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Probing the deep Earth's interior by synergistic use of observations of the magnetic and gravity fields, and of the rotation of the Earth.

Periodic Reporting for period 2 - 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.)

Período documentado: 2022-03-01 hasta 2023-08-31

What is the problem/issue being addressed?
GRACEFUL is a project that addresses fundamental shortcomings in the current understanding of the deep interior of our planet, in the fluid core in particular, with a long-term vision to open new frontiers and foster research on Earth’s interior. It combines most up-to-date observations of the Earth gravity field, the magnetic field and the Earth rotation, as well as by developing precise modeling of the core flows.
The key scientific questions we want to address with this novel approach are:
- What are the flow motions in the liquid core that can explain the observed magnetic field, its secular variation and its jerks, and that can explain the observed LOD variations at decadal and sub-decadal timescale?
- What kind of global motion in the core is acting at these timescales?
- What is the impact of core flow near the CMB on temporal gravity and LOD variations?
- What is the role of the core, the core-mantle boundary and the lower mantle in explaining observed gravity, magnetic field and LOD variations?
- Can we predict the changes in the flow of the core as well as in the magnetic field, LOD variations and core contributions on gravity?

Why is it important for society?
One needs to understand why the magnetic field is changing these days (even more rapidly than expected) and one needs to know the rotation and orientation of the Earth very precisely in space, and therefore understand what is going on inside the core and at the core-mantle boundary.
The study of the Earth's deep interior is a discipline still in a period of important evolution and of great interest for the public. Only indirect observations are available on the dynamics of the Earth’s fluid iron-rich outer core. In addition to seismic observation, the magnetic and gravity fields, as well as the rotation of the Earth (in particular the Length Of Day, so-called LOD) provide invaluable information on processes occurring in the Earth's deep interior. The magnetic field, originating mainly within the Earth’s fluid outer core, and its temporal variations can be used to infer motions of the fluid core on decadal and sub-decadal time-scales. Temporal variations in the Earth's gravity field, which reflect changes in the mass distribution within the Earth and at its surface, occur on a broad range of time-scales, in particular in the decadal and interannual band. Such gravity changes are largely dominated by surface contributions related to the global water cycle and climate-driven land ice loss but they also include the gravitational signature of flow inside the core deforming the core-mantle boundary and penetrating/infiltrating the lower mantle. Changes in the Earth’s rotation, i.e. LOD variations, also occur on similar timescales, and are generally assumed to be caused by the motions in the fluid core through exchange of angular momentum at the core-mantle boundary. Scientific progress is most effective when combining observations and models.

What are the overall objectives?
To understand the processes involved in the deep interior of the Earth and explaining its evolution, in particular the dynamics of the Earth’s fluid iron-rich outer core, only indirect satellite and ground observations are available. They each provide invaluable information about the core flow but are incomplete on their own:
- The time dependent magnetic field, originating mainly within the core, can be used to infer the motions of the fluid at the top of the core on decadal and sub-decadal time scales.
- The time dependent gravity field variations that reflect changes in the mass distribution within the Earth and at its surface occur on a broad range of time scales. Decadal and interannual variations include the signature of the flow inside the core, though they are largely dominated by surface contributions related to the global water cycle and climate-driven land ice loss.
- Earth rotation changes (or variations in the length of the day) also occur on these timescales, and are largely related to the core fluid motions through exchange of angular momentum between the core and the mantle.
The GRACEFUL project goes beyond the potential of individual satellite and ground observations and combines the information about the core deduced from the gravity field, from the magnetic field and from the Earth rotation in synergy, in order to examine in unprecedented depth the dynamical processes occurring inside the core and at the core-mantle boundary.
We are developing cutting-edge algorithms to process observational data and we are using up-to-date numerical models of the core flow to infer its dynamics. This interdisciplinary approach challenges our current understanding of the core dynamics that is modelled in full detail and provides a step change in our understanding of the deep Earth’s interior.
We have modelled core flows for a fluid, rotating, viscous core, immersed in a magnetic field. Our model is a fully coupled, self-consistent, core-mantle model (numerical, fully 3-D) (see Figure).
In that frame, we have extended the theory of planetary inertial waves — resulting from the restoring effect of the Coriolis force. We have demonstrated how inertial waves are related to the free planetary rotational modes (e.g. the Free Core Nutation) and how the fluid motions change when viscosity and magnetic field are taken into account. We have shown the important role of viscous and ohmic dissipation, occurring mainly close to the core boundary. We have also incorporated the possibility of a stratification in the core in our codes and we are working on the effects of a topography at the core-mantle boundary. We are presently exploiting the results of our codes in terms of geodynamics.
We are in parallel analyzing the data from the magnetic field, the gravity field and the Earth rotation in order to isolate the core-related contributions. We have noted a period around 6 years that is entering all the geophysical fluids. We are studying these signals to deduce constraints on the core flow from which we will further build realistic core flows.
1. Data processing of the 3 observables (removal of core-unrelated physical signals):
Geomagnetic field Data from CHAMP and the Swarm missions
Gravity field Data from GRACE and GRACE-FO
Length-Of-Day (LOD) Data from the International Earth Rotation Service
--> Residual time series of core magnetic field, of gravity residuals and of core angular momentum

2. Identification of the spatio-temporal correlations (gravity/magnetic fields; LOD/magnetic field):
Geomagnetic and gravity fields - spatio-temporal correlations of Gravity and Magnetic fields --> spatial structures & time series of the two fields
Magnetic field, core flow, and LOD - Temporal correlation between LOD and magnetic anomalies --> Observed LOD versus reconstructed flow using magnetic field at CMB

3. Modelling dynamical processes in the core and at the Core Mantle Boundary:
Computation and representation of the magnetic field at CMB: secular variations and secular acceleration of magnetic field at CMB
Computation of the fluid flow at the top of the core
Modelling of core processes: hydromagnetic modes and interaction with rotational modes
Use of magnetic field, gravity field, and LOD ‘residual’ observations to constrain core processes, including core-mantle interactions
coremodel.jpg