出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/01/27 16:02:02」(JST)
The North Magnetic Pole is the point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downwards (in other words, if a magnetic compass needle is allowed to rotate about a horizontal axis, it will point straight down). There is only one location where this occurs, near (but distinct from) the Geographic North Pole and the Geomagnetic North Pole.
The North Magnetic Pole moves over time due to magnetic changes in the Earth's core.[1] In 2001, it was determined by the Geological Survey of Canada to lie near Ellesmere Island in northern Canada at 81°18′N 110°48′W / 81.3°N 110.8°W / 81.3; -110.8 (Magnetic North Pole 2001). It was situated at 83°06′N 117°48′W / 83.1°N 117.8°W / 83.1; -117.8 (Magnetic North Pole 2005 est) in 2005. In 2009, while still situated within the Canadian Arctic territorial claim at 84°54′N 131°00′W / 84.9°N 131.0°W / 84.9; -131.0 (Magnetic North Pole 2009),[2] it was moving toward Russia at between 34 and 37 miles (55 and 60 km) per year.[3] As of 2012, the pole is projected to have moved beyond the Canadian Arctic territorial claim to 85°54′N 147°00′W / 85.9°N 147.0°W / 85.9; -147.0 (Magnetic North Pole 2012 est).[2]
Its southern hemisphere counterpart is the South Magnetic Pole. Since the Earth's magnetic field is not exactly symmetrical, the North and South Magnetic Poles are not antipodal: i.e., a line drawn from one to the other does not pass through the geometric centre of the Earth.
The Earth's North and South Magnetic Poles are also known as Magnetic Dip Poles, with reference to the vertical "dip" of the magnetic field lines at those points.[4]
All magnets have two poles that are distinguished by the direction of the magnetic flux. These poles could have any names, for example, "+" and "−"; but the convention in early compasses was to call the end of the needle pointing to the Earth's North Magnetic Pole the "north pole" (or "north-seeking pole") and the other end the "south pole" (the names are often abbreviated to "N" and "S"). Because opposite poles attract, this definition implies that the Earth's North Magnetic Pole is a magnetic south pole and the Earth's South Magnetic Pole is a magnetic north pole.[5][6]
Early European navigators believed that compass needles were attracted to a "magnetic mountain" or "magnetic island" somewhere in the far north (see Rupes Nigra), or to the Pole Star.[7][8] The idea that the Earth itself acts as a giant magnet was first proposed in 1600 by the English physician and natural philosopher William Gilbert. 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 nowadays, though it would be a few hundred years before the nature of the Earth's magnetic field was understood properly.[7]
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 in 1947.[9]
At the start of the Cold War, the United States Department of War recognized a need for a comprehensive survey of the North American Arctic and asked the United States Army to undertake the task. An assignment was made in 1946 for the newly-formed Army’s Air Corps Strategic Air Command to explore the entire Arctic Ocean area. The exploration was conducted by the 46th (later re-designated the 72nd) Photo Reconnaissance Squadron and reported on as a classified Top Secret mission named Project Nanook. This project in turn was divided into many separate, but identically classified, projects, one of which was Project Polaris, which was a radar, photographic (trimetrogon, or three-angle, cameras) and visual study of the entire Canadian Archipelago. A Canadian officer observer was assigned to accompany each flight.
Directing Project Polaris was its navigation leader, 1st Lieutenant Frank O. Klein, a World War II combat veteran. Incidental to the project and taken up at his own initiative was a study of northern terrestrial magnetism. The study was prompted by the surprise that the fluxgate compass did not behave erratically as expected. It oscillated no more than 1 to 2 degrees over much of the region.[10][11] With the cooperation of many of his squadron teammates in obtaining many hundreds of statistical readings, startling results were revealed:
The center of the north magnetic dip pole was on Prince of Wales Island some 250 miles NNW of the positions determined by Amundsen and Ross, and the dip pole occupied a larger elliptical area, with foci about 250 miles apart on Boothia Peninsula and Bathurst Island.
