Multidimensional simulations of magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma collision

The energetic electromagnetic eruptions observed during the prompt phase of gamma-ray bursts are attributed to synchrotron emissions. The internal shocks moving through the ultrarelativistic jet, which is ejected by an imploding supermassive star, are the likely source of this radiation. Synchrotron emissions at the observed strength require the simultaneous presence of powerful magnetic fields and highly relativistic electrons. We explore with one and three-dimensional relativistic particle-in-cell simulations the transition layer of a shock, that evolves out of the collision of two plasma clouds at a speed 0.9c and in the presence of a quasi-parallel magnetic field. The cloud densities vary by a factor of 10. The number densities of ions and electrons in each cloud, which have the mass ratio 250, are equal. The peak Lorentz factor of the electrons is determined in the 1D simulation, as well as the orientation and the strength of the magnetic field at the boundary of the two colliding clouds. The relativistic masses of the electrons and ions close to the shock transition layer are comparable as in previous work. The 3D simulation shows rapid and strong plasma filamentation behind the transient precursor. The magnetic field component orthogonal to the initial field direction is amplified in both simulations to values that exceed those expected from the shock compression by over an order of magnitude. The forming shock is quasi-perpendicular due to this amplification. The simultaneous presence of highly relativistic electrons and strong magnetic fields will give rise to significant synchrotron emissions.Comment: 8 pages, 5 figures. This work was presented at 21st International Conference on Numerical Simulation of Plasmas (ICNSP'09). Accepted for publication IEEE Trans. on Plasma Scienc

Particle-in-cell simulation of a mildly relativistic collision of an electron-ion plasma carrying a quasi-parallel magnetic field: Electron acceleration and magnetic field amplification at supernova shocks

Plasma processes close to SNR shocks result in the amplification of magnetic fields and in the acceleration of electrons, injecting them into the diffusive acceleration mechanism. The acceleration of electrons and the B field amplification by the collision of two plasma clouds, each consisting of electrons and ions, at a speed of 0.5c is investigated. A quasi-parallel guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of 25 keV are considered. A quasi-planar shock forms at the front of the dense plasma cloud. It is mediated by a circularly left-hand polarized electromagnetic wave with an electric field component along the guiding magnetic field. Its propagation direction is close to that of the guiding field and orthogonal to the collision boundary. It has a low frequency and a wavelength that equals several times the ion inertial length, which would be indicative of a dispersive Alfven wave close to the ion cyclotron resonance frequency of the left-handed mode (ion whistler), provided that the frequency is appropriate. However, it moves with the super-alfvenic plasma collision speed, suggesting that it is an Alfven precursor or a nonlinear MHD wave such as a Short Large-Amplitude Magnetic Structure (SLAMS). The growth of the magnetic amplitude of this wave to values well in excess of those of the quasi-parallel guiding field and of the filamentation modes results in a quasi-perpendicular shock. We present evidence for the instability of this mode to a four wave interaction. The waves developing upstream of the dense cloud give rise to electron acceleration ahead of the collision boundary. Energy equipartition between the ions and the electrons is established at the shock and the electrons are accelerated to relativistic speeds.Comment: 16 pages, 18 figures, Accepted for publication by Astron & Astrophy

Stress response and hypothetical genes in Desulfovibrio vulgaris Hildenborough

The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file.Thesis advisor: Dr. Judy D. Wall."December 2008"Thesis (M.S.) University of Missouri-Columbia 2008.The sulfate-reducing bacteria have a significant impact on the environment and the economy, necessitating further investigation of their physiology to harness their positive attributes and to minimize the damaging byproducts. Through the use of in vitro and in silico experiments, I have examined some of the metabolic pathways of Desulfovibrio vulgaris Hildenborough. The information obtained may contribute to an application of the SRB, D. vulgaris in particular, as an effective and economical means of bioremediation and also may play a role in controlling their activity in corrosion of metals and concrete. During my work on this project, I have developed software tools to expedite the creation of targeted deletion mutants through marker exchange, protein tagging, and to identify the insertion sites of randomly integrated transposons. A substantial amount of microarray data from stressed cultures has been generated by our collaborators that I have analyzed for trends across the experimental conditions. My analysis has yielded new insights to the general and specific stress response systems of D. vulgaris. An interesting subset of data, the massive subset of hypothetical genes, offers many tantalizing opportunities for further study with the hints revealed by my analysis. Evidence for the translation of hypothetical genes, revised annotations of the functional descriptions, and clustering according to gene activation was compiled and used to assist in clarification of the role of some of the hypothetical genes identified within the genome sequence. Finally, physiological characterization of select deletion mutants that I constructed has also revealed interesting involvement of the specific gene products in the stress responses. The tools that have been developed and the studies undertaken have yielded immediate results that have increased the knowledge base of the SRB. In addition, further questions became evident whose answers will hopefully lead to critical breakthroughs to reach the ultimate goal of a natural bioremediation tool.Includes bibliographical reference

