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Numerical simulations of the geodynamo are used to investigate the transfer of angular momentum between the core and the mantle, in the presence of both an electro-magnetic torque and a gravitational torque between the inner core and the mantle. Over a broad range of periods, angular momentum changes are accurately explained when deriving geostrophic motions (organized as cylinders co-axial with the Earth’s rotation vector) from core surface motions. Inverted core surface flows can thus be used to infer the core angular momentum budget. In the dynamo, the largest part of the EM torque variations is associated with the evolution of the solid body rotation, with little contribution from electrical currents originating deep inside the core (the "leakage torque”, formerly associated with the westward drift of the geomagnetic field). A 1D model of geostrophic motions is furthermore used to estimate the efficiency of torsional eigenmodes to couple with the solid Earth. From this we revisit the estimate of the mantle conductance from 1D simulations of geostrophic motions stochastically forced in the volume. We then discuss the ability of torsional waves to trigger the interannual oscillations observed in the length-of-day series, and the relative importance of forcing versus resonances in angular momentum changes at various time-scales., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Investigating the dynamics of liquid iron at the top of the core requires an inversion process from magnetic field measurements. Due to possible leakage, traditional spectral techniques are not optimal for regional investigations or studies of Secular Variation (first time derivative of magnetic field, SV) on short-time periods. Improved data coverage and quality from satellite missions have provided insight into short-period dynamics (such as jerks and waves), and improved understanding of localised features. However, there continues to be a need for new techniques for better spatio-temporal models of flow motions at the top of the core, especially for regional investigations.The aim of this work is to investigate regional dynamics at the Core-Mantle Boundary (CMB) using the SOLA (Subtractive Optimally Localised Averages) methodology. This method allows us to create point estimates of radial field SV at the CMB by considering global satellite measurements. The SOLA methodology allows for a local estimate of radial field SV at any desired location directly at the CMB, which opens the way to investigations of regional dynamics. Producing localised-average estimates at the CMB bypasses many problems encountered when using only a subset of magnetic satellite data, downward continuing data, and models based on spherical harmonics.Finally, we discuss how the SOLA methodology can be incorporated into the PyGeodyn code to produce core flow models. This approach can provide additional information on wave-like flow motions at the top of the Earth’s core where these are most prominent, and permits new investigations of shorter period phenomenon.&nbsp, The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

We now benefit of more than two decades of global geomagnetic surveys from low-Earth orbiting satellites. They have revealed repeated interannual changes, more intense towards the equator. In order to reconstruct the dynamics at the surface of the core, these observational constraints are introduced in the “pygeodyn” data assimilation tool. This algorithm incorporates spatio-temporal constraints derived from geodynamo numerical simulations approaching Earth’s conditions. We discovered recurrent non-axisymmetric flow patterns presenting a period of about 7 yr. They propagate equatorward throughout the fluid core, and present their strongest amplitude (~3 km/yr) at the equator, where they show a coherent westward drift at phase speeds of about 1,500 km/yr. We interpret and model these flows as the signature of Magneto-Coriolis waves. Using synthetic data from dynamo simulations, we show that the recovery of such transient motions depends mostly on the data coverage and on their magnitude. The identification of Magneto-Coriolis waves offers a way to probe the cylindrical radial component of the dynamo field inside Earth’s core, and possibly to sample lateral variations in the electrical conductivity of the lower mantle. It follows from our work that there is no need for a stratified layer at the top of the core to account for these geomagnetic field changes. Such waves most likely also exist on longer time-scales, calling for long-lived magnetic records from space., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

International audience; 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.

Inferring the core dynamics responsible for the observed geomagnetic secular variation requires knowledge of the magnetic field at the core mantle boundary together with its associated model covariances. However, all currently available field models have been built using regularization conditions, which force the expansions in the spatial and time domains to converge, but also hinders the calculation of reliable second order statistics. To tackle this issue, we propose a stochastic approach that integrates, through time covariance functions, some prior information on the time evolution of the geomagnetic field. We consider the time series of spherical harmonic coefficients as realizations of a continuous and differentiable stochastic process. Our specific choice of process, such that it is not twice differentiable, mainly relies on two properties of magnetic observatory records (time spectra, existence of geomagnetic jerks). In addition, the required characteristic times for the low degree coefficients are obtained from available models of the magnetic field and its secular variation based on satellite data. We construct the new family COV-OBS of field models spanning the observatory and satellite era of 1840-2010. These models include the external dipole and permit sharper time changes of the internal field compared to previous regularized reconstructions. The a posteriori covariance matrix displays correlations in both space and time, which should be accounted for through the secular variation error model in core flow inversions and geomagnetic data assimilation studies.