Periodic Reporting for period 4 - GRACEFUL (Probing the deep Earth's interior by synergistic use of observations of the magnetic and gravity fields, and of the rotation of the Earth.)
Période du rapport: 2025-03-01 au 2025-08-31
What is the problem/issue being addressed?
GRACEFUL is a project that tackles fundamental gaps in our understanding of Earth’s deep interior, particularly the fluid core. Its long-term vision is to open new research frontiers and advance knowledge of Earth’s internal dynamics. The project integrates the latest observations of Earth’s gravity field, magnetic field, and rotation, while also developing precise models of core flows.
The key scientific questions this novel approach seeks to address are:
- What flow motions in the liquid core can account for the observed magnetic field, its secular variation, and magnetic jerks, as well as the observed variations in length of day (LOD) on decadal and sub-decadal timescales?
- What types of global motions occur in the core at these timescales?
- How does core flow near the core–mantle boundary (CMB) influence temporal variations in gravity and LOD?
- What roles do the core, the core–mantle boundary, and the lower mantle play in explaining observed variations in gravity, the magnetic field, and LOD?
- Can we predict changes in core flow, and consequently in the magnetic field, LOD variations, and the core’s contribution to gravity?
Why is it important for society?
It is crucial to understand why Earth’s magnetic field is currently changing—at times more rapidly than expected—and to precisely determine Earth’s rotation and orientation in space, which are essential for satellite-based applications. Achieving this requires deeper knowledge of the processes occurring within the core and at the core–mantle boundary.
The study of Earth’s deep interior remains a rapidly evolving scientific discipline and continues to attract significant public interest. Because direct access to the fluid, iron-rich outer core is impossible, only indirect observations provide insights into its dynamics. Alongside seismic data, Earth’s magnetic and gravity fields, as well as variations in its rotation (particularly changes in the length of day, or LOD), offer invaluable information about processes occurring deep within the planet.
The magnetic field, generated primarily in the fluid outer core, and its temporal variations allow scientists to infer fluid motions on decadal and sub-decadal timescales. Similarly, temporal variations in Earth’s gravity field reflect changes in mass distribution both within the planet and at its surface. While much of this variability is dominated by surface processes such as the global water cycle and climate-driven ice loss, the gravity field also carries the signature of core flows that deform the core–mantle boundary and interact with the lower mantle.
Variations in Earth’s rotation, including LOD changes and polar motion, occur on comparable timescales and are thought to be primarily driven by fluid core motions through angular momentum exchange at the core–mantle boundary. Scientific progress is most effective when observations and models are combined, offering the most comprehensive understanding of these fundamental processes.
What are the overall objectives?
Understanding the processes within Earth’s deep interior—particularly the dynamics of the fluid, iron-rich outer core—is fundamental to explaining the planet’s evolution. Because direct access is impossible, only indirect observations from satellites and ground-based measurements are available. Each provides valuable but incomplete insights:
- Magnetic field variations: Generated primarily in the core, their temporal changes can be used to infer fluid motions at the top of the core on decadal and sub-decadal timescales.
- Gravity field variations: Reflect changes in Earth’s internal and surface mass distribution across a wide range of timescales. While decadal and interannual variations are dominated by surface processes such as the global water cycle and climate-driven ice loss, they also contain the signature of core flows that deform the core–mantle boundary.
- Earth rotation changes: Variations in the length of day (LOD) and polar motion occur on comparable timescales and are strongly influenced by fluid core motions through angular momentum exchange with the mantle.
The GRACEFUL project transcends the limitations of individual observation types by integrating information from magnetic field, gravity field, and Earth rotation data in a synergistic framework. This enables us to investigate, at unprecedented depth, the dynamical processes taking place inside the core and at the core–mantle boundary.
We are developing state-of-the-art algorithms to process observational data and applying advanced numerical models of core flow to infer its dynamics. This interdisciplinary approach not only challenges current models of core dynamics but also represents a step change in our understanding of Earth’s deep interior.
GRACEFUL is a project that tackles fundamental gaps in our understanding of Earth’s deep interior, particularly the fluid core. Its long-term vision is to open new research frontiers and advance knowledge of Earth’s internal dynamics. The project integrates the latest observations of Earth’s gravity field, magnetic field, and rotation, while also developing precise models of core flows.
The key scientific questions this novel approach seeks to address are:
- What flow motions in the liquid core can account for the observed magnetic field, its secular variation, and magnetic jerks, as well as the observed variations in length of day (LOD) on decadal and sub-decadal timescales?
- What types of global motions occur in the core at these timescales?
- How does core flow near the core–mantle boundary (CMB) influence temporal variations in gravity and LOD?
- What roles do the core, the core–mantle boundary, and the lower mantle play in explaining observed variations in gravity, the magnetic field, and LOD?
- Can we predict changes in core flow, and consequently in the magnetic field, LOD variations, and the core’s contribution to gravity?
Why is it important for society?
It is crucial to understand why Earth’s magnetic field is currently changing—at times more rapidly than expected—and to precisely determine Earth’s rotation and orientation in space, which are essential for satellite-based applications. Achieving this requires deeper knowledge of the processes occurring within the core and at the core–mantle boundary.
The study of Earth’s deep interior remains a rapidly evolving scientific discipline and continues to attract significant public interest. Because direct access to the fluid, iron-rich outer core is impossible, only indirect observations provide insights into its dynamics. Alongside seismic data, Earth’s magnetic and gravity fields, as well as variations in its rotation (particularly changes in the length of day, or LOD), offer invaluable information about processes occurring deep within the planet.
