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The Geo dynamo theory

There are three requisites for a dynamo to operate:

  • An electrically conductive fluid medium
  • Kinetic energy provided by planetary rotation
  • An internal energy source to drive convective motions within the fluid.

In the case of the Earth, the magnetic field is induced and constantly maintained by the convection of liquid iron in the outer core. A requirement for the induction of field is a rotating fluid. Rotation in the outer core is supplied by the Coriolis effect caused by the rotation of the Earth.

By: Narmina

It was once believed that the dipole, which comprises much of the Earth's magnetic field and is misaligned along the rotation axis by 11.3 degrees, was caused by permanent magnetization of the materials in the earth. This means that dynamo theory was originally used to explain the Sun's magnetic field in its relationship with that of the Earth.

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Tidal heating

Tidal forces between celestial orbiting bodies cause friction that heats up their interiors. This is known as tidal heating, and it helps keep the interior liquid. A liquid interior that can conduct electricity is required to produce a dynamo. Saturn's Enceladus and Jupiter's Io have enough tidal heating to liquify their inner cores, but they may not create a dynamo because they cannot conduct electricity. Mercury, despite its small size, has a magnetic field, because it has a conductive liquid core created by its iron composition and friction resulting from its highly elliptical orbit. It is theorized that the Moon once had a magnetic field, based on evidence from magnetized lunar rocks, due to its short-lived closer distance to Earth creating tidal heating. An orbit and rotation of a planet helps provide a liquid core, and supplements kinetic energy that supports a dynamo action.

What is the Geodynamo theory?

The dynamo theory, is the geophysical theory that explains the origin of Earth’s main magnetic field in terms of a self-sustaining dynamo. In this dynamo mechanism, fluid motion in Earth’s outer core moves conducting material (liquid iron) across an already existing weak magnetic field and generates an electric current. The electric current, in turn, produces a magnetic field that also interacts with the fluid motion to create a secondary magnetic field. Together, the two fields are stronger than the original and lie essentially along the axis of Earth’s rotation.

Magnetic reversals

Aurora Borealis

Paleomagnetic records indicate that the geomagnetic field has existed for at least three billion years