Dynamics of Earth’s core from multi-satellite observations

Project Information

CoreSat

Grant agreement ID: 772561

DOI

10.3030/772561

Project closed

EC signature date 21 February 2018

Start date 1 March 2018

End date 31 August 2023

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 828 708,00

EU contribution

€ 1 828 708,00

Coordinated by

DANMARKS TEKNISKE UNIVERSITET
Denmark

Periodic Reporting for period 4 - CoreSat (Dynamics of Earth’s core from multi-satellite observations)

Reporting period: 2022-09-01 to 2023-08-31

Summary of the context and overall objectives of the project

Earth’s magnetic field plays a fundamental role in our planetary habitat, controlling interactions between the Earth and the solar wind. Yet observations show a region of persistently weak field in the South Atlantic that has grown in size in recent decades. Pinning down the core dynamics responsible for this behaviour is essential if we are to understand the present geodynamo, and to forecast future magnetic field changes. Global magnetic observations provided by multiple satellites, today provide us with the means to probe Earth's magnetic field, and the underlying core dynamics, in exquisite detail and allow us to test hypotheses concerning the origin of field changes in the South Atlantic.

The objectives of the CoreSat project are to use multi-satellite magnetic field measurements to

1. Reveal small scales and rapid time changes of the core-generated magnetic field
2. Test whether rotation-dominated core convection can explain the recent time-dependence of the South Atlantic Anomaly.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Objective 1 Reveal small scales and rapid time changes of the core-generated magnetic field

A new technique has been developed for separating out the lithospheric field signal and isolating the small scale core field signal in satellite observations (Otzen et al. 2023b). It uses of prior information from core dynamo and lithospheric magnetization simulations and provides a posterior probability density for the core-generated field. Validation tests were carried out using synthetic data and the technique was applied to derive a new high resolution core field model. This was used to study the evolution of reversed flux features at the core-mantle boundary that underlie the South Atlantic Anomaly (Otzen et al. 2023b).

Regarding rapid time variations, a novel technique for parameterising and co-estimating fields due to ionospheric currents, with a special focus on the challenging polar region has been developed (Kloss et al. 2023). Improved instrumentation has been installed at ground observatories in Greenland that are important for modelling both core and ionospheric fields. Substorm dynamics were accounted for using a ground-based index. Using this technique field models with reduced temporal smoothing have been derived and core field variations on timescales down to 1 year have been analyzed.

New models of the core-generated magnetic field have been derived using data from the three Swarm satellites, the CHAMP, Oersted, SAC-C and Cryosat-2 satellites and ground observatory data (Finlay et al., 2020). The resulting field model was analysed with a specific focus on changes in the South Atlantic Anomaly weak field region and underlying field features on the core surface.

Main results / dissemination:

[1a] Imaging the small-scale core-generated magnetic field

Otzen et al. (2023a) IUGG General Assembly, Berlin 2023, https://doi.org/10.57757/IUGG23-2850​
Otzen et al. (2023b) Under review at Geophys. J. Int., minor revisions - Sept 2023.

[1b] Co-estimation of fields due to polar ionospheric currents and investigations of rapid core field variations

Kloss et al. (2023) Geophys. J. Int., https://doi.org/10.1093/gji/ggad325

[1c] Discovery of secondary minimum in the strength of the Earth’s magnetic field that has developed South-west of Africa over the past decade

Finlay et al., (2020) Earth, Planets, Space, https://doi.org/10.1186/s40623-020-01252-9

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Objective 2: Test whether rotation-dominated core convection can explain the recent time-dependence of the South Atlantic Anomaly

A numerical code for modelling rapidly-rotating (quasi-geostrophic, QG) core convection was extended to treat the temperature field in 3D and to allow for inhomogeneous heat flux boundary conditions (Barrois et al. 2022a). A new high performance computing cluster for running this code was established. A systematic study of core convection was carried out as a function of control parameters and scaling laws tested (Barrois et al. 2022a). The hybrid code was further coupled to a 3D magnetic field (Barrois et al. 2022b) and a survey of dynamo action as a function of control parameters carried out. Only multipolar, reversing dynamos with weak magnetic compared with kinetic energy were found (Barrois et al. 2023).

Expressions for the field gradient elements at target locations at satellite altitude (virtual observtories) were derived and estimated at 300 sites with 4-monthly cadence based on data from the CHAMP and Swarm satellites. These were shown to be suitable for geomagnetic field modelling (Hammer et al. 2022). A Singular Evolutive Extended Kalman (SEEK) filter data assimilation algorithm was implemented and tested using synthetic data at virtual observatories sites (Hammer et al. 2023). The algorithm was extended to work with our hybrid QG-3D dynamo model, but the fields produced by this dynamo have so far not been suitable for explaining Earth's magnetic field and the South Atlantic Anomaly.

Main results / dissemination:

[2a] Core convection models where the flow is treated as 2D (quasi-geostrophic, QG), while the temperature is treated in 3D, closely reproduce full 3D convection patterns, provided forcing is moderate

Barrois, et al. (2022a) Geophys. J. Int, https://doi.org/10.1093/gji/ggac141

[2b] Quasi-geostrophic flows in a spherical shell can act as dynamos, but resulting magnetic fields are weak and non-dipolar so not suitable for explaining Earth's magnetic field and the South Atlantic Anomaly.

Barrois et al. (2022b) SEDI Symposium Zurich, 2022, https://sedi2022.earth.sinica.edu.tw
Barrois et al. (2023) Under review at Geophys. J. Int., moderate revisions - Sept 2023

[2c] Satellite magnetic data can be combined into field gradient elements at target locations (virtual observatories). These are less effected by along-track correlated errors than conventional observations, and suitable for data assimilation studies

Hammer et al. (2022) Geophys. J. Int, https://doi.org/10.1093/gji/ggac004

[2d] A data assimilation algorithm (the Singular Evolutive Extended Kalman filter) data has been implemented for satellite-based virtual observatories and state vectors from the QG-3D dynamo model.

Hammer et al. (2023) IUGG General Assembly, Berlin 2023, https://doi.org/10.57757/IUGG23-2863

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

New methods for separating the lithospheric and ionospheric magnetic signals that obscure signals from Earth's core have been developed. These go beyond previous methods for modelling Earth's magnetic field by better accounting for the physics of the sources. They provide a new way to study small scales and rapid time variations of the core field.

Using multi-satellite data new models of the South Atlantic weak field anomaly, and its evolution at Earth's surface, have been constructed. We identified a secondary minimum in field intensity that has developed south-west of Africa over the past decade. This unexpected finding suggests that the South Atlantic weak field anomaly is not a single entity but the combined result of numerous evolving flux features at the core surface.

A novel hybrid (QG-3D) dynamo simulation tool has been developed. This has demonstrated that rapidly rotating (quasi-geostrophic) convective flows can act as dynamos and produce self-sustained magnetic fields. However, so far, it has not produced Earth-like dipole-dominated dynamos, a negative result that likely points to missing ingredients in quasi-geostrophic flows.

Magnetic Field Intensity at Earth's surface, showing secondary minimum of South Atlantic Anomaly