At intervals, the polarity of the Earth's magnetic field reverses, The timescale for this reversal is unclear -- in fact, it seems to depend on the latitude of the site from which the geological data are extracted. At the Earth's surface, the planetary magnetic field can vary on timescales that range over more than 18 orders of magnitude -- from less than a millisecond to more than 100 million years. The most dramatic of these changes are magnetic-field reversals, in which the geomagnetic poles swaphemispheres. Several hundred such reversals have been documented from geological records. They seem to occur randomly in time, with the shortest interval between successive reversals being 20,000-30,000 years and the longest about 50 million years. It is 40 years since the reality of field reversals became widely accepted; by now one might expect that what happens to the magnetic field during a reversal would be understood.
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