5,035 research outputs found
Enhancement of small-scale turbulent dynamo by large-scale shear
Small-scale dynamos are ubiquitous in a broad range of turbulent flows with
large-scale shear, ranging from solar and galactic magnetism to accretion
disks, cosmology and structure formation. Using high-resolution direct
numerical simulations we show that in non-helically forced turbulence with zero
mean magnetic field, large-scale shear supports small-scale dynamo action,
i.e., the dynamo growth rate increases with shear and shear enhances or even
produces turbulence, which, in turn, further increases the dynamo growth rate.
When the production rates of turbulent kinetic energy due to shear and forcing
are comparable, we find scalings for the growth rate of the
small-scale dynamo and the turbulent velocity with shear rate
that are independent of the magnetic Prandtl number: and
. For large fluid and magnetic Reynolds numbers,
, normalized by its shear-free value, depends only on shear. Having
compensated for shear-induced effects on turbulent velocity, we find that the
normalized growth rate of the small-scale dynamo exhibits the scaling,
, arising solely from the induction
equation for a given velocity field.Comment: Improved version submitted to the Astrophysical Journal Letters, 6
pages, 5 figure
Canonical structure of the E10 model and supersymmetry
A coset model based on the hyperbolic Kac-Moody algebra E10 has been
conjectured to underly eleven-dimensional supergravity and M theory. In this
note we study the canonical structure of the bosonic model for finite- and
infinite-dimensional groups. In the case of finite-dimensional groups like
GL(n) we exhibit a convenient set of variables with Borel-type canonical
brackets. The generalisation to the Kac-Moody case requires a proper treatment
of the imaginary roots that remains elusive. As a second result, we show that
the supersymmetry constraint of D=11 supergravity can be rewritten in a
suggestive way using E10 algebra data. Combined with the canonical structure,
this rewriting explains the previously observed association of the canonical
constraints with null roots of E10. We also exhibit a basic incompatibility
between local supersymmetry and the K(E10) `R symmetry', that can be traced
back to the presence of imaginary roots and to the unfaithfulness of the spinor
representations occurring in the present formulation of the E10 worldline
model, and that may require a novel type of bosonisation/fermionisation for its
resolution. This appears to be a key challenge for future progress with E10.Comment: 1+39 pages. v2: small corrections. Version to appear in PR
Decay of helical and non-helical magnetic knots
We present calculations of the relaxation of magnetic field structures that
have the shape of particular knots and links. A set of helical magnetic flux
configurations is considered, which we call -foil knots of which the trefoil
knot is the most primitive member. We also consider two nonhelical knots;
namely, the Borromean rings as well as a single interlocked flux rope that also
serves as the logo of the Inter-University Centre for Astronomy and
Astrophysics in Pune, India. The field decay characteristics of both
configurations is investigated and compared with previous calculations of
helical and nonhelical triple-ring configurations. Unlike earlier nonhelical
configurations, the present ones cannot trivially be reduced via flux
annihilation to a single ring. For the -foil knots the decay is described by
power laws that range form to , which can be as slow as
the behavior for helical triple-ring structures that were seen in
earlier work. The two nonhelical configurations decay like , which is
somewhat slower than the previously obtained behavior in the decay
of interlocked rings with zero magnetic helicity. We attribute the difference
to the creation of local structures that contain magnetic helicity which
inhibits the field decay due to the existence of a lower bound imposed by the
realizability condition. We show that net magnetic helicity can be produced
resistively as a result of a slight imbalance between mutually canceling
helical pieces as they are being driven apart. We speculate that higher order
topological invariants beyond magnetic helicity may also be responsible for
slowing down the decay of the two more complicated nonhelical structures
mentioned above.Comment: 11 pages, 27 figures, submitted to Phys. Rev.
