2,886 research outputs found
The Evolution of Modern Sovereign Debt Litigation: Vultures, Alter Egos, and Other Legal Fauna
Pulsars With Jets May Harbor Dynamically Important Accretion Disks
For many astrophysical sources with jets, there is evidence for the
contemporaneous presence of disks. In contrast, pulsars such as the Crab and
Vela show jets but have not yet revealed direct evidence for accretion disks.
Here we show that for such pulsars, an accretion disk radiating below
detectable thresholds may simultaneously account for (1) observed deviations in
the braking indices from that of the simple dipole, (2) observed pulsar timing
ages, and (3) possibly even the jet morphology via a disk outflow that
interacts with the pulsar wind within, collimating and/or redirecting it.Comment: 10 pages, 2 figs., in press, ApJ. Let
Graduate-entry medical student variables that predict academic and clinical achievement
A hypothetical model was formulated to explore factors that influenced academic and clinical achievement for graduate-entry medical students completing their third year of university studies. Nine latent variables were considered including the students' background, previous successes with their undergraduate and postgraduate studies and their assessed ability to study graduate-entry medicine based on their scholastic aptitude and their interview selection scores. The academic and clinical achievement of 99 graduate-entry medical students were estimated by measuring their performance on two separate assessment procedures, a 150 item multiple choice examination and a 20 item objectively structured clinical evaluation (OSCE) test. These two assessments were taken across two years (to include two student groups) and were equated using Rasch scaling procedures. Models identifying causal pathways leading to academic and clinical achievement were tested using Partial Least Squares Path Analysis (PLSPAT). The study's results suggest that medical student achievement can be predicted by variables, which account for 6 to 22 percent of the variance of scores that assess academic achievement and clinical performance at the third year level respectively. The most significant predictors and those which had direct influence on graduate-entry medical student achievement were: (a) student gender, undergraduate grade point average scores, type of undergraduate studies undertaken, and where those studies were carried out that were related to the OSCE scores, and (b) whether or not the graduate-entry medical students had pursued other studies prior to undertaking the medical course and age that were both negatively related to achievement on the multiple choice examination. Measures of performance at interview and student scores for GAMSAT that were used in the selection process were not related to the performance outcomes assessed. [Author abstract
New mechanism of generation of large-scale magnetic field in a sheared turbulent plasma
A review of recent studies on a new mechanism of generation of large-scale
magnetic field in a sheared turbulent plasma is presented. This mechanism is
associated with the shear-current effect which is related to the W x J-term in
the mean electromotive force. This effect causes the generation of the
large-scale magnetic field even in a nonrotating and nonhelical homogeneous
sheared turbulent convection whereby the alpha effect vanishes. It is found
that turbulent convection promotes the shear-current dynamo instability, i.e.,
the heat flux causes positive contribution to the shear-current effect.
However, there is no dynamo action due to the shear-current effect for small
hydrodynamic and magnetic Reynolds numbers even in a turbulent convection, if
the spatial scaling for the turbulent correlation time is k^{-2}, where k is
the small-scale wave number. We discuss here also the nonlinear mean-field
dynamo due to the shear-current effect and take into account the transport of
magnetic helicity as a dynamical nonlinearity. The magnetic helicity flux
strongly affects the magnetic field dynamics in the nonlinear stage of the
dynamo action. When the magnetic helicity flux is not small, the saturated
level of the mean magnetic field is of the order of the equipartition field
determined by the turbulent kinetic energy. The obtained results are important
for elucidation of origin of the large-scale magnetic fields in astrophysical
and cosmic sheared turbulent plasma.Comment: 7 pages, Planetory and Space Science, in pres
Probing the causes of thermal hysteresis using tunable N-agg micelles with linear and brush-like thermoresponsive coronas
Self-assembled thermoresponsive polymers in aqueous solution have great potential as smart, switchable materials for use in biomedical applications. In recent years, attention has turned to the reversibility of these polymers’ thermal transitions, which has led to debate over what factors influence discrepancies in the transition temperature when heating the system compared to the temperature obtained when cooling the system, known as the thermal hysteresis. Herein, we synthesize micelles with tunable aggregation numbers (Nagg) whose cores contain poly(n-butyl acrylate-co-N,N-dimethylacrylamide) (p(nBA-co-DMA)) and four different thermoresponsive corona blocks, namely poly(N-isopropylacrylamide) (pNIPAM), poly(N,N-diethylacrylamide) (pDEAm), poly(diethylene glycol monomethyl ether methacrylate) (pDEGMA) and poly(oligo(ethylene glycol) monomethyl ether methacrylate) (pOEGMA). By studying their thermoresponsive behavior, we elucidate the effects of changing numerous important characteristics both in the thermoresponsive chain chemistry and architecture, and in the structure of their self-assemblies. Our findings demonstrate large deviations in the reversibility between the self-assemblies and the corresponding thermoresponsive homopolymers; specifically we find that micelles whose corona consist of polymers with a brush-like architecture (pDEGMA and pOEGMA) exhibit irreversible phase transitions at a critical chain density. These results lead to a deeper understanding of stimuli-responsive self-assemblies and demonstrate the potential of tunable Nagg micelles for uncovering structure–property relationships in responsive polymer systems
Current helicity of active regions as a tracer of large-scale solar magnetic helicity
We demonstrate that the current helicity observed in solar active regions
traces the magnetic helicity of the large-scale dynamo generated field. We use
an advanced 2D mean-field dynamo model with dynamo saturation based on the
evolution of the magnetic helicity and algebraic quenching. For comparison, we
also studied a more basic 2D mean-field dynamo model with simple algebraic
alpha quenching only. Using these numerical models we obtained butterfly
diagrams both for the small-scale current helicity and also for the large-scale
magnetic helicity, and compared them with the butterfly diagram for the current
helicity in active regions obtained from observations. This comparison shows
that the current helicity of active regions, as estimated by
evaluated at the depth from which the active region arises, resembles the
observational data much better than the small-scale current helicity calculated
directly from the helicity evolution equation. Here and are
respectively the dynamo generated mean magnetic field and its vector potential.