Cosmic ray diffusive acceleration at shock waves with finite upstream and downstream escape boundaries

In the present paper we discuss the modifications introduced into the first-order Fermi shock acceleration process due to a finite extent of diffusive regions near the shock or due to boundary conditions leading to an increased particle escape upstream and/or downstream the shock. In the considered simple example of the planar shock wave we idealize the escape phenomenon by imposing a particle escape boundary at some distance from the shock. Presence of such a boundary (or boundaries) leads to coupled steepening of the accelerated particle spectrum and decreasing of the acceleration time scale. It allows for a semi-quantitative evaluation and, in some specific cases, also for modelling of the observed steep particle spectra as a result of the first-order Fermi shock acceleration. We also note that the particles close to the upper energy cut-off are younger than the estimate based on the respective acceleration time scale. In Appendix A we present a new time-dependent solution for infinite diffusive regions near the shock allowing for different constant diffusion coefficients upstream and downstream the shock.Comment: LaTeX, 14 pages, 4 postscript figures; Solar Physics (accepted

Listen to the band! How sound can realize group identity and enact intergroup domination

Recent research suggests that sound appraisal can be moderated by social identity. We validate this finding, and also extend it, by examining the extent to which sound can also be understood as instrumental in intergroup relations. We interviewed nine members of a Catholic enclave in predominantly Protestant East Belfast about their experiences of an outgroup (Orange Order) parade, where intrusive sound was a feature. Participants reported experiencing the sounds as a manifestation of the Orange Order identity and said that it made them feel threatened and anxious because they felt it was targeted at them by the outgroup (e.g., through aggressive volume increases). There was also evidence that the sounds produced community disempowerment, which interviewees explicitly linked to the invasiveness of the music. Some interviewees described organising to collectively ‘drown out’ the bands’ sounds, an activity which appeared to be uplifting. These findings develop the elaborated social identity model of empowerment, by showing that intergroup struggle and collective self-objectification can operate through sound as well as through physical actions

A survey of UK Special School Speech & Language Therapists (SLT) and school staff regarding the use of mealtime mats to provide eating and drinking guidelines

Purpose: A survey of UK Special School Speech & Language Therapists (SLT) and school staff regarding the use of mealtime mats to provide eating and drinking guidelines. Method: Two questionnaires were developed to collect experiences of creating and using mealtime mats within UK special schools including benefits and disadvantages of this method, training provided and improvement suggestions. SLTs (11) completed an online survey and education staff members of a UK special school (25) completed a paper questionnaire. The data was analysed using descriptive statistics and reviewing the free text comments. Results: SLTs reported usage of mealtime mats with many utilising similar formats and areas of advice covered. Some aspects received less focus e.g. communication and environment. SLTs reported difficulties in continuing to provide mealtime mats, despite being positive about their usage, due to increased service pressures. This was often without active evaluation of the effectiveness of mealtime mats. Only 2 school staff members reported using mealtime mat and they had a different name. Other school staff were aware of the mats but did not support children with complex dysphagia. However they reported that a mealtime mat resource for children with autism would be useful. Conclusions: This survey has shown that mealtime mats are being utilised within UK special schools and are viewed positively. However there is variety in usage in terms of name, format, content and evaluation. Research to determine the effectiveness of mealtime mats in special schools and other settings, including how to implement them, would be beneficial

Inverse Compton Emission from Galactic Supernova Remnants: Effect of the Interstellar Radiation Field

The evidence for particle acceleration in supernova shells comes from electrons whose synchrotron emission is observed in radio and X-rays. Recent observations by the HESS instrument reveal that supernova remnants also emit TeV gamma-rays; long awaited experimental evidence that supernova remnants can accelerate cosmic rays up to the ``knee'' energies. Still, uncertainty exists whether these gamma-rays are produced by electrons via inverse Compton scattering or by protons via neutral pion decay. The multi-wavelength spectra of supernova remnants can be fitted with both mechanisms, although a preference is often given to neutral pion decay due to the spectral shape at very high energies. A recent study of the interstellar radiation field indicates that its energy density, especially in the inner Galaxy, is higher than previously thought. In this paper we evaluate the effect of the interstellar radiation field on the inverse Compton emission of electrons accelerated in a supernova remnant located at different distances from the Galactic Centre. We show that contribution of optical and infra-red photons to the inverse Compton emission may exceed the contribution of cosmic microwave background and in some cases broaden the resulted gamma-ray spectrum. Additionally, we show that if a supernova remnant is located close to the Galactic Centre its gamma-ray spectrum will exhibit a ``universal'' cutoff at very high energies due to the Klein-Nishina effect and not due to the cut-off of the electron spectrum. As an example, we apply our calculations to the supernova remnants RX J1713.7-3946 and G0.9+0.1 recently observed by HESS.Comment: 4 pages, 4 figures. Uses emulateapj.cls. Accepted by ApJ