16,792 research outputs found
What can we say about seed fields for galactic dynamos?
We demonstrate that a quasi-uniform cosmological seed field is a much less
suitable seed for a galactic dynamo than has often been believed. The age of
the Universe is insufficient for a conventional galactic dynamo to generate a
contemporary galactic magnetic field starting from such a seed, accepting
conventional estimates for physical quantities. We discuss modifications to the
scenario for the evolution of galactic magnetic fields implied by this result.
We also consider briefly the implications of a dynamo number that is
significantly larger than that given by conventional estimates
Magnetic fields near the peripheries of galactic discs
Magnetic fields are observed beyond the peripheries of optically detected
galactic discs, while numerical models of their origin and the typical
magnitudes are still absent. Previously, studies of galactic dynamo have
avoided considering the peripheries of galactic discs because of the very
limited (though gradually growing) knowledge about the local properties of the
interstellar medium. Here we investigate the possibility that magnetic fields
can be generated in the outskirts of discs, taking the Milky Way as an example.
We consider a simple evolving galactic dynamo model in the "no-z" formulation,
applicable to peripheral regions of galaxies, for various assumptions about the
radial and vertical profiles of the ionized gas disc. The magnetic field may
grow as galaxies evolve, even in the more remote parts of the galactic disc,
out to radii of 15 to 30 kpc, becoming substantial after times of about 10 Gyr.
This result depends weakly on the adopted distributions of the half thickness
and surface density of the ionized gas component. The model is robust to
changes in the amplitude of the initial field and the position of its maximum
strength. The magnetic field in the remote parts of the galactic disc could be
generated in situ from a seed field by local dynamo action. Another possibility
is field production in the central regions of a galaxy, followed by transport
to the disc's periphery by the joint action of the dynamo and turbulent
diffusivity. Our results demonstrate the possibilities for the appearance and
strengthening of magnetic fields at the peripheries of disc galaxies and
emphasize the need for observational tests with new and anticipated radio
telescopes (LOFAR, MWA, and SKA).Comment: 8 pages, 5 figure
Magnetic fields in barred galaxies. II. Dynamo models
We study the generation and maintenance of large-scale magnetic fields in
barred galaxies. We take a velocity field (with strong noncircular components)
from a published gas dynamical simulation of Athanassoula (1992), and use this
as input to a galactic dynamo calculation. Our work is largely motivated by
recent high quality VLA radio observations of the barred galaxy NGC 1097, and
we compare our results in detail with the regular magnetic fields deduced from
these observations. We are able to reproduce most of the conspicuous
large-scale features of the observed regular field, including the field
structure in the central regions, by using a simple mean-field dynamo model in
which the intensity of interstellar turbulence (more precisely, the turbulent
diffusivity) is enhanced by a factor of 2-6 in the dust lanes and near the
circumnuclear ring. We argue that magnetic fields can be dynamically important,
and therefore should be included in models of gas flow in barred galaxies.Comment: 16 pages, 13 figures, submitted to Astronomy and Astrophysics.
Revised version (changes shown in bold face
Applications and Sexual Version of a Simple Model for Biological Ageing
We use a simple model for biological ageing to study the mortality of the
population, obtaining a good agreement with the Gompertz law. We also simulate
the same model on a square lattice, considering different strategies of
parental care. The results are in agreement with those obtained earlier with
the more complicated Penna model for biological ageing. Finally, we present the
sexual version of this simple model.Comment: For Int.J.Mod.Phys.C Dec. 2001; 11 pages including 6 fig
Magnetic field reversals and galactic dynamos
We argue that global magnetic field reversals similar to those observed in
the Milky Way occur quite frequently in mean-field galactic dynamo models that
have relatively strong, random, seed magnetic fields that are localized in
discrete regions. The number of reversals decreases to zero with reduction of
the seed strength, efficiency of the galactic dynamo and size of the spots of
the seed field. A systematic observational search for magnetic field reversals
in a representative sample of spiral galaxies promises to give valuable
information concerning seed magnetic fields and, in this way, to clarify the
initial stages of galactic magnetic field evolution
Towards A Mean-Field Formulation Of The Babcock-Leighton Type Solar Dynamo. I. Alpha Coefficient Versus Durney's Double Ring Approach
We develop a model of the solar dynamo in which, on the one hand, we follow
the Babcock-Leighton approach to include surface processes like the production
of poloidal field from the decay of active regions, and, on the other hand, we
attempt to develop a mean field theory that can be studied in quantitative
detail. One of the main challenges in developing such models is to treat the
buoyant rise of toroidal field and the production of poloidal field from it
near the surface. We build up a dynamo model with two contrasting methods of
treating buoyancy. In one method, we incorporate the generation of the poloidal
field near the solar surface by Durney's procedure of double ring eruption. In
the second method, the poloidal field generation is treated by a positive
alpha-effect concentrated near the solar surface, coupled with an algorithm for
handling buoyancy. The two methods are found to give qualitatively similar
results.Comment: 32 pages, 27 figures, uses aastex.cls and epsfig.st
Numerical investigation of friction in inflaton equations of motion
The equation of motion for the expectation value of a scalar quantum field
does not have the local form that is commonly assumed in studies of
inflationary cosmology. We have recently argued that the true, temporally
non-local equation of motion does not possess a time-derivative expansion and
that the conversion of inflaton energy into particles is not, in principle,
described by the friction term estimated from linear response theory. Here, we
use numerical methods to investigate whether this obstacle to deriving a local
equation of motion is purely formal, or of some quantitative importance. Using
a simple scalar-field model, we find that, although the non-equilibrium
evolution can exhibit significant damping, this damping is not well described
by the local equation of motion obtained from linear response theory. It is
possible that linear response theory does not apply to the situation we study
only because thermalization turns out to be slow, but we argue that that the
large discrepancies we observe indicate a failure of the local approximation at
a more fundamental level.Comment: 13 pages, 7 figure
A Possible Resolution of the Black Hole Information Puzzle
The problem of information loss is considered under the assumption that the
process of black hole evaporation terminates in the decay of the black hole
interior into a baby universe. We show that such theories can be decomposed
into superselection sectors labeled by eigenvalues of the third-quantized baby
universe field operator, and that scattering is unitary within each
superselection sector. This result relies crucially on the quantum-mechanical
variability of the decay time. It is further argued that the decay rate in the
black hole rest frame is necessarily proportional to , where
is the total entropy produced during the evaporation process,
entailing a very long-lived remnant.Comment: 15 pages, 3 uuencoded figures. Revised version contains some
notational simplification
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