Earth's Magnetic Field Magnetic Field Reversals See Also

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 than north magnetic pole is from the north geographic pole.

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 magnetosphereA magnetosphere is the region around an astronomical object, in which phenomena are dominated by its magnetic field. Earth is surrounded by a magnetosphere, as are the magnetized planets Jupiter, Saturn, Uranus and Neptune. Mercury is magnetized, but too. At large distances from the planet, this dominates the surface magnetic field. In addition, the magnetized elementGenerally, an element is a basic part that is the foundation of something. For a long time, elements classical element were believed (by the Pythagoreans and alchemists for example) to be the building blocks of all matter in the universe. Similarly, Chines within the planetA planet (from the Greek , planetes or "wanderers") is a body of considerable mass that orbits a star and that produces very little or no energy through nuclear fusion. Prior to the 1990s only nine were known (all of them in our own solar system); as of 3ary coreScience In biology the core of a fruit contains its seeds. In planetary science the core of a planet contains its innermost layer(s). Due to planetary differentiation, such layers tend to be more dense than outer layers. In archaeology a core is a distinc are undergoing rotationThis article is about rotation as a movement of a physical body. For other meanings, see rotation (disambiguation). Rotation is the movement of a body in such a way that any given point of that body remains at a constant distance from some other fixed poi and are not static.

1 Magnetic field reversals

The Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of yearA year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, "Martian year". Seasonal year A seasonal year is the time between successive recurrencess, 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-Matayama reversalThe Brunhes-Matayama Reversal was a geologic event approximately 780,000 years ago when the Earth's magnetic field last underwent 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 normalThe Cretaceous long normal was the long period of stability in the Earth's magnetic field when no field reversals occurred. It lasted between 120 83 million years ago.).

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.

2 See also

3 References

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

4 External links

Magnetism

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North Magnetic Pole[1]	(2001) 81.3°N 110.8°W	(2004 est) 82.3°N 113.4°W
South Magnetic Pole[2]	(1998) 64.6°S 138.5°E.	(2004 est) 63.5°S 138°E