1,082 research outputs found
Earth's Inner Core dynamics induced by the Lorentz force
Seismic studies indicate that the Earth's inner core has a complex structure
and exhibits a strong elastic anisotropy with a cylindrical symmetry. Among the
various models which have been proposed to explain this anisotropy, one class
of models considers the effect of the Lorentz force associated with the
magnetic field diffused within the inner core. In this paper we extend previous
studies and use analytical calculations and numerical simulations to predict
the geometry and strength of the flow induced by the poloidal component of the
Lorentz force in a neutrally or stably stratified growing inner core, exploring
also the effect of different types of boundary conditions at the inner core
boundary (ICB). Unlike previous studies, we show that the boundary condition
that is most likely to produce a significant deformation and seismic anisotropy
is impermeable, with negligible radial flow through the boundary. Exact
analytical solutions are found in the case of a negligible effect of buoyancy
forces in the inner core (neutral stratification), while numerical simulations
are used to investigate the case of stable stratification. In this situation,
the flow induced by the Lorentz force is found to be localized in a shear layer
below the ICB, which thickness depends on the strength of the stratification,
but not on the magnetic field strength. We obtain scaling laws for the
thickness of this layer, as well as for the flow velocity and strain rate in
this shear layer as a function of the control parameters, which include the
magnitude of the magnetic field, the strength of the density stratification,
the viscosity of the inner core, and the growth rate of the inner core. We find
that the resulting strain rate is probably too small to produce significant
texturing unless the inner core viscosity is smaller than about Pa.s.Comment: submitted to Geophysical Journal Internationa
Parametric instability of the helical dynamo
We study the dynamo threshold of a helical flow made of a mean (stationary)
plus a fluctuating part. Two flow geometries are studied, either (i) solid body
or (ii) smooth. Two well-known resonant dynamo conditions, elaborated for
stationary helical flows in the limit of large magnetic Reynolds numbers, are
tested against lower magnetic Reynolds numbers and for fluctuating flows (zero
mean). For a flow made of a mean plus a fluctuating part the dynamo threshold
depends on the frequency and the strength of the fluctuation. The resonant
dynamo conditions applied on the fluctuating (resp. mean) part seems to be a
good diagnostic to predict the existence of a dynamo threshold when the
fluctuation level is high (resp. low).Comment: 37 pages, 8 figure
Rossby-wave turbulence in a rapidly rotating sphere
We use a quasi-geostrophic numerical model to study the turbulence of rotating flows in a sphere, with realistic Ekman friction and bulk viscous dissipation. The forcing is caused by the destabilization of an axisymmetric Stewartson shear layer, generated by differential rotation, resulting in a forcing at rather large scales. <P> The equilibrium regime is strongly anisotropic and inhomogeneous but exhibits a steep <i>m<sup>-5</sup></i> spectrum in the azimuthal (periodic) direction, at scales smaller than the injection scale. This spectrum has been proposed by Rhines for a Rossby wave turbulence. For some parameter range, we observe a turbulent flow dominated by a large scale vortex located in the shear layer, reminding us of the Great Red Spot of Jupiter
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The structural effect of Methyl substitution on the binding of Polypyridyl Ru-dppz Complexes to DNA
ABSTRACT: Polypyridyl ruthenium complexes have been intensively studied and possess photophysical properties which are both interesting and useful. They can act as probes for DNA, with a substantial enhancement in emission when bound, and can induce DNA damage upon photoirradiation and therefore, the synthesis and characterization of DNA binding of new complexes is an area of intense research activity. Whilst knowledge of how the binding of derivatives compares to the parent compound is highly desirable, this information can be difficult to obtain. Here we report the synthesis of three new methylated complexes, [Ru(TAP)2(dppz-10-Me).2Cl, [Ru(TAP)2(dppz-10,12-Me2)].2Cl and [Ru(TAP)2(dppz-11-Me)].2Cl, and examine the consequences for DNA binding through the use of atomic resolution X-ray crystallography. We find that the methyl groups are located in discrete positions with a complete directional preference. This may help to explain the quenching behavior which is found in solution for analogous [Ru(phen)2(dppz)]2+ derivatives
Experimental study of super-rotation in a magnetostrophic spherical Couette flow
We report measurements of electric potentials at the surface of a spherical
container of liquid sodium in which a magnetized inner core is differentially
rotating. The azimuthal angular velocities inferred from these potentials
reveal a strong super-rotation of the liquid sodium in the equatorial region,
for small differential rotation. Super-rotation was observed in numerical
simulations by Dormy et al. [1]. We find that the latitudinal variation of the
electric potentials in our experiments differs markedly from the predictions of
a similar numerical model, suggesting that some of the assumptions used in the
model - steadiness, equatorial symmetry, and linear treatment for the evolution
of both the magnetic and velocity fields - are violated in the experiments. In
addition, radial velocity measurements, using ultrasonic Doppler velocimetry,
provide evidence of oscillatory motion near the outer sphere at low latitude:
it is viewed as the signature of an instability of the super-rotating region
Is inner core seismic anisotropy a marker for plastic flow of cubic iron?
