Systematic and Stochastic Variations in Pulsar Dispersion Measures

We analyze deterministic and random temporal variations in dispersion measure (DM) from the full three-dimensional velocities of pulsars with respect to the solar system, combined with electron-density variations on a wide range of length scales. Previous treatments have largely ignored the pulsar's changing distance while favoring interpretations involving the change in sky position from transverse motion. Linear trends in pulsar DMs seen over 5-10~year timescales may signify sizable DM gradients in the interstellar medium (ISM) sampled by the changing direction of the line of sight to the pulsar. We show that motions parallel to the line of sight can also account for linear trends, for the apparent excess of DM variance over that extrapolated from scintillation measurements, and for the apparent non-Kolmogorov scalings of DM structure functions inferred in some cases. Pulsar motions through atomic gas may produce bow-shock ionized gas that also contributes to DM variations. We discuss possible causes of periodic or quasi-periodic changes in DM, including seasonal changes in the ionosphere, annual variation of the solar elongation angle, structure in the heliosphere-ISM boundary, and substructure in the ISM. We assess the solar cycle's role on the amplitude of ionospheric and solar-wind variations. Interstellar refraction can produce cyclic timing variations from the error in transforming arrival times to the solar system barycenter. We apply our methods to DM time series and DM gradient measurements in the literature and assess consistency with a Kolmogorov medium. Finally, we discuss the implications of DM modeling in precision pulsar timing experiments.Comment: 24 pages, 17 figures, published in Ap

Mammals of the Northern Great Plains

For the purposes of this book, the Northern Great Plains are defined as the states of Nebraska, North Dakota, and South Dakota. As a physiographic concept, the northern part of the great interior grasslands of North America is, of course, much broader in geographic extent than the Dakotas and Nebraska, but the three states lie in the heart of the region, and thus the title for this work seems appropriate. Our expectations in writing Mammals of the Northern Great Plains were to provide a comprehensive, yet semitechnical, treatmeat of free-living mammals that would prove useful to specialist and nonspecialist alike. The content and style therefore were developed in a way we hope will interest the inquiring high-school student, on the one hand, and provide a point of reference for the professional mammalogist on the other. Between these extremes, wildlife biologists, conservationists, environmental specialists, college students of vertebrate zoology, and others interested in mammalian natural history should find the present treatment useful for their needs as well. In the accounts that follow, species of each genus are listed in alphabetical order. Genera, families, and orders are arranged in conventional phylogenetic sequence, and treatment of these higher taxa is deliberately brief. Readers desiring more detail should consult the synopsis by Anderson and Jones (1967) for orders and families and that of Walker et al. (1964 and subsequent editions) for genera. Both scientific and vernacular names of species generally follow Jones, Carter, and Genoways (1979). Information in the accounts of species is organized under five headings: Name, Distribution, Description, Natural History, and Selected References. In the first section, comment is made on the derivation of the scientific name of the species; often, alternative vernacular names are provided. The section on distribution describes the general geographic and ecological range of a species. Subspecies (if recognized) are listed in this section. The geographic ranges of most species are mapped, based on the currently known distribution; the maps, however, are deliberately conservative, and additional fieldwork in various parts of the region certainly will extend the known limits of many mammals. In several cases where too few specimens of a species have been reported to allow the distribution in the tristate region to be shaded with confidence, only the actual localities of record are indicated

Sensory exploitation, sexual dimorphism, and human voice pitch

Selection for low male voice pitch is generally assumed to occur because it is a valid cue of formidability. Here we summarize recent empirical challenges to this hypothesis. We also outline an alternative account in which selection for low male voice pitch is a byproduct of sensory exploitation

