5.07 - Geomagnetism

Abstract

Interaction of temporal fluctuations in Earth's external magnetic field with the electrically conductive rocks of the planet generates internal secondary electric and magnetic fields. Measurements of the primary and secondary fields may then be used to probe the conductivity structure over depths from a few meters to the lower mantle. Conductivity in turn may be used to infer physical and chemical properties, such as temperature, melt content, water and volatile content, mineral type, and anisotropic fabric. There are three basic steps necessary to obtain useful information from geomagnetic induction studies: estimation of electromagnetic impedances from observations of the electric and magnetic fields; modeling of these impedances in both forward (conductivity to impedance) and inverse (impedance to conductivity) directions; and finally the use of laboratory studies to relate conductivity to the relevant physical and chemical properties. Each of these steps represent significant scientific challenges, and all of them are the subject of current research and recent progress. Clearly, the results of each of these components depend on the quality of the preceding step, and conclusions about the state of Earth's interior depend on the reliability of all three parts of the process.