Earth's magnetic field
The cause of Earth's magnetic field (the surface magnetic field) isn't known for certain, but is possibly explained by dynamo theory. The magnetic field extends several tens of thousands of kilometers into space. There are two magnetic poles, one in the south and one in the north; the poles are approximately 11.3° from the poles.The field is, approximately, a magnetic dipole, similar to the field of a bar magnet[?], but this similarity is superficial. In a bar magnet, the magnetic field 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. Although the Earth appears externally like a bar magnet, its magnetic field is caused mostly by electric currents, see: dynamo theory.
The Earth's magnetic field reverses at intervals, ranging from tens of thousands to hundreds of thousands of years. It is believed that this last occurred some 600,000 years ago (Comins[?] - DEU p.84). The overall geomagnetic field is becoming weaker at a rate which will cause the field to disappear by about 4000 AD.1 Other sources have put the date of field collapse as early as 3000 AD.
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.
See also: ionosphere, Sherwood machine[?], Edward Sabine.
References
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brooding and sulking as he does. He resents any interest being
shot me by accident."
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with me - if you can spare us both."
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dropped down the river on Thursday morning - the Private at the
Subaltern, who respected the reserve of the Private.
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said no more till bedtime.
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On
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