1,772 research outputs found
Kinematic dynamo wave in the vicinity of the solar poles
We consider a dynamo wave in the solar convective shell for the kinematic
-dynamo model. The spectrum and eigenfunctions of the
corresponding equations are derived analytically with the aid of the WKB
method. Our main aim here is to investigate the dynamo wave behavior in the
vicinity of the solar poles. Explicit expressions for the incident and
reflected waves are obtained. The reflected wave is shown to be relatively weak
in comparison to the incident wave. The phase shifts and the ratio of
amplitudes of the two waves are found.Comment: 20 pages, 2 EPS figure
Wavy stripes and squares in zero P number convection
A simple model to explain numerically observed behaviour of chaotically
varying stripes and square patterns in zero Prandtl number convection in
Boussinesq fluid is presented. The nonlinear interaction of mutually
perpendicular sets of wavy rolls, via higher mode, may lead to a competition
between the two sets of wavy rolls. The appearance of square patterns is due to
the secondary forward Hopf bifurcation of a set of wavy rolls.Comment: 8 pages and 3 figures, late
Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbers
This paper is a detailed report on a programme of simulations used to settle
a long-standing issue in the dynamo theory and demonstrate that the fluctuation
dynamo exists in the limit of large magnetic Reynolds number Rm>>1 and small
magnetic Prandtl number Pm<<1. The dependence of the critical Rm_c vs. the
hydrodynamic Reynolds number Re is obtained for 1<Re<6700. In the limit Pm<<1,
Rm_c is ~3 times larger than for Pm>1. The stability curve Rm_c(Re) (and, it is
argued, the nature of the dynamo) is substantially different from the case of
the simulations and liquid-metal experiments with a mean flow. It is not as yet
possible to determine numerically whether the growth rate is ~Rm^{1/2} in the
limit Re>>Rm>>1, as should be the case if the dynamo is driven by the
inertial-range motions. The magnetic-energy spectrum in the low-Pm regime is
qualitatively different from the Pm>1 case and appears to develop a negative
spectral slope, although current resolutions are insufficient to determine its
asymptotic form. At 1<Rm<Rm_c, the magnetic fluctuations induced via the
tangling by turbulence of a weak mean field are investigated and the
possibility of a k^{-1} spectrum above the resistive scale is examined. At low
Rm<1, the induced fluctuations are well described by the quasistatic
approximation; the k^{-11/3} spectrum is confirmed for the first time in direct
numerical simulations.Comment: IoP latex, 27 pages, 25 figures, 3 tables. Accepted by New J. Physic
Low-Prandtl-number B\'enard-Marangoni convection in a vertical magnetic field
The effect of a homogeneous magnetic field on surface-tension-driven
B\'{e}nard convection is studied by means of direct numerical simulations. The
flow is computed in a rectangular domain with periodic horizontal boundary
conditions and the free-slip condition on the bottom wall using a
pseudospectral Fourier-Chebyshev discretization. Deformations of the free
surface are neglected. Two- and three-dimensional flows are computed for either
vanishing or small Prandtl number, which are typical of liquid metals. The main
focus of the paper is on a qualitative comparison of the flow states with the
non-magnetic case, and on the effects associated with the possible
near-cancellation of the nonlinear and pressure terms in the momentum equations
for two-dimensional rolls. In the three-dimensional case, the transition from a
stationary hexagonal pattern at the onset of convection to three-dimensional
time-dependent convection is explored by a series of simulations at zero
Prandtl number.Comment: 26 pages, 9 figure
Influence of topography on tide propagation and amplification in semi-enclosed basins
An idealized model for tide propagation and amplification in semi-enclosed rectangular basins is presented, accounting for depth differences by a combination of longitudinal and lateral topographic steps. The basin geometry is formed by several adjacent compartments of identical width, each having either a uniform depth or two depths separated by a transverse topographic step. The problem is forced by an incoming Kelvin wave at the open end, while allowing waves to radiate outward. The solution in each compartment is written as the superposition of (semi)-analytical wave solutions in an infinite channel, individually satisfying the depth-averaged linear shallow water equations on the f plane, including bottom friction. A collocation technique is employed to satisfy continuity of elevation and flux across the longitudinal topographic steps between the compartments. The model results show that the tidal wave in shallow parts displays slower propagation, enhanced dissipation and amplified amplitudes. This reveals a resonance mechanism, occurring when\ud
the length of the shallow end is roughly an odd multiple of the quarter Kelvin wavelength. Alternatively, for sufficiently wide basins, also PoincarĂ© waves may become resonant. A transverse step implies different wavelengths of the incoming and reflected Kelvin wave, leading to increased amplitudes in shallow regions and a shift of amphidromic points in the direction of the deeper part. Including the shallow parts near the basinâs closed end (thus capturing the Kelvin resonance mechanism) is essential to reproduce semi-diurnal and diurnal\ud
tide observations in the Gulf of California, the Adriatic Sea and the Persian Gulf
Quasiperiodic waves at the onset of zero Prandtl number convection with rotation
We show the possibility of quasiperiodic waves at the onset of thermal
convection in a thin horizontal layer of slowly rotating zero-Prandtl number
Boussinesq fluid confined between stress-free conducting boundaries. Two
independent frequencies emerge due to an interaction between a stationary
instability and a self-tuned wavy instability in presence of coriolis force, if
Taylor number is raised above a critical value. Constructing a dynamical system
for the hydrodynamical problem, the competition between the interacting
instabilities is analyzed. The forward bifurcation from the conductive state is
self-tuned.Comment: 9 pages of text (LaTex), 5 figures (Jpeg format
Stochastic excitation of acoustic modes in stars
For more than ten years, solar-like oscillations have been detected and
frequencies measured for a growing number of stars with various characteristics
(e.g. different evolutionary stages, effective temperatures, gravities, metal
abundances ...).
