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Relating core-mantle interactions and the geomagnetic field

Final Activity Report Summary - CORE-MANTLE-BR (Relating core-mantle interactions and the geomagnetic field)

We investigated the impact of the heterogeneous mantle on the working of Earth’s dynamo over long timescales. We combined numerical dynamos, seismic tomography models, core flow inversions from geomagnetic data and thermal wind theory to assess the possibility of identifying mantle control on the geodynamo and to detect regions where this could be observed on the core-mantle boundary.

We firstly used a seismic tomographic pattern to impose heterogeneous outer boundary heat flux on numerical dynamos. We conducted a parametric study and found a significant likelihood of finding mantle control in core flow models time-averaged over the historical geomagnetic record (Aubert et al., 2007).

We then looked for such mantle control in time-average core flow inferred from geomagnetic secular variation inversions. We designed a simple method to predict the steady core flow which was driven by mantle heterogeneity based on thermal wind theory and some inferences about the radial shear of the flow from numerical dynamos. We implemented the thermal wind method on the mantle tomography model and compared the non-zonal thermal wind flow with the non-zonal time-average flow inferred from geomagnetism. The comparison revealed interesting agreements below the southern Atlantic and Asia, as well as some disagreements below the western hemisphere (Amit et al., 2008).

We also used numerical dynamos with tomographic outer heat flux pattern to show that the inner-core growth was coupled to the lower mantle heterogeneity via outer core convection. We showed that high heat flux structure on the core-mantle boundary below Asia produced a vortex at the same region at the top of the core, in agreement with a similar flow structure in time-average core flow models. Due to Coriolis forces, this vortex extended into the core parallel to the rotation axis, creating a large buoyancy flux at the inner-core boundary below equatorial Asia, in good agreement with recent observations of seismic heterogeneity at the top of the inner-core. Our model suggested that mantle control accounted for non-axisymmetric observations of the long-term magnetic field, flow at the top of the core, and inner-core heterogeneity (Aubert et al., 2008).

Mantle control could also be responsible for magnetic phenomena in other planets. Mantle convection models of Mars suggested single-hemisphere heat flux heterogeneity at the Martian core-mantle boundary. Numerical dynamos with such imposed boundary condition showed a strong north-south hemispherical asymmetry in the intensity of the surface field, in agreement with the observations of the Martian lithospheric magnetic field. Mantle control on the past Martian dynamo might therefore solve one of the planet’s greatest enigmas (Langlais and Amit, 2008).

The second part of the proposal involved studying the kinematic mechanisms that were responsible for changes in the geomagnetic tilt. We developed a new theory to identify and quantify dynamo mechanisms of dipole change. We found that most of the dipole tilt change, including the recent rapid decrease event, could be explained by advection. The remaining unexplained signal potentially indicated the effects of radial diffusion on tilt variations (Amit and Olson, 2008).

Two additional projects were also performed. We firstly used numerical dynamos to model magnetic diffusion in order to improve core flow inversions from geomagnetic secular variation without assuming frozen-flux. This led to weaker Atlantic-Pacific hemispherical dichotomy in core flow activity and stronger cyclones below Asia and North America (Amit and Christensen, 2008). Those cyclones were consistent with the upper inner-core seismic structure and the intense steady magnetic field below North America and could be understood as mantle control effects along the lines of our previous Aubert et al. study which was elaborated in 2008. We secondly sought precursors for magnetic reversals using numerical dynamos. Detailed comparisons between reversing and non-reversing dipole collapse events suggested that the present geomagnetic field did not meet the conditions for an imminent reversal, though these conditions could change in the future (Olson et al., in press by the time of the project completion).