7,055 research outputs found
Minimax Current Density Coil Design
'Coil design' is an inverse problem in which arrangements of wire are
designed to generate a prescribed magnetic field when energized with electric
current. The design of gradient and shim coils for magnetic resonance imaging
(MRI) are important examples of coil design. The magnetic fields that these
coils generate are usually required to be both strong and accurate. Other
electromagnetic properties of the coils, such as inductance, may be considered
in the design process, which becomes an optimization problem. The maximum
current density is additionally optimized in this work and the resultant coils
are investigated for performance and practicality. Coils with minimax current
density were found to exhibit maximally spread wires and may help disperse
localized regions of Joule heating. They also produce the highest possible
magnetic field strength per unit current for any given surface and wire size.
Three different flavours of boundary element method that employ different basis
functions (triangular elements with uniform current, cylindrical elements with
sinusoidal current and conic section elements with sinusoidal-uniform current)
were used with this approach to illustrate its generality.Comment: 24 pages, 6 figures, 2 tables. To appear in Journal of Physics D:
Applied Physic
The combinatorics of neurite self-avoidance
During neural development in Drosophila, the ability of neurite branches to recognize whether they are from the same or different neurons depends crucially on the molecule Dscam1. In particular, this recognition depends on the stochastic acquisition of a unique combination of Dscam1 isoforms out of a large set of possible isoforms. To properly interpret these findings, it is crucial to understand the combinatorics involved, which has previously been attempted only using stochastic simulations for some specific parameter combinations. Here we present closed-form solutions for the general case. These reveal the relationships among the key variables and how these constrain possible biological scenarios
Comments on the continuing widespread and unnecessary use of a defective emission equation in field emission related literature
Field electron emission (FE) has relevance in many different technological
contexts. However, many related technological papers use a physically defective
elementary FE equation for local emission current density (LECD). This equation
takes the tunneling barrier as exactly triangular, as in the original FE theory
of 90 years ago. More than 60 years ago, it was shown that the so-called
Schottky-Nordheim (SN) barrier, which includes an image-potential-energy term
(that models exchange-and-correlation effects) is better physics. For a
metal-like emitter with work-function 4.5 eV, the SN-barrier-related
Murphy-Good FE equation predicts LECD values that are higher than the
elementary equation values by a large factor, often between around 250 and
around 500. By failing to mention/apply this 60-year-old established science,
or to inform readers of the large errors associated with the elementary
equation, many papers (aided by defective reviewing) spread a new kind of
"pathological science", and create a modern research-integrity problem. The
present paper aims to enhance author and reviewer awareness by summarizing
relevant aspects of FE theory, by explicitly identifying the misjudgment in the
original 1928 Fowler-Nordheim paper, by explicitly calculating the size of the
resulting error, and by showing in detail why most FE theoreticians regard the
1950s modifications as better physics. Suggestions are made, about nomenclature
and about citation practice, that may help to diminish misunderstandings.Comment: Submitted for publication; in v2 a correction to historical
information (with no numerical consequences) has been made in Appendix
Sinking particle fluxes from the euphotic zone over the continental slope of an eastern boundary current region
We analyze data from sediment traps and current meters moored at two locations 100 km apart over the Vancouver Island continental slope during the spring and summer of 1990. Time-series of sinking particle fluxes, major biogenic components (biogenic silica, calcium carbonate, and particulate organic carbon and nitrogen), and stable isotopic composition (δ13Corganic and δ15Ntotal) were determined on samples obtained with sequential sediment traps moored at 200–250 m depth. Associated water property data were obtained from CTD/Rosette profiles taken during trap service periods and from current meters positioned in the surface layer and near the sediment trap. These data indicate that the two locations (a southern site J and a northern site NJ) were hydrographically distinct during the investigation. At site J, we found evidence for frequent upwelling events and more variability in the upper layer water properties. The main difference in the sinking fluxes of particles between the two sites was the occurrence of a one-week event at the end of May at J that contributed about one third of the total particle flux during the sampling period. Otherwise, the total flux collected during the study and the flux of major biogenic particles were similar at both sites. Silica shells dominated the flux of particles, particularly during the spring and early summer period. At both sites, particulate organic carbon rather than calcium carbonate was the main contributor to particulate carbon fluxes. The δ13Corganic showed marked variations during the sampling period at both sites likely due to variations in the growth rate of phytoplankton and in species composition. In comparison, variations in nitrate availability appear to dominate the changes in δ15Ntotal
Discovery of a Boxy Peanut Shaped Bulge in the Near Infrared
We report on the discovery of a boxy/peanut shaped bulge in the highly
inclined barred Seyfert 2 galaxy NGC~7582. The peanut shape is clearly evident
in near infrared images but obscured by extinction from dust in visible
images. This suggests that near infrared imaging surveys will discover a
larger number of boxy/peanut morphologies than visible surveys, particularly in
galaxies with heavy extinction such as NGC~7582. The bulge in NGC~7582 exhibits
strong boxiness compared to other boxy/peanut shaped bulges. If the starburst
was mediated by the bar, then it is likely that the bar formed in less than a
few bar rotation periods or a few years ago. If the bar also
caused the peanut, then the peanut would have formed quickly; on a timescale of
a few bar rotation periods.Comment: AAS Latex and Postcript Figures, accepted for publication in Ap
The Nature of LINERs
We present -band (m) spectroscopy of a sample of nine
galaxies showing some degree of LINER activity (classical LINERs, weak-[O {\sc
i}] LINERs and transition objects), together with -band spectroscopy for
some of them. A careful subtraction of the stellar continuum allows us to
obtain reliable [Fe {\sc ii}]m/Pa line ratios. We conclude
that different types of LINERs (i.e., photoionized by a stellar continuum or by
an AGN) cannot be easily distinguished based solely on the [Fe {\sc ii}]m/Pa line ratio.
