244 Inner core rotation, waves in Earth’s core and length-of-day variations

Job title:

244 Inner core rotation, waves in Earth’s core and length-of-day variations

Company:

Centre National d’Etudes Spatiales

Job description

25-244 Inner core rotation, waves in Earth’s core and length-of-day variationsPostuler25-244 Inner core rotation, waves in Earth’s core and length-of-day variations

• Doctorat, 36 mois
• Temps plein
• Indifférent
• Maitrise, IEP, IUP, Bac+4
• Internal geophysics, geodynamics and geodesy

PostulerMissionMotions 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] and be sensitive to the inner core. 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 ground-based magnetic data [6]. 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 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 also 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. She/he will interact with members of the “geodynamo” team of ISTerre.[1] Finlay et al. Nature Reviews Earth & Environment, 4(6), 377-392 (2023)[2] Yang and Song. Nature Geoscience, 16, 182-187 (2023)[3] Wang et al. Nature, 631, 340-343 (2024)[4] Gillet et al. Proc. Nat. Acad. Sci. 119(13), e2115258119 (2022)[5] Aubert et al. Geophys. J. Int., 231(1), 650-672 (2022)[6] Gillet et al. Nature, 465(7294), 74-77 (2010)[7] Lau et al. Nature, 551, 321-326 (2017)[8] Lecomte et al. Geophys. Res. Lett. 50(23), e2023GL104790 (2023)[9] Schaeffer et al. Geophys. J. Int. 211, 1-29 (2017)For more Information about the topics and the co-financial partner (found by the lab !);contact Directeur de thèse –Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online before March 14th, 2025 Midnight Paris time !ProfilThe candidate should have a master in one of the following topics: (geo)physics, applied maths, or scientific computing.

Expected salary

Location

Gières, Isère

Job date

Wed, 05 Feb 2025 05:20:46 GMT

To help us track our recruitment effort, please indicate in your email/cover letter where (vacanciesin.eu) you saw this job posting.