9,257 research outputs found
First record of an Odontaspidid shark in Ascension Island waters
The occurrence of the poorly understood shark species Odontapsis ferox is reported at an oceanic seamount in the central south Atlantic, within the Exclusive Economic Zone of Ascension Island. The presence of the species at this location is confirmed by the discovery of a tooth embedded in scientific equipment, and footage of at least one animal on autonomous underwater video. The new record of this shark species at this location demonstrates the knowledge gaps which still exist at many remote, oceanic structures and their candidacy for status as important conservation areas.info:eu-repo/semantics/publishedVersio
On-Line Dependability Enhancement of Multiprocessor SoCs by Resource Management
This paper describes a new approach towards dependable design of homogeneous multi-processor SoCs in an example satellite-navigation application. First, the NoC dependability is functionally verified via embedded software. Then the Xentium processor tiles are periodically verified via on-line self-testing techniques, by using a new IIP Dependability Manager. Based on the Dependability Manager results, faulty tiles are electronically excluded and replaced by fault-free spare tiles via on-line resource management. This integrated approach enables fast electronic fault detection/diagnosis and repair, and hence a high system availability. The dependability application runs in parallel with the actual application, resulting in a very dependable system. All parts have been verified by simulation
Effective Actions, Boundaries and Precision Calculations of Casimir Energies
We perform the matching required to compute the leading effective boundary
contribution to the QED lagrangian in the presence of a conducting surface,
once the electron is integrated out. Our result resolves a confusion in the
literature concerning the interpretation of the leading such correction to the
Casimir energy. It also provides a useful theoretical laboratory for
brane-world calculations in which kinetic terms are generated on the brane,
since a lot is known about QED near boundaries.Comment: 5 pages. revtex; Added paragraphs describing finite-conductivity
effects and effects due to curvatur
Scaling theory for vortices in the two-dimensional inverse energy cascade
B.H.B. is supported by the Natural Environment Research Council grant NE/M014983/1.We propose a new similarity theory for the two-dimensional inverse energy cascade and the coherent vortex population it contains when forced at intermediate scales. Similarity arguments taking into account enstrophy conservation and a prescribed constant energy injection rate such that EâŒt yield three length scales, lÏ, lE and lÏ, associated with the vorticity field, energy peak and streamfunction, and predictions for their temporal evolutions, t1/2, t and t3/2, respectively. We thus predict that vortex areas grow linearly in time, AâŒl2ÏâŒt, while the spectral peak wavenumber kE ⥠2Ïlâ1E ⌠tâ1. We construct a theoretical framework involving a three-part, time-evolving vortex number density distribution, n(A) ⌠tαiAâri, i â 1,2,3. Just above the forcing scale (i =1) there is a forcing-equilibrated scaling range in which the number of vortices at fixed A is constant and vortex âself-energyâ Evcm = (2D)â1â«Ïv2A2n(A) dA is conserved in A-space intervals [ÎŒA0(t), A0(t)] comoving with the growth in vortex area, A0(t) ⌠t. In this range, α1 = 0 and n(A) ⌠Aâ3. At intermediate scales (i = 2) sufficiently far from the forcing and the largest vortex, there is a range with a scale-invariant vortex size distribution. We predict that in this range the vortex enstrophy Zvcm = (2D)â1â« Ïv2An(A)dA is conserved and n(A) ⌠tâ1Aâ1. The final range (i = 3), which extends over the largest vortex-containing scales, conserves Ïvcm = (2D)â1â« Ïv2n(A)dA. If Ïv2 is constant in time, this is equivalent to conservation of vortex number Nvcm =â« n(A)dA. This regime represents a âfrontâ of sparse vortices, which are effectively point-like; in this range we predict n(A) ⌠tr3â1Aâr3. Allowing for time-varying Ïv2 results in a small but significant correction to these temporal dependences. High-resolution numerical simulations verify the predicted vortex and spectral peak growth rates, as well as the theoretical picture of the three scaling ranges in the vortex population. Vortices steepen the energy spectrum E(k) past the classical kâ5/3 scaling in the range k â [kf , kv], where kv is the wavenumber associated with the largest vortex, while at larger scales the slope approaches â5/3. Though vortices disrupt the classical scaling, their number density distribution and evolution reveal deeper and more complex scale invariance, and suggest an effective theory of the inverse cascade in terms of vortex interactions.PostprintPeer reviewe
Potential vorticity fronts and the late-time evolution of large-scale quasi-geostrophic flows
Funding: BHB acknowledges support for this research from a Leverhulme Trust Early Career Fellowship.The late-time behaviour of freely evolving quasi-geostrophic flows with initial characteristic length scale L0 larger than or equal to the deformation radius LD, L0/LDâ„1 , is studied. At late time the flows are dominated by large multi-level vortices consisting of ascending terraces of well-mixed potential vorticity (PV), i.e. PV staircases. We examine how the number of mixed PV levels depends on the initial conditions, in particular L0/LD . For sufficiently large values of L0/LDâ5 , a complete staircase with regular steps forms, but as L0/LD decreases, the staircase becomes more irregular, with fewer mixed levels and the appearance of a large step centred on zero PV, corresponding to large regions of near-zero PV separating the multi-level vortices. This occurs because weak PV features in the initial field with scales smaller than LD undergo filamentation and are coarse-grained away or homogenised. For all values of L0/LD considered, inverse cascades of potential energy commence at sufficiently late times. The onset of these cascades, even when the flow is initialised well within the âasymptotic modelâ (AM) regime, suggests that the AM regime is not self-consistent: when potential vorticity fronts are well-resolved, frontal dynamics eventually drive ongoing flow evolution.Publisher PDFPeer reviewe
Quasiperpendicular high Mach number Shocks
Shock waves exist throughout the universe and are fundamental to
