Description du sujet de thèse

Motions in the Earth's core are primarily estimated from the analysis of the time changes in the Earth's magnetic field [1]. Satellite magnetic data have enabled us to investigate the temporal evolution of the fluid motions at the core surface, from 2000 to the present. At the same time, the differential rotation between the solid inner core and the mantle may be estimated from the study of repeated seismic waves traveling through the inner core. It has been suggested recently that this differential rotation has changed from prograde to retrograde around 2008 [2, 3]. Such a behavior necessarily involves motions in the fluid core, as these are magnetically coupled to the solid inner core. The goal of the thesis is to put together these two types of data in a dynamical model of the Earth's core.
On the magnetic side, the recent accumulation of observations makes this work timely. Data from the Swarm mission of ESA, which followed up the German CHAMP mission, are now complemented by data from the Macau MSS satellite. This latter should help us to improve to about one year or less the time resolution of magnetic field (and thus core flow) models.
The analysis of satellite magnetic data has led to the discovery of axially invariant wave-like motions in the Earth's fluid core, with period around 7 years. For these non axisymmetric (2-D) hydromagnetic waves the restoring mechanism mixes the magnetic and Coriolis forces. Although mapped in the equatorial region of the Earth's core [4], they may be present in the entire fluid volume [5]. There also exists axisymmetric waves with similar periods, for which the restoring force is purely magnetic - therefore called torsional Alfvén waves. They had been revealed previously from data collected in ground magnetic observatories [6]. The torsional Alfvén waves propagate within the entire fluid core and are coupled with the axial rotation of the inner core.
The rotation of the inner core relative to the mantle and the waves in the fluid core are also coupled to the rotation of the mantle by transfer of angular momentum. Its time variations are reflected in the changes of the length-of-day (LOD) accurately known from geodesy. Interannual LOD oscillations have been detected, and associated to waves in the core [6]. However, the underlying coupling mechanism responsible for observed LOD variations remains debated. The electromagnetic torque between the Earth's fluid core and the mantle has been much studied. The gravitational coupling between the inner core and the mantle constitutes an alternative, since the latter presents lateral density variations [7]. Interestingly there may be a signature of the inner core oscillations relative to the mantle in the GRACE and GRACE-FO data [8]. In the gravitational coupling scenario, the fluid core (which carries most of the core angular momentum) is tightly coupled to the solid inner core (which brings the mantle coupling) via Lorentz forces. Observed resonances in the LOD series may then sign free oscillations of an inner core magnetically coupled to fluid motions, and/or inner core oscillations forced by hydromagnetic waves traveling throughout the fluid core.
The PhD candidate will investigate simultaneously waves within the fluid core, differential rotation of the solid inner core relative to the mantle, and the angular momentum balance. She/he will employ the 3-D numerical code XSHELLS developed by N. Schaeffer at ISTerre [9] and possibly develop 1-D and 2-D reduced models that take advantage of the invariance of the motions parallel to the rotation axis. Predictions of the LOD changes will be another tool to validate the models constructed by the PhD candidate. Specifically, despite the accurate magnetic coverage from space available over the past 25 years, no improvement has been observed in the angular momentum balance between the core and the mantle. The work of the PhD candidate may help to solve this puzzle.
References:
[1] Finlay et al. Gyres, jets and waves in the Earth’s core. Nature Reviews Earth & Environment, 4(6), 377-392 (2023).
[2] Yang and Song. Multidecadal variation of the Earth’s inner core rotation. Nature Geoscience, 16, 182-187 (2023)
[3] Wang et al. Inner core backtracking by seismic waveform change reversals. Nature, 631, 340-343 (2024).
[4] Gillet et al. Satellite magnetic data reveal interannual waves in Earth’s core. Proc. Nat. Acad. Sci. 119(13), e2115258119 (2022).
[5] Aubert et al. A taxonomy of simulated geomagnetic jerks. Geophys. J. Int., 231(1), 650-672 (2022).
[6] Gillet et al. Fast torsional waves and strong magnetic field within the Earth’s core. Nature, 465(7294), 74-77 (2010).
[7] Lau et al. Tidal tomography constrains Earth’s deep mantle buoyancy. Nature, 551, 321-326 (2017).
[8] Lecomte et al. Gravitational constraints on the Earth's inner core differential rotation. Geophys. Res. Lett., 50(23), e2023GL104790 (2023).
[9] Schaeffer et al. Turbulent geodynamo simulations. A leap towards Earth’s core. Geophys. J. Int., 211, 1-29 (2017).
The candidate should have a Master level with some background in (geo)physics, applied maths and/or scientific computing.
The following skills are expected: applied maths or (geo)physics, scientific computing, teamwork, autonomy.

Contexte de travail

Centre national de la recherche scientifique is one of the world's leading research institutions. To meet the major challenges of today and tomorrow, its scientists explore life, matter, the Universe and the workings of human society. Internationally recognized for the excellence of its scientific work, the CNRS is a benchmark in the world of research and development, as well as for the general public.
The thesis will be at ISTerre laboratory, a Joint Research Unit of Université Grenoble Alpes, CNRS, USMB, IRD and Université Gustave Eiffel, located 1381 rue de la Piscine 38400 Saint-Martin d'Hères and on the Bourget du Lac Science Campus.
It is part of the Observatoire des Sciences de l'Univers de Grenoble (OSUG) and the PAGE research cluster of the Université Grenoble Alpes (UGA). It employs around 300 people and has an average annual budget of €7 million.
It is organized around 10 research and service teams, with the scientific objective of studying the physics and chemistry of planet Earth, with a particular focus on coupling observations of natural objects with experimentation and modeling of the associated complex processes.
ISTerre also carries out solid Earth observation missions, and hosts and maintains national fleets of geophysical instruments, as well as a data center.
The thesis will be carried out in the geodynamo team, with 14 people (researchers, teacher-researchers, post-docs, PhD students and engineers).
The doctoral school will be STEP (Sciences de la Terre, de l'environnement et des planètes) at UGA

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