DEFINITION OF GEOMAGNETISM

The Earth has a magnetic field that is generated

within the core of the planet. The field is generally

approximated as a dipole, with north and south poles, and

magnetic field lines that emerge from the Earth at the south

pole and reenter at the north pole. The field is characterized

at each place on the planet by an inclination and a declination.

The inclination is a measure of how steeply inclined the

field lines are with respect to the surface, with low inclinations

near the surface, and steep inclinations near the poles.

The declination measures the apparent angle between the

rotational north pole and the magnetic north pole.

The magnetic field originates in the liquid outer core of

the Earth and is thought to result from electrical currents

generated by convective motions of the iron-nickel alloy that

the outer core is made from. The formation of the magnetic

field by motion of the outer core is known as the geodynamo

theory, pioneered by Walter M. Elsasser of Johns Hopkins

University in the 1940s. The basic principle for the generation

of the field is that the dynamo converts mechanical energy

from the motion of the liquid outer core, which is an

electrical conductor, into electromagnetic energy of the magnetic

field. The convective motion of the outer core, maintained

by thermal and gravitational forces, is necessary to

maintain the field. If the convection stopped or if the outer

core solidified, the magnetic field would cease to be generated.

Secular variations in the magnetic field have been well

documented by examination of the paleomagnetic record in

the seafloor, lava flows, and sediments. Every few thousand

years the magnetic field changes intensity and reverses, with

the north and south poles abruptly flipping.

See also PALEOMAGNETISM.

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