2,097 research outputs found
Medical education on fitness to drive : a survey of all UK medical schools
Aim: To identify the extent to which medical aspects of fitness to drive (FTD) are taught within UK medical schools.
Methods: A survey of all 32 UK medical schools. In-depth interviews with a range of staff at two medical schools; telephone survey of 30 schools.
Results: Two thirds of schools reported specific teaching on medical aspects of FTD but few covered it in any depth or in relation to specific medical conditions. Only one school taught FTD in relation to elderly medicine. FTD was an examination topic at only 12 schools.
Conclusion: Teaching on FTD is inconsistent across UK medical schools. Many new doctors will graduate with limited knowledge of medical aspects of FTD
Test results for composite specimens and elements containing joints and cutouts
A program was conducted to develop the technology for joints and cutouts in a composite fuselage that meets all design requirements of a large transport aircraft for the 1990s. An advanced trijet derivative of the DC-10 was selected as the baseline aircraft. Design and analysis of a 30-foot-long composite fuselage barrel provided a realistic basis for the test effort. The primary composite material was Hexcel F584 resin on 12 K IM6 fiber, in tape and broadgoods form. Fiberglass broadgoods were used in E-glass and S-glass fiber form in the cutout region of some panels. Additionally, injection-molded chopped graphite fiber/PEEK was used for longeron-to-frame shear clips. The test effort included four groups of test specimens, beginning with coupon specimens of mono-layer and cross-piled laminates, progressing through increasingly larger and more complex specimens, and ending with two 4- by 5-foot curved fuselage side panels. One of the side panels incorporated a transverse skin splice, while the second included two cabin window cutouts
Stretching of the toroidal field and generation of magnetosonic waves in differentially rotating plasma
We evaluate the generation of magnetosonic waves in differentially rotating
magnetized plasma. Differential rotation leads to an increase of the azimuthal
field by winding up the poloidal field lines into the toroidal field lines. An
amplification of weak seed perturbations is considered in this time-dependent
background state. It is shown that seed perturbations can be amplified by
several orders of magnitude in a differentially rotating flow. The only
necessary condition for this amplification is the presence of a non-vanishing
component of the magnetic field in the direction of the angular velocity
gradient.Comment: 5 pages, 5 figure
The Magnetohydrodynamics of Convection-Dominated Accretion Flows
Radiatively inefficient accretion flows onto black holes are unstable due to
both an outwardly decreasing entropy (`convection') and an outwardly decreasing
rotation rate (the `magnetorotational instability'; MRI). Using a linear
magnetohydrodynamic stability analysis, we show that long-wavelength modes are
primarily destabilized by the entropy gradient and that such `convective' modes
transport angular momentum inwards. Moreover, the stability criteria for the
convective modes are the standard Hoiland criteria of hydrodynamics. By
contrast, shorter wavelength modes are primarily destabilized by magnetic
tension and differential rotation. These `MRI' modes transport angular momentum
outwards. The convection-dominated accretion flow (CDAF) model, which has been
proposed for radiatively inefficient accretion onto a black hole, posits that
inward angular momentum transport and outward energy transport by
long-wavelength convective fluctuations are crucial for determining the
structure of the accretion flow. Our analysis suggests that the CDAF model is
applicable to a magnetohydrodynamic accretion flow provided the magnetic field
saturates at a sufficiently sub-equipartition value (plasma beta >> 1), so that
long-wavelength convective fluctuations can fit inside the accretion disk.
Numerical magnetohydrodynamic simulations are required to determine whether
such a sub-equipartition field is in fact obtained.Comment: 17 pages including 3 figures. Accepted for publication in ApJ. New
appendix and figure were added; some changes of the text were made in
response to the referee
First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
We report the first direct detection of a strong, 5 kG magnetic field on the
surface of an active brown dwarf. LSR J1835+3259 is an M8.5 dwarf exhibiting
transient radio and optical emission bursts modulated by fast rotation. We have
detected the surface magnetic field as circularly polarized signatures in the
819 nm sodium lines when an active emission region faced the Earth. Modeling
Stokes profiles of these lines reveals the effective temperature of 2800 K and
log gravity acceleration of 4.5. These parameters place LSR J1835+3259 on
evolutionary tracks as a young brown dwarf with the mass of 554 M and age of 224 Myr. Its magnetic field is at least 5.1 kG and covers
at least 11% of the visible hemisphere. The active region topology recovered
using line profile inversions comprises hot plasma loops with a vertical
stratification of optical and radio emission sources. These loops rotate with
the dwarf in and out of view causing periodic emission bursts. The magnetic
field is detected at the base of the loops. This is the first time that we can
quantitatively associate brown dwarf non-thermal bursts with a strong, 5 kG
surface magnetic field and solve the puzzle of their driving mechanism. This is
also the coolest known dwarf with such a strong surface magnetic field. The
young age of LSR J1835+3259 implies that it may still maintain a disk, which
may facilitate bursts via magnetospheric accretion, like in higher-mass T
Tau-type stars. Our results pave a path toward magnetic studies of brown dwarfs
and hot Jupiters.Comment: ApJ, in pres
Microwave performance of high-density bulk MgB2
We have performed microwave measurements on superconducting
hot-isostatically- pressed (HIPed) bulk MgB2 using a parallel-plate resonator
technique. The high density and strength of the HIPed material allowed
preparation of samples with mirror-like surfaces for microwave measurements.
