North Magnetic Pole

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Part of the Carta Marina of 1539 by Olaus Magnus, depicting the location of magnetic north vaguely conceived as "Insula Magnetu[m]" (Latin for "Island of Magnets") off modern day Murmansk. The man holding the rune staffs is the Norse hero Starkad.

The Earth's North Magnetic Pole is the wandering point on the Earth's surface at which the Earth's magnetic field points vertically downwards (i.e. the "dip" is 90°). The North Magnetic Pole should not be confused with the lesser known North Geomagnetic Pole, described later in this article.

In 2001, the North Magnetic Pole was determined by the Geological Survey of Canada to lie near Ellesmere Island in northern Canada at 81.3° N 110.8° W. It was estimated to be at 82.7° N 114.4° W in 2005.

Its southern hemisphere counterpart is the South Magnetic Pole. Because the Earth's magnetic field is not exactly symmetrical, the North and South Magnetic Poles are not antipodal: a line drawn from one to the other does not pass through the centre of the Earth (it actually misses by about 530 km or 329.3 mi).

1 Polarity
2 History
3 Expeditions and measurements
4 Magnetic North and Magnetic Declination
5 North Geomagnetic Pole
6 Geomagnetic reversal
7 References
8 See also
9 External links

[edit] Polarity

In physics, all magnets have two poles that are distinguished by the direction of the magnetic flux. In principle these poles could be labelled in any way; for example, as "+" and "−", or "A" and "B". However, based on the early use of magnets in compasses they were named the "north pole" ("N") and the "south pole" ("S"), with the north pole being the pole that pointed north. When it was later understood that opposite poles attract, a terminological dilemma arose: the Earth's North Magnetic Pole and the pole of the magnet that was attracted to it could not have the same polarity. By convention, the "north pole" of a magnet remained defined as the one attracted to the Earth's North Magnetic Pole,^[1]^[2] and by this definition the Earth's North Magnetic Pole is physically a magnetic south pole.^[3] Conversely, the Earth's South Magnetic Pole is physically a magnetic north pole.

[edit] History

In early times European navigators believed that compass needles were attracted either to a "magnetic mountain" or "magnetic island" somewhere in the far north, or to the Pole Star. The idea that the Earth acts as a giant magnet was first proposed in 1600 by Sir William Gilbert, a courtier of Queen Elizabeth I. He was also the first to define the North Magnetic Pole as the point where the Earth's magnetic field points vertically downwards. This is the definition used today, though it would take several hundred years before the nature of the Earth's magnetic field was properly understood.^[4]

[edit] Expeditions and measurements

The first expedition to reach the North Magnetic Pole was led by James Clark Ross, who found it at Cape Adelaide on the Boothia Peninsula on June 1, 1831. Roald Amundsen found the North Magnetic Pole in a slightly different location in 1903. The third observation was by Canadian government scientists Paul Serson and Jack Clark, of the Dominion Astrophysical Observatory, who found the pole at Allen Lake on Prince of Wales Island.^[5]

The Canadian government has made several measurements since, which show that the North Magnetic Pole is continually moving northwest. In 1996 an expedition certified its location by magnetometer and theodolite at 78°35.7′N, 104°11.9′W.^[6] Its estimated 2005 position was 82.7° N 114.4° W, to the west of Ellesmere Island, the biggest of the Queen Elizabeth Islands, in Canada.^[7] During the 20th century it has moved 1100 km, and since 1970 its rate of motion has accelerated from 9 km/year to 41 km/year (2001–2003 average; see also Polar drift). If it maintained its present speed and direction it would reach Siberia in about 50 years, but it is expected to veer from its present course and slow down.

This movement is on top of a daily or diurnal variation in which the North Magnetic Pole describes a rough ellipse, with a maximum deviation of 80 km from its mean position.^[8] This effect is due to disturbances of the geomagnetic field by charged particles from the Sun.

North Magnetic Pole ^[7]	(2001) 81.3° N 110.8° W	(2004 est) 82.3° N 113.4° W	(2005 est) 82.7° N 114.4° W
South Magnetic Pole ^[9]	(1998) 64.6° S 138.5° E.	(2004 est) 63.5° S 138.0° E

The first team of novices to reach the Magnetic North Pole did so in 1996 led by David Hempleman-Adams. It included the first British woman and first Swedish woman to reach the Pole. The team also successfully tracked the location of the Magnetic North Pole on behalf of the University of Ottawa.

The biennial Polar Race takes place between Resolute Bay northern Canada and the 1996-certified location of the North Magnetic Pole at 78°35.7′N, 104°11.9′W. On 25 July 2007, a Top Gear Polar Challenge Special was aired on BBC Two in the United Kingdom, in which Jeremy Clarkson and James May became the first people in history to reach this location in an automobile.^[10]

[edit] Magnetic North and Magnetic Declination

Main article: Magnetic declination. See also: Earth's magnetic field

Magnetic declination from true north in 2000.

The direction in which a compass needle points is known as magnetic north. In general, this is not exactly the direction of the North Magnetic Pole (or of any other consistent location). Instead, the compass aligns itself to the local geomagnetic field, which varies in a complex manner over the Earth's surface, as well as over time. The angular difference between magnetic north and true north (defined in reference to the Geographic North Pole), at any particular location on the Earth's surface, is called the magnetic declination. Most map coordinate systems are based on true north, and magnetic declination is often shown on map legends so that the direction of true north can be determined from north as indicated by a compass.

Magnetic declination has been measured in many countries, including the U.S. The line of zero declination (the agonic line) in the U.S. runs from the North Magnetic Pole through Lake Superior and across the western panhandle of Florida. Along this line, true north is the same as magnetic north. West of the line of zero declination, a compass will give a reading that is east of true north. Conversely, east of the line of zero declination, a compass reading will be west of true north.

Magnetic declination is still very important for certain types of navigation that have traditionally made heavy use of the magnetic compass; see the main article (Magnetic declination) for details.

[edit] North Geomagnetic Pole

As a first-order approximation, the Earth's magnetic field can be modelled as a simple dipole (like a bar magnet), tilted about 11° with respect to the Earth's rotation axis and centered at the Earth's centre. The residuals form the nondipole field. 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 therefore coincide with the Magnetic Poles. However, the approximation is in fact far from perfect, so in reality the Magnetic and Geomagnetic Poles lie some distance apart.

Like the North Magnetic Pole, the North Geomagnetic Pole attracts the north pole of a bar magnet and so is in a physical sense actually a south magnetic pole. It is the centre of the region of the magnetosphere in which the Aurora Borealis can be seen. As of 2005 it was located at approximately 79.74° N 71.78° W, off the northwest coast of Greenland,^[11] but it is now drifting away from North America and toward Siberia.

[edit] Geomagnetic reversal

Main article: Geomagnetic reversal

Over geological timescales, the orientation of Earth's magnetic field (and that of other planets) can flip over, so that magnetic north becomes magnetic south and vice versa – an event known as a geomagnetic reversal. The Earth's magnetic field has done this repeatedly throughout history. It is thought that reversals occur when the circulation of liquid nickel/iron in the Earth's outer core is disrupted and then reestablishes itself in the opposite direction. It is not known what causes these disruptions. Evidence of geomagnetic reversals can be seen at mid-ocean ridges where tectonic plates move apart and the sea bed is filled in with magma. As the magma seeps out of the mantle the magnetic particles contained within it are oriented in the direction of the magnetic field at the time the magma cools and solidifies.