The magnetic field, generated primarily in the fluid outer core, and its temporal variations allow scientists to infer fluid motions on decadal and sub-decadal timescales. Similarly, temporal variations in Earth’s gravity field reflect changes in mass distribution both within the planet and at its surface. While much of this variability is dominated by surface processes such as the global water cycle and climate-driven ice loss, the gravity field also carries the signature of core flows that deform the core–mantle boundary and interact with the lower mantle.
Variations in Earth’s rotation, including LOD changes and polar motion, occur on comparable timescales and are thought to be primarily driven by fluid core motions through angular momentum exchange at the core–mantle boundary. Scientific progress is most effective when observations and models are combined, offering the most comprehensive understanding of these fundamental processes.
What are the overall objectives?
Understanding the processes within Earth’s deep interior—particularly the dynamics of the fluid, iron-rich outer core—is fundamental to explaining the planet’s evolution. Because direct access is impossible, only indirect observations from satellites and ground-based measurements are available. Each provides valuable but incomplete insights:
- Magnetic field variations: Generated primarily in the core, their temporal changes can be used to infer fluid motions at the top of the core on decadal and sub-decadal timescales.
- Gravity field variations: Reflect changes in Earth’s internal and surface mass distribution across a wide range of timescales. While decadal and interannual variations are dominated by surface processes such as the global water cycle and climate-driven ice loss, they also contain the signature of core flows that deform the core–mantle boundary.
- Earth rotation changes: Variations in the length of day (LOD) and polar motion occur on comparable timescales and are strongly influenced by fluid core motions through angular momentum exchange with the mantle.
The GRACEFUL project transcends the limitations of individual observation types by integrating information from magnetic field, gravity field, and Earth rotation data in a synergistic framework. This enables us to investigate, at unprecedented depth, the dynamical processes taking place inside the core and at the core–mantle boundary.
We are developing state-of-the-art algorithms to process observational data and applying advanced numerical models of core flow to infer its dynamics. This interdisciplinary approach not only challenges current models of core dynamics but also represents a step change in our understanding of Earth’s deep interior.
We have developed and implemented numerical models of core flows in a rotating, viscous, and electrically conducting fluid immersed in a magnetic field. Our model is a fully coupled, self-consistent 3D core–mantle system (see Figure KORE).
In parallel, we have analysed and modelled magnetic field, gravity field, and Earth rotation data to isolate the contributions specifically related to core dynamics (see Figure Teams).
These combined approaches have significantly improved our understanding of processes in the fluid core, particularly the role of core modes such as magneto-Coriolis waves and torsional Alfvén waves.
Significant work has also been devoted to studying the six-year cycle in Earth’s climate (a discovery of the project) and its connection to the previously known six-year cycles in Earth’s rotation, magnetic field, and fluid core dynamics.
In parallel, we have analysed and modelled magnetic field, gravity field, and Earth rotation data to isolate the contributions specifically related to core dynamics (see Figure Teams).
These combined approaches have significantly improved our understanding of processes in the fluid core, particularly the role of core modes such as magneto-Coriolis waves and torsional Alfvén waves.
Significant work has also been devoted to studying the six-year cycle in Earth’s climate (a discovery of the project) and its connection to the previously known six-year cycles in Earth’s rotation, magnetic field, and fluid core dynamics.
Using geomagnetic field data from geomagnetic observatories, CHAMP and Swarm satellites, gravity field data from GRACE and GRACE-FO, and Length-of-Day (LOD) measurements from the International Earth Rotation Service, we identified a previously unknown 6-year oscillation affecting the entire Earth system. This oscillation is expressed in fluid core motions, the core magnetic field, Earth’s rotation, the gravity field, atmospheric circulation, land hydrology, and other climate-related parameters. It reflects complex angular momentum exchanges between the core, mantle, and atmosphere, offering new insight into Earth’s internal dynamics and their surprising connections to climate processes.
We also investigated mass redistributions at the core–mantle boundary (CMB) and their links to geomagnetic and gravimetric variations. A striking example is a rapid gravity anomaly detected near the Atlantic–African boundary in 2007, which evolved over 2–3 years. As this signal cannot be explained by surface hydrological or oceanic mass changes, it points to a solid Earth origin.
The GRACEFUL project has pioneered fluid core models capable of reproducing core modes, using a newly developed numerical code (KORE, see Figure KORE). This breakthrough has enabled the discovery and characterization of magneto-Coriolis, torsional, and inertial modes—persistent yet damped oscillations with global impacts. These numerical models of the liquid core have revealed key core dynamics and CMB processes that drive coupling between the core and mantle.
In addition, our research highlights the critical role of CMB topography and the background magnetic field in shaping these interactions, deepening our understanding of the fundamental processes governing Earth’s interior.
We also investigated mass redistributions at the core–mantle boundary (CMB) and their links to geomagnetic and gravimetric variations. A striking example is a rapid gravity anomaly detected near the Atlantic–African boundary in 2007, which evolved over 2–3 years. As this signal cannot be explained by surface hydrological or oceanic mass changes, it points to a solid Earth origin.
The GRACEFUL project has pioneered fluid core models capable of reproducing core modes, using a newly developed numerical code (KORE, see Figure KORE). This breakthrough has enabled the discovery and characterization of magneto-Coriolis, torsional, and inertial modes—persistent yet damped oscillations with global impacts. These numerical models of the liquid core have revealed key core dynamics and CMB processes that drive coupling between the core and mantle.
In addition, our research highlights the critical role of CMB topography and the background magnetic field in shaping these interactions, deepening our understanding of the fundamental processes governing Earth’s interior.