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The geomagnetic poles are where the axis of a best-fitting dipole intersects the surface of Earth. This theoretical dipole is equivalent to a powerful bar magnet at the inner core, and comes closer than any other point dipole model to describing the magnetic field observed at Earth's surface. In contrast, the magnetic poles of the actual Earth are not antipodes; that is, the line on which they lie does not pass through Earth's center.
Owing to the motion of fluid in the Earth's outer core, the actual magnetic poles are constantly moving (secular variation). 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 (i.e., north switches place with south) although the time frame of this switching can be anywhere from every 10 thousand years to every 50 million years. The poles also swing in an oval of around in diameter daily due to solar wind deflecting the magnetic field.
Although the geomagnetic pole is only theoretical and cannot be located directly, it arguably is of more practical relevance than the magnetic (dip) pole. This is because the poles describe a great deal about the Earth's magnetic field, determining for example where can be observed. The dipole model of the Earth's magnetic field consists of the location of geomagnetic poles and the dipole moment, which describes the strength of the field.
| +Recent locations of Earth's geomagnetic (auroral) poles, IGRF-13 fit |
The South Geomagnetic Pole is the point where the axis of this best-fitting tilted dipole intersects the Earth's surface in the southern hemisphere. , it is located at , whereas in 2005, it was calculated to be located at , near Vostok Station.
Because the Earth's actual magnetic field is not an exact dipole, the (calculated) North and South Geomagnetic Poles do not coincide with the North and South Magnetic Poles. If the Earth's magnetic fields were exactly dipolar, the north pole of a magnetic compass needle would point directly at the North Geomagnetic Pole. In practice, it does not because the geomagnetic field that originates in the core has a more complex non-dipolar part, and magnetic anomaly in the Earth's crust also contribute to the local field.
The locations of geomagnetic poles are calculated by a statistical fit to measurements of the Earth's field by satellites and in geomagnetic observatories. This can be the International Geomagnetic Reference Field (covering a wide time-span in history) or the U.S. World Magnetic Model (only covering a five-year period).
Over several thousand years, the average location of the geomagnetic poles coincides with the geographical poles. Paleomagnetism have long relied on the geocentric axial dipole (GAD) hypothesis, which states that — aside from during geomagnetic reversals — the time-averaged position of the geomagnetic poles has always coincided with the geographic poles. There is considerable paleomagnetic evidence supporting this hypothesis.
Movement
The geomagnetic poles move over time because the geomagnetic field is produced by motion of the molten iron alloys in the Earth's outer core. (See geodynamo.) Over the past 150 years, the poles have moved westward at a rate of 0.05° to 0.1° per year and closer to the true poles at 0.01° per year.
Geomagnetic reversal
Over the life of the Earth, the orientation of Earth's magnetic field has reversed many times, with geomagnetic north becoming geomagnetic south and vice versa – an event known as a geomagnetic reversal. Evidence of geomagnetic reversals can be seen at where move apart. As magma seeps out of the mantle and solidifies to become new ocean floor, the magnetic minerals in it are magnetized in the direction of the magnetic field. The study of this remanence is called palaeomagnetism. Thus, starting at the most recently formed ocean floor, one can read out the direction of the magnetic field in previous times as one moves farther away to older ocean floor.
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