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https://en.wikipedia.org/wiki/Geomagnetic_secular_variation
Four centuries of geomagnetic secular variation from historical records
Jackson, A., Jonkers, A. R. T. and Walker, M. R.
Phil. Trans. Roy. Soc. A (2000) Vol. 358, pp957-990
Some of the data and modelling procedures were also described in the
earlier papers:
Time-dependent mapping of the magnetic field at the core-mantle boundary
Bloxham, J. and Jackson, A.
Journal of Geophysical Research Vol. 97, B13, pp.19537-19563
Geomagnetic Secular Variation
Bloxham, J., Gubbins, D. and Jackson, A.
Phil. Trans. Roy. Soc. A (1989) Vol. 329 pp415-502
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