1. Jones, C. A. in Treatise on Geophysics 1st edn, Vol. 8 (ed. Schubert, G.) Ch. 5 (Elsevier, 2007).
2. Jackson, A. & Finlay, C. in Treatise on Geophysics 2nd edn, Vol. 5 (ed. Schubert, G.) Ch. 5 (Elsevier, 2015).
3. Bloxham, J. & Gubbins, D. The secular variation of the Earth’s magnetic field. Nature 317, 777–781 (1985).
4. Jackson, A., Jonkers, A. R. T. & Walker, M. R. Four centuries of geomagnetic secular variation from historical records. Philos. Trans. R. Soc. Lond. A 358, 957–990 (2000).
5. Fisk, H. W. Isopors and isoporic movements. Intern. Geodetic Geophys. Union Sect. Terrest. Magnet. Elec. Bull. Stockholm 8, 280–292 (1931).