11,783 research outputs found
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
HyRec: A fast and highly accurate primordial hydrogen and helium recombination code
We present a state-of-the-art primordial recombination code, HyRec, including
all the physical effects that have been shown to significantly affect
recombination. The computation of helium recombination includes simple analytic
treatments of hydrogen continuum opacity in the He I 2 1P - 1 1S line, the He
I] 2 3P - 1 1S line, and treats feedback between these lines within the
on-the-spot approximation. Hydrogen recombination is computed using the
effective multilevel atom method, virtually accounting for an infinite number
of excited states. We account for two-photon transitions from 2s and higher
levels as well as frequency diffusion in Lyman-alpha with a full radiative
transfer calculation. We present a new method to evolve the radiation field
simultaneously with the level populations and the free electron fraction. These
computations are sped up by taking advantage of the particular sparseness
pattern of the equations describing the radiative transfer. The computation
time for a full recombination history is ~2 seconds. This makes our code well
suited for inclusion in Monte Carlo Markov chains for cosmological parameter
estimation from upcoming high-precision cosmic microwave background anisotropy
measurements.Comment: Version accepted by PRD. Numerical integration switches adapted to be
well behaved for a wide range of cosmologies (Sec. V E). HyRec is available
at http://www.tapir.caltech.edu/~yacine/hyrec/hyrec.htm
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
Site Condition Assessments of Welsh SAC and SSSI Standing Water Features
This report was commissioned by the Countryside Council for Wales (CCW) in 2005 and
provides an assessment of the conservation status of Welsh Special Areas of Conservation
(SACs) and Sites of Special Scientific Interest (SSSIs). It details the site condition
assessments of 43 individual standing water features and provides overall assessments of the
13 SACs and 11 SSSIs in which they lie.
Site condition is assessed using Common Standards Monitoring (CSM) methods, where
specific habitat feature attributes are assessed against targets corresponding to ‘favourable’
condition. To make these assessments, data from CCW Contract Science Report no. 704
(Goldsmith et al. 2006) is employed, alongside further chemical and biological data collected
by ENSIS Ltd. and the Environment Agency (EA) between 2003-2005. Data from previous
reports and surveys is also utilised where available to provide a longer-term perspective and
possible evidence of trends.
The results of the site condition assessments are discussed in terms of general categories of
impact (e.g. acidification or eutrophication). Where sites were in unfavourable condition,
recommendations for further investigation and / or management are made. Reference is also
made to Water Framework Directive (WFD) Risk Assessments and some attempt is made to
relate condition assessment outcomes to the probability of sites failing to meet good
ecological status by 2015 in accordance with the objectives of Article 4 of the WFD.
Condition assessments for the oligotrophic to mesotrophic Welsh lake SACs (23 lakes) and
SSSIs (7 lakes) with vegetation of the Littorelletea uniflorae and / or of the IsoëtoNanojuncetea, indicate that approximately 80 % of lakes of this type are currently in
‘unfavourable’ (60 %) or ‘unfavourable, recovering’ (20 %) condition. Only one SAC -
Cadair Idris (3 lakes) – and three lakes within two other SACs are classified as ‘favourable’.
Acidification is the primary reason for failure to meet favourable condition targets,
particularly for SAC lakes. The recovery trends observed at a number of acid-impacted lakes
most likely relate to reductions in atmospheric deposition of sulphur and nitrogen. It is
expected that alkalinisation trends will continue provided that atmospheric deposition
stabilises or continues to decrease. Nutrient enrichment, grazing pressure, sediment inwash,
forestry operations and drawdown are further pressures that result in unfavourable condition
assessment outcomes. Eutrophication is of particular concern amongst SSSI lakes of this type.
All eleven Welsh lake SACs and SSSIs of the naturally eutrophic type (with Magnopotamion
or Hydrocharition-type vegetation) or hard oligo-mesotrophic Chara spp. type are classified
as ‘unfavourable’ (70 %) or ‘unfavourable, recovering’ (20 %), with one lake classified as
‘unfavourable, declining’. Eutrophication is the primary reason for failure to meet favourable
condition targets. However, unlike acidification, eutrophication may come from both point
and diffuse sources, and its effects may be exacerbated by local management practices such as
grazing and fish stocking.
For many eutrophied lakes there is scope to identify and reduce diffuse sources of nutrients
within the catchment. However, residual sediment nutrient concentrations may be
problematic, as may inappropriate fish communities resulting from past stocking practices.
Eutrophication can dramatically alter the structure and function of a lake ecosystem; therefore carefully constructed management plans must be implemented if favourable condition is to be
a realistic future target for impacted naturally eutrophic and hard-water Welsh lake SACs and
SSSIs.
Only one SAC in Wales is notified for the dystrophic lakes feature (2 lakes). This feature was
provisionally classified as unfavourable. However, the targets for this habitat type may
require refinement.
The report concludes by discussing uncertainty in lake classification, data confidence
concerns, CSM issues relating to survey methodology and the appropriateness of targets used
for condition assessment. Comparisons between the CSM approach and other lake assessment
methodologies are also considered. Overall recommendations for future monitoring and
assessment are provided
On non-perturbative corrections to the Kahler potential
We present the results of a detailed investigation into the consequences of
adding specific string motivated non-perturbative corrections to the usual tree
level Kahler potential in dilaton dominated scenarios. The success of the model
is judged through our ability to obtain a realistic VEV for the dilaton < Re S
> ~ 2, corresponding to the true minima of the scalar potential and being
associated with a reasonable value for the SUSY breaking scale via the
gravitino mass. The status of the so-called moduli problem is also reviewed in
each of the ansatze studied. Those include previous proposals made in the
context of both the chiral and the linear multiplet formalisms to describe
gaugino condensation, and a new ansatz which shows explicitly the equivalence
between the two.Comment: 11 pages, LaTex, uses psfig.sty with 4 figure
Scaling Solutions to 6D Gauged Chiral Supergravity
We construct explicitly time-dependent exact solutions to the field equations
of 6D gauged chiral supergravity, compactified to 4D in the presence of up to
two 3-branes situated within the extra dimensions. The solutions we find are
scaling solutions, and are plausibly attractors which represent the late-time
evolution of a broad class of initial conditions. By matching their near-brane
boundary conditions to physical brane properties we argue that these solutions
(together with the known maximally-symmetric solutions and a new class of
non-Lorentz-invariant static solutions, which we also present here) describe
the bulk geometry between a pair of 3-branes with non-trivial on-brane
equations of state.Comment: Contribution to the New Journal of Physics focus issue on Dark
Energy; 28 page
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
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, Kiełpińskie and Lidzbarskie, and
lower resolution diatom data from a further seven Polish lakes: Dąbrowa Wielka, Dąbrowa Mała,
Grądy, Tarczyńskie, 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
Derivation of the Semi-circle Law from the Law of Corresponding States
We show that, for the transition between any two quantum Hall states, the
semi-circle law and the existence of a duality symmetry follow solely from the
consistency of the law of corresponding states with the two-dimensional scaling
flow. This puts these two effects on a sound theoretical footing, implying that
both should hold exactly at zero temperature, independently of the details of
the microscopic electron dynamics. This derivation also shows how the
experimental evidence favours taking the two-dimensional flow seriously for the
whole transition, and not just near the critical points.Comment: 4 pages, 1 figure, typeset in LaTeX (uses revtex
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