166 research outputs found
Planetary gyre, time-dependent eddies, torsional waves, and equatorial jets at the Earth's core surface
We report a calculation of time-dependent quasi-geostrophic core flows for
1940-2010. Inverting recursively for an ensemble of solutions, we evaluate the
main source of uncertainties, namely the model errors arising from interactions
between unresolved core surface motions and magnetic fields. Temporal
correlations of these uncertainties are accounted for. The covariance matrix
for the flow coefficients is also obtained recursively from the dispersion of
an ensemble of solutions. Maps of the flow at the core surface show, upon a
planetary-scale gyre, time-dependent large-scale eddies at mid-latitudes and
vigorous azimuthal jets in the equatorial belt. The stationary part of the flow
predominates on all the spatial scales that we can resolve. We retrieve
torsional waves that explain the length-of-day changes at 4 to 9.5 years
periods. These waves may be triggered by the nonlinear interaction between the
magnetic field and sub-decadal non-zonal motions within the fluid outer core.
Both the zonal and the more energetic non-zonal interannual motions were
particularly intense close to the equator (below 10 degrees latitude) between
1995 and 2010. We revise down the amplitude of the decade fluctuations of the
planetary scale circulation and find that electromagnetic core-mantle coupling
is not the main mechanism for angular momentum exchanges on decadal time scales
if mantle conductance is 3 10 8 S or lower
Stochastic modelling of regional archaeomagnetic series
SUMMARY We report a new method to infer continuous time series of the
declination, inclination and intensity of the magnetic field from
archeomagnetic data. Adopting a Bayesian perspective, we need to specify a
priori knowledge about the time evolution of the magnetic field. It consists in
a time correlation function that we choose to be compatible with present
knowledge about the geomagnetic time spectra. The results are presented as
distributions of possible values for the declination, inclination or intensity.
We find that the methodology can be adapted to account for the age
uncertainties of archeological artefacts and we use Markov Chain Monte Carlo to
explore the possible dates of observations. We apply the method to intensity
datasets from Mari, Syria and to intensity and directional datasets from Paris,
France. Our reconstructions display more rapid variations than previous studies
and we find that the possible values of geomagnetic field elements are not
necessarily normally distributed. Another output of the model is better age
estimates of archeological artefacts
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
Pressure torque of torsional Alfvén modes acting on an ellipsoidal mantle
We investigate the pressure torque between the fluid core and the solid mantle arising from magnetohydrodynamic modes in a rapidly rotating planetary core. A two-dimensional reduced model of the core fluid dynamics is developed to account for the non-spherical core-mantle boundary. The simplification of such a quasi-geostrophic model rests on the assumption of invariance of the equatorial components of the fluid velocity along the rotation axis. We use this model to investigate and quantify the axial torques of linear modes, focusing on the torsional Alfvén modes (TM) in an ellipsoid. We verify that the periods of these modes do not depend on the rotation frequency. Furthermore, they possess angular momentum resulting in a net pressure torque acting on the mantle. This torque scales linearly with the equatorial ellipticity. We estimate that for the TM calculated here topographic coupling to the mantle is too weak to account for the variations in the Earth’s length-of-day
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
Relations between dynamo-region geometry and the magnetic behavior of stars and planets
The geo and solar magnetic fields have long been thought to be very different
objects both in terms of spatial structure and temporal behavior. The recently
discovered field structure of a fully convective star is more reminiscent of
planetary magnetic fields than the Sun's magnetic field (Donati J.-F. et al.,
Science, 311 (2006) 633), despite the fact that the physical and chemical
properties of these objects clearly differ. This observation suggests that a
simple controlling parameter could be responsible for these different
behaviors. We report here the results of three-dimensional simulations which
show that varying the aspect ratio of the active dynamo region can yield sharp
transition from Earth-like steady dynamos to Sun-like dynamo waves
Single Nanoparticle Plasmonic Spectroscopy for Study of the Efflux Function of Multidrug ABC Membrance Transporters of Single Live Cells
ATP-binding cassette (ABC) membrane transporters exist in all living organisms and play key roles in a wide range of cellular and physiological functions. The ABC transporters can selectively extrude a wide variety of structurally and functionally unrelated substrates, leading to multidrug resistance. Despite extensive study, their efflux molecular mechanisms remain elusive. In this study, we synthesized and characterized purified silver nanoparticles (Ag NPs) (97 ± 13 nm in diameter), and used them as photostable optical imaging probes to study efflux kinetics of ABC membrane transporters (BmrA) of single live cells (B. subtilis). The NPs with concentrations up to 3.7 pM were stable (non-aggregated) in a PBS buffer and biocompatible with the cells. We found a high dependence of accumulation of the intracellular NPs in single live cells (WT, Ct-BmrAEGFP, ΔBmrA) upon the cellular expression level of BmrA and NP concentration (0.93, 1.85 and 3.7 pM), showing the highest accumulation of intracellular NPs in ΔBmrA (deletion of BmrA) and the lowest ones in Ct-BmrA-EGFP (over-expression of BmrA). Interestingly, the accumulation of intracellular NPs in ΔBmrA increases nearly proportionally with the NP concentration, while those in WT and Ct-BmrA-EGFP do not. This result suggests that the NPs enter the cells via passive diffusion driven by concentration gradients across the cellular membrane and they are extruded out of cells by BmrA transporters, similar to conventional pump substrates (antibiotics). This study shows that such large substrates (84-100 nm NPs) can enter into the live cells and be extruded out of the cells by BmrA, and the NPs can serve as nm-sized optical imaging probes to study the size-dependent efflux kinetics of membrane transporters in single live cells in real time
Viral Mimicry of Cdc2/Cyclin-Dependent Kinase 1 Mediates Disruption of Nuclear Lamina during Human Cytomegalovirus Nuclear Egress
The nuclear lamina is a major obstacle encountered by herpesvirus nucleocapsids in their passage from the nucleus to the cytoplasm (nuclear egress). We found that the human cytomegalovirus (HCMV)-encoded protein kinase UL97, which is required for efficient nuclear egress, phosphorylates the nuclear lamina component lamin A/C in vitro on sites targeted by Cdc2/cyclin-dependent kinase 1, the enzyme that is responsible for breaking down the nuclear lamina during mitosis. Quantitative mass spectrometry analyses, comparing lamin A/C isolated from cells infected with viruses either expressing or lacking UL97 activity, revealed UL97-dependent phosphorylation of lamin A/C on the serine at residue 22 (Ser22). Transient treatment of HCMV-infected cells with maribavir, an inhibitor of UL97 kinase activity, reduced lamin A/C phosphorylation by approximately 50%, consistent with UL97 directly phosphorylating lamin A/C during HCMV replication. Phosphorylation of lamin A/C during viral replication was accompanied by changes in the shape of the nucleus, as well as thinning, invaginations, and discrete breaks in the nuclear lamina, all of which required UL97 activity. As Ser22 is a phosphorylation site of particularly strong relevance for lamin A/C disassembly, our data support a model wherein viral mimicry of a mitotic host cell kinase activity promotes nuclear egress while accommodating viral arrest of the cell cycle
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