Revistas

Understanding Earth's core dynamics over millennial timescales requires models that jointly describe the evolution of the geomagnetic field and core surface flow, while accommodating the sparse, irregular, and uncertain nature of archaeomagnetic and palaeomagnetic data. We present a new Bayesian core field and core flow modelling framework that utilises archaeo/palaeomagnetic data directly, combining a reduced stochastic representation of core surface dynamics derived from numerical geodynamo statistics with a probabilistic treatment of observational and chronological uncertainties. A key innovation is an efficient discrete marginalisation of age uncertainties, which avoids the convergence difficulties associated with co-estimating ages in high-dimensional Hamiltonian Monte Carlo inversions. The framework aims to reconstruct the coupled evolution of the geomagnetic field and core surface flow over the past 9000 years while preserving dynamical correlations implied by the prior geodynamo time series. Tests using synthetic data generated from an Earth-like geodynamo demonstrate that the method reliably recovers large-scale geomagnetic field variations and key aspects of core dynamics, including long-term westward drift and the evolution of planetary-scale eccentric gyres. These results show that, when combined with physically informed priors, archaeo/palaeomagnetic data can constrain millennial-scale core flow, paving the way for reconstructions based on real data.

Nilsson, Andreas

We investigate how the radial profile σ(r) of the lower mantle electrical conductivity affects the downward continuation of the time-varying magnetic field to the core surface and the resulting inverted core motions. We compare core flow predictions to the length-of-day (LOD) with geodetic records, in order to assess how plausible the considered conductivity profiles are. The core flow inverse problem, mixing the information carried by single spherical harmonic magnetic coefficients, makes it non trivial to infer the delay expected for LOD predictions. Our results indicate that the timescale characteristic of the mantle filter in the low-frequency limit yields an integral measure of σ(r) allowing us to select admissible conductivity models. Models of σ(r) inferred from magnetospheric and tidal sources over the satellite era involve mantle filter lags less than a couple of months and provide the best fit to LOD variations. Other conductivity profiles constructed based on mineralogical properties and iron partitioning inferred for deep mantle rocks (i.e., σ increasing from a few S/m at 1200 km depth up to some tens of S/m ~ 300 km above the core surface, with a more conducting D'' layer) are acceptable. A highly conducting layer of thickness O(10 km) or thinner cannot be excluded.

Gillet, Nicolas