26,589 research outputs found
Effects of Defects on Friction for a Xe Film Sliding on Ag(111)
The effects of a step defect and a random array of point defects (such as
vacancies or substitutional impurities) on the force of friction acting on a
xenon monolayer film as it slides on a silver (111) substrate are studied by
molecular dynamic simulations and compared with the results of lowest order
perturbation theory in the substrate corrugation potential. For the case of a
step, the magnitude and velocity dependence of the friction force are strongly
dependent on the direction of sliding respect to the step and the corrugation
strength. When the applied force F is perpendicular to the step, the film is
pinned forF less than a critical force Fc. Motion of the film along the step,
however, is not pinned. Fluctuations in the sliding velocity in time provide
evidence of both stick-slip motion and thermally activated creep. Simulations
done with a substrate containing a 5 percent concentration of random point
defects for various directions of the applied force show that the film is
pinned for the force below a critical value. The critical force, however, is
still much lower than the effective inertial force exerted on the film by the
oscillations of the substrate in experiments done with a quartz crystal
microbalance (QCM). Lowest order perturbation theory in the substrate potential
is shown to give results consistent with the simulations, and it is used to
give a physical picture of what could be expected for real surfaces which
contain many defects.Comment: 13 pages, 17 figures, latex plus postscript files for figure
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 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
Parity fluctuations in stellar dynamos
Observations of the solar butterfly diagram from sunspot records suggest
persistent fluctuation in parity, away from the overall, approximately dipolar
structure. We use a simple mean-field dynamo model with a solar-like rotation
law, and perturb the -effect. We find that the parity of the magnetic
field with respect to the rotational equator can demonstrate what we describe
as resonant behaviour, while the magnetic energy behaves in a more or less
expected way. We discuss possible applications of the phenomena in the context
of various deviations of the solar magnetic field from dipolar symmetry, as
reported from analysis of archival sunspot data. We deduce that our model
produces fluctuations in field parity, and hence in the butterfly diagram, that
are consistent with observed fluctaions in solar behaviour
Galactic dynamo action in presence of stochastic alpha and shear
Using a one-dimensional -dynamo model appropriate to galaxies,
we study the possibility of dynamo action driven by a stochastic alpha effect
and shear. To determine the field evolution, one needs to examine a large
number of different realizations of the stochastic component of . The
net growth or decay of the field depends not only on the dynamo parameters but
also on the particular realization, the correlation time of the stochastic
compared to turbulent diffusion timescale and the time over which the
system is evolved. For dynamos where both a coherent and fluctuating
are present, the stochasticity of can help alleviate catastrophic
dynamo quenching, even in the absence of helicity fluxes. One can obtain final
field strengths up to a fraction of the equipartition field for dynamo numbers , while fields comparable to
require much larger degree of fluctuations or shear. This type of
dynamo may be particularly useful for amplifying fields in the central regions
of disk galaxies.Comment: Accepted for publication in MNRAS Letter
Current status of turbulent dynamo theory: From large-scale to small-scale dynamos
Several recent advances in turbulent dynamo theory are reviewed. High
resolution simulations of small-scale and large-scale dynamo action in periodic
domains are compared with each other and contrasted with similar results at low
magnetic Prandtl numbers. It is argued that all the different cases show
similarities at intermediate length scales. On the other hand, in the presence
of helicity of the turbulence, power develops on large scales, which is not
present in non-helical small-scale turbulent dynamos. At small length scales,
differences occur in connection with the dissipation cutoff scales associated
with the respective value of the magnetic Prandtl number. These differences are
found to be independent of whether or not there is large-scale dynamo action.
However, large-scale dynamos in homogeneous systems are shown to suffer from
resistive slow-down even at intermediate length scales. The results from
simulations are connected to mean field theory and its applications. Recent
work on helicity fluxes to alleviate large-scale dynamo quenching, shear
dynamos, nonlocal effects and magnetic structures from strong density
stratification are highlighted. Several insights which arise from analytic
considerations of small-scale dynamos are discussed.Comment: 36 pages, 11 figures, Spa. Sci. Rev., submitted to the special issue
"Magnetism in the Universe" (ed. A. Balogh
Radio Observations of the Magnetic Fields in Galaxies
After a short introduction on how we get information of the magnetic fields
from radio observations I discuss the results concerning the magnetic field
structure in galaxies: Large-scale regular magnetic field pattern of spiral
structure exist in grand-design spirals, flocculent and even irregular
galaxies. The regular field in spirals is aligned along the optical spiral arms
but strongest in the interarm region, sometimes forming 'magnetic arms'. The
strongest total field is found in the optical arms, but mainly irregular. The
large-scale regular field is best explained by some kind of dynamo action. Only
a few galaxies show a dominant axisymmetric field pattern, most field
structures seem to be a superposition of different dynamo modes or rather
reveal more local effects related to density waves, bars or shocks.
Observations of edge-on galaxies show that the magnetic fields are mainly
parallel to the disk except in some galaxies with strong star formation and
strong galactic winds as e.g. NGC 4631.Comment: 9 pages with 4 figures. To be published in Acta Astronomica Sinica
Vol. 44, 2003 (Conf. Proc. "Radio Studies of Galactic Objects, Galaxies and
AGNs", eds. J.L. Han et al.). Final published version also available at
http://www.bao.ac.cn/bao/hjl/xian/proceedings
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