Overlength modes of transferred-electron oscillators

Simulations of n+?n?n+ GaAs devices show three régimes where the operating frequency substantially exceeds the normal transit frequency. The efficiency/frequency characteristics and the effects of length and carrier density differ from predictions for an idealised I.s.a. mode.     

Introduction

It is embarrassing that 95% of the Universe is unaccounted for. Galaxies and larger-scale cosmic structures are composed mainly of "dark matter", whose nature is still unknown. Favoured candidates are weakly interacting particles that have survived from the very early Universe, but more exotic options cannot be excluded. (There are strong arguments that the dark matter is not composed of baryons.) Intensive experimental searches are being made for the "dark" particles (which pervade our entire Galaxy), but we have indirect clues to their nature too. Inferences from galactic dynamics and gravitational lensing allow astronomers to "map" the dark-matter distribution; comparison with numerical simulations of Galaxy formation can constrain (for example) the particle velocities and collision cross-sections; and, of course, progress in understanding the extreme physics of the ultra-early Universe could offer clues to what particles might have existed then, and how many would have survived. The mean cosmic density of dark matter (plus baryons) is now pinned down to be only ca.30% of the so-called critical density corresponding to a "flat" Universe. However, other recent evidence-microwave background anisotropies, complemented by data on distant supernovae-reveals that our Universe actually is "flat", but that its dominant ingredient (ca.70% of the total mass energy) is something quite unexpected: "dark energy" pervading all space, with negative pressure. We now confront two mysteries. (i) Why does the Universe have three quite distinct basic ingredients-baryons, dark matter and dark energy-in the proportions (roughly) 5%, 25% and 70%? (ii) What are the (almost certainly profound) implications of the "dark energy" for fundamental physics?     

Improving the accuracy of low-cost GPS measurements for remote sensing applications

Field work in remote sensing often requires that the absolute position of a point on the Earth's surface can be determined with an accuracy of better than 100 m, for example to locate a Ground Control Point or to identify the land cover at a point corresponding to a particular pixel in an image. This paper assesses the extent to which low-cost, hand-held GPS (Global Positioning System) receivers can be used for this purpose. The nominal accuracy of GPS is 100 m, meaning that a single position measurement should yield a result within 100 m of the true position at least 95% of the time. Here, we investigate the extent to which this performance can be improved upon. Two approaches are studied: averaging several measurements, and correcting the measurement at one receiver using the known error at another. The first approach involves an investigation of the temporal correlation properties of the errors in a measured position. The correlation time is shown to be about 4 min for the horizontal coordinates and about 11 min for the vertical coordinate. Simple models are derived to allow the accuracy of an average measurement to be estimated, and it is shown that accuracies of typically 15 m (horizontally) and 30 m (vertically) can be achieved from 30 min of observation. The second approach involves an investigation of the spatial correlation properties of the errors. It is shown that these are inadequate to provide any improvement in accuracy.     

Potential operational multi-satellite sensor mapping of snow cover in maritime sub-polar regions

Scottish snow cover as an example of a maritime sub-polar region has two principal problems for an operational monitoring programme: the often ephemeral nature of the snow cover, and the loss of direct access to snow imagery due to clouds. At present only the NOAA AVHRR series provides images with the required temporal and spatial resolutions. Based on the availability of data from the Dundee satellite data receiving station a range of NOAA-12, -14 and -15 day and night passes were collected and processed. Three snow cover products were produced from the NOAA AVHRR/2 data: snow area based on channel 134 ISODATA classifications, percentage snow cover based on multi-temporal Normalized Difference Vegetation Index (NDVI), and daily maximum snow surface temperature maps using split-window combinations of thermal channels. Noted improvements were evident in the accuracy and resolution of snow cover classifications based on provisional testing of AVHRR/3 data. Maximum snow surface temperature maps indicated a potential for mapping areas of snow melt. The principal limitation in the operational snow cover mapping with AVHRR, however, remains the loss of temporal resolution due to cloud cover.     

Interleukin-1 mediates a rapid inflammatory response after injection of adenoviral vectors into the brain

Adenovirus-mediated gene transfer into the brain is associated with significant inflammation and activation of anti-vector and anti-transgene immune responses that curtail the gene delivery of adenoviruses and therapeutic efficacy. Elucidating the molecular mediators of inflammatory and immune responses to adenoviruses injected into the brain should allow us to inhibit their inflammatory actions, thereby reducing vector clearance and enhance adenoviral-mediated gene transfer into the CNS. Cytokines are primary mediators of the immune response and are released during inflammation. Here we report for the first time that injection of replication-deficient adenovirus vectors into the cerebral ventricles of rats causes a rapid increase in body temperature. This fever response precedes any vector-encoded transgene expression and occurs with vectors encoding no transgene, as well as with vectors encoding a therapeutic transgene i.e., HSV1-thymidine kinase. No fever is detected after infection of the striatum, an important brain target in studies on neurodegeneration. After infection of the brain ventricles, CSF levels of immunoreactive tumor necrosis factor (TNF)-alpha and interleukin (IL)-1beta increase significantly (up to 300-fold). In the hypothalamus, the locus of thermoregulation in the brain, only IL-1beta and IL-6 are significantly elevated. A neutralizing TNF-alpha antibody has no effect on adenovirus-induced fever. However, pretreatment with either the IL-1 receptor antagonist or the cyclooxygenase inhibitor flurbiprofen completely abolishes adenovirus-induced fever, suggesting that IL-1 and prostaglandins are direct mediators of this response. These results are the first to demonstrate that IL-1, but not TNF-alpha, is the main mediator of a very early inflammatory response to adenovirus in the brain.     

