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Small-scale structure of the geodynamo inferred from Oersted and Magsat satellite data


The ‘geodynamo’ in the Earth's liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the Earth's surface1,2. Here we use data from the currently operating Danish Oersted3 satellite, and from the US Magsat2 satellite that operated in 1979/80, to identify and interpret variations in the magnetic field over the past 20 years, down to length scales previously inaccessible. Projected down to the surface of the Earth's core, we found these variations to be small below the Pacific Ocean, and large at polar latitudes and in a region centred below southern Africa. The flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs (and retrograde vortices with lows) in the historical (400-year average) magnetic field4,5. This pattern is analogous to those seen in a large class of numerical dynamo simulations6, except for its longitudinal asymmetry. If this asymmetric state was reached often in the past, it might account for several persistent patterns observed in the palaeomagnetic field7,8,9,10. We postulate that it might also be a state in which the geodynamo operates before reversing.

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Figure 1: Spectra of the Oersted and Magsat models11, of their error, of their difference, and of how well flows predict this difference.
Figure 2: Polar (north and south) and Hammer views of the small-scale structure of the geodynamo at the core surface.
Figure 3: Axisymmetric component of the flow at the core surface.


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We thank those involved in the Oersted project for their contribution to this work. The Oersted project is funded by the Danish Ministry of Transport, the Ministry of Research and Information Technology, and the Ministry of Trade and Industry of Denmark. Additional support for Oersted came from NASA, the Centre National d’Etudes Spatiales (CNES) and DARA. We also thank R. Holme for comments and suggestions.

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Correspondence to Gauthier Hulot.

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Hulot, G., Eymin, C., Langlais, B. et al. Small-scale structure of the geodynamo inferred from Oersted and Magsat satellite data. Nature 416, 620–623 (2002).

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