Learning by Research

Thesis Title: Heterogeneous heat flux from mantle convection simulations: impact on the geodynamo and magnetic reversals

The Earth's magnetic field has been generated for billions of years in the Earth's core, a shell composed mainly of liquid iron located between 2890 km and 5200 km deep. Within the core, the flow of liquid iron sustain this magnetic field through a dynamo effect. Records of the past magnetic field in rocks show that the magnetic field has varied in intensity and geometry over geological time and that the magnetic poles reverse at a frequency that has varied greatly. During this thesis, we focused on one of the mechanisms that could explain these variations in the magnetic field, based on changes in thermal conditions between the core and the Earth's mantle, located above the core. More precisely, we focused on heterogeneities in heat transfer between the core and the mantle. These heterogeneities affect the flows in the core, which in turn can modify the magnetic field.
To determine realistic thermal conditions between the core and the mantle, we used recent numerical models of mantle convection. The thermal conditions can be extracted in the form of temperature maps at the core-mantle boundary. An important contribution of this thesis was to correct in these models the true polar wander, corresponding to global rotations of the Earth's mantle relative to the Earth's rotation axis. This correction is important to perform, as it can redistribute the "hot" or "cold" zones above the core between the equator and the poles, modifying the expected effect on the flows in the core.

We also used numerical models reproducing the flows in the core as well as the magnetic field to study the effect of heterogeneous thermal conditions at the top of the core on magnetic reversals. These simulations showed that reversals are favored by a colder mantle at the equator than at the poles, or when one hemisphere is warmer than the other.
These results help better understand magnetic reversals and the processes that may affect their likelihood. More generally, this thesis contributes to a more complete understanding of variations in the Earth's magnetic field, which plays a major role on Earth by deflecting solar wind particles, but also in modern navigation technologies.
Key words: Geomagnetism, Geodynamics, Geophysics, Deep Earth dynamics, Magnetic reversals

Doctoral School: ED STEP - Earth, Environmental and Planetary Sciences
Research laboratory: Institut des Sciences de la Terre (ISTerre - UGA/CNRS/USMB/IRD/UGE)
Thesis supervision: Henri-Claude NATAF and Stéphane LABROSSE

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