307 research outputs found
A Magnetic Alpha-Omega Dynamo in Active Galactic Nuclei Disks: I. The Hydrodynamics of Star-Disk Collisions and Keplerian Flow
A magnetic field dynamo in the inner regions of the accretion disk
surrounding the supermassive black holes in AGNs may be the mechanism for the
generation of magnetic fields in galaxies and in extragalactic space. We argue
that the two coherent motions produced by 1) the Keplerian motion and 2)
star-disk collisions, numerous in the inner region of AGN accretion disks, are
both basic to the formation of a robust, coherent dynamo and consequently the
generation of large scale magnetic fields. They are frequent enough to account
for an integrated dynamo gain, e^{10^{9}} at 100 gravitational radii of a
central black hole, many orders of magnitude greater than required to amplify
any seed field no matter how small. The existence of extra-galactic, coherent,
large scale magnetic fields whose energies greatly exceed all but massive black
hole energies is recognized. In paper II (Pariev, Colgate, and Finn 2006) we
argue that in order to produce a dynamo that can access the free energy of
black hole formation and produce all the magnetic flux in a coherent fashion
the existence of these two coherent motions in a conducting fluid is required.
The differential winding of Keplerian motion is obvious, but the disk structure
depends upon the model of "alpha", the transport coefficient of angular
momentum chosen. The counter rotation of driven plumes in a rotating frame is
less well known, but fortunately the magnetic effect is independent of the disk
model. Both motions are discussed in this paper, paper I. The description of
the two motions are preliminary to two theoretical derivations and one
numerical simulation of the alpha-omega dynamo in paper II. (Abridged)Comment: 34 pages, 1 figure, accepted by Ap
On the inverse cascade of magnetic helicity
We study the inverse cascade of magnetic helicity in conducting fluids by
investigating the detailed transfer of helicity between different spherical
shells in Fourier space in direct numerical simulations of three-dimensional
magnetohydrodynamics (MHD). Two different numerical simulations are used, one
where the system is forced with an electromotive force in the induction
equation, and one in which the system is forced mechanically with an ABC flow
and the magnetic field is solely sustained by a dynamo action. The magnetic
helicity cascade at the initial stages of both simulations is observed to be
inverse and local (in scale space) in the large scales, and direct and local in
the small scales. When saturation is approached most of the helicity is
concentrated in the large scales and the cascade is non-local. Helicity is
transfered directly from the forced scales to the largest scales. At the same
time, a smaller in amplitude direct cascade is observed from the largest scale
to small scales.Comment: Submitted to PR
Dynamo quenching due to shear flow
We provide a theory of dynamo (α effect) and momentum transport in three-dimensional magnetohydrodynamics. For the first time, we show that the α effect is reduced by the shear even in the absence of magnetic field. The α effect is further suppressed by magnetic fields well below equipartition (with the large-scale flow) with different scalings depending on the relative strength of shear and magnetic field. The turbulent viscosity is also found to be significantly reduced by shear and magnetic fields, with positive value. These results suggest a crucial effect of shear and magnetic field on dynamo quenching and momentum transport reduction, with important implications for laboratory and astrophysical plasmas, in particular, for the dynamics of the Sun
How astrophysical mean field dynamos can circumvent existing quenching constraints
Mean field dynamo theory is a leading candidate to explain the observed large
scale magnetic fields of galaxies and stars. However, controversy arises over
the extent of premature quenching by the backreaction of the growing field. We
distinguish between rapid mean field dynamo action, which is required by
astrophysical systems, and resistively limited action. We show how the flow of
magnetic helicity is important for rapid action. Existing numerical and
analytic work suggesting that mean field dynamos are prematurely quenched and
resistively limited include approximations or boundary conditions which
suppress the magnetic helicity flow from the outset. Thus they do not
unambiguously reveal whether real astrophysical mean field dynamos are
dynamically suppressed when the helicity flow is allowed. An outflow of
helicity also implies an outflow of magnetic energy and so active coronae or
winds should accompany mean field dynamos. Open boundaries alone may not be
sufficient for rapid dynamo action and the additional physics of buoyancy and
outflows may be required. Possible simulation approaches to test some of the
principles are briefly discussed. Some limitations of the ``Zeldovich
relation'' are also addressed.Comment: 19 pages LaTex, invited submission to Physics of Plasmas, APS/DPP
Quebec 2000 Meeting Proceeding
Direct democracy and intergenerational conflicts in ageing societies
To evaluate the potential effects of population ageing on the outcomes of direct democracy, we analyze the effect of age on voting decisions in public referendums. In a case study of the Stuttgart 21 referendum on one of the largest infrastructure projects in Germany, we find that support for the project decreased significantly in age. A quan‐ titative review of the relevant literature affirms that similar lifecycle patterns appear to be the norm in referendums on projects that require initial expenditures and pay off in the long run. Population ageing, thus, presents a potential threat to investment‐like reform projects
Large-Scale Magnetic-Field Generation by Randomly Forced Shearing Waves
A rigorous theory for the generation of a large-scale magnetic field by
random non-helically forced motions of a conducting fluid combined with a
linear shear is presented in the analytically tractable limit of low Rm and
weak shear. The dynamo is kinematic and due to fluctuations in the net
(volume-averaged) electromotive force. This is a minimal proof-of-concept
quasilinear calculation aiming to put the shear dynamo, a new effect recently
found in numerical experiments, on a firm theoretical footing. Numerically
observed scalings of the wavenumber and growth rate of the fastest growing
mode, previously not understood, are derived analytically. The simplicity of
the model suggests that shear dynamo action may be a generic property of
sheared magnetohydrodynamic turbulence.Comment: Paper substantially rewritten, results changed (relative to v1).
