1,091 research outputs found
A novel type of intermittency in a nonlinear dynamo in a compressible flow
The transition to intermittent mean--field dynamos is studied using numerical
simulations of isotropic magnetohydrodynamic turbulence driven by a helical
flow. The low-Prandtl number regime is investigated by keeping the kinematic
viscosity fixed while the magnetic diffusivity is varied. Just below the
critical parameter value for the onset of dynamo action, a transient
mean--field with low magnetic energy is observed. After the transition to a
sustained dynamo, the system is shown to evolve through different types of
intermittency until a large--scale coherent field with small--scale turbulent
fluctuations is formed. Prior to this coherent field stage, a new type of
intermittency is detected, where the magnetic field randomly alternates between
phases of coherent and incoherent large--scale spatial structures. The
relevance of these findings to the understanding of the physics of mean--field
dynamo and the physical mechanisms behind intermittent behavior observed in
stellar magnetic field variability are discussed.Comment: 19 pages, 13 figure
Observation and Modeling of the Solar-Cycle Variation of the Meridional Flow
We present independent observations of the solar-cycle variation of flows
near the solar surface and at a depth of about 60 Mm, in the latitude range
. We show that the time-varying components of the meridional flow
at these two depths have opposite sign, while the time-varying components of
the zonal flow are in phase. This is in agreement with previous results. We
then investigate whether the observations are consistent with a theoretical
model of solar-cycle dependent meridional circulation based on a flux-transport
dynamo combined with a geostrophic flow caused by increased radiative loss in
the active region belt (the only existing quantitative model). We find that the
model and the data are in qualitative agreement, although the amplitude of the
solar-cycle variation of the meridional flow at 60 Mm is underestimated by the
model.Comment: To be published in Solar Physcis Topical Issue "Helioseismology,
Asteroseismology, and MHD Connections
ProMoT : Modular Modeling for Systems Biology
Summary: PROMOT is a software designed to support efficient and comprehensible modeling, visualization and analysis of complex and large-scale models. In recent years it has been improved in many aspects. New functionality especially tailored for Systems Biology has been added. It is now a very convenient tool for modular modeling. Availability: PROMOT is an open source project and freely available at http://www.mpi-magdeburg.mpg.de/projects/promot/download.html
Minnestoa Sheep Research Notes
This report provides condensed summaries of research projects conducted by researches at the University of Minnesota
Acoustic-gravity wave propagation characteristics in 3D radiation hydrodynamic simulations of the solar atmosphere
There has been tremendous progress in the degree of realism of
three-dimensional radiation magneto-hydrodynamic simulations of the solar
atmosphere in the past decades. Four of the most frequently used numerical
codes are Bifrost, CO5BOLD, MANCHA3D, and MURaM. Here we test and compare the
wave propagation characteristics in model runs from these four codes by
measuring the dispersion relation of acoustic-gravity waves at various heights.
We find considerable differences between the various models. The height
dependence of wave power, in particular of high-frequency waves, varies by up
to two orders of magnitude between the models, and the phase difference spectra
of several models show unexpected features, including phase
jumps.Comment: 19 pages, 15 figure
Sizes and fluorescence of cadmium sulfide quantum dots
Cadmium sulfide quantum dots have been synthesized by wet chemical deposition from an aqueous solution. The sizes of the quantum dots determined by dynamic light scattering directly in the colloidal solution and by intermittent-contact atomic force microscopy in the dry sediment agree with each other. It has been found that splitting of the fluorescence peaks of the quantum dots can be affected by the disorder of the atomic structure of cadmium sulfide quantum dots. © 2013 Pleiades Publishing, Ltd
Origin of solar torsional oscillations
Helioseismology has revealed many details of solar differential rotation and
also its time variation, known as torsional oscillations. So far there is no
generally accepted theoretical explanation for torsional oscillations, even
though a close relation to the solar activity cycle is evident. On the
theoretical side non-kinematic dynamo models (including the Lorentz force
feedback on differential rotation) have been used to explain torsional
oscillations. In this paper we use a slightly different approach by forcing
torsional oscillations in a mean field differential rotation model. Our aim is
not a fully self-consistent model but rather to point out a few general
properties of torsional oscillations and their possible origin that are
independent from a particular dynamo model. We find that the poleward
propagating high latitude branch of the torsional oscillations can be explained
as a response of the coupled differential rotation / meridional flow system to
periodic forcing in mid-latitudes, of either mechanical (Lorentz force) or
thermal nature. The speed of the poleward propagation sets constraints on the
value of the turbulent viscosity in the solar convection zone to be less than
3x10^8 m^2/s. We also show that the equatorward propagating low latitude branch
is very unlikely a consequence of mechanical forcing (Lorentz force) alone, but
rather of thermal origin due to the Taylor-Proudman theorem.Comment: 11 pages, 7 figures. accepted by Astrophys.
Flux-transport dynamos with Lorentz force feedback on differential rotation and meridional flow: Saturation mechanism and torsional oscillations
In this paper we discuss a dynamic flux-transport dynamo model that includes
the feedback of the induced magnetic field on differential rotation and
meridional flow. We consider two different approaches for the feedback:
meanfield Lorentz force and quenching of transport coefficients such as
turbulent viscosity and heat conductivity. We find that even strong feedback on
the meridional flow does not change the character of the flux-transport dynamo
significantly; however it leads to a significant reduction of differential
rotation. To a large degree independent from the dynamo parameters, the
saturation takes place when the toroidal field at the base of the convection
zone reaches between 1.2 an 1.5 T, the energy converted intomagnetic energy
corresponds to about 0.1 to 0.2% of the solar luminosity. The torsional
oscillations produced through Lorentz force feedback on differential rotation
show a dominant poleward propagating branch with the correct phase relation to
the magnetic cycle. We show that incorporating enhanced surface cooling of the
active region belt (as proposed by Spruit) leads to an equatorward propagating
branch in good agreement with observations.Comment: 15 pages, 12 figures, Accepted for publication in ApJ August 10
issue; corrected typos, corrected referenc
On the Fredholm property of bisingular pseudodifferential operators
For operators belonging either to a class of global bisingular
pseudodifferential operators on or to a class of bisingular
pseudodifferential operators on a product of two closed smooth
manifolds, we show the equivalence of their ellipticity (defined by the
invertibility of certain associated homogeneous principal symbols) and their
Fredholm mapping property in associated scales of Sobolev spaces. We also prove
the spectral invariance of these operator classes and then extend these results
to the even larger classes of Toeplitz type operators.Comment: 21 pages. Expanded sections 3 and 4. Corrected typos. Added
reference
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