The cause of Earth's magnetic field (the surface magnetic field) is not known for certain, but is possibly explained by dynamo theory. The magnetic field extends several tens of thousands of kilometres into space.

The field is approximately a magnetic dipole, with one pole near the geographic north pole and the other near the geographic south pole. An imaginary line joining the magnetic poles would be inclined by approximately 11.3° from the planet's axis of rotation. The location of the magnetic poles is not static but wanders as much as several miles a year. The two poles wander independently of each other and are not at exact opposite positions on the globe. Currently the south magnetic pole is further from the geographic south pole than the north magnetic pole is from the north geographic pole.

Magnetic pole positions

The strength of the field at the Earth's surface at this time ranges from less than 30 microtesla (0.3 gauss) in an area including most of South America and South Africa to over 60 microtesla (0.6 gauss) around the magnetic poles in northern Canada and south of Australia, and in part of Siberia.

The field is similar to that of a bar magnet, but this similarity is superficial. The magnetic field of a bar magnet, or any other type of permanent magnet, is created by the coordinated motions of electrons (negatively charged particles) within iron atoms. The Earth's core, however, is hotter than 1043 K, the temperature at which the orientations of electron orbits within iron become randomized. Such randomization tends to cause the substance to lose its magnetic field. Therefore the Earth's magnetic field is caused not by magnetised iron deposits, but mostly by electric currents (known as telluric currents).

Another feature that distinguishes the Earth magnetically from a bar magnet is its magnetosphere. At large distances from the planet, this dominates the surface magnetic field. In addition, the magnetized elements within the planetary core are undergoing rotation and are not static.

Magnetic field reversals

The Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 250,000 years. It is believed that this last occurred some 780,000 years ago, referred to as the Brunhes-Matuyama reversal. Past field reversals are recorded in the "frozen" magnetic domains of solidified lava that has welled up along spreading ocean floor ridges; since the sea floor spreads at a fairly constant rate, this results in broad "stripes" of sea floor from which the past magnetic field direction can be read. At least once in Earth's history, the magnetic field held a constant direction for as long as 30 million years (see: Cretaceous long normal).

The mechanism responsible for geomagnetic reversals is not well understood. Some scientists have produced models for the core of the Earth wherein the magnetic field is only quasi-stable and the poles can spontaneously migrate from one orientation to the other over the course of a few hundred to a few thousand years. Other scientists propose that the geodynamo first turns itself off, either spontaneously or through some external action like a comet impact, and then restarts itself with the North pole pointing either up or down. When the North reappears in the opposite direction, we would interpret this as a reversal, whereas turning off and returning in the same direction is called a geomagnetic excursion.

At present, the overall geomagnetic field is becoming weaker at a rate which would, if it continues, cause the field to disappear, albeit temporarily, by about 4000 AD.¹ Other sources have put the date of field collapse as early as 3000 AD. The deterioration began roughly 150 years ago and has accelerated in the past several years. So far the strength of the earth's field has decreased by 10 to 15 percent. However, one should note that no one knows if field decay will continue in the future. Also, since a magnetic field reversal has never been observed by humans and the mechanism of field generation is not well understood, it is difficult to say what the characteristics of the magnetic field might be leading up to such a reversal.

See also

• Dynamo theory
• Geophysics
• Schumann resonance
• Ionosphere
• Magnetohydrodynamics
• Edward Sabine
• Sherwood machine
• South Atlantic Magnetic Anomaly.

References

• Discovering the Essential Universe by Neil F. Comins (2001)
• Introduction to Geomagnetically Trapped Radiation by Martin Walt (1994)

External links

• "Will Compasses Point South?", New York Times article published July 13, 2004 (http://www.nytimes.com/2004/07/13/science/13magn.html?ex=1247457600&en=e8f37e14d213ba16&ei=5090&partner=rssuserland)
• "Why Does Earth's Magnetic Field Flip?", National Geographic (http://news.nationalgeographic.com/news/2004/09/0927_040927_field_flip.html)
• PBS NOVA: Magnetic Storm (about pole reversals) (http://www.pbs.org/wgbh/nova/magnetic/)