Earth's magnetic field

Earth's magnetic field (the surface magnetic 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 cause of the field is probably explained by dynamo theory. The magnetic field extends several tens of thousands of kilometres into space as the magnetosphere.

Magnetic poles

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 directly 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

Field characteristics

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 Curie point 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.

Electric currents induced in the ionosphere also generate magnetic fields. Such a field is always generated near where the atmosphere is closest to the Sun, causing daily alterations which can deflect surface magnetic fields by as much as one degree.

Magnetic field variations

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

Magnetic field reversals

Recent geomagnetic 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).

Magnetic reversals are used by paleontologists as a basis for dating fossils. This is generally used in combination with radiometric dating. Radiometric dating generally provides an absolute date with a wide margin of error, while magnetic reversals can provide a much smaller margin of error when comparing two locations, but no absolute date.

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 North or South. 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 at least 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. The present decrease and strength are in the normal range of variation, as shown by study of magnetic fields in rocks.

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. Some speculate that a greatly diminished magnetic field during a reversal period will expose the surface of the earth to a substantial and potentially damaging increase in solar radiation. However homo erectus and their ancestors certainly survived many previous reversals.

See also

• Dynamo theory
• Edward Sabine
• Geophysics
• Ionosphere
• Magnetohydrodynamics
• Schumann resonance
• Sherwood machine
• Solar variation
• 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

• USGS Geomagnetism Program (http://geomag.usgs.gov)
• NOAA page on geomagnetism (http://www.ngdc.noaa.gov/seg/geomag)
• "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/)de:Erdmagnetfeld

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