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Satellite missions Oersted, CHAMP and Swarm now offer almost two decades of continuous monitoring of the geomagnetic field from space. This enables us to investigate interannual variations of core motions with an unprecedented accuracy. For this purpose, we rely on magnetic field models constructed using prior information compatible with the occurrence of geomagnetic jerks, which allows us to benefit from estimates of field changes uncertainties. These are required to reduce as much as possible biases in the core flow inverse problem. We focus on the analysis of time-dependent flow at the top of the core between 1998 and 2017. We find a minimum (resp. maximum) of the radial (resp. azimuthal) flow on interannual timescales at the location of the tangent cylinder (the virtual cylinder aligned with the rotation axis that contains the inner core). This observation is consistent with the quasi-geostrophic assumption, which assumes that the Coriolis acceleration dominates the force balance. It suggests that interannual core motions are only weakly sensitive to a proposed stratified layer at the top of the core inferred from seismology. We discuss implications concerning the density structure and mixing below the core surface.
A Quasi-Geostrophic Magnetoconvection Model of the Decadal Zonal Flow Dynamics in Earth’s Core
Colin More, Mathieu Dumberry
University of Alberta, Canada
Geomagnetic data allows the reconstruction of fluid flow in Earth’s outer core. These reconstructions predict mean zonal accelerations on several timescales. The coupling of angular momentum between the core and mantle confirms the presence of such accelerations through the strong agreement between predicted and observed length-of-day changes.
Scaling arguments for Earth suggest that mean zonal accelerations on both interannual and decadal timescales should be forced by magnetic, rather than inertial, forces. However, numerical models of the geodynamo have not been able to enter the parameter regime necessary to test this theory.
We have constructed a quasi-geostrophic model of magnetoconvection, with thermally-driven flows perturbing a steady, imposed background magnetic field. The Taylor-Proudman theorem is used to justify this two-dimensional approach, in which velocities vary little parallel to the rotation axis. The reduction in dimensionality allows access to different areas of parameter space compared to three-dimensional models.
At Alfven numbers similar those in Earth’s core, an analysis of the force balance responsible for mean zonal accelerations shows that magnetic forces dominate. We reproduce in our model both the free Alfven waves and forced accelerations, the latter occuring at a larger timescale. Our model shows that decadal zonal accelerations can be produced by the underlying convective dynamics in Earth’s core.
Gravity Signal of the Crust-Mantle Boundary and Density Structures in this Region
Bart Root1, Wouter van der Wal1, Jörg Ebbing2
1TUDelft, Netherlands, The; 2University of Kiel, Germany
The mass sources that are responsible for the long-wavelength gravity field of the Earth are not yet fully understood. The biggest candidate to describe these anomalies is mantle convection, yet models that can explain the mantle convection and the gravity field are not available. Another candidate is the core-mantle boundary with a density contrast of 4500 kg/m3 between the ferrous core and the silicate mantle. Due to the high density contrast variations in the core-mantle boundary can have a great impact on the gravity field. Also, density anomalies above the core-mantle boundary are large enough to be seen in the gravity field. The geometry of these anomalies amount to hundreds of km’s with density contrast up to 100 kg/m3.
Sensitivity tests using similar dimensions and density contrasts show that gravity anomalies of 10-100 mGal can be computed, which is in the same as the long-wavelength part of the gravity field. We argue that the gravity signal from structures at the deepest part of the lower mantle should be part of any global analysis. During the 3D Earth project, we will use seismic models of the core-mantle region to simulate more realistic geometries and density anomalies to study their effect on the global gravity field.
This presentation is part of the ESA Support To Science Element - 3D Earth – which is presented by Jörg Ebbing