Transient core surface dynamics from ground and satellite geomagnetic data

Abstract

Summary We present an update of the geomagnetic data assimilation tool pygeodyn, use it to analyze ground and satellite-based geomagnetic datasets, and report new findings on the dynamics of the Earth’s outer core on interannual to decadal time-scales. Our results support the idea that quasi-geostrophic Magneto-Coriolis waves, recently discovered at a period of 7 years, also operate on both shorter and longer time-scales, specifically in period bands centered around 3.5 and 15 years. We revisit the source of interannual variations in the length of day and argue that both geostrophic torsional Alfvén waves and quasi-geostrophic Magneto-Coriolis waves can possibly contribute to spectral lines that have been isolated around 8.5 and 6 years. A significant improvement to our ensemble Kalman filter algorithm comes from accounting for cross-correlations between variables of the state vector forecast, using the ‘Graphical lasso’ method to help stabilize the correlation matrices. This allows us to avoid spurious shrinkage of the model uncertainties while (i) conserving important information contained in off-diagonal elements of the forecast covariance matrix, and (ii) considering a limited number of realizations, thus reducing the computational cost. Our updated scheme also permits us to use observations either in the form of Gauss coefficient data or more directly as ground-based and satellite-based virtual observatory series. It is thanks to these advances that we are able to place global constraints on core dynamics even at short periods.

Wave-like motions and torques in Earth's core as inferred from geomagnetic data: A synthetic study

“…For wave-like motions with periods respectively [2 − 4.5]τ A and [6 − 13.5]τ A , we have estimated the outward wave velocity C to be about U A /10 and U A /15. We can compare these values with the geophysical estimate by Istas et al (2023). They found the wave velocity to be about 150 km/yr (U A /7) and 75 km/yr (U A /15) at periods about 7 yr (3.5 τ A ) and 15 yr (7.5 τ A ).…”

“…The changes in core angular momentum carried by geostrophic motions, as estimated from core surface velocities, do balance the changes in mantle angular momentum inferred from LOD data for periods ranging from ≈ 5 to 60 years (Jault et al, 1988;Jackson et al, 1993;Baerenzung et al, 2018;Gillet et al, 2019;Istas et al, 2023). The transfer of angular momentum between the core and the mantle is necessarily mediated by one or several torques, such as the electro-magnetic (EM), gravitational, pressure or possibly the viscous torque (Buffett, 1996;Jault, 2003;Roberts and Aurnou, 2012).…”

“…unrealistic decrease of the ensemble spread). We thus followIstas et al (2023), and use the 'Graphical Lasso' method (e.g.,Friedman et al, 2007), which mitigates the impact of noise, while keeping as much as possible the information contained in the empirical cross-covariances. This method involves a tuning parameter that controls the sparsity of the forecast covariance matrices.…”

State and evolution of the geodynamo from numerical models reaching the physical conditions of Earth’s core

“…The new quasi-geostrophic (QG) magnetoinertial (Alfv én) and magneto-Coriolis waves exhibited that way could successfully explain the occurrence of geomagnetic jerks, or abrupt changes in the acceleration of Earth's magnetic field (Aubert & Finlay 2019 ;Aubert et al 2022 ). A tight interconnection between numerical and observational research also enabled the first theoretical characterization (Gerick et al 2021 ) and observation (Gillet et al 2022 ;Istas et al 2023 ) of rapid, QG magneto-Coriolis (QGMC) waves in core flows inferred from geomagnetic variations.…”

Dynamic Mode Decomposition of the Core Surface Flow Inverted From Geomagnetic Field Models

“…(QGMC) waves. This scenario is further discussed by Istas et al (2023) through the inversion of the core flow using prior information from numerical dynamos. In summary, previous studies have suggested that geomagnetic jerks are likely associated with wave-like fluid motions within the Earth's core.…”