Turboelectric distributed propulsion protection system design trades

The NASA N3-X blended-wing body with turboelectric distributed propulsion concept is being studied to achieve N+3 goals such as reduced noise, emissions, and improved energy efficiency. The electrical distribution system is cryogenic in order to maximize its efficiency and increase the power density of all associated components, while the motors, generators, and transmission lines are superconducting. The protection of a superconducting DC network poses unique electrical and thermal challenges due to the low impedance of the superconductor and operation in the superconducting or quenched states. For a given TeDP electrical system architecture with fixed power ratings, conventional and solid-state circuit breakers combined with superconducting fault-current limiters are examined with both voltage and current source control to limit and interrupt the fault current. To estimate the protection system weight and losses, scalable models of cryogenic bidirectional current-source converters, cryogenic bidirectional IGBT solid-state circuit breakers, and resistive-type superconducting fault current limiters are developed to assess how the weight and losses of these components vary as a function of nominal voltage and current and fault current ratings. The scalable models are used to assess the protection system weight for several trade-offs. System studies include the trade-off in fault-current limiting capability of SFCL on CB mass, alongside the fault-current limiting capability of the converter and its impact on CB fault-current interruption ratings and weight

Understanding the diverse needs of subtitle users in a rapidly evolving media landscape.

Audiences are increasingly using services such as video on demand and the web to watch television programmes. Broadcasters need to make subtitles available across all these new platforms. These platforms also create new design opportunities for subtitles along with the ability to customise them to an individual's needs. To explore these new opportunities for subtitles we have begun the process of reviewing the guidance for subtitles on television and evaluating the original user research. We have found that existing guidelines have been shaped by a mixture of technical constraints, industry practice and user research, constrained by existing technical standards. This paper provides an overview of the subtitle research at BBC R&D over the past two years. Our research is revealing significant diversity in the needs and preferences of frequent subtitle users, and points to the need for personalisation in the way subtitles are displayed. We are developing a new approach to the authoring and display of subtitles that can respond to the user requirements by adjusting the subtitle layout in the client device

Resource Control for Synchronous Cooperative Threads

We develop new methods to statically bound the resources needed for the execution of systems of concurrent, interactive threads. Our study is concerned with a \emph{synchronous} model of interaction based on cooperative threads whose execution proceeds in synchronous rounds called instants. Our contribution is a system of compositional static analyses to guarantee that each instant terminates and to bound the size of the values computed by the system as a function of the size of its parameters at the beginning of the instant. Our method generalises an approach designed for first-order functional languages that relies on a combination of standard termination techniques for term rewriting systems and an analysis of the size of the computed values based on the notion of quasi-interpretation. We show that these two methods can be combined to obtain an explicit polynomial bound on the resources needed for the execution of the system during an instant. As a second contribution, we introduce a virtual machine and a related bytecode thus producing a precise description of the resources needed for the execution of a system. In this context, we present a suitable control flow analysis that allows to formulte the static analyses for resource control at byte code level

A pre-design sensitivity analysis tool for consideration of full electrical aircraft propulsion electrical power system architectures

Turbo-electric distributed power (TeDP) systems proposed for hybrid wing body (HWB) N3-X aircraft are complex, superconducting electrical networks, which must be developed to meet challenging weight, efficiency and propulsor power requirements. An integrated system sensitivity analysis tool is presented, which can be used to support rapid appraisal studies of architectures, protection systems and redundancy requirements for TeDP systems. The use of this tool can help direct future research on TeDP systems towards the key challenges relevant to meeting the stringent weight and efficiency targets set out for N+3 aircraft concepts

Comparison of candidate architectures for future distributed propulsion aircraft

Turbine engine driven distributed electrical aircraft power systems (also referred to as Turboelectric Distributed Propulsion (TeDP)) are proposed for providing thrust for future aircraft with superconducting components operating at 77K in order for performance and emissions targets to be met. The proposal of such systems presents a radical change from current state-of-the-art aero-electrical power systems. Central to the development of such power systems are architecture design trades which must consider system functionality and performance, system robustness and fault ride-through capability, in addition to the balance between mass and efficiency. This paper presents a quantitative comparison of the three potential candidate architectures for TeDP electrical networks. This analysis provides the foundations for establishing the feasibility of these different architectures subject to design and operational constraints. The findings of this paper conclude that a purely AC synchronous network performs best in terms of mass and efficiency, but similar levels of functionality and controllability to an architecture with electrical decoupling via DC cannot readily be achieved. If power electronic converters with cryocoolers are found to be necessary for functionality and controllability purposes, then studies show that a significant increase in the efficiency of solid state switching components is necessary to achieve specified aircraft performance targets