understanding the nature of collisionless plasmas. Reformation is a process,
driven by microphysics, which typically occurs at high Mach number
supercritical shocks. While ongoing studies have investigated this process
extensively both theoretically and via simulations, their observations remain
few and far between. In this letter we present a study of very high Mach number
shocks in a parameter space that has been poorly explored and we identify
reformation using in situ magnetic field observations from the Cassini
spacecraft at 10 AU. This has given us an insight into quasi-perpendicular
shocks across two orders of magnitude in Alfven Mach number (MA) which could
potentially bridge the gap between modest terrestrial shocks and more exotic
astrophysical shocks. For the first time, we show evidence for cyclic
reformation controlled by specular ion reflection occurring at the predicted
timescale of ~0.3 {\tau}c, where {\tau}c is the ion gyroperiod. In addition, we
experimentally reveal the relationship between reformation and MA and focus on
the magnetic structure of such shocks to further show that for the same MA, a
reforming shock exhibits stronger magnetic field amplification than a shock
that is not reforming.Comment: Accepted and Published in Physical Review Letters (2015
Diatom analysis of Polish cores
The purpose of this project is to supply high resolution diatom data to the Norwegian Institute of Water Research for cores from three Polish lakes: Lakes Rumian, KieĆpiĆskie and Lidzbarskie, and
lower resolution diatom data from a further seven Polish lakes: DÄ
browa Wielka, DÄ
browa MaĆa,
GrÄ
dy, TarczyĆskie, Zwiniarz, Zarybinek and Hartowieckie. The data will feed into a
palaeolimnological project which also includes analysis of algal pigments and radiometric dating of
the cores. The study aims to assess shifts in the diatom assemblages and to determine the nature of the baseline assemblages. Additionally the project aims to apply an existing diatom-phosphorus (P)
transfer function to the diatom data in order to infer the trophic histories of the lakes
Ultrafast effective multi-level atom method for primordial hydrogen recombination
Cosmological hydrogen recombination has recently been the subject of renewed
attention because of its importance for predicting the power spectrum of cosmic
microwave background anisotropies. It has become clear that it is necessary to
account for a large number n >~ 100 of energy shells of the hydrogen atom,
separately following the angular momentum substates in order to obtain
sufficiently accurate recombination histories. However, the multi-level atom
codes that follow the populations of all these levels are computationally
expensive, limiting recent analyses to only a few points in parameter space. In
this paper, we present a new method for solving the multi-level atom
recombination problem, which splits the problem into a computationally
expensive atomic physics component that is independent of the cosmology, and an
ultrafast cosmological evolution component. The atomic physics component
follows the network of bound-bound and bound-free transitions among excited
states and computes the resulting effective transition rates for the small set
of "interface" states radiatively connected to the ground state. The
cosmological evolution component only follows the populations of the interface
states. By pre-tabulating the effective rates, we can reduce the recurring cost
of multi-level atom calculations by more than 5 orders of magnitude. The
resulting code is fast enough for inclusion in Markov Chain Monte Carlo
parameter estimation algorithms. It does not yet include the radiative transfer
or high-n two-photon processes considered in some recent papers. Further work
on analytic treatments for these effects will be required in order to produce a
recombination code usable for Planck data analysis.Comment: Version accepted by Phys. Rev. D. Proof of equivalence of effective
and standard MLA methods moved to the main text. Some rewording
On the spacing of meandering jets in the strong-stair limit
Based on an assumption of strongly inhomogeneous potential vorticity mixing in quasi-geostrophic -plane turbulence, a relation is obtained between the mean spacing of latitudinally meandering zonal jets and the total kinetic energy of the flow. The relation applies to cases where the Rossby deformation length is much smaller than the Rhines scale, in which kinetic energy is concentrated within the jet cores. The relation can be theoretically achieved in the case of perfect mixing between regularly spaced jets with simple meanders, and of negligible kinetic energy in flow structures other than in jets. Incomplete mixing or unevenly spaced jets will result in jets being more widely separated than the estimate, while significant kinetic energy outside the jets will result in jets closer than the estimate. An additional relation, valid under the same assumptions, is obtained between the total kinetic and potential energies. In flows with large-scale dissipation, the two relations provide a means to predict the jet spacing based only on knowledge of the energy input rate of the forcing and dissipation rate, regardless of whether the latter takes the form of frictional or thermal damping. Comparison with direct numerical integrations of the forced system shows broad support for the relations, but differences between the actual and predicted jet spacings arise both from the complex structure of jet meanders and the non-negligible kinetic energy contained in the turbulent background and in coherent vortices lying between the jets.PostprintPeer reviewe
Fermionic alpha-vacua
A spin one-half particle propagating in a de Sitter background has a one
parameter family of states which transform covariantly under the isometry group
of the background. These states are the fermionic analogues of the alpha-vacua
for a scalar field. We shall show how using a point-source propagator for a
fermion in an alpha-state produces divergent perturbative corrections. These
corrections cannot be used to cancel similar divergences arising from scalar
fields in bosonic alpha-vacua since they have an incompatible dependence on the
external momenta. The theory can be regularized by modifying the propagator to
include an antipodal source.Comment: 13 pages, 3 eps figures, uses RevTe
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