A research program will be carried out to reconstruct the three dimensional variability of the electrical conductivity of the mantle. First, a set of recently published globally-distributed 'response functions' will be transformed into a global-scale mantle model with greater detail than previously possible. Following this, geomagnetic observatory time series data will be analyzed with the goal of devising new methods to extract an even higher resolution view of mantle conductivity, particularly at frequencies where the Earth's external magnetic field is dominated by the diurnal (and harmonic) Sq (solar-quiet) magnetospheric and ionospheric electrical current systems. This promises to provide a framework to improve the spatial resolving power for future upper mantle investigations, and to improve our view of the geodynamics of the region most closely associated with plate tectonics.
Scientific and technical merit. The electrical conductivity of the mantle depends on temperature, composition (e.g. Fe content), state (e.g. partial melt fraction), and the presence of volatiles. Mantle conductivity primarily increases radially with depth, but a growing body of evidence indicates there is significant lateral heterogeneity in the upper and mid-mantle. A research program is proposed that will lead to significant improvements in imaging these 3-d variations, and in interpreting and understanding the geodynamic significance of global scale lateral conductivity variations. External electrical current systems in the Earth's ionosphere and magnetosphere ("source currents"), generated by variations in solar wind activity, in turn induce electrical current to flow in the Earth's interior. This is a source of signal that may be extracted in the form of electromagnetic 'response functions', which in turn contain information about the distribution of electrical conduction within the Earth's interior. Empirical orthogonal function analysis of geomagnetic observatory data simplifies the task of interpreting the configuration of the source currents, and improves the quality of the resulting response functions, which are inverted to obtain a higher resolution model of the mantle's electrical properties.
Broader impacts. New constraints on the 3-d structure and physical state of the Earth's deep interior are of great importance to the broad community of researchers working to understand the dynamics and evolution of the Earth. Co-registered electrical and seismic models can be used to separate temperature and compositional effects in 3-d tomographic images, and to develop equations of state relevant to deep Earth materials. A global 3-d conductivity reference model can be used to provide boundary conditions, as well as large scale context, for modeling and inversion of data from regional electromagnetic studies, such as those supported by the NSF Continental Dynamics Program, the Margins Program, and Earthscope (www.earthscope.org/links_pubs/index.html). A global 3-d reference model can be used by academic, government and industry groups to address the problem of contamination and biased interpretation of data in situations where the induced internal magnetic variations are a source of noise, e.g., in studies of crustal magnetization with satellite data, or long-baseline aeromagnetic data. The improved characterization of magnetospheric and ionospheric source fields, as viewed from an Earth fixed reference frame will be very useful to researchers working with satellite data, e.g., to provide improved external field corrections for modeling of the main or crustal fields, and for studies of ionospheric and magnetospheric physics. Improved characterization of source fields is essential to progress in satellite-based induction studies.
