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Geomagnetic pole
The geomagnetic poles are antipodal points where the axis of a best-fitting dipole intersects the Earth's surface. This dipole is equivalent to a powerful bar magnet at the center of the Earth, and it is this theoretical dipole that comes closer than any other to accounting for the magnetic field observed at the Earth's surface. In contrast, the actual Earth's magnetic poles are not antipodal—that is, they do not lie on a line passing through the center of the Earth.
Owing to motion of fluid in the Earth's outer core, the actual magnetic poles are constantly moving. However, over thousands of years their direction averages to the Earth's rotation axis. On the order of once every half a million years, the poles ''reverse'' (north changes place with south).
==Definition==
As a first-order approximation, the Earth's magnetic field can be modelled as a simple dipole (like a bar magnet), tilted about 11.3° with respect to the Earth's rotation axis (which defines the Geographic North and Geographic South Poles) and centered at the Earth's center.〔(【引用サイトリンク】title=Geomagnetism Frequently Asked Questions )〕 The North and South Geomagnetic Poles are the antipodal points where the axis of this theoretical dipole intersects the Earth's surface. If the Earth's magnetic field were a perfect dipole then the field lines would be vertical at the Geomagnetic Poles, and they would coincide with the North and South magnetic poles. However, the approximation is imperfect, and so the Magnetic and Geomagnetic Poles lie some distance apart.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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