confined to evolution of localised initial perturbations upon an otherwise uniform wave train. For a breather to emerge out of an initial hump, a certain integral over the hump, which we refer to as the “area”, should exceed a certain critical value. It is shown that the breathers produced by the critical and slightly supercritical initial perturbations are described by the Peregrine soliton which represents a spatially localised breather with only one oscillation in time and thus captures the main feature of freak waves: a propensity to appear out of nowhere and disappear without trace. The maximal amplitude of the Peregrine soliton equals exactly three times the amplitude of the unperturbed uniform wave train. It is found that, independently of the proximity to criticality, all small-amplitude supercritical humps generate the Peregrine solitons to leading order. Since the criticality condition requires the spatial scale of the initially small perturbation to be very large (inversely proportional to the square root of the smallness of the hump magnitude), this allows one to predict a priori whether a freak wave could develop judging just by the presence/absence of the corresponding scales in the initial conditions. If a freak wave does develop, it will be most likely the Peregrine soliton with the peak amplitude close to three times the background level. Hence, within the framework of the one-dimensional NLS equation the Peregrine soliton describes the most likely freak-wave patterns. The prospects of applying the findings to real-world freak waves are also discussed.     

A technique for constructing highly available services

This paper describes a general method for constructing a highly available service for use in a distributed system. It gives a specific implementation of the method and proves the implementation correct. The service consists of replicas that reside at several different locations in a network. It presents its clients with a consistent view of its state, but the view may contain old information. Clients can indicate how recent the information must be. The method can be used in applications satisfying certain semantic constraints. For applications that can use it, the method performs better than other replication techniques.