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