Calculation of mixing in nonlinear circuits

A novel scheme is described for calculating the harmonic response of a nonlinear circuit driven by two or more signals incommensurate in frequency. The method, which is a natural extension of the harmonic balance technique, is not limited by the degree of nonlinearity or signal strengths nor by their proximity in frequency.     

Optical switching of a GaAs triangular barrier switch grown by MBE

A 100 ?m-diameter GaAs triangular barrier switch (TBS) has been optically switched using a GaAlAs laser diode. The resulting transient response has been measured to have a leading edge of ~ 295 ps.     

Pyruvate prevents peroxide-induced injury of in vitro preimplantation bovine embryos

The impact of oxidative stress on the in vitro development of bovine embryos in synthetic oviduct fluid medium (mSOF) was assessed by using H2O2 as a stress inducer. In a preliminary experiment, a chemiluminescent method was used to measure the antioxidative capacity of the mSOF culture medium. Pyruvate was the mSOF component displaying the highest H2O2 degrading ability. Essential and nonessential amino acids also significantly reduced the H2O2 concentration, whereas lactate and glutamine were ineffective. The effect on further development of a short exposure of zygotes, 9-16-cell stage embryos and blastocysts to 0 M; 10(-7) M ; 10(-6) M, and 10(-5) M H2O2 in pyruvate-free mSOF was evaluated. Developmental rates of the H2O2-treated zygotes to the 5-8-cell or blastocyst stages and survival of H2O2-treated blastocysts were reduced in a dose-dependent manner whereas the 9-16-cell embryos were unaffected by those treatments. Blastocysts treated with H2O2 also tended to have lower numbers of bisbenzimide-stained nuclei and showed increased nuclear fragmentation. Including pyruvate in the mSOF culture medium during a 10(-5) M H2O2 pulse highly reduced the H2O2 concentration as measured by chemiluminescence and improved zygote and blastocyst development, but failed to prevent blastocyst nuclei degradation. These experiments suggest that bovine embryos show developmental change in sensitivity to exogenous H2O2, the 9-16-cell embryos being more resistant than zygotes and blastocysts and that H2O2 and its toxic effects can be attenuated by including pyruvate in the medium.     

Nonlinear gas turbine modeling using NARMAX structures

The estimation of a nonlinear autoregressive moving average with exogenous inputs (NARMAX) model of an aircraft gas turbine is presented. A method is proposed whereby periodic signals with certain harmonic content are used to qualify the nature of the nonlinearity of the engine in the frequency domain. The static behavior of the engine is investigated in the time domain to approximate the order of nonlinearity and this information is used a priori to restrict the search space of the potential NARMAX models. A forward-regression orthogonal estimation algorithm is then employed to select the model terms using the error reduction ratio. The performance of the estimated NARMAX model is illustrated against a range of small- and large-signal engine tests     

Longitudinal trends in river functioning: Patterns of nutrient and carbon processing in three Australian rivers

Understanding longitudinal trends in the processing of carbon in rivers represents a much conceptualised, but infrequently tested, issue in aquatic ecology. In this study, we conducted concurrent longitudinal examinations of three very different rivers in eastern Australia to determine whether general principles in river functioning exist across broad geographic and hydrologic scales. Specifically, we examined trends in ambient basic water chemistry, nutrient concentrations, dissolved organic carbon (DOC), extracellular enzymes and food web structure and functioning and conducted bioassays to examine the degree to which DOC and nutrients limit heterotrophic bacterial respiration. These parameters revealed striking similarities across all sites. For metazoan communities, stable isotope analysis showed that algal carbon was the dominant basal resource utilised by consumers in all three rivers, suggesting that in‐stream primary producers strongly underpin trophic pathways regardless of the position within a catchment or catchment condition. Analyses of extracellular enzymes revealed that microbial communities are actively utilising DOC at all sites. In fact, heterotrophic microbial respiration was strongly limited by DOC at all sites, with nutrient additions resulting in only relatively minor increases in respiration. Ultimately, this study demonstrates that DOC and algal carbon are critically important drivers of ecosystem processes in Australian riverine ecosystems. Furthermore, across all of our sites and rivers, ambient nutrient concentrations did not influence carbon processing. The consistent longitudinal trends in river function identified in this study provide useful insights for catchment managers and modellers with respect to identifiying key principles that underpin ecosystem functioning in Australian rivers. Copyright © 2009 John Wiley & Sons, Ltd.