Short Timescale Core Dynamics: TheoryandObservations

Fluid motions in the Earth's core produce changes in the geomagnetic field (secular variation) and are also an important ingredient in the planet's rotational dynamics. In this article we review current understanding of core dynamics focusing on short timescales of years to centuries. We describe both theoretical models and what may be inferred from geomagnetic and geodetic observations. The kinematic concepts of frozen flux and magnetic diffusion are discussed along with relevant dynamical regimes of magnetostrophic balance, tangential geostrophy, and quasi-geostrophy. An introduction is given to free modes and waves that are expected to be present in Earth's core including axisymmetric torsional oscillations and non-axisymmetric Magnetic-Coriolis waves. We focus on important recent developments and promising directions for future investigation

The transition to Earth-like torsional oscillations in magnetoconvection simulations

Evidence for torsional oscillations (TOs) operating within the Earth's fluid outer core has been found in the secular variation of the geomagnetic field. These waves arise via disturbances to the predominant (magnetostrophic) force balance believed to exist in the core. The coupling of the core and mantle allow TOs to affect the length-of-day of the Earth via angular momentum conservation. Encouraged by previous work, where we were able to observe TOs in geodynamo simulations, we perform 3-D magnetoconvection simulations in a spherical shell in order to reach more Earth-like parameter regimes that proved hitherto elusive. At large Ekman numbers we find that TOs can be present but are typically only a small fraction of the overall dynamics and are often driven by Reynolds forcing at various locations throughout the domain. However, as the Ekman number is reduced to more Earth-like values, TOs become more apparent and can make up the dominant portion of the short timescale flow. This coincides with a transition to regimes where excitation is found only at the tangent cylinder, is delivered by the Lorentz force and gives rise to a periodic Earth-like wave pattern, approximately operating on a 4 to 5 year timescale. The core travel times of our waves also become independent of rotation at low Ekman number with many converging to Earth-like values of around 4 years

An accelerating high-latitude jet in Earth's core

Observations of the change in Earth's magnetic field, the secular variation, provide information on the motion of liquid metal within the core that is responsible for its generation. The very latest high-resolution observations from ESA's Swarm satellite mission show intense field change at high-latitude localised in a distinctive circular daisy-chain configuration centred on the north geographic pole. Here we explain this feature with a localised, non-axisymmetric, westwards jet of 420 km width on the tangent cylinder, the cylinder of fluid within the core that is aligned with the rotation axis and tangent to the solid inner core. We find that the jet has increased in magnitude by a factor of three over the period 2000--2016 to about 40 km/yr, and is now much stronger than typical large-scale flows inferred for the core. The current accelerating phase may be a part of a longer term fluctuation of the jet causing both eastwards and westwards movement of magnetic features over historical periods, and may contribute to recent changes in torsional wave activity and the rotation direction of the inner core

The limited contribution from outer core dynamics to global deformations at the Earth’s surface

International audiencePlanetary scale interannual deformations of the Earth's surface, of millimetric amplitude, have recently been related to both geomagnetic field changes and motion within the fluid outer core. We calculate the temporal variations of the dynamical pressure at the surface of the core associated with core flow models inverted from geomagnetic observations. From these we compute predictions of the changes in Earth's topography in response to elastic deformations in the mantle. We show that at decadal periods, the predicted changes in Earth's topography are at most of the order of 0.3 mm. Focused at interannual periods between 4 and 9.5 yr, the predicted topography variations are smaller than 0.05 mm, at least an order of magnitude smaller than the reported observations. These amplitudes are only weakly sensitive to the choice of hypothesis used to reconstruct fluid motions at the core surface. We conclude that surface deformations induced by dynamical pressure changes in the core are below the detection level at present-day. Alternative geophysical sources must be sought to explain the observed millimetric interannual variations of the planetary scale topography, and its associated gravity variations. We currently see no justification for a physical relationship between interannual fluctuations of the geomagnetic Downloaded fro

Short Timescale Core Dynamics: Theory and Observations

ISSN:1572-9672ISSN:0038-630

Short Timescale Core Dynamics: Theory and Observations

International audienceFluid motions in the Earth's core produce changes in the geomagnetic field (secular variation) and are also an important ingredient in the planet's rotational dynamics. In this article we review current understanding of core dynamics focusing on short timescales of years to centuries. We describe both theoretical models and what may be inferred from geomagnetic and geodetic observations. The kinematic concepts of frozen flux and magnetic diffusion are discussed along with relevant dynamical regimes of magnetostrophic balance, tangential geostrophy, and quasi-geostrophy. An introduction is given to free modes and waves that are expected to be present in Earth's core including axisymmetric torsional oscillations and non-axisymmetric Magnetic-Coriolis waves. We focus on important recent developments and promising directions for future investigations

Constraints on the coupling at the core-mantle and inner core boundaries inferred from nutation observations

International audienceWe present an inversion of nutation observations in terms of parameters characterizing the Earth's interior properties. We use a Bayesian inversion in the time-domain, allowing us to take fully into account non-linearities in the nutation model and to reduce the loss of information occurring in frequency-domain inversions. Among the parameters we retrieve are two complex parameters, KCMB and KICB, referred to as `coupling constants', characterizing the mechanical coupling at the core-mantle boundary (CMB) and the inner core boundary (ICB), respectively. Based on a joint inversion of nutation observations provided by different analysis centres, we find Im(KCMB) = (-1.78 +/- 0.02)10-5,Re(KICB) = (1.01 +/- 0.02)10-3 and Im(KICB) = (-1.09 +/- 0.03)10-3 (where the errors correspond to 99.7 per cent confidence intervals). While our value of Im(KCMB) is similar to previous estimates, our new values of Re(KICB) and Im(KICB) are significantly different. This is mainly because of the different inversion strategy that we use and also because of the lengthier record of observation available. We show that, based on existing coupling models, neither viscous nor electromagnetic coupling alone can explain our new values of Re(KICB) and Im(KICB). A combination of these two mechanisms is required and necessitates a radial magnetic field at the ICB of total rms strength between 6 and 7 mT and a kinematic viscosity of the fluid core at the ICB should be between 10 and 30 m2s-1, depending on the exact partition between viscous and electromagnetic coupling

On the coupling between magnetic field and nutation in a numerical integration approach

Nutation amplitudes are computed in a displacement field approach that incorporates the influence of a prescribed magnetic field inside the Earth's core. The existence of relative nutational motions between the liquid core and its surrounding solid parts induces a shearing of the magnetic field. An incremental magnetic field is then created, which in return perturbs the nutations themselves. This problem has already been addressed within a nutation model computed from an angular momentum budget approach. Here we incorporate the magnetic field influence directly in the motion equation and in the boundary conditions used in precise nutation theory, and a new strategy to compute nutations is established. As in previous studies, we assume that the root-mean-square of the radial magnetic field amplitude at the core-mantle boundary is 6.9 Gauss, that the magnetic diffusivity at the bottom of the mantle and in the fluid outer core side is 1.6 m 2/s, and that the thickness of the conductive layer at the bottom of the mantle is 200 m. The Coriolis force is included in this work. The results show that the free core nutation period decreases by 0.38 days, and that the out-of-phase (in-phase) amplitudes of the retrograde 18.6 year and the retrograde annual nutations increase (decrease) by 20 and 39 μas, respectively. Comparisons of these results with previous studies are made, and discussions are also presented on the contribution of Coriolis force and the prescribed magnetic field on the coupling constants