Mid-latitude and equatorial core surface flow variations derived from observatory and satellite magnetic data

Abstract

Summary A series of models of the Earth magnetic field and core surface flow have been simultaneously and sequentially co-estimated from year 1999 to 2022. The models were derived from magnetic satellite and ground observatory data using a linear Kalman filter approach and prior statistics based on numerical dynamo simulations. The core field and secular variation model components present the same characteristics as the most recent core field models with slightly higher resolution in time. A principal component analysis of the core surface flow series of models shows that the largest flow variations are observed at high latitudes and under the western part of the Pacific Ocean. Filtering out the flow variation periods longer than ∼11.5 years leads to a filtered azimuthal flow that presents ∼7 years periodicities with patterns propagating westward by ∼60o longitude per year. These patterns are present mainly at mid- and equatorial latitudes. They are compatible with a perturbation of the main flow made of small columnar flows with rotation axis intersecting the core-mantle boundary between 10o to 15o latitudes, and flow speed of less than 5km/y. Present at all longitudes, these columnar flows are particularly strong under the Pacific Ocean after 2013. They can also be clearly identified under the Atlantic Ocean from 2005 to 2015.

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

“…5 ∼ 0 and indeed remains constant between the two runs. From 71 per cent of the path onwards, the results are in agreement with typical periods of 7 yr and amplitudes of about 7 km yr −1 retrieved from recent geomagnetic variations by Gillet et al ( 2022 ) and Ropp & Lesur ( 2023 ).…”

Gyres, jets and waves in the Earth’s core

“…The planetary gyre structure becomes clearer on time-averaging and if only the large length scale flow is considered. Flow variations on interannual to decadal timescales, especially at low latitudes, also turn out to be particularly well constrained by the observations and show evidence for waves [75][76][77] .…”

“…The flow is therefore often assumed to be large scale 124,125 or else the impact of unresolved scales is included as an error term 5,62,63,120 . Despite these challenges there is now considerable agreement across a variety of approaches concerning the planetary-scale flow inferred from satellite observations 62,63,75,76,79,111,117,126 . The planetary gyre structure becomes clearer on time-averaging and if only the large length scale flow is considered.…”

“…A major advance since 1999 has been the availability of high quality satellite observations of the geomagnetic field [72][73][74] . This has enabled global high resolution studies of the flow in Earth's core [75][76][77] providing important new insights and constraints on interannual to decadal core dynamics.…”

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

“…Another difficulty for the core flow inversion is how to take into account unresolved small‐scale magnetic fields, which contribute to the observed SV through interacting with the core flow (Gillet et al., 2009; Hulot et al., 1992). Nevertheless, many core surface flow models have been derived in the satellite‐era using different assumptions, inversion schemes and data set (e.g., Eymin & Hulot, 2005; Gillet et al., 2022; Kloss & Finlay, 2019; Olsen & Mandea, 2008; Pais & Jault, 2008; Ropp & Lesur, 2023; Whaler et al., 2022). These models have revealed very similar large‐scale core surface flow structures such as a planetary‐scale gyre and a high‐latitude jet (Finlay et al., 2023; Holme, 2015), though some details are different.…”