Satellite magnetic field measurements: Applications in studying the deep Earth

Abstract

Following a 20 years hiatus, there are several magnetometry satellites in near-Earth orbit providing a global view of the geomagnetic field and how it changes. The measured magnetic field is an admixture of all field sources, among which one must identify the contributions of interest, namely (1) the field generated in Earth's core, and (2) the fields induced in Earth's mantle by external magnetic variations used in studies of electrical conductivity. Models of the core field can be downward continued to the core surface under the assumption that Earth's mantle is a source free region with zero electrical conductivity. Additional assumptions are invoked to estimate the fluid flow at the core surface. New satellite measurements provide an unprecedented view of changes in the core over the past 20 years; further measurements will clarify the temporal spectrum of the secular variation. Secular changes are coupled to changes in length of day, and recent modeling of torsional oscillations in the core can provide an explanation for the abrupt changes in the field known as geomagnetic jerks. Mantle induction studies require a comprehensive approach to magnetic field modeling. Unwanted internal field contributions are removed to yield time series of external variations and their induced counterparts: improved modeling, combined with the increased data accuracy, and longer term magnetic measurements make conductivity studies feasible. One-dimensional global conductivity responses have been estimated under strong assumptions about the structure of the source field. Ongoing improvements to this work will take account of more complicated source-field structure, three-dimensional Earth structure, and spatio-temporal aliasing due to satellite motion. Modeling of three-dimensional near surface conductivity structure, and the use of time-domain rather than frequency-domain techniques to estimate the 3-D Earth response are needed. Progress could be furthered by future magnetometer missions that involve multiple satellite configurations.