MHD in Astro- and Geophysics, Département de Physique, LRA, Ecole Normale Supérieure, France
The origin of the Earth's magnetic field is a long-standing issue, which has captured the attention of many renowned scientists. If William Gilbert, André-Marie Ampère, René Descartes, Edmond Halley, Karl-Friedrich Gauss, Lord Blackett, and many others who contributed to the development of science, have worked on this problem, it is mainly because it was related to a very practical issue of critical importance: navigation at sea. This is not so true anymore, now that satellites provide the precise latitude and longitude without the need for us to rely on the Earth's internal magnetism. Yet the question of the origin of the Earth's magnetic field is so natural that it is still the object of very competitive research. Nobody can ignore that the compass needle points toward the north, and it is a bit irritating that we still cannot offer a complete physical understanding of why it is so. The problem has therefore become an active field of fundamental research in which significant progress has been made in the last few years using combined theoretical, experimental and numerical approaches. By its very nature, the problem is interdisciplinary and lies at the interface of physics, geophysics and applied mathematics. This problem has recently received considerable attention in the press because of the concern of a possible reversal of polarity of the Earth's magnetic field in the near future. Considering that we risk seeing our planet unshielded from the solar wind, understanding the field generation mechanism again appears to be a societal concern and a legitimate goal of environmental research. I will summarise here the basics of our present understanding of the generation of the Earth's magnetic field and then ponder on the scientific clues supporting the possibility of an approaching or imminent polarity reversal.