International audienceThis paper investigates whether observations of seismic anisotropy are compatible with a cubic structure of the inner core Fe alloy.We assume that anisotropy is the result of plastic deformation within a large scale flow induced by preferred growth at the inner core equator. Based on elastic moduli from the literature, bcc- or fcc-Fe produce seismic anisotropy well below seismic observations (). A Monte-Carlo approach allows us to generalize this result to any form of elastic anisotropy in a cubic system. Within our model, inner core global anisotropy is not compatible with a cubic structure of Fe alloy.Hence, if the inner core material is indeed cubic, large scale coherent anisotropic structures, incompatible with plastic deformation induced by large scale flow, must be present
Numerical Simulations of Dynamos Generated in Spherical Couette Flows
We numerically investigate the efficiency of a spherical Couette flow at
generating a self-sustained magnetic field. No dynamo action occurs for
axisymmetric flow while we always found a dynamo when non-axisymmetric
hydrodynamical instabilities are excited. Without rotation of the outer sphere,
typical critical magnetic Reynolds numbers are of the order of a few
thousands. They increase as the mechanical forcing imposed by the inner core on
the flow increases (Reynolds number ). Namely, no dynamo is found if the
magnetic Prandtl number is less than a critical value .
Oscillating quadrupolar dynamos are present in the vicinity of the dynamo
onset. Saturated magnetic fields obtained in supercritical regimes (either
or ) correspond to the equipartition between magnetic and
kinetic energies. A global rotation of the system (Ekman numbers ) yields to a slight decrease (factor 2) of the critical magnetic
Prandtl number, but we find a peculiar regime where dynamo action may be
obtained for relatively low magnetic Reynolds numbers (). In this
dynamical regime (Rossby number , spheres in opposite direction) at
a moderate Ekman number (), a enhanced shear layer around the inner
core might explain the decrease of the dynamo threshold. For lower
() this internal shear layer becomes unstable, leading to small
scales fluctuations, and the favorable dynamo regime is lost. We also model the
effect of ferromagnetic boundary conditions. Their presence have only a small
impact on the dynamo onset but clearly enhance the saturated magnetic field in
the ferromagnetic parts. Implications for experimental studies are discussed
Zonal shear and super-rotation in a magnetized spherical Couette flow experiment
We present measurements performed in a spherical shell filled with liquid
sodium, where a 74 mm-radius inner sphere is rotated while a 210 mm-radius
outer sphere is at rest. The inner sphere holds a dipolar magnetic field and
acts as a magnetic propeller when rotated. In this experimental set-up called
DTS, direct measurements of the velocity are performed by ultrasonic Doppler
velocimetry. Differences in electric potential and the induced magnetic field
are also measured to characterize the magnetohydrodynamic flow. Rotation
frequencies of the inner sphere are varied between -30 Hz and +30 Hz, the
magnetic Reynolds number based on measured sodium velocities and on the shell
radius reaching to about 33. We have investigated the mean axisymmetric part of
the flow, which consists of differential rotation. Strong super-rotation of the
fluid with respect to the rotating inner sphere is directly measured. It is
found that the organization of the mean flow does not change much throughout
the entire range of parameters covered by our experiment. The direct
measurements of zonal velocity give a nice illustration of Ferraro's law of
isorotation in the vicinity of the inner sphere where magnetic forces dominate
inertial ones. The transition from a Ferraro regime in the interior to a
geostrophic regime, where inertial forces predominate, in the outer regions has
been well documented. It takes place where the local Elsasser number is about
1. A quantitative agreement with non-linear numerical simulations is obtained
when keeping the same Elsasser number. The experiments also reveal a region
that violates Ferraro's law just above the inner sphere.Comment: Phys Rev E, in pres
Shortening of the Short Refractory Periods in Short QT Syndrome.
BACKGROUND: Diagnosis of short QT syndrome (SQTS) remains difficult in case of borderline QT values as often found in normal populations. Whether some shortening of refractory periods (RP) may help in differentiating SQTS from normal subjects is unknown. METHODS AND RESULTS: Atrial and right ventricular RP at the apex and right ventricular outflow tract as determined during standard electrophysiological study were compared between 16 SQTS patients (QTc 324±24 ms) and 15 controls with similar clinical characteristics (QTc 417±32 ms). Atrial RP were significantly shorter in SQTS compared with controls at 600- and 500-ms basic cycle lengths. Baseline ventricular RP were significantly shorter in SQTS patients than in controls, both at the apex and right ventricular outflow tract and for any cycle length. Differences remained significant for RP of any subsequent extrastimulus at any cycle length and any pacing site. A cut-off value of baseline RP <200 ms at the right ventricular outflow tract either at 600- or 500-ms cycle length had a sensitivity of 86% and a specificity of 100% for the diagnosis of SQTS. CONCLUSIONS: Patients with SQTS have shorter ventricular RP than controls, both at baseline during various cycle lengths and after premature extrastimuli. A cut-off value of 200 ms at the right ventricular outflow tract during 600- and 500-ms basic cycle length may help in detecting true SQTS from normal subjects with borderline QT values
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