A global model of the neutral thermosphere in magnetic coordinates based on AE-C data

An empirical model of the global atomic oxygen and helium distributions in the thermosphere is developed in a magnetic coordinate system and compared to similar models which are expanded in geographic coordinates. The advantage of using magnetic coordinates is that fewer terms are needed to make predictions which are nearly identical to those which would be obtained from a geographic model with longitudinal and universal time corrections. Magnetic coordinates are more directly related to the major energy inputs in the polar regions than geographic coordinates and are more convenient to use in studies of high latitude energy deposition processes. This is important for comparison with theoretical models where the number of coordinates is limited. The effect of magnetic activity on the atomic oxygen distribution in the morning sector of the high latitude thermosphere in the auroral zone is also considered. A magnetic activity indicator (ML) based on an auroral electrojet index (AL) and the 3 hour ap index are used to relate the atomic oxygen density variations to magnetic activity in this region

Alternative packet switch architectures for a 30/20 GHz FDMA/TDMA geostationary communication satellite network

This study has investigated alternatives for realizing a packet-based network switch for deployment on a communication satellite. The emphasis was on the avoidance of contention problems that can occur due to the simultaneous arrival of an excessive number of packets destined for the same downlink dwell. The study was to look ahead, beyond the current Advanced Communications Technology Satellite (ACTS) capability, to the next generation of satellites. The study has not been limited by currently available technology, but has used university and commercial research efforts as a basis for designs that can be readily constructed and launched within the next five years. Tradeoffs in memory requirement, power requirement, and architecture have been considered as a part of our study

Technical support for digital systems technology development. Task order 1: ISP contention analysis and control

Alternatives for realizing a packet-based network switch for use on a frequency division multiple access/time division multiplexed (FDMA/TDM) geostationary communication satellite were investigated. Each of the eight downlink beams supports eight directed dwells. The design needed to accommodate multicast packets with very low probability of loss due to contention. Three switch architectures were designed and analyzed. An output-queued, shared bus system yielded a functionally simple system, utilizing a first-in, first-out (FIFO) memory per downlink dwell, but at the expense of a large total memory requirement. A shared memory architecture offered the most efficiency in memory requirements, requiring about half the memory of the shared bus design. The processing requirement for the shared-memory system adds system complexity that may offset the benefits of the smaller memory. An alternative design using a shared memory buffer per downlink beam decreases circuit complexity through a distributed design, and requires at most 1000 packets of memory more than the completely shared memory design. Modifications to the basic packet switch designs were proposed to accommodate circuit-switched traffic, which must be served on a periodic basis with minimal delay. Methods for dynamically controlling the downlink dwell lengths were developed and analyzed. These methods adapt quickly to changing traffic demands, and do not add significant complexity or cost to the satellite and ground station designs. Methods for reducing the memory requirement by not requiring the satellite to store full packets were also proposed and analyzed. In addition, optimal packet and dwell lengths were computed as functions of memory size for the three switch architectures

The Foreign Direct Investment Location Decision: A Contingency Model of the Foreign Direct Investment Location Decision-Making Process

