PhD in geosciences (M/F) – High-latitude core dynamics from the geomagnetic secular variation and numerical dynamos with heterogeneous outer boundary heat flux

The present-day geomagnetic secular variation (SV) on the core-mantle boundary (CMB) is strongest at high latitudes of the northern hemisphere.  Regional analysis indicates a westward jet at the top of the core there (Livermore et al., 2017). Although rapid rotation effects are expected to yield equatorial symmetry in the core flow, the SV at high latitudes of the southern hemisphere is much weaker, possibly due to field-aligned flow that does not induce advective SV there. Alternatively, the equatorial symmetry in the core flow is broken, e.g. by boundary heterogeneity (Lézin et al., 2023).

This thesis involves two complimentary approaches to shed light on core dynamics at high latitudes. The first approach concerns a regional analysis of the geomagnetic SV at high latitudes. In order to model the westward jet, the student will apply different mathematical descriptions of the core flow at high latitudes of the northern hemisphere, including previously proposed regional core flow models (Olson and Aurnou, 1999; Livermore et al., 2017) as well as original models that will be developed in the thesis. These mathematical descriptions will be inserted into the radial induction equation on the CMB at high latitudes of the southern hemisphere. Comparison with the observed SV will reveal whether the observed weak SV at southern high latitudes is due to field-aligned flow or evidence for a break of equatorial symmetry in the core flow. In a second approach, the student will analyze numerical dynamo simulations with imposed heterogeneous outer boundary heat flux inferred from lower mantle seismic tomography models (Terra-Nova et al., 2019). The student will compare northern vs. southern high-latitude azimuthal flows and establish the relation between the hemispherical dichotomy of high-latitude core dynamics, the control parameters of the dynamo simulations and the imposed CMB heat flux spatial distribution.

Objectives:

• Infer core dynamics at high latitudes from a regional analysis of the geomagnetic SV at high latitudes. 
• Examine break of equatorial symmetry in the core flow. 
• Analyze numerical dynamo simulations with imposed heterogeneous outer boundary heat flux inferred from lower mantle seismic tomography models in order to compare northern vs. southern high-latitude azimuthal flows. 
• Establish the relation between the hemispherical dichotomy of high-latitude core dynamics, the control parameters of the dynamo simulations and the imposed CMB heat flux spatial distribution.

Good level of mathematics and programming, good background in geophysics.