North Pole, South Pole: the quest to understand the mystery of Earth's magnetism

The story of the quest to understand Earth’s magnetic field is one of the longest and richest in the history of science. It weaves together Greek philosophy, Chinese mysticism, the development of the compass and navigation, the physics of electromagnetism and the jig-saw like piecing together of the internal structure of the planet beneath our feet. The story begins with Magnes, an old shepherd, trudging up the mountainside after a violent thunder storm, astonished at how the iron studs in his boots stick to the rocks. It was Alexander von Humboldt who, three millennia on, pointed to lightning as the source of such magnetization. The first compass was made 2000 years ago in China - to divine the ways of feng shui - a guide to planting crops, planning streets, orienting buildings and more. It reached Europe as a navigational tool in the 12th century - no-one is quite sure how, but en route it changed from south-pointing to the north-pointing compasses of today. The earliest truly scientific experiments and writings concerned magnets and geomagnetism: Petrus Peregrinus’ Epistola of 1269, and William Gilbert’s De Magnete of1600, in which he declared Magnus magnes globus terrestris ipse est - the Earth itself is a great magnet. By then it was recognized that the compass didn’t point exactly north, and the discrepancy varied from place to place and changed over time - something of a problem for Gilbert’s idea of a geocentric axial dipole. However declination and secular variation were problems well known to Edmund Halley, who, in 1700, charted the angle of declination over the Atlantic Ocean, and in the process introduced the Halleyan line - the contour. Many of the world’s greatest scientists have turned their minds to the problem of magnetism and geomagnetism in particular - Coulomb, Gauss, Faraday, Maxwell - yet in 1905, Einstein described geomagnetism as “one of the great unsolved problems of physics”. In the mid-late nineteenth century new areas of geophysics emerged: geodesy and seismology, and from these came the discoveries of the liquid iron outer core and the inner core. Later, with the recognition and validation of the palaeomagnetic method came the amazing discovery that as well as the gradual secular variation, the polarity of the field has reversed, not once but many times over history. The idea of a simply connected, self-sustaining hydromagnetic dynamo was first proposed by Larmor in 1919, but through most of the 20th century attempts to demonstrate its feasibility were hampered by lack of computational power. When, in the 1990s, supercomputers burst onto the scene it became possible to programme, albeit with some compromises, the many calculations needed to simulate Earth’s core - its motion, electric currents and magnetic fields over a significant part of the life of the Earth. The result was a model that reproduced in character the predominant geocentric axial dipole, the secular variation, and, finally the ability to reverse polarity - the Earth itself is a great hydromagnetic dynamo. The story is told in a new book, published by Awa Press, New Zealand this year, and which is scheduled for publication in the United States early in 2011. Written for a wide audience, it is readily accessible to non-experts and students of any area of earth science.