Revised versio
The origin of grand minima in the sunspot cycle
One of the most striking aspects of the 11-year sunspot cycle is that there
have been times in the past when some cycles went missing, a most well-known
example of this being the Maunder minimum during 1645-1715. Analyses of
cosmogenic isotopes (C14 and Be10) indicated that there were about 27 grand
minima in the last 11,000 yr, implying that about 2.7% of the solar cycles had
conditions appropriate for forcing the Sun into grand minima. We address the
question how grand minima are produced and specifically calculate the frequency
of occurrence of grand minima from a theoretical dynamo model. We assume that
fluctuations in the poloidal field generation mechanism and the meridional
circulation produce irregularities of sunspot cycles. Taking these fluctuations
to be Gaussian and estimating the values of important parameters from the data
of last 28 solar cycles, we show from our flux transport dynamo model that
about 1-4% of the sunspot cycles may have conditions suitable for inducing
grand minima.Comment: Accepted for publication in Physical Review Letter
Direct Measurement of Effective Magnetic Diffusivity in Turbulent Flow of Liquid Sodium
The first direct measurements of effective magnetic diffusivity in turbulent
flow of electro-conductive fluids (the so-called beta-effect) under magnetic
Reynolds number Rm >> 1 are reported. The measurements are performed in a
nonstationary turbulent flow of liquid sodium, generated in a closed toroidal
channel. The peak level of the Reynolds number reached Re \approx 3 10^6, which
corresponds to the magnetic Reynolds number Rm \approx 30. The magnetic
diffusivity of the liquid metal was determined by measuring the phase shift
between the induced and the applied magnetic fields. The maximal deviation of
magnetic diffusivity from its basic (laminar) value reaches about 50% .Comment: 5 pages, 6 figuser, accepted in PR
Thermodynamics of MHD flows with axial symmetry
We present strategies based upon extremization principles, in the case of the
axisymmetric equations of magnetohydrodynamics (MHD). We study the equilibrium
shape by using a minimum energy principle under the constraints of the MHD
axisymmetric equations. We also propose a numerical algorithm based on a
maximum energy dissipation principle to compute in a consistent way the
equilibrium states. Then, we develop the statistical mechanics of such flows
and recover the same equilibrium states giving a justification of the minimum
energy principle. We find that fluctuations obey a Gaussian shape and we make
the link between the conservation of the Casimirs on the coarse-grained scale
and the process of energy dissipation
Dynamo effect in parity-invariant flow with large and moderate separation of scales
It is shown that non-helical (more precisely, parity-invariant) flows capable
of sustaining a large-scale dynamo by the negative magnetic eddy diffusivity
effect are quite common. This conclusion is based on numerical examination of a
large number of randomly selected flows. Few outliers with strongly negative
eddy diffusivities are also found, and they are interpreted in terms of the
closeness of the control parameter to a critical value for generation of a
small-scale magnetic field. Furthermore, it is shown that, for parity-invariant
flows, a moderate separation of scales between the basic flow and the magnetic
field often significantly reduces the critical magnetic Reynolds number for the
onset of dynamo action.Comment: 44 pages,11 figures, significantly revised versio
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