Earth's magnetic field
From Freepedia
Earth's magnetic field (and 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.
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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
| North Magnetic Pole [1] | (2001) 81.3° N 110.8° W | (2004 est) 82.3° N 113.4° W | (2005 est) 82.7° N 114.4° W |
| South Magnetic Pole [2] | (1998) 64.6° S 138.5° E. | (2004 est) 63.5° S 138.0° E |
References
- ^ Geomagnetism, North Magnetic Pole. Natural Resources Canada, 2005-03-13.
- ^ South Magnetic Pole. Commonwealth of Australia, Australian Antarctic Division, 2002.
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 in the liquid outer core.
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.
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.
Magnetometers detect minute deviations in the Earth's magnetic field caused by iron artefacts, kilns, some types of stone structures, and even ditches and middens in geophysical survey. Using the magnetic instruments adapted from airborne devices developed during World War II to detect submarines, the magnetic variations across the ocean floor were mapped. The basalt -- the iron-rich, volcanic rock making up the ocean floor -- contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. The distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials recorded the Earth's magnetic field at the time
In October 2003, the Earth's magnetosphere was hit by a solar flare causing a brief but intense geomagnetic storm, provoking unusual displays of northern lights.
Magnetic field reversals
- Main article: geomagnetic reversal
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.
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 3000-4000 AD. The rapid deterioration began at least 150 years ago and has accelerated in the past several years, with a total decrease of 10-15% over these 150 years. This change is within the normal range of variation, as shown by study of magnetic fields in rocks, and need not necessarily lead to a reversal.
See also
Image:Birkeland-anode-globe-fig259.jpg
Field characteristics and phenonomena
- Ionosphere : Part of the atmosphere that is ionized by solar radiation.
- Schumann resonance : Set of spectrum peaks in the ELF portion of the Earth's electromagnetic field spectrum.
- Solar variation : Fluctuations in the amount of energy emitted by the Sun. Small variations have been measured from satellites during recent decades.
- South Atlantic Magnetic Anomaly : The region where Earth's inner van Allen radiation belt makes its closest approach to the planet's surface.
Disciplines
- Geophysics : Study of the earth by quantitative physical methods, especially by seismic reflection and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods.
- Magnetohydrodynamics : Academic discipline which studies the dynamics of electrically conducting fluids.
Theories
- Dynamo theory : Mechanism by which a celestial body such as the Earth generates a magnetic field.
People
- Edward Sabine : Extensively researched the Earth's magnetic field.
References
- Discovering the Essential Universe by Neil F. Comins (2001)
- Introduction to Geomagnetically Trapped Radiation by Martin Walt (1994)
External links
- USGS Geomagnetism Program. Real time monitoring of the Earth's magnetic field. U.S. Department of the Interior, U.S. Geological Survey, February 17, 2005.
- Geomagnetism. National Geophysical Data Center, NOAA. Apr-2005.
- BGS Geomagnetism. Information on monitoring and modelling the geomagnetic field. British Geological Survey, August 2005.
- William J. Broad, "Will Compasses Point South?". New York Times, July 13, 2004.
- John Roach, "Why Does Earth's Magnetic Field Flip?". National Geographic, September 27, 2004.
- "Magnetic Storm". PBS NOVA, 2003. (ed. about pole reversals)