A theoretical interpretation of these results is given.Comment: 11 pages, 5 figures, revised versio
Accretion Disks and Dynamos: Toward a Unified Mean Field Theory
Conversion of gravitational energy into radiation in accretion discs and the
origin of large scale magnetic fields in astrophysical rotators have often been
distinct topics of research. In semi-analytic work on both problems it has been
useful to presume large scale symmetries, necessarily resulting in mean field
theories. MHD turbulence makes the underlying systems locally asymmetric and
nonlinear. Synergy between theory and simulations should aim for the
development of practical mean field models that capture essential physics and
can be used for observational modeling. Mean field dynamo (MFD) theory and
alpha-viscosity accretion theory exemplify such ongoing pursuits. 21st century
MFD theory has more nonlinear predictive power compared to 20th century MFD
theory, whereas accretion theory is still in a 20th century state. In fact,
insights from MFD theory are applicable to accretion theory and the two are
artificially separated pieces of what should be a single theory. I discuss
pieces of progress that provide clues toward a unified theory. A key concept is
that large scale magnetic fields can be sustained via local or global magnetic
helicity fluxes or via relaxation of small scale magnetic fluctuations, without
the kinetic helicity driver of 20th century textbooks. These concepts may help
explain the formation of large scale fields that supply non-local angular
momentum transport via coronae and jets in a unified theory of accretion and
dynamos. In diagnosing the role of helicities and helicity fluxes in disk
simulations, each disk hemisphere should be studied separately to avoid being
misled by cancelation that occurs as a result of reflection asymmetry. The
fraction of helical field energy in disks is expected to be small compared to
the total field in each hemisphere as a result of shear, but can still be
essential for large scale dynamo action.Comment: For the Proceedings of the Third International Conference and
Advanced School "Turbulent Mixing and Beyond," TMB-2011 held on 21 - 28
August 2011 at the Abdus Salam International Centre for Theoretical Physics,
Trieste, http://users.ictp.it/~tmb/index2011.html Italy, To Appear in Physica
Scripta (corrected small items to match version in print
Comparison of photo- and thermally initiated polymerization-induced self-assembly : a lack of end group fidelity drives the formation of higher order morphologies
Polymerization-induced self-assembly (PISA) is an emerging industrially relevant technology, which allows the preparation of defined and predictable polymer self-assemblies with a wide range of morphologies. In recent years, interest has turned to photoinitiated PISA processes, which show markedly accelerated reaction kinetics and milder conditions, thereby making it an attractive alternative to thermally initiated PISA. Herein, we attempt to elucidate the differences between these two initiation methods using isothermally derived phase diagrams of a well-documented poly(ethylene glycol)-b-(2-hydroxypropyl methacrylate) (PEG-b-HPMA) PISA system. By studying the influence of the intensity of the light source used, as well as an investigation into the thermodynamically favorable morphologies, the factors dictating differences in the obtained morphologies when comparing photo- and thermally initiated PISA were explored. Our findings indicate that differences in a combination of both reaction kinetics and end group fidelity led to the observed discrepencies between the two techniques. We find that the loss of the end group in photoinitiated PISA drives the formation of higher order structures and that a morphological transition from worms to unilamellar vesicles could be induced by extended periods of light and heat irradiation. Our findings demonstrate that PISA of identical block copolymers by the two different initiation methods can lead to structures that are both chemically and morphologically distinct
- …