Steve | Magnetic D component at the Earth's surface (AE Map) 1590-1990 @SteveFlatEarth | Uploaded September 2017 | Updated October 2024, 4 hours ago.
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