Fanning out of the -mode in presence of nonuniform magnetic fields
We show that in the presence of a harmonically varying magnetic field the
fundamental or -mode in a stratified layer is altered in such a way that it
fans out in the diagnostic diagram, but with mode power also within
the fan. In our simulations, the surface is defined by a temperature and
density jump in a piecewise isothermal layer. Unlike our previous work (Singh
et al. 2014) where a uniform magnetic field was considered, we employ here a
nonuniform magnetic field together with hydromagnetic turbulence at length
scales much smaller than those of the magnetic fields. The expansion of the
-mode is stronger for fields confined to the layer below the surface. In
some of those cases, the diagram also reveals a new class of low
frequency vertical stripes at multiples of twice the horizontal wavenumber of
the background magnetic field. We argue that the study of the -mode
expansion might be a new and sensitive tool to determining subsurface magnetic
fields with longitudinal periodicity.Comment: 6 pages, 4 figures, submitted to Astrophysical Journal Letter
Visibility of Cold Atomic Gases in Optical Lattices for Finite Temperatures
In nearly all experiments with ultracold atoms time-of-flight pictures are
the only data available. In this paper we present an analytical strong-coupling
calculation for those time-of-flight pictures of bosons in an optical lattice
in the Mott phase. This allows us to determine the visibility, which quantifies
the contrast of peaks in the time-of-flight pictures, and we suggest how to use
it as a thermometer.Comment: Author Information under
http://www.theo-phys.uni-essen.de/tp/ags/pelster_dir
Coupled-resonator optical waveguide: a proposal and analysis
We propose a new type of optical waveguide that consists of a sequence of coupled high- Q resonators. Unlike other types of optical waveguide, waveguiding in the coupled-resonator optical waveguide (CROW) is achieved through weak coupling between otherwise localized high- Q optical cavities. Employing a formalism similar to the tight-binding method in solid-state physics, we obtain the relations for the dispersion and the group velocity of the photonic band of the CROW's and find that they are solely characterized by coupling factor k 1 . We also demonstrate the possibility of highly efficient nonlinear optical frequency conversion and perfect transmission through bends in CROW's
Population Aging, Savings Behavior and Capital Markets
Population aging is just beginning to hit the industrialized countries in full force, and it will have a tremendous impact on capital markets. In this paper, we argue that the capital market effects of population aging are particularly strong in continental European economies such as Germany, France, and Italy, with their large and ailing pay-as-you-go public pension systems, relatively thin capital markets, and poor capital performance. The younger generations in these countries are quite aware of the need to provide for more retirement income through own private saving, and these effects will be accentuated by fundamental pension reforms that aim at more pre-funding. Population aging changes households' savings behavior and portfolio composition, and much more assets will be invested on the stock market. Capital markets will grow in size, and active institutional investors such as pension funds will become more important in continental European countries. These changes are likely to have beneficial side effects in terms of improved capital efficiency, total factor productivity, and growth. Looking at the effects of population aging on savings behavior and capital markets therefore adds a new dimension to the continuing debate about advantages and disadvantages of pay-as-you-go and fully funded pension systems.
Properties of - and -modes in hydromagnetic turbulence
With the ultimate aim of using the fundamental or -mode to study
helioseismic aspects of turbulence-generated magnetic flux concentrations, we
use randomly forced hydromagnetic simulations of a piecewise isothermal layer
in two dimensions with reflecting boundaries at top and bottom. We compute
numerically diagnostic wavenumber-frequency diagrams of the vertical velocity
at the interface between the denser gas below and the less dense gas above. For
an Alfv\'en-to-sound speed ratio of about 0.1, a 5% frequency increase of the
-mode can be measured when -, where is the
horizontal wavenumber and is the pressure scale height at the
surface. Since the solar radius is about 2000 times larger than ,
the corresponding spherical harmonic degree would be 6000-8000. For weaker
fields, a -dependent frequency decrease by the turbulent motions becomes
dominant. For vertical magnetic fields, the frequency is enhanced for
, but decreased relative to its nonmagnetic value for
.Comment: 17 pages, 22 figures, Version accepted in MNRA
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