International audienceForced longitudinal librations are oscillatory perturbations of the rotation rate of a planet resulting from a gravitational coupling with orbital partners. In the present study we report the first experimental evidence that a librating cylindrical container can viscously drive mean azimuthal flows in the liquid interior, hereafter referred to as zonal flows. Using a novel laser Doppler velocimetry system, the current work extends upon the study of libration-driven flows by Noir et al. (2009). We investigate the different mechanisms underlying the zonal flow generation. It is found that zonal flows in the interior result primarily from non-linearities in the Ekman boundary layer. Furthermore, the zonal flow scales as the square of the libration amplitude and is independent of the Ekman number. This scaling implies that forced longitudinal libration in an axisymmetric container (purely viscous coupling) will drive unobservably small zonal flows at planetary conditions. Thus, purely viscous librational coupling will not generate significant energy dissipation in a planetary fluid layer. It follows that any observed phase lag between the gravitational forcing and the orbital response of a planet requires non-viscous coupling mechanisms to account for the energy dissipatio

Progressive crystallisation of Earth's inner core over geological times drives convection in the outer core and the generation of the Earth's magnetic field. Resolving the rate and pattern of inner core growth is thus crucial to understanding the evolution of the geodynamo. The growth history of Earth's inner core is likely recorded in the distribution and strength of seismic anisotropy arising from 19 deformation texturing constrained by boundary conditions at the inner-core solid-20 fluid boundary. Travel times of seismic body waves indicate that seismic anisotropy 21 increases with depth. Here we find that the strongest anisotropy is offset from Earth's rotation axis. Using geodynamic growth models and mineral physics calculations, we simulate the development of inner core anisotropy in a self-consistent manner. We show for the first time that an inner core model composed of hexagonally closepacked iron-nickel alloy, deformed by a combination of preferential equatorial growth and slow translation can match the seismic observations without requiring the introduction of hemispheres with sharp boundaries. We find a model of the inner core 28 growth history compatible with external constraints from outer core dynamics, supporting arguments for a relatively young inner core (~0.5-1.5 Ga) and a viscosity >10 18 Pa-s.The presence of seismic anisotropy -the dependence of seismic wavespeed on direction of propagation -in the inner core (IC) was proposed over 30 years ago to explain the early arrival times of IC sensitive seismic body waves (PKPdf) travelling on paths parallel to the Earth's rotation axis 1,2 and anomalous splitting of core-sensitive free oscillations 3 . This anisotropy is thought to result from alignment of iron crystals caused by deformation in a flow field induced by the evolution of the core, i.e. deformation texturing. In previous work, different geodynamic 4 and plastic deformation mechanisms 5 were explored to explain the variation of PKPdf travel times with angle of the ray path with respect to the rotation axis.Here, for the first time, we combine geodynamic modelling of the evolution of flow in the IC, allowing for slow lateral translation, with presently available knowledge on the mineralogy and deformation mechanisms proposed for the IC to explain spatially varying patterns of observed seismic travel times in an updated global dataset.