The microwave surface resistance decreased by about 40% at 20 K when the
root-mean-square surface roughness was reduced from 220 nm to 110 nm through
surface-polishing and ion-milling. The surface resistance was independent of
surface microwave magnetic field at least up to 4 Oe and below 30 K. We
attribute this behavior, and the overall low surface resistance (~0.8 mOhms at
10 GHz and 20 K), to the high density of our samples and the absence of weak
links between grains
On the Nature of Angular Momentum Transport in Nonradiative Accretion Flows
The principles underlying a proposed class of black hole accretion models are
examined. The flows are generally referred to as ``convection-dominated,'' and
are characterized by inward transport of angular momentum by thermal convection
and outward viscous transport, vanishing mass accretion, and vanishing local
energy dissipation. We examine the viability of these ideas by explicitly
calculating the leading order angular momentum transport of axisymmetric modes
in magnetized, differentially rotating, stratified flows. The modes are
destabilized by the generalized magnetorotational instability. There are no
inward transporting modes at all unless the magnitude of the (imaginary) Brunt
frequency is comparable to the epicyclic frequency, a condition requiring
substantial levels of dissipation. When inward transporting modes do exist,
they appear at long wavelengths, unencumbered by magnetic tension. Moreover,
very general thermodynamic principles prohibit the complete recovery of
irreversible dissipative energy losses, a central feature of
convection-dominated models. Dissipation of the free energy of differential
rotation inevitably requires outward angular momentum transport. Our results
are in good agreement with global MHD simulations, which find significant
levels of outward transport and energy dissipation, whether or not
destabilizing entropy gradients are present.Comment: To appear in ApJ v. 573, 14 pages, 2 figures, AAS Latex macros 4.
Three-dimensional MHD Simulations of Radiatively Inefficient Accretion Flows
We present three-dimensional MHD simulations of rotating radiatively
inefficient accretion flows onto black holes. In the simulations, we
continuously inject magnetized matter into the computational domain near the
outer boundary, and we run the calculations long enough for the resulting
accretion flow to reach a quasi-steady state. We have studied two limiting
cases for the geometry of the injected magnetic field: pure toroidal field and
pure poloidal field. In the case of toroidal field injection, the accreting
matter forms a nearly axisymmetric, geometrically-thick, turbulent accretion
disk. The disk resembles in many respects the convection-dominated accretion
flows found in previous numerical and analytical investigations of viscous
hydrodynamic flows. Models with poloidal field injection evolve through two
distinct phases. In an initial transient phase, the flow forms a relatively
flattened, quasi-Keplerian disk with a hot corona and a bipolar outflow.
However, when the flow later achieves steady state, it changes in character
completely. The magnetized accreting gas becomes two-phase, with most of the
volume being dominated by a strong dipolar magnetic field from which a thermal
low-density wind flows out. Accretion occurs mainly via narrow slowly-rotating
radial streams which `diffuse' through the magnetic field with the help of
magnetic reconnection events.Comment: 35 pages including 3 built-in plots and 14 separate jpg-plots;
version accepted by Ap
Towards a New Standard Model for Black Hole Accretion
We briefly review recent developments in black hole accretion disk theory,
emphasizing the vital role played by magnetohydrodynamic (MHD) stresses in
transporting angular momentum. The apparent universality of accretion-related
outflow phenomena is a strong indicator that large-scale MHD torques facilitate
vertical transport of angular momentum. This leads to an enhanced overall rate
of angular momentum transport and allows accretion of matter to proceed at an
interesting rate. Furthermore, we argue that when vertical transport is
important, the radial structure of the accretion disk is modified at small
radii and this affects the disk emission spectrum. We present a simple model
demonstrating how energetic, magnetically-driven outflows modify the emergent
disk emission spectrum with respect to that predicted by standard accretion
disk theory. A comparison of the predicted spectra against observations of
quasar spectral energy distributions suggests that mass accretion rates
inferred using the standard disk model may severely underestimate their true
values.Comment: To appear in the Fifth Stromlo Symposium Proceedings special issue of
ApS
Extending the Shakura-Sunyaev approach to a strongly magnetized accretion disc model
We develop a model of thin turbulent accretion discs supported by magnetic
pressure of turbulent magnetic fields. This applies when the turbulent kinetic
and magnetic energy densities are greater than the thermal energy density in
the disc. Whether such discs survive in nature or not remains to be determined,
but here we simply demonstrate that self-consistent solutions exist when the
alpha-prescription for the viscous stress, similar to that of the original
Shakura-Sunyaev model, is used. We show that \alpha \sim 1 for the strongly
magnetized case and we calculate the radial structure and emission spectra from
the disc in the regime when it is optically thick. Strongly magnetized
optically thick discs can apply to the full range of disc radii for objects <
10^{-2} of the Eddington luminosity or for the outer parts of discs in higher
luminosity sources. In the limit that the magnetic pressure is equal to the
thermal or radiation pressure, our strongly magnetized disc model transforms
into the Shakura-Sunyaev model with \alpha=1. Our model produces spectra quite
similar to those of standard Shakura-Sunyaev models. In our comparative study,
we also discovered a small discrepancy in the spectral calculations of Shakura
and Sunyaev (1973).Comment: 27 pages, 11 figures, Astron. Astroph. in press; shortened version
accepted by A&A, all calculations and conclusions are unchange
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