Excitation of such oscillations is attributed to turbulent convection and
takes place in the uppermost part of the convective envelope. Since the
pioneering work of Goldreich & Keely (1977), more sophisticated theoretical
models of stochastic excitation were developed, which differ from each other
both by the way turbulent convection is modeled and by the assumed sources of
excitation. We review here these different models and their underlying
approximations and assumptions.
We emphasize how the computed mode excitation rates crucially depend on the
way turbulent convection is described but also on the stratification and the
metal abundance of the upper layers of the star. In turn we will show how the
seismic measurements collected so far allow us to infer properties of turbulent
convection in stars.Comment: Notes associated with a lecture given during the fall school
organized by the CNRS and held in St-Flour (France) 20-24 October 2008 ; 39
pages ; 11 figure
Activation of mTOR coincides with autophagy during ligation-induced atrophy in the rat submandibular gland
Salivary gland atrophy is a common consequence of pathology, including Sjögren's syndrome, irradiation therapy and obstructive sialadenitis. During severe atrophy of the rat submandibular gland caused by excretory duct ligation, the majority of acinar cells disappear through apoptosis, whereas ductal cells proliferate and dedifferentiate; yet, the gland can survive in the atrophic state almost indefinitely, with an ability to fully recover if deligated. The control mechanisms governing these observations are not well understood. We report that âŒ10% of acinar cells survive in ligation-induced atrophy. Microarray and quantitative real-time PCR analysis of ligated glands indicated sustained transcription of acinar cell-specific genes, whereas ductal-specific genes were reduced to background levels. After 3 days of ligation, activation of the mammalian target of rapamycin (mTOR) pathway and autophagy occurred as shown by phosphorylation of 4E-BP1 and expression of autophagy-related proteins. These results suggest that activation of mTOR and the autophagosomal pathway are important mechanisms that may help to preserve acinar cells during atrophy of salivary glands after injury
The chemical composition of Ultracompact Dwarf Galaxies in the Virgo and Fornax Clusters
We present spectroscopic observations of ultra compact dwarf (UCD) galaxies
in the Fornax and Virgo Clusters made to measure and compare their stellar
populations. The spectra were obtained on the Gemini-North (Virgo) and
Gemini-South (Fornax) Telescopes using the respective Gemini Multi-Object
Spectrographs.
We estimated the ages, metallicities and abundances of the objects from mea-
surements of Lick line-strength indices in the spectra; we also estimated the
ages and metallicities independently using a direct spectral fitting technique.
Both methods re- vealed that the UCDs are old (mean age 10.8 \pm 0.7 Gyr) and
(generally) metal-rich (mean [Fe/H] = -0.8 \pm 0.1). The alpha-element
abundances of the objects measured from the Lick indices are super-Solar.
We used these measurements to test the hypothesis that UCDs are formed by the
tidal disruption of present-day nucleated dwarf elliptical galaxies. The data
are not consistent with this hypothesis because both the ages and abundances
are significantly higher than those of observed dwarf galaxy nuclei (this does
not exclude disruption of an earlier generation of dwarf galaxies). They are
more consistent with the properties of globular star clusters, although at
higher mean metallicity. The UCDs display a very wide range of metallicity
(-1.7 <[Fe/H]< 0.0), spanning the full range of both globular clusters and
dwarf galaxy nuclei.
We confirm previous reports that most UCDs have high metalliticities for
their luminosities, lying significantly above the canonical
metallicitiy-luminosity relation followed by early-type galaxies. In contrast
to previous work we find that there is no significant difference in either the
mean ages or the mean metallicities of the Virgo and Fornax UCD populations.Comment: 15 pages (including references and appendix), 8 figures (including
appendix
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