Geomagnetic pole
Encyclopedia
The geomagnetic poles are antipodal point
s where the axis of a theoretical dipole
intersects the Earth's surface. This dipole is equivalent to a powerful bar magnet
at the center of the Earth
, and it is the dipole that comes closest to accounting for the magnetic field observed at the Earth's surface. Owing to motion of fluid in the Earth's outer core
, the poles are constantly moving. However, over thousands of years their direction averages to the Earth's rotation axis. Over millions of years, the poles reverse (north changes place with south).
, 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 (which defines the Geographic North
and Geographic South Pole
s) and centered at the Earth's center. The North and South Geomagnetic Poles are the antipodal point
s 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 line
s 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.
, 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
, and is drifting away from North America toward Siberia.
The locations of geomagnetic poles are predicted by the International Geomagnetic Reference Field, a statistical fit to measurements of the Earth's field by satellites and in geomagnetic observatories. If the Earth's field 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 anomalies
in the Earth's crust also contribute to the local field.
(see geodynamo). Over the past 150 years the poles have moved westward at a rate of 0.05° to 0.1° per year, with little net north or south motion.
Over several thousand years, the average location of the geomagnetic poles coincides with the geographical poles. Paleomagnetists
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.
. Evidence of geomagnetic reversals can be seen at mid-ocean ridge
s where tectonic plates 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. 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 further away to older ocean floor.
Antipodal point
In mathematics, the antipodal point of a point on the surface of a sphere is the point which is diametrically opposite to it — so situated that a line drawn from the one to the other passes through the centre of the sphere and forms a true diameter....
s where the axis of a theoretical dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
intersects the Earth's surface. This dipole is equivalent to a powerful bar magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
at the center of the Earth
Inner core
The inner core of the Earth, its innermost hottest part as detected by seismological studies, is a primarily solid ball about in radius, or about 70% that of the Moon...
, and it is the dipole that comes closest to accounting for the magnetic field observed at the Earth's surface. Owing to motion of fluid in the Earth's outer core
Outer core
The outer core of the Earth is a liquid layer about 2,266 kilometers thick composed of iron and nickel which lies above the Earth's solid inner core and below its mantle. Its outer boundary lies beneath the Earth's surface...
, the poles are constantly moving. However, over thousands of years their direction averages to the Earth's rotation axis. Over millions of years, the poles reverse (north changes place with south).
Definition
As a first-order approximationOrders of approximation
In science, engineering, and other quantitative disciplines, orders of approximation refer to formal or informal terms for how precise an approximation is, and to indicate progressively more refined approximations: in increasing order of precision, a zeroth order approximation, a first order...
, the Earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
can be modelled as a simple dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
(like a bar magnet), tilted about 11° with respect to the Earth's rotation axis (which defines the Geographic North
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
and Geographic South Pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...
s) and centered at the Earth's center. The North and South Geomagnetic Poles are the antipodal point
Antipodal point
In mathematics, the antipodal point of a point on the surface of a sphere is the point which is diametrically opposite to it — so situated that a line drawn from the one to the other passes through the centre of the sphere and forms a true diameter....
s 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 line
Field line
A field line is a locus that is defined by a vector field and a starting location within the field. Field lines are useful for visualizing vector fields, which are otherwise hard to depict...
s would be vertical at the Geomagnetic Poles, and they would coincide with the North
North Magnetic Pole
The Earth's North Magnetic Pole is the point on the surface of the Northern Hemisphere at which the Earth's magnetic field points vertically downwards . Though geographically in the north, it is, by the direction of the magnetic field lines, physically the south pole of the Earth's magnetic field...
and South
South Magnetic Pole
The Earth's South Magnetic Pole is the wandering point on the Earth's surface where the geomagnetic field lines are directed vertically upwards...
magnetic poles. However, the approximation is imperfect, and so the Magnetic and Geomagnetic Poles lie some distance apart.
Location
Like the North Magnetic PoleNorth Magnetic Pole
The Earth's North Magnetic Pole is the point on the surface of the Northern Hemisphere at which the Earth's magnetic field points vertically downwards . Though geographically in the north, it is, by the direction of the magnetic field lines, physically the south pole of the Earth's magnetic field...
, the North Geomagnetic Pole attracts the north pole of a bar magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
and so is in a physical sense actually a south magnetic pole. It is the centre of the region of the magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
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
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
, and is drifting away from North America toward Siberia.
The locations of geomagnetic poles are predicted by the International Geomagnetic Reference Field, a statistical fit to measurements of the Earth's field by satellites and in geomagnetic observatories. If the Earth's field were exactly dipolar, the north pole of a magnetic compass
Compass
A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...
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 anomalies
Magnetic anomaly
In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying material...
in the Earth's crust also contribute to the local field.
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 coreOuter core
The outer core of the Earth is a liquid layer about 2,266 kilometers thick composed of iron and nickel which lies above the Earth's solid inner core and below its mantle. Its outer boundary lies beneath the Earth's surface...
(see geodynamo). Over the past 150 years the poles have moved westward at a rate of 0.05° to 0.1° per year, with little net north or south motion.
Over several thousand years, the average location of the geomagnetic poles coincides with the geographical poles. Paleomagnetists
Paleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
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.
Geomagnetic reversal
Over the life of the Earth, the orientation of Earth's magnetic field has reversed several times, with geomagnetic north becoming geomagnetic south and vice versa – an event known as a geomagnetic reversalGeomagnetic reversal
A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse...
. Evidence of geomagnetic reversals can be seen at mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
s where tectonic plates move apart. As magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
seeps out of the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
and solidifies to become new ocean floor, the magnetic minerals in it are magnetized in the direction of the magnetic field. 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 further away to older ocean floor.