17,129 research outputs found
Simulations of a Magnetic Fluctuation Driven Large Scale Dynamo and Comparison with a Two-scale Model
Models of large scale (magnetohydrodynamic) dynamos (LSD) which couple large
scale field growth to total magnetic helicity evolution best predict the
saturation of LSDs seen in simulations. For the simplest so called "{\alpha}2"
LSDs in periodic boxes, the electromotive force driving LSD growth depends on
the difference between the time-integrated kinetic and current helicity
associated with fluctuations. When the system is helically kinetically forced
(KF), the growth of the large scale helical field is accompanied by growth of
small scale magnetic (and current) helicity which ultimately quench the LSD.
Here, using both simulations and theory, we study the complementary
magnetically forced(MF) case in which the system is forced with an electric
field that supplies magnetic helicity. For this MF case, the kinetic helicity
becomes the back-reactor that saturates the LSD. Simulations of both MF and KF
cases can be approximately modeled with the same equations of magnetic helicity
evolution, but with complementary initial conditions. A key difference between
KF and MF cases is that the helical large scale field in the MF case grows with
the same sign of injected magnetic helicity, whereas the large and small scale
magnetic helicities grow with opposite sign for the KF case. The MF case can
arise even when the thermal pressure is approximately smaller than the magnetic
pressure, and requires only that helical small scale magnetic fluctuations
dominate helical velocity fluctuations in LSD driving. We suggest that LSDs in
accretion discs and Babcock models of the solar dynamo are actually MF LSDs.Comment: 12 pages, 34 figure
Effect of Fractional Kinetic Helicity on Turbulent Magnetic Dynamo Spectra
Magnetic field amplification in astrophysics ultimately requires an
understanding of magnetohydrodynamic turbulence. Kinetic helicity has long been
known to be important for large scale field growth in forced MHD turbulence,
and has been recently demonstrated numerically to be asymptotically consistent
with slow mean field dynamo action in a periodic box. Here we show numerically
that the magnetic spectrum at and below the forcing scale is also strongly
influenced by kinetic helicity. We identify a critical value,
above which the magnetic spectrum develops maxima at wavenumber scale
{\it and} at the forcing scale, For the field peaks only at the
resistive scale. Kinetic helicity may thus be important not only for generating
a large scale field, but also for establishing observed peaks in magnetic
spectra at the forcing scale. The turbulent Galactic disk provides an example
where both large scale ( supernova forcing scale) fields and small scale
( forcing scale, with peak at forcing scale) fields are observed. We
discuss this, and the potential application to the protogalaxy, but also
emphasize the limitations in applying our results to these systems.Comment: version accepted to ApJL, 10 pages, 3 fig
Current Developments in Services for People with Intellectual Disabilities
[Taken from Executive Summary]
This literature review is the culmination of the Saskatchewan Community Living Division jurisdictional study which began in the autumn of 2003. Following a brief survey of developments in providing services to people with intellectual disabilities (hitherto the People) for creating the questionnaire for this study, information was gleaned from the provinces and territories on their services. The CLD Jurisdictional Project was completed in the spring of 2005. Subsequently, a thorough search and examination of pertinent resources for serving this People and for policy development was conducted. From over 800 documents about 350 were selected for this literature review.
The material is recorded in the following chapters: Public Consultation and Policy Development; Social Philosophy: the philosophical influence on contemporary social issues; Definition of disabilities; Needs assessment systems; Human Rights; Advocacy; Community services & Deinstitutionalization; Issues and Influences; Citizenship; Inclusion; Self-determination; Person-centered planning; Supports; Respite; Individualized funding; Canadian governmental initiatives; Provincial Services
Identifying Deficiencies of Standard Accretion Disk Theory: Lessons from a Mean-Field Approach
Turbulent viscosity is frequently used in accretion disk theory to replace
the microphysical viscosity in order to accomodate the observational need for
in- stabilities in disks that lead to enhanced transport. However, simply
replacing the microphysical transport coefficient by a single turbulent
transport coeffi- cient hides the fact that the procedure should formally arise
as part of a closure in which the hydrodynamic or magnetohydrodynamic equations
are averaged, and correlations of turbulent fluctuations are replaced by
transport coefficients. Here we show how a mean field approach leads quite
naturally two transport coefficients, not one, that govern mass and angular
momentum transport. In particular, we highlight that the conventional approach
suffers from a seemingly inconsistent neglect of turbulent diffusion in the
surface density equation. We constrain these new transport coefficients for
specific cases of inward, outward, and zero net mass transport. In addition, we
find that one of the new transport terms can lead to oscillations in the mean
surface density which then requires a constant or small inverse Rossby number
for disks to maintain a monotonic power-law surface density.Comment: 11 page
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