ALBERT

Description: 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, whose 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 10 12 Pa s.

Description: Inner core translation, with solidification on one hemisphere and melting on the other, provides a promising basis for understanding the hemispherical dichotomy of the inner core, as well as the anomalous stable layer observed at the base of the outer core—the so-called F-layer—which might be sustained by continuous melting of inner core material. In this paper, we study in details the dynamics of inner core thermal convection when dynamically induced melting and freezing of the inner core boundary (ICB) are taken into account. If the inner core is unstably stratified, linear stability analysis and numerical simulations consistently show that the translation mode dominates only if the viscosity is large enough, with a critical viscosity value, of order ~3  x 10 18 Pa s, depending on the ability of outer core convection to supply or remove the latent heat of melting or solidification. If is smaller, the dynamic effect of melting and freezing is small. Convection takes a more classical form, with a one-cell axisymmetric mode at the onset and chaotic plume convection at large Rayleigh number. being poorly known, either mode seems equally possible. We derive analytical expressions for the rates of translation and melting for the translation mode, and a scaling theory for high Rayleigh number plume convection. Coupling our dynamic models with a model of inner core thermal evolution, we predict the convection mode and melting rate as functions of inner core age, thermal conductivity, and viscosity. If the inner core is indeed in the translation regime, the predicted melting rate is high enough, according to Alboussière et al. 's experiments, to allow the formation of a stratified layer above the ICB. In the plume convection regime, the melting rate, although smaller than in the translation regime, can still be significant if is not too small. Thermal convection requires that a superadiabatic temperature profile is maintained in the inner core, which depends on a competition between extraction of the inner core internal heat by conduction and cooling at the ICB. Inner core thermal convection appears very likely with the low thermal conductivity value proposed by Stacey & Loper, but nearly impossible with the much higher thermal conductivity recently put forward by Sha & Cohen, de Koker et al. and Pozzo et al. We argue however that the formation of an iron-rich layer above the ICB may have a positive feedback on inner core convection: it implies that the inner core crystallized from an increasingly iron-rich liquid, resulting in an unstable compositional stratification which could drive inner core convection, perhaps even if the inner core is subadiabatic.

Description: Mitochondrial DNA damage induces apoptosis in senescent cells Cell Death and Disease 4, e727 (July 2013). doi:10.1038/cddis.2013.199 Authors: R-M Laberge, D Adler, M DeMaria, N Mechtouf, R Teachenor, G B Cardin, P-Y Desprez, J Campisi & F Rodier

Description: SUMMARY The discovery of torsional Alfvén waves (geostrophic Alfvén waves) in the Earth’s core (Gillet et al. 2010) calls for a better understanding of their properties. We present the first experimental observations of torsional Alfvén waves, performed in the DTS-Ω set-up. In this set-up, 50 L of liquid sodium are confined between an inner sphere (ri = 74 mm) and an outer shell (ro = 210 mm). The inner sphere houses a permanent magnet, imposing a dipolar magnetic field (Bmax = 345 mT). Both the inner sphere and the outer shell can rotate around the vertical axis. Alfvén waves are triggered by a sudden jerk of the inner sphere. We study the propagation of these waves when the fluid is initially at rest, and when it spins at a rotation rate up to 15 Hz. We measure the azimuthal magnetic field of the wave at different radii inside the fluid with magnetometers installed in a sleeve. We also record the electric potential signature on the outer shell at several latitudes. Besides, we probe the associated azimuthal velocity field using ultrasound Doppler velocimetry. With a 15 Hz rotation rate, the dynamical regimes we achieve are characterized by dimensionless numbers in the following ranges: Lundquist number 0.5 〈 Lu 〈 12, Lehnert number 0.01 〈 Le 〈 0.26, Rossby number Ro ∼ 0.1. We observe that the magnetic signal propagates away from the inner sphere, strongly damped by magnetic diffusion. Rotation affects the magnetic signature in a subtle way. Its effect is more pronounced on the surface electric potentials, which are sensitive to the actual fluid velocity of the wave. The ultrasound Doppler probes provide the first experimental measurement of the fluid velocity of an Alfvén wave. To complement these observations, we ran numerical simulations, using the XSHELLS pseudospectral code with parameters as close as possible to the experimental ones. The synthetic magnetic and electric signals match our measurements. The meridional snapshots of the synthetic azimuthal velocity field reveal the formation of geostrophic cylinders expected for torsional Alfvén waves. We establish scaling laws for the magnetic and kinetic energies of Alfvén waves with and without rotation. In both cases, we find that the magnetic energy EM saturates at a level proportional to $Rm_{ m jerk}^2$, where Rmjerk = Ujerkro/η is the magnetic Reynolds number built with the maximum azimuthal velocity of the inner sphere during the jerk. The $E_K^{ m max}/E_M^{ m max}$ ratio (where $E_K^{ m max}$ is the maximum kinetic energy), close to 1 for very quick jerks, increases linearly with the jerk duration.