2,615 research outputs found
Robustly Unstable Eigenmodes of the Magnetoshearing Instability in Accretion Disk
The stability of nonaxisymmetric perturbations in differentially rotating
astrophysical accretion disks is analyzed by fully incorporating the properties
of shear flows. We verify the presence of discrete unstable eigenmodes with
complex and pure imaginary eigenvalues, without any artificial disk edge
boundaries, unlike Ogilvie & Pringle(1996)'s claim. By developing the
mathematical theory of a non-self-adjoint system, we investigate the nonlocal
behavior of eigenmodes in the vicinity of Alfven singularities at
omega_D=omega_A, where omega_D is the Doppler-shifted wave frequency and
omega_A=k_// v_A is the Alfven frequency. The structure of the spectrum of
discrete eigenmodes is discussed and the magnetic field and wavenumber
dependence of the growth rate are obtained. Exponentially growing modes are
present even in a region where the local dispersion relation theory claims to
have stable eigenvalues. The velocity field created by an eigenmode is
obtained, which explains the anomalous angular momentum transport in the
nonlinear stage of this stability.Comment: 11pages, 11figures, to be published in ApJ. For associated eps files,
see http://dino.ph.utexas.edu/~knoguchi
Access to recreational physical activities by car and bus : an assessment of socio-spatial inequalities in mainland Scotland
Obesity and other chronic conditions linked with low levels of physical activity (PA) are associated with deprivation. One reason for this could be that it is more difficult for low-income groups to access recreational PA facilities such as swimming pools and sports centres than high-income groups. In this paper, we explore the distribution of access to PA facilities by car and bus across mainland Scotland by income deprivation at datazone level. GIS car and bus networks were created to determine the number of PA facilities accessible within travel times of 10, 20 and 30 minutes. Multilevel negative binomial regression models were then used to investigate the distribution of the number of accessible facilities, adjusting for datazone population size and local authority. Access to PA facilities by car was significantly (p<0.01) higher for the most affluent quintile of area-based income deprivation than for most other quintiles in small towns and all other quintiles in rural areas. Accessibility by bus was significantly lower for the most affluent quintile than for other quintiles in urban areas and small towns, but not in rural areas. Overall, we found that the most disadvantaged groups were those without access to a car and living in the most affluent areas or in rural areas
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XML-structured documents: Retrievable units and inheritance
We consider the retrieval of XML-structured documents, and of passages from such documents, defined as elements of the XML structure. These are considered from the point of view of passage retrieval, as a form of document retrieval. A retrievable unit (an element chosen as defining suitable passages for retrieval) is a textual document in its own right, but may inherit information from the other parts of the same document. Again, this inheritance is defined in terms of the XML structure. All retrievable units are mapped onto a common field structure, and the ranking function is a standard document retrieval function with a suitable field weighting. A small experiment to demonstrate the idea, using INEX data, is described
Use of synthetic fractures in the analysis of natural fracture apertures
Numerically synthesized models of rough fractures in rocks are promising alternatives to the expensive profilometry process in different applications. The authors present a new powerful and flexible method targeted for the generation of high-quality synthetic fracture models. This method has been implemented in SynFracTM software, developed by authors. Statistical analysis of suites of synthetic fractures has been performed. As result of the analysis, the dependence of the mean fracture aperture on the main parameters of bounding surfaces has been obtained. This technique and the developed software are not restricted to use with rock surfaces, but can be applied for imaging and modelling of any rough surfaces in any material
A translational framework for public health research
<p><b>Background</b></p>
<p>The paradigm of translational medicine that underpins frameworks such as the Cooksey report on the funding of health research does not adequately reflect the complex reality of the public health environment. We therefore outline a translational framework for public health research.</p>
<p><b>Discussion</b></p>
