8,125 research outputs found
Sustained Magnetorotational Turbulence in Local Simulations of Stratified Disks with Zero Net Magnetic Flux
We examine the effects of density stratification on magnetohydrodynamic
turbulence driven by the magnetorotational instability in local simulations
that adopt the shearing box approximation. Our primary result is that, even in
the absence of explicit dissipation, the addition of vertical gravity leads to
convergence in the turbulent energy densities and stresses as the resolution
increases, contrary to results for zero net flux, unstratified boxes. The ratio
of total stress to midplane pressure has a mean of ~0.01, although there can be
significant fluctuations on long (>~50 orbit) timescales. We find that the time
averaged stresses are largely insensitive to both the radial or vertical aspect
ratio of our simulation domain. For simulations with explicit dissipation, we
find that stratification extends the range of Reynolds and magnetic Prandtl
numbers for which turbulence is sustained. Confirming the results of previous
studies, we find oscillations in the large scale toroidal field with periods of
~10 orbits and describe the dynamo process that underlies these cycles.Comment: 13 pages, 18 figures, submitted to Ap
Coronae as Consequence of Large Scale Magnetic Fields in Turbulent Accretion Disks
Non-thermal X-ray emission in compact accretion engines can be interpreted to
result from magnetic dissipation in an optically thin magnetized corona above
an optically thick accretion disk. If coronal magnetic field originates in the
disk and the disk is turbulent, then only magnetic structures large enough for
their turbulent shredding time to exceed their buoyant rise time survive the
journey to the corona. We use this concept and a physical model to constrain
the minimum fraction of magnetic energy above the critical scale for buoyancy
as a function of the observed coronal to bolometric emission. Our results
suggest that a significant fraction of the magnetic energy in accretion disks
resides in large scale fields, which in turn provides circumstantial evidence
for significant non-local transport phenomena and the need for large scale
magnetic field generation. For the example of Seyfert AGN, for which of order
30 per cent of the bolometric flux is in the X-ray band, we find that more than
20 per cent of the magnetic energy must be of large enough scale to rise and
dissipate in the corona.Comment: submitted to ApJL, 2 fig
Superconductivity of the One-Dimensional d-p Model with p-p transfer
Using the numerical diagonalization method, we investigate the
one-dimensional - model, simulating a Cu-O linear chain with strong
Coulomb repulsions. Paying attention to the effect of the transfer energy
between the nearest neighbor oxygen-sites, we calculate the critical
exponent of correlation functions based on the Luttinger liquid
relations and the ground state energy as a function of an external
flux . We find that the transfer increases the charge
susceptibility and the exponent in cooperation with the repulsion
at Cu-site. We also show that anomalous flux quantization occurs for
. The superconducting region is presented on a phase diagram of
vs. plane.Comment: 4 pages, RevTex + 5 PS figures include
Generic differential geometry of hyperbolic plane curves
We study hyperbolic invariants of hyperbolic plane curves as applications of the singularity theory of smooth function
Mapeamento de ĂĄreas de campo limpo Ășmido no Distrito Federal a partir de fusĂŁo de imagens multiespectrais.
Elastic Convection in Vibrated Viscoplastic Fluids
We observe a new type of behavior in a shear thinning yield stress fluid:
freestanding convection rolls driven by vertical oscillation. The convection
occurs without the constraint of container boundaries yet the diameter of the
rolls is spontaneously selected for a wide range of parameters. The transition
to the convecting state occurs without hysteresis when the amplitude of the
plate acceleration exceeds a critical value. We find that a non-dimensional
stress, the stress due to the inertia of the fluid normalized by the yield
stress, governs the onset of the convective motion.Comment: 4 pages, 6 figure
The Effect of the Hall Term on the Nonlinear Evolution of the Magnetorotational Instability: I. Local Axisymmetric Simulations
The effect of the Hall term on the evolution of the magnetorotational
instability (MRI) in weakly ionized accretion disks is investigated using local
axisymmetric simulations. First, we show that the Hall term has important
effects on the MRI when the temperature and density in the disk is below a few
thousand K and between 10^13 and 10^18 cm^{-3} respectively. Such conditions
can occur in the quiescent phase of dwarf nova disks, or in the inner part
(inside 10 - 100 AU) of protoplanetary disks. When the Hall term is important,
the properties of the MRI are dependent on the direction of the magnetic field
with respect to the angular velocity vector \Omega. If the disk is threaded by
a uniform vertical field oriented in the same sense as \Omega, the axisymmetric
evolution of the MRI is an exponentially growing two-channel flow without
saturation. When the field is oppositely directed to \Omega, however, small
scale fluctuations prevent the nonlinear growth of the channel flow and the MRI
evolves into MHD turbulence. These results are anticipated from the
characteristics of the linear dispersion relation. In axisymmetry on a field
with zero-net flux, the evolution of the MRI is independent of the size of the
Hall term relative to the inductive term. The evolution in this case is
determined mostly by the effect of ohmic dissipation.Comment: 31 pages, 3 tables, 12 figures, accepted for publication in ApJ,
postscript version also available from
http://www.astro.umd.edu/~sano/publications
Electronic States and Superconducting Transition Temperature based on the Tomonaga-Luttinger liquid in PrBaCuO
An NQR experiment revealed superconductivity of
PrBaCuO (Pr247) to be realized on CuO double chain
layers and suggests possibility of novel one-dimensional(1D) superconductivity.