Despite considerable prior research into foreign direct investment (FDI) location decisions, our understanding of the processes underlying such decisions is still limited. Findings from work based in the economics and behavioral theories of the multinational enterprise (MNE) both acknowledge that FDI is not a point-of-time decision but a gradual process that yields important changes over its duration. However, these competing traditions both fall short when attempting to portray the actual process by which FDI location decisions are made by managers in MNEs. This gap has been recently attributed to two interrelated limitations. Firstly, level of analysis concerns have artificially separated managerial decision-making processes from the organizational and environmental structures within which they are made. Secondly, because of the complexity inherent in the FDI location decision environment, the study of these decisions has not taken contextual factors into consideration. This study addresses three important questions in order to build our understanding of the FDI location decision-making processes: (1) What are the decision-making processes that lead to FDI location choice? (2) What is the impact of contextual variables on FDI location decision-making processes at different levels of analysis, and are there any patterns of variation in decision processes under different decision conditions? (3) What factors drive final FDI location choice, and can a useful framework or theory be developed that links FDI location decision-making processes and context to drivers of FDI location choice? In order to address level of analysis concerns, the study places the manager at the center of the FDI location decision in modeling and in research, a strategy recommended by an emerging stream of behavioral-focused international business research (Aharoni, 2010; Buckley et al., 2007; Devinney, 2011). By examining FDI location decisions from the perspective of the managers who implement them, it is possible to clarify the nature of processes that lead to FDI location choice, and identify the impact of different elements of decision maker, firm and environmental context on such processes. The conceptual framework builds on Aharoni’s (1966) pivotal research while incorporating findings from broader behavioral managerial decision models and international business research. The framework is based on the assumption that FDI location decision-making processes and final choice are contingent upon interactions between the environmental, firm and decision maker context under which the decision is made. The research was undertaken in three phases. Phase 1 included a literature review that covered research on the MNE, internationalization, and decision making. The findings of the review identified key aspects of FDI location decision context and led to the development of an initial contingency framework of strategic decision making. Phase 2 consisted of an exploratory case study of twenty four FDI location decisions. The initial contingency framework developed during the literature review was used during this stage to identify the relationship between decision-making processes and contextual variables at the case decisions. By drawing on results from the exploratory research, an initial conceptual model and a set of propositions were developed. In Phase 3, twenty case studies were theoretically sampled from a pool of MNEs of varying size and parent-country nationality within the knowledge-based industries. The data collection and analysis followed a process, event-driven approach to case study research involving the mapping of key sequences of events as well as within- and cross-case analysis. The results identify the key elements of the decision process that explain FDI location behavior and develop a framework that links them together and makes them sensible. The four key elements of the FDI location decision that comprise the framework include: (i) the process, (ii) the context, (iii) patterns, and (iv) location. Research findings show the FDI location decision process as comprising of five broad stages, the content of each driven by a dynamic and evolving interpretation of maximum subjective expected utility. Utility preferences are identified as the consequence of shifting and opaque goals, founded upon imperfect information, operating in an environment marked by uncertainty. Five variations in the overall orientation of utility at case decisions, classified in the study as ‘decision rules,’ proved to be more useful predictors of decision-making behavior than traditional notions of bounded rationality seeking rent extraction and profitability. Decision processes were found to vary in five prototypical patterns, according to clusters of contextual variables that together moderated the level of decision-maker autonomy, hierarchical centralization, rule formalization, commitment to strategy, and politicization of the decision. Patterns are described as FDI location decision-making models, and proposed as an initial step towards the development of a taxonomy of FDI location decision-making processes. Because of the dynamic and staged nature of the process, findings showed that factors that were important at one stage of the decision were not as important at the next. As such, the task of identifying universal drivers of FDI location was deemed an unfeasible one. In place of universal drivers, the initiating force of the investment, the purpose of investment and information sources and networks are identified as the key context-specific determinants of location in FDI decisions. Bounded by uncertainty, chance, the dynamics of the process and decision-maker effects, each of these aspects of the decision served to limit the possible consideration set for investment, and formed the value basis and measures from which to select the most attractive location choice. Despite the contextual differences in these drivers, however, the study revealed a strong pattern that showed that the importance of specific location considerations differed in much the same way across case decisions. During the first stage of case decisions primarily strategic aspects of locations were considered; during the second, considerations relating to the system; operational concerns in the third; implementation concerns in the fourth; and added value factors in the final choice. How each of these concerns was interpreted to reach final location choice differed according to the drivers mentioned previously, although the patterns were the same. This study develops a contingency framework for examining the FDI location decision-making processes of MNEs under different operating conditions. By identifying the four key components of the FDI location decision, their interrelationships and many sources of variance, this thesis shows that despite its complexity, the FDI location decision is amenable to useful conceptual structuring. From an academic standpoint, the framework answers Aharoni’s most recent call to action in ‘Behavioral Elements in Foreign Direct Investment’ (2010) by developing a replicable structure within which to think about incorporating managerial decision models and context into the theory of the MNE. These findings enhance understandings of decision making at MNEs, reconcile a number of inconsistencies between opposing perspectives of MNE theory, and thereby update extant theory so that it has greater relevance in today’s diverse international business environment. From a managerial standpoint, the thesis helps managers to recognize the opportunities and limitations posed by different aspects of decision context so that they are able to tailor their FDI location decision strategies to best suit their needs. Finally, from the perspective of policy markers, research findings provide great support for the use of investment attraction schemes through the use of targeted location marketing and investment incentives.