Klein called the two foci local poles, for their importance to navigation in emergencies when using a "homing" procedure. About 3 months after Klein's findings were officially reported, a Canadian ground expedition was sent into the Archipelago to locate the position of the magnetic pole. R. Glenn Madill, Chief of Terrestrial Magnetism, Department of Mines and Resources, Canada, wrote to Lt. Klein on 21 July 1948:
…we can agree on one point, and that is the presence of what we can call the main magnetic pole on northwestern Prince of Wales Island. I have accepted as a purely preliminary value, the position Latitude 72 degrees N and 100 degrees Longitude West. Your value of 73 degrees 15' and 99 degrees 45’ W is in excellent agreement, and I suggest you use your value by all means.
—R. Glenn Madill
(The positions were less than 20 miles apart.)[12]
The Canadian government has made several measurements since, which show that the North Magnetic Pole is moving continually northwestward. In 2001, an expedition located the pole at 81°18′N 110°48′W / 81.3°N 110.8°W / 81.3; -110.8 (Magnetic North Pole 2001 location). In 2007, the latest survey found the pole at 83°57′N 120°43′W / 83.95°N 120.72°W / 83.95; -120.72 (Magnetic North Pole 2007 location).[13] During the 20th century it moved 1100 km, and since 1970 its rate of motion has accelerated from 9 km/year to approximately 52 km/year (2001–2007 average; see also Polar drift). Members of the 2007 expedition to locate the magnetic north pole wrote that such expeditions have become logistically difficult, as the pole moves further away from inhabited locations. They expect that in the future, the magnetic pole position will be obtained from satellite data instead of ground surveys.[13]
This general movement is in addition to 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.[14] This effect is due to disturbances of the geomagnetic field by charged particles from the Sun.
| North Magnetic Pole[15] | (2001) 81°18′N 110°48′W / 81.3°N 110.8°W / 81.3; -110.8 | (2004 est) 82°18′N 113°24′W / 82.3°N 113.4°W / 82.3; -113.4 | (2007) 83°57′N 120°43′W / 83.95°N 120.72°W / 83.95; -120.72[13] |
| South Magnetic Pole | (1998) 64°36′S 138°30′E / 64.6°S 138.5°E / -64.6; 138.5 | (2004 est) 63°30′S 138°00′E / 63.5°S 138.0°E / -63.5; 138.0 | (2007) 64°29′49″S 137°41′02″E / 64.497°S 137.684°E / -64.497; 137.684[16] |
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, and certified its location by magnetometer and theodolite at 78°35.7′N 104°11.9′W / 78.5950°N 104.1983°W / 78.5950; -104.1983 (Magnetic North Pole 1996).[17]
The biennial Polar Race takes place between Resolute Bay in northern Canada and the 1996-certified location of the North Magnetic Pole at 78°35.7′N 104°11.9′W / 78.5950°N 104.1983°W / 78.5950; -104.1983 (Magnetic North Pole 1996).[citation needed] On 25 July 2007, the Top Gear Polar Challenge Special was broadcast on BBC Two in the United Kingdom, in which Jeremy Clarkson and James May (and their support and camera team) became the first people in history to reach this location in a car.[18]
The direction 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 local angular difference between magnetic north and true north 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 southward into the Gulf of Mexico. 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 much use of magnetic compasses; see Magnetic declination for details.
As a first-order approximation, the Earth's magnetic field can be modelled as a simple dipole (like a bar magnet), tilted about 10° with respect to the Earth's rotation axis (which defines the Geographic North and Geographic South Poles) and centered at the Earth's centre. 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 Magnetic Poles. However, the approximation is imperfect, and so the Magnetic and Geomagnetic Poles lie some distance apart.[19]
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°44′N 71°47′W / 79.74°N 71.78°W / 79.74; -71.78 (Geomagnetic North Pole 2005 est), off the northwest coast of Greenland,[19] but it is now drifting away from North America and toward Siberia.
Over the life of the Earth, the orientation of Earth's magnetic field has reversed several times, with magnetic north becoming magnetic south and vice versa – an event known as a geomagnetic reversal. Evidence of geomagnetic reversals can be seen at mid-ocean ridges where tectonic plates move apart and the seabed 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.[20]
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