1. Arfken G.B., Weber H.J.: Mathematical Methods for Physicists, 6th edn. Elsevier Academic Press, Amsterdam (2005)
2. Backus G.: Poloidal and toroidal fields in geomagnetic field modeling. Rev. Geophys. 24, 75–109 (1986)
3. Backus G., Parker R., Constable C.: Foundations of Geomagnetism. Cambridge University Press, Cambridge (1996)
4. Cain, J.C.: POGO (OGO-2, −4 and −6 spacecraft). In: Gubbins, D., Herrero-Bervera, E. (eds.) Encyclopedia of Geomagnetism and Paleomagnetism, pp. 828–829, Springer, Heidelberg (2007)
5. Christensen A., Rajagopalan S.: The magnetic vector and gradient tensor in mineral and oil exploration. Preview 84, 77 (2000)