<p>Our framework redefines the objective of translation from that of institutionalising effective interventions to that of improving population health by influencing both individual and collective determinants of health. It incorporates epidemiological perspectives with those of the social sciences, recognising that many types of research may contribute to the shaping of policy, practice and future research. It also identifies a pivotal role for evidence synthesis and the importance of non-linear and intersectoral interfaces with the public realm.</p>
<p><b>Summary</b></p>
<p>We propose a research agenda to advance the field and argue that resources for 'applied' or 'translational' public health research should be deployed across the framework, not reserved for 'dissemination' or 'implementation'.</p>
An alpha theory of time-dependent warped accretion discs
The non-linear fluid dynamics of a warped accretion disc was investigated in
an earlier paper by developing a theory of fully non-linear bending waves in a
thin, viscous disc. That analysis is here extended to take proper account of
thermal and radiative effects by solving an energy equation that includes
viscous dissipation and radiative transport. The problem is reduced to simple
one-dimensional evolutionary equations for mass and angular momentum, expressed
in physical units and suitable for direct application. This result constitutes
a logical generalization of the alpha theory of Shakura & Sunyaev to the case
of a time-dependent warped accretion disc. The local thermal-viscous stability
of such a disc is also investigated.Comment: 16 pages, 3 figures, to be published in MNRA
Surface water monitoring in small water bodies: potential and limits of multi-sensor Landsat time series
Hydrometric monitoring of small
water bodies (1–10 ha) remains rare, due to their limited size and
large numbers, preventing accurate assessments of their agricultural
potential or their cumulative influence in watershed hydrology. Landsat
imagery has shown its potential to support mapping of small water bodies, but
the influence of their limited surface areas, vegetation growth, and rapid
flood dynamics on long-term surface water monitoring remains unquantified. A
semi-automated method is developed here to assess and
optimize the potential of
multi-sensor Landsat time series to monitor surface water extent and mean
water availability in these small water bodies. Extensive hydrometric field
data (1999–2014) for seven small reservoirs within the Merguellil catchment
in central Tunisia and SPOT imagery are used to calibrate the method and
explore its limits. The Modified Normalised Difference Water Index (MNDWI) is shown out of six
commonly used water detection indices to provide high overall accuracy and
threshold stability during high and low floods, leading to a mean surface
area error below 15 %. Applied to 546 Landsat 5, 7, and 8 images over
1999–2014, the method reproduces surface water extent variations across
small lakes with high skill (R2 = 0.9) and a mean root mean square error
(RMSE) of 9300 m2. Comparison with published global water datasets
reveals a mean RMSE of 21 800 m2 (+134 %) on the same lakes
and highlights the value of a tailored MNDWI approach to improve hydrological
monitoring in small lakes and reduce omission errors of flooded vegetation.
The rise in relative errors due to the larger proportion and influence of
mixed pixels restricts surface water monitoring below 3 ha with
Landsat (Normalised RMSE  =  27 %). Interferences from
clouds and scan line corrector failure on ETM+ after 2003 also decrease the
number of operational images by 51 %, reducing performance on lakes with
rapid flood declines. Combining Landsat observations with 10 m
pansharpened Sentinel-2 imagery further reduces RMSE to 5200 m2,
displaying the increased opportunities for surface water monitoring in small
water bodies after 2015.</p
An Incoherent Dynamo in Accretion Disks
We use the mean-field dynamo equations to show that an incoherent alpha
effect in mirror-symmetric turbulence in a shearing flow can generate a large
scale, coherent magnetic field. We illustrate this effect with simulations of a
few simple systems. In accretion disks, this process can lead to axisymmetric
magnetic domains whose radial and vertical dimensions will be comparable to the
disk height. This process may be responsible for observations of dynamo
activity seen in simulations of dynamo-generated turbulence involving, for
example, the Balbus-Hawley instability. In this case the magnetic field
strength will saturate at times the ambient pressure in real
accretion disks. The resultant dimensionless viscosity will be of the same
order. In numerical simulations the azimuthal extent of the simulated annulus
should be substituted for . We compare the predictions of this model to
numerical simulations previously reported by Brandenburg et al. (1995). In a
radiation pressure dominated environment this estimate for viscosity should be
reduced by a factor of due to magnetic buoyancy.Comment: 23 pages, uses aaste
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