The emission line properties of many LINERs can be explained in terms of an
aging starburst. The optical line ratios of these LINERs are reproduced by a
model with a metal-rich H {\sc ii} region component photoionized with a single
stellar temperature K, plus a supernova remnant (SNR) component.
The [Fe {\sc ii}] line is predominantly excited by shocks produced by SNRs in
starbursts and starburst-dominated LINERs, while Pa tracks H {\sc ii}
regions ionized by massive young stars. The contribution from SNRs to the
overall emission line spectrum is constrained by the [Fe {\sc ii}]m/Pa line ratio. Although our models for aging starbursts are
constrained only by these infrared lines, they consistently explain the optical
spectra of the galaxies also.
The LINER-starburst connection is tested by predicting the time dependence of
the ratio of the ionizing luminosity () to the supernova rate
(SNr), /(SNr). We predict the relative number of starbursts to
starburst-dominated LINERs (aging starbursts) and show that it is in
approximate agreement with survey findings for nearby galaxies.Comment: Accepted in ApJ (19 pages, 8 figures, uses emulateapj.sty
Photochemical production and loss rates of ozone at Sable Island, Nova Scotia during the North Atlantic Regional Experiment (NARE) 1993 summer intensive
Three weeks of summertime surface‐based chemical and meteorological observations at Sable Island, Nova Scotia during the North Atlantic Regional Experiment (NARE) 1993 summer intensive are used to study instantaneous photochemical production and loss rates of ozone by means of a numerical photochemical model. Results are most sensitive to the averaging scheme of data used to constrain the model and the ambient variability of the measurements. Model simulations driven by a time series of 5 min averaged data, most representative of the chemistry at the site, yield an average net photochemical ozone production of 3.6 ppbv/d. Estimates of net ozone production designed to filter out local sources, by using 1000–1400 LT median values of observations to drive the model and by excluding short‐lived hydrocarbons, give values ranging from 1 to 4 ppbv/d. These positive values of net ozone production within the marine boundary layer over Sable Island demonstrate the impact of polluted continental plumes on the background photochemistry of the region during the intensive. The dominant ambient variables controlling photochemical production and loss rates of ozone at the site during the measurement campaign appear to be levels of nitrogen oxides, ozone, nonmethane hydrocarbons, and solar intensity determined by cloud cover. The model partitioning of nitrogen oxides agrees for the most part with measurements, lending credence to calculated photochemical production and loss rates of ozone as well as inferred levels of peroxy radicals not measured at the site. Discrepancies, however, often occur during episodes of intermittent cloud cover, fog, and rain, suggesting the influence of cloud processes on air masses reaching the site
Probing Spectral Line Gradients Beyond One Effective Radius in NGC 3610
The outer region (0.75--1.25 r_e in the B-band) of the merger-remnant
elliptical NGC 3610 is studied using extremely high signal to noise Keck
spectra, with a supplementary spectrum of the galaxy center. Stellar population
parameters -- age, [Z/H], [/Fe] -- are measured in several apertures
along the slit. Using the multi-index simultaneous fitting method of Proctor et
al. (2004), no significant stellar population gradients are detected in the
outer parts of the galaxy. The overall gradients relative to the galaxy center
are consistent with those found in many other early-type galaxies, though the
metallicity gradient is much steeper than would be expected if NGC 3610 formed
in a major merger event. Standard analysis methods using the H index are
found to produce spurious radially variable gradients.Comment: 15 pages, 9 figures, accepted by A
Killer-cell immunoglobulin-like receptors and malaria caused by Plasmodium falciparum in The Gambia
The relevance of innate immune responses to Plasmodium falciparum infection, in particular the central role of natural killer (NK) cell-derived interferon gamma (IFN-γ), is becoming increasingly recognised. Recently, it has been shown that IFN-γ production in response to P. falciparum antigens is in part regulated by killer-cell immunoglobulin-like receptor (KIR) genes, and a study from malaria-exposed Melanesians suggested an association between KIR genotypes and susceptibility to infection. This prompted us to determine and compare the frequencies of 15 KIR genes in Gambian children presenting with either severe malaria (n = 133) or uncomplicated malaria (n = 188) and in cord-blood population control samples (n = 314) collected from the same area. While no significant differences were observed between severe and uncomplicated cases, proportions of individuals with KIR2DS2+C1 and KIR2DL2+C1 were significantly higher among malaria cases overall than in population control samples. In an exploratory analysis, activating KIR genes KIR2DS2, KIR3DS1 and KIR2DS5 were slightly higher in children in disease subgroups associated with the highest mortality. In addition, our data suggest that homozygosity for KIR genotype A might be associated with different malaria outcomes including protection from infection and higher blood parasitaemia levels in those that do get infected. These findings are consistent with a probable role of KIR genes in determining susceptibility to malaria, and further studies are warranted in different populations
Spatially-resolved potential measurement with ion crystals
We present a method to measure potentials over an extended region using
one-dimensional ion crystals in a radio frequency (RF) ion trap. The
equilibrium spacings of the ions within the crystal allow the determination of
the external forces acting at each point. From this the overall potential, and
also potentials due to specific trap features, are calculated. The method can
be used to probe potentials near proximal objects in real time, and can be
generalized to higher dimensions.Comment: 7 pages (double spaced), 3 figure
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