Review of the Nieman Foundation with special reference to the report of the committee to review the Nieman Foundation

Thesis (M.S.)--Boston University, 195

Senescence can play an essential role in modelling and estimation of vector based epidemiological indicators: demographical approach

In the paper basic epidemiological indicators, produced by an aging population of vectors, are calculated. In the study we follow two lines: calculations for demographically structured population and individual life-history approach. We discuss the advantages and limitations of these approaches and compare the results of our calculations with epidemiological indicators obtained for non-aging population of vectors.Gibraltar, age effect, disease control, gerontology

Thermal stability and topological protection of skyrmions in nanotracks

Magnetic skyrmions are hailed as a potential technology for data storage and other data processing devices. However, their stability against thermal fluctuations is an open question that must be answered before skyrmion-based devices can be designed. In this work, we study paths in the energy landscape via which the transition between the skyrmion and the uniform state can occur in interfacial Dzyaloshinskii-Moriya finite-sized systems. We find three mechanisms the system can take in the process of skyrmion nucleation or destruction and identify that the transition facilitated by the boundary has a significantly lower energy barrier than the other energy paths. This clearly demonstrates the lack of the skyrmion topological protection in finite-sized magnetic systems. Overall, the energy barriers of the system under investigation are too small for storage applications at room temperature, but research into device materials, geometry and design may be able to address this

Modeling of airblast propagation through an enclosed structure

The ability to model explosively formed shock waves propagating through a structure is of particular interest to engineers concerned with structural responses to blasts. Accurate peak pressure and impulse values are critical to understanding blast loads on structures and predicting the resulting structural behavior, but are sometimes difficult to determine analytically. Experiments are necessary to determine the true structural response, but the experiments alone may not identify all the details involved in an explosive event that may be important for design purposes. When tied to experiments, computational modeling of explosive events can be an invaluable tool for an engineer. The most difficult part of modeling structural response to a close-in explosive event is capturing the fluid-structure interaction of the resulting flow of the detonation products. In this paper, we compare the results of numerical simulations of an explosive experimental event in an enclosed structure, or “attic space”, using two different computational codes, CTH and DYSMAS. Both adequately model the explosive event in attic space when compared to the experiment. We also compare the two codes’ ability to produce explosive-induced pressure-time histories in the free field. The advantage of using a coupled code like DYSMAS is that structural response can be more accurately captured than by using a hydrocode like CTH alone. The differences between the two codes’ ability to model the event are analyzed and described as well as a general description of the shock wave propagation in the attic space

The ion-induced charge-exchange X-ray emission of the Jovian Auroras: Magnetospheric or solar wind origin?

A new and more comprehensive model of charge-exchange induced X-ray emission, due to ions precipitating into the Jovian atmosphere near the poles, has been used to analyze spectral observations made by the Chandra X-ray Observatory. The model includes for the first time carbon ions, in addition to the oxygen and sulfur ions previously considered, in order to account for possible ion origins from both the solar wind and the Jovian magnetosphere. By comparing the model spectra with newly reprocessed Chandra observations, we conclude that carbon ion emission provides a negligible contribution, suggesting that solar wind ions are not responsible for the observed polar X-rays. In addition, results of the model fits to observations support the previously estimated seeding kinetic energies of the precipitating ions (~0.7-2 MeV/u), but infer a different relative sulfur to oxygen abundance ratio for these Chandra observations.Comment: 11 pages, 2 figures, 2 tables, submitted to ApJ Lette

Spin transfer switching of spin valve nanopillars using nanosecond pulsed currents

Spin valve nanopillars are reversed via the mechanism of spin momentum transfer using current pulses applied perpendicular to the film plane of the device. The applied pulses were varied in amplitude from 1.8 mA to 7.8 mA, and varied in duration within the range of 100 ps to 200 ns. The probability of device reversal is measured as a function of the pulse duration for each pulse amplitude. The reciprocal pulse duration required for 95% reversal probability is linearly related to the pulse current amplitude for currents exceeding 1.9 mA. For this device, 1.9 mA marks the crossover between dynamic reversal at larger currents and reversal by thermal activation for smaller currents