Bond angle distribution in amorphous germania and silica

The distribution of Ge-O-Ge and Si-O-Si bond angles alpha in amorphous germania and silica is re-determined on the basis of diffraction experiments. The bond angle alpha joining adjacent tetrahedra is the central parameter of any continuous random network description (CRN) of these glasses. New high energy photon diffraction experiments on amorphous germania (at photon energies of 97 and 149 keV) are presented, covering the momentum transfer 0.6-33.5 AA^{-1}. In photon diffraction experiments on GeO2 the contribution of the OO pairs is very small. To obtain a similar information for amorphous SiO2, high energy photon diffraction experiments have been combined with neutron diffraction data on amorphous silica in order to eliminate the OO- partial structure factor. With this technique it is shown that the Si-O-Si angle distribution is fairly narrow (sigma=7.5 degree) and in fact comparable in width to the Ge-O-Ge angle distribution (sigma=8.3 degree), a result which differs from current opinion. The narrower distribution found in this study are in much better agreement to the determinations based on 29Si-MAS-NMR. Among the various models relating the chemical shift to the bond angle, best agreement is found with those models based on the secant model. Sharp components in the bond angle distribution can be excluded within the reached real space resolution of 0.09 AA.Comment: 12 pages LATEX, 13 Postscript figures, experimental data includes as LATEX comment

Direct numerical simulation of gas transfer across the air-water interface driven by buoyant convection

A series of direct numerical simulations of mass transfer across the air-water interface driven by buoyancy-induced convection has been carried out to elucidate the physical mechanisms that play a role in the transfer of heat and atmospheric gases. The buoyant instability is caused by the presence of a thin layer of cold water situated on top of a body of warm water. In time, heat and atmospheric gases diff use into the uppermost part of the thermal boundary layer and are subsequently transported down into the bulk by falling sheets and plumes of cold water. Using a specifically-designed numerical code for the discretization of scalar convection and diffusion, it was possible to accurately resolve this buoyant instability induced transport of atmospheric gases into the bulk at a realistic Prandtl number (Pr = 6) and Schmidt numbers ranging from Sc = 20 to Sc = 500. The simulations presented here provided a detailed insight into instantaneous gas transfer processes. The falling plumes with highly gas-saturated fluid in their core were found to penetrate deep inside the bulk. With an initial temperature difference between the water surface and the bulk of slightly above 2 K peaks in the instantaneous heat flux in excess of 1600 W/m² were observed, proving the potential effectiveness of buoyant convective heat and gas transfer. Furthermore, the validity of the scaling law for the ratio of gas and heat transfer velocities K_L / H_L ∼ (Pr/Sc)^0:5 for the entire range of Schmidt numbers considered was confirmed. A good time-accurate approximation of K_L was found using surface information such as velocity fluctuations and convection cell size or surface divergence. A reasonable time-accuracy for the K_L estimation was obtained using the horizontal integral length scale and the root-mean-square of the horizontal velocity fluctuations in the upper part of the bulk.The German Research Foundation (DFG grant UH242/6-1). Additional funding by the Helmholtz Water Network

Constraints on Muon Decay Parameters from Neutrino Mass

We derive model-independent constraints on chirality-changing terms in the muon decay Lagrangian using limits on neutrino mass. We consider all dimension-six operators invariant under the gauge symmetry of the Standard Model which contribute to either a Dirac neutrino mass or muon decay. Taking an upper limit on neutrino mass of 1 eV, we derive limits on the contributions of chirality-changing operators to the Michel parameters four orders of magnitude tighter than the current experimental constraints. We also identify two operators which, due to their flavor structure, are not constrained by neutrino mass. If near-future experiments find contributions to muon decay from these operators, it could indicate interesting flavor structure in physics beyond the SM.Comment: 4 pages, contribution to the proceedings of CIPANP 2006 (May 30-June 3, 2006), Rio Grande, Puerto Ric

Cellular excitability and the regulation of functional neuronal identity: from gene expression to neuromodulation

The intrinsic properties of a neuron determine the translation of synaptic input to axonal output. It is this input– output relationship that is the heart of all nervous system activity. As such, the overall regulation of the intrinsic excitability of a neuron directly determines the output of that neuron at a given point in time, giving the cell a unique “functional identity.” To maintain this distinct functional output, neurons must adapt to changing patterns of synaptic excitation. These adaptations are essential to prevent neurons from either falling silent as synaptic excitation falls or becoming saturated as excitation increases. In the absence of stabilizing mechanisms, activity-dependent plasticity could drive neural activity to saturation or quiescence. Furthermore, as cells adapt to changing patterns of synaptic input, presumably the overall balance of intrinsic conductances of the cell must be maintained so that reliable output is achieved (Daoudal and Debanne, 2003; Turrigiano and Nelson, 2004; Frick and Johnston, 2005). Although these regulatory phenomena have been well documented, the molecular and physiological mechanisms involved are poorly understood

The Option Conundrum in Tax Law: After All These Years, What Exactly is an Option?

Some of the latest financial products that have become prevalent on Wall Street defy easy categorization for tax purposes. Certain products, such as economic derivatives or weather derivatives, bear the trappings of options, but lack an underlying property component. Other products, such as credit default swaps, have option-type payouts, but are cast in the form of financial swaps. Which of these products are truly options and why? When and how to tax these instruments depends on proper resolution of this fundamental classification issue. With respect to credit default swaps, arguably the single most important product innovation on Wall Street in the last 20 years, hundreds of millions of dollars in potential tax liability (on cross-border swap payments) are riding on a resolution of this issue. This article tackles a longstanding philosophical question which has modern-day resonance in the ongoing debate over the appropriate taxation of the latest generation of financial products. Relying on contract-law principles and economic risk analysis, the article revisits longstanding case law to fashion a modern-day definition for what constitutes an option. Ultimately, it resolves the question of whether the concept of an option properly encompasses non-property-based options, such as economic or weather derivatives, as well as contingent options, such as credit default swaps. The article concludes by proposing a basis for differentiating between options and financial swaps, two product categories which have overlapping reach, allowing taxpayers to avoid undesired tax results by the simple expedient of selecting the desired form of transaction. While the article’s suggested approach would deprive taxpayers of the flexibility inherent in current law’s ambiguity, this flexibility conflicts with the government’s asserted interest in promoting neutrality in the taxation of financial products. Equally important, the financial products sector is more likely to thrive when uncertainties in the taxation of economically useful transaction are satisfactorily resolved

Intensive Parenting: Does it Have the Desired Impact on Child Outcomes?

Although parental involvement is generally thought to be beneficial for children, it is unclear whether the intensive level of parenting that has become commonplace results in improved child outcomes. Intensive parenting may involve the desire to anticipate and solve children’s problems as well as to enroll them in numerous, structured activities that might enhance their physical, cognitive, and social abilities. We surveyed 241 parents to assess intensive parenting beliefs, anticipatory problem solving (APS), enrollment in structured activities, and developmental outcomes of their children ages 2–5. Using structural equation modeling, we found that intensive parenting beliefs predicted more APS, which predicted greater enrollment in creative and physical activities. However, enrollment in structured activities did not predict children’s developmental outcomes. Although parents may believe that expensive and time-consuming activities are the keys to ensuring their children’s health, happiness, and success, this study does not support this assumption

340 years of atmospheric circulation characteristics reconstructed from an eastern Antarctic Peninsula ice core

Copyright @ 2006 American Geophysical Union (AGU)Precipitation delivery mechanisms for Dolleman Island (DI), located off the east coast of the Antarctic Peninsula, are investigated using reanalysis and back trajectory data. The Southern Annular Mode (SAM) and ENSO are both shown to influence precipitation delivery and event size. Precipitation delivery variability is compared against the interannual variation of chemical data from two DI ice cores. Nitrate concentration in the cores is strongly linked with the ratio of easterly to westerly back trajectories arriving at DI, as described by a Cross-Peninsula Index (CPI) defined in this paper. This CPI is used subsequently to reconstruct the atmospheric circulation characteristics for the 340-year ice core record. The analysis highlights a period of increased easterlies during 1720–1780 and an increase in westerlies for 1950–1980, the latter concomitant with a positive SAM trend and western Peninsula warming. The reconstruction also reveals periods when polynyas may have been present in the Weddell Sea

Triumph Over Tragedy, Second Edition: A Curriculum for Extension Professionals Responding to Disasters and Terrorism

We describe Triumph Over Tragedy, Second Edition: A Community Response to Managing Trauma in Times of Disaster and Terrorism, a curriculum designed to assist Extension professionals and other community stakeholders in helping their communities prepare for, and respond to, trauma associated with natural and human-made disasters. In the post-September 11, 2001 environment, it is imperative that community professionals become involved in planning for the psychological impact of these events

Toward a general theory of exploitation of fish populations

A possible form of a theory of exploitation of fish populations was examined. The exploitation theory was derived from a theory of community dynamics that represents the interactions between populations in a biological community with complex interrelated systems of isoclines on phase planes. The isocline systems are deduced, with a graphical calculus, from response functions that represent the biological characteristics of each of the interacting populations. The time-invariant systems of isoclines and the response functions from which they are deduced underlie, determine, and so explain time-variant population performances. A fish population was defined as a member of a simple community. Isocline systems were deduced to represent and provide an integrated explanation of the effects of the following factors on the dynamics and persistence of the fish populations: changes in the levels of environmental factors such as light energy and fishing effort, competition for food with another fish population, and foraging by the fish population on two prey species. The impact of these factors on the magnitude and form of the recruitment, production, and yield curves of the fish population was also determined. In general increases in light energy input rate and other environmental factors such as plant nutrients, and the addition of another prey species to the diet of the fish population, were found to increase the magnitude of these curves. Competition reduced their magnitude. With the theoretical approach developed here, causal-deterministic explanations of the dynamics of exploited fish populations can be developed. These explanations couple the dynamics of the fish population to the dynamics of other populations in the biological community and to external environmental factors. This approach thus permits broad understanding of the performances of exploited fish populations as they interact with their co-extensive environmental systems