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Session Chair: Stefan Maus Session Chair: Erwan Thébault
11:30am - 11:45am
Temporal Variability In Core Surface Flows Inferred From Satellite ‘Virtual Observatory’ Secular Variation
Kathy Whaler1, Magnus Hammer2, Chris Finlay2, Nils Olsen2
1University of Edinburgh, United Kingdom; 2DTU Space, Denmark
We derive the temporal variability of the flow at the core surface explaining the geomagnetic secular variation (SV) over the period of the CHAMP and Swarm missions using satellite-derived ‘virtual observatory’ (VO) data. Time series of monthly values of the vector field components, with their uncertainties, have been derived from data from sectors of satellite paths crossing regularly arranged volumes. The data are reduced to a single point value at the centre of the volume using a local harmonic expansion of the magnetic scalar potential. First differences of field values provide SV estimates. We invert the data assuming the main field is known, specified by the CHAOS model. We allow the maximum amount of temporal variability by solving for a series of flows steady over 3 months, and also with the minimum amount of variability by restricting variability from epoch to epoch. Flows are regularised spatially. We compare the flows with those inferred from observatory data, and examine them for features associated with geomagnetic jerks. Future plans include deriving and taking account of a more sophisticated data covariance matrix, and using spherical Slepian functions to localise the flow e.g. into regions inside and outside the tangent cylinder, and outside the ambiguous area for tangentially geostrophic flows.
11:45am - 12:00pm
Keynote: Earth Dynamics: The big picture
Trond Helge Torsvik
University of Oslo, Norway
The Earth is a stable degree-2 planet dominated by two antipodal large low shear-wave velocity provinces in the lower mantle beneath Africa (TUZO) and the Pacific (JASON). TUZO and JASON are probably both denser and hotter in the lowermost parts and the Earth’s residual geoid is largely a result of buoyant upwellings above them. Subduction zones show a predominantly large-scale pattern, especially the “ring of fire” circling the entire Pacific and thus slabs sinking all the way to the lowermost mantle also relate to long-wavelength lower mantle structure dominated by degree-2.
Conceptually, the link between plate tectonics and the deep Earth's mantle can be viewed as a simple mass-balance: subducted lithosphere slabs restore mass to the mantle and trigger the return flow toward the surface ─ including mantle plumes ─ rising from the margins of TUZO and JASON. The surface manifestations of plumes are hotspot lavas, kimberlites and large igneous provinces (LIPs), which punctuate plate tectonics by creating new plate boundaries as well as driving rapid climate changes. This realistic model of surface-mantle interaction has emerged after the recognition of a remarkable correlation between reconstructed LIPs and the position of deep mantle structures, showing that TUZO and JASON have been stable for at least 300 Myrs, and probably much longer.
True Polar Wander (TPW) is a process whereby the entire solid Earth (mantle and lithosphere) rotates with respect to the spin-axis in response to changes of the planetary moment of inertia arising from redistribution of density heterogeneities within the mantle. The pattern of TPW can be interpreted as slow oscillatory swings around an axis close to the centres of TUZO and JASON, and adding dense subducted material to the upper mantle at intermediate to high latitudes is a primary cause for TPW. Whilst TUZO and JASON stabilize the moment of inertia through time, their presence may lead to significant non-dipole field contributions. Many hotspots (e.g. Hawaii and Reunion) are rooted in the margin of TUZO and JASON and imaged from the core-mantle boundary to the surface; surface volcanics are associated with pronounced inclination (latitude) anomalies attributed to strong lateral variations in core–mantle boundary heat-flow.
12:00pm - 12:15pm
Simultaneous Inversion of Magnetospheric and Tidal Satellite Signals Constrain Electrical Conductivity from Crust to Mid-Mantle.
Alexander Grayver1, Alexey Kuvshinov1, Terence Sabaka2
1ETH Zurich, Instutute of Geophysics, Switzerland; 2Planetary Geodynamics Laboratory, NASA/GSFC, USA
Magnetospheric time series along with tidal magnetic signals have been shown to carry information on electrical conductivity of the Earth’s mantle and thus its composition and physical state. However, both signals sense different parts of the mantle due to limited frequency range. Specifically, tidal magnetic signals are sensitive to conductivity distribution in the upper mantle, whereas magnetospheric signals lack resolution in the upper mantle, but can constrain regions across transition zone and lower mantle. This prompted us to invert these data simultaneously and derive a single, consistent electrical conductivity model . This can potentially enhance existing mantle conductivity L2-Cat product.
12:15pm - 12:30pm
Global Shallow-Earth Structure Models Hand in Hand with Satellite Gravimetry
Josef Sebera1, Roger Haagmans2, Rune Floberghagen1, Diego Fernández Prieto1, Joerg Ebbing3
1ESA, ESRIN, Italy; 2ESA, ESTEC, The Netherlands; 3Christian-Albrechts-Universität zu Kiel, Germany
Apart from the non-trivial conversion of seismic velocities to the volumetric mass density, the global Earth's density distribution from seismic tomography is still being improved in the accuracy, lateral as well as the vertical resolution. In our contribution, we look at the gravitational signal generated by the model Litho1.0 and the signal obtained from the satellite gravimetry. Litho1.0 provides about 10% of the total Earth's gravitational acceleration and sheds light on the spectral properties between the main “players” – the crust, the lithosphere and the remaining upper mantle. It will be shown that GOCE gravitational gradients can provide a strong feedback to density distribution models. The gradients better localize a possible problem in the density than the gravity vector and are less sensitive to distant sources in the mantle. Thus, the next logic step is to use these gravimetric constraints in the seismic inversion or in density structure modelling. Essentially the same thinking applies to the magnetic susceptibility distribution in the magnetic lithosphere and the satellite magnetometry represented now by the ESA’s mission Swarm.