To clarify the nature of the 1D superconductivity, we calculate the band
dispersions of Pr247 by using the generalized gradient approximation(GGA). It
indicates that Fermi surface of CuO double chains is well described to the
electronic structure of a quasi-1D system.
Assuming the zigzag Hubbard chain model to be an effective model of the
system, we derive tight binding parameters of the model from a fit to the
result of GGA. Based on the Tomonaga-Luttinger liquid theory, we estimate
transition temperature () of the quasi-1D zigzag Hubbard model from the
calculated value of the Luttinger liquid parameter . The result of
is consistent with that of experiments in Pr247 and it suggests that the
mechanism of the superconductivity is well understood within the concept of the
Tomonaga-Luttinger liquid.Comment: 4 pages, 5 figure
Angular Momentum Transport by MHD Turbulence in Accretion Disks: Gas Pressure Dependence of the Saturation Level of the Magnetorotational Instability
The saturation level of the magnetorotational instability (MRI) is
investigated using three-dimensional MHD simulations. The shearing box
approximation is adopted and the vertical component of gravity is ignored, so
that the evolution of the MRI is followed in a small local part of the disk. We
focus on the dependence of the saturation level of the stress on the gas
pressure, which is a key assumption in the standard alpha disk model. From our
numerical experiments it is found that there is a weak power-law relation
between the saturation level of the Maxwell stress and the gas pressure in the
nonlinear regime; the higher the gas pressure, the larger the stress. Although
the power-law index depends slightly on the initial field geometry, the
relationship between stress and gas pressure is independent of the initial
field strength, and is unaffected by Ohmic dissipation if the magnetic Reynolds
number is at least 10. The relationship is the same in adiabatic calculations,
where pressure increases over time, and nearly-isothermal calculations, where
pressure varies little with time. Our numerical results are qualitatively
consistent with an idea that the saturation level of the MRI is determined by a
balance between the growth of the MRI and the dissipation of the field through
reconnection. The quantitative interpretation of the pressure-stress relation,
however, may require advances in the theoretical understanding of non-steady
magnetic reconnection.Comment: 45 pages, 5 tables, 17 figures, accepted for publication in Ap
On the Saturation of the Magnetorotational Instability via Parasitic Modes
We investigate the stability of incompressible, exact, non-ideal
magnetorotational (MRI) modes against parasitic instabilities. Both
Kelvin-Helmholtz and tearing-mode parasitic instabilities may occur in the
dissipative regimes accessible to current numerical simulations. We suppose
that a primary MRI mode saturates at an amplitude such that its fastest
parasite has a growth rate comparable to its own. The predicted alpha parameter
then depends critically on whether the fastest primary and parasitic modes fit
within the computational domain and whether non-axisymmetric parasitic modes
are allowed. Hence even simulations that resolve viscous and resistive scales
may not saturate properly unless the numerical domain is large enough to allow
the free evolution of both MRI and parasitic modes. To minimally satisfy these
requirements in simulations with vertical background fields, the vertical
extent of the domain should accommodate the fastest growing MRI mode while the
radial and azimuthal extents must be twice as large. The fastest parasites have
horizontal wavelengths roughly twice as long as the vertical wavelength of the
primary.Comment: 5 pages, 2 figures, uses emulateapj. Modified text in order to
address referee's comments and suggestions. References added. ApJ Letters
accepte
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