Severe pneumonia due to Parachlamydia acanthamoebae following intranasal inoculation: a mice model.

Parachlamydia acanthamoebae is an obligate intracellular bacterium naturally infecting free-living amoebae. The role of this bacterium as an agent of pneumonia is suggested by sero-epidemiological studies and molecular surveys. Furthermore, P. acanthamoebae may escape macrophages microbicidal effectors. Recently, we demonstrated that intratracheal inoculation of P. acanthamoebae induced pneumonia in 100% of infected mice. However, the intratracheal route of infection is not the natural way of infection and we therefore developed an intranasal murine model. Mice inoculated with P. acanthamoebae by intranasal inoculation lost 18% of their weight up to 8 days post-inoculation. All mice presented histological signs of pneumonia at day 2, 4, 7, and 10 post-inoculation, whereas no control mice harboured signs of pneumonia. A 5-fold increase in bacterial load was observed from day 0 to day 4 post-inoculation. Lungs of inoculated mice were positive by Parachlamydia-specific immunohistochemistry 4 days post-inoculation, and P. acanthamoebae were localized within macrophages. Thus, we demonstrated that P. acanthamoebae induce a severe pneumonia in mice. This animal model (i) further supports the role of P. acanthamoebae as an agent of pneumonia, confirming the third Koch postulate, and (ii) identified alveolar macrophages as one of the initial cells where P. acanthamoebae is localized following infection

Formation of Episodic Magnetically Driven Radiatively Cooled Plasma Jets in the Laboratory

We report on experiments in which magnetically driven radiatively cooled plasma jets were produced by a 1 MA, 250 ns current pulse on the MAGPIE pulsed power facility. The jets were driven by the pressure of a toroidal magnetic field in a ''magnetic tower'' jet configuration. This scenario is characterized by the formation of a magnetically collimated plasma jet on the axis of a magnetic ''bubble'', confined by the ambient medium. The use of a radial metallic foil instead of the radial wire arrays employed in our previous work allows for the generation of episodic magnetic tower outflows which emerge periodically on timescales of ~30 ns. The subsequent magnetic bubbles propagate with velocities reaching ~300 km/s and interact with previous eruptions leading to the formation of shocks.Comment: 6 pages, 5 figures. Accepted for publication in Astrophysics & Space Scienc

Magnetohydrodynamic modeling of the accretion shocks in classical T Tauri stars: the role of local absorption on the X-ray emission

We investigate the properties of X-ray emission from accretion shocks in classical T Tauri stars (CTTSs), generated where the infalling material impacts the stellar surface. Both observations and models of the accretion process reveal several aspects that are still unclear: the observed X-ray luminosity in accretion shocks is below the predicted value, and the density versus temperature structure of the shocked plasma, with increasing densities at higher temperature, deduced from the observations, is at odds with that proposed in the current picture of accretion shocks. To address these open issues we investigate whether a correct treatment of the local absorption by the surrounding medium is crucial to explain the observations. To this end, we describe the impact of an accretion stream on a CTTS by considering a magnetohydrodynamic model. From the model results we synthesize the X-ray emission from the accretion shock by producing maps and spectra. We perform density and temperature diagnostics on the synthetic spectra, and we directly compare the results with the observations. Our model shows that the X-ray fluxes inferred from the emerging spectra are lower than expected because of the complex local absorption by the optically thick material of the chromosphere and of the unperturbed stream. Moreover, our model including the effects of local absorption explains in a natural way the apparently puzzling pattern of density versus temperature observed in the X-ray emission from accretion shocks.Comment: Accepted for publication in Astrophysical Journal Letters; 5 pages, 4 figure

Tidally distorted accretion discs in binary stars

The non-axisymmetric features observed in the discs of dwarf novae in outburst are usually considered to be spiral shocks, which are the non-linear relatives of tidally excited waves. This interpretation suffers from a number of problems. For example, the natural site of wave excitation lies outside the Roche lobe, the disc must be especially hot, and most treatments of wave propagation do not take into account the vertical structure of the disc. In this paper I construct a detailed semi-analytical model of the non-linear tidal distortion of a thin, three-dimensional accretion disc by a binary companion on a circular orbit. The analysis presented here allows for vertical motion and radiative energy transport, and introduces a simple model for the turbulent magnetic stress. The m=2 inner vertical resonance has an important influence on the amplitude and phase of the tidal distortion. I show that the observed patterns find a natural explanation if the emission is associated with the tidally thickened sectors of the outer disc, which may be irradiated from the centre. According to this hypothesis, it may be possible to constrain the physical parameters of the disc through future observations.Comment: 13 pages, 3 figures, to be published in MNRA

The early spectral evolution of SN 2004dt

Aims. We study the optical spectroscopic properties of Type Ia Supernova (SN Ia) 2004dt, focusing our attention on the early epochs. Methods. Observation triggered soon after the SN 2004dt discovery allowed us to obtain a spectrophotometric coverage from day -10 to almost one year (~353 days) after the B band maximum. Observations carried out on an almost daily basis allowed us a good sampling of the fast spectroscopic evolution of SN 2004dt in the early stages. To obtain this result, low-resolution, long-slit spectroscopy was obtained using a number of facilities. Results. This supernova, which in some absorption lines of its early spectra showed the highest degree of polarization ever measured in any SN Ia, has a complex velocity structure in the outer layers of its ejecta. Unburnt oxygen is present, moving at velocities as high as ~16,700 km/s, with some intermediate-mass elements (Mg, Si, Ca) moving equally fast. Modeling of the spectra based on standard density profiles of the ejecta fails to reproduce the observed features, whereas enhancing the density of outer layers significantly improves the fit. Our analysis indicates the presence of clumps of high-velocity, intermediate-mass elements in the outermost layers, which is also suggested by the spectropolarimetric data.Comment: 13 pages, 15 figures, accepted for pubblication in Astronomy and Astrophysic

High-Velocity Features: a ubiquitous property of Type Ia SNe

Evidence of high-velocity features such as those seen in the near-maximum spectra of some Type Ia Supernovae (eg SN 2000cx) has been searched for in the available SNIa spectra observed earlier than one week before B maximum. Recent observational efforts have doubled the number of SNeIa with very early spectra. Remarkably, all SNeIa with early data (7 in our RTN sample and 10 from other programmes) show signs of such features, to a greater or lesser degree, in CaII IR, and some also in SiII 6255A line. High-velocity features may be interpreted as abundance or density enhancements. Abundance enhancements would imply an outer region dominated by Si and Ca. Density enhancements may result from the sweeping up of circumstellar material by the highest velocity SN ejecta. In this scenario, the high incidence of HVFs suggests that a thick disc and/or a high-density companion wind surrounds the exploding white dwarf, as may be the case in Single Degenerate systems. Large-scale angular fluctuations in the radial density and abundance distribution may also be responsible: this could originate in the explosion, and would suggest a deflagration as the more likely explosion mechanism. CSM-interaction and surface fluctuations may coexist, possibly leaving different signatures on the spectrum. In some SNe the HVFs are narrowly confined in velocity, suggesting the ejection of blobs of burned material.Comment: 12 pages, 2 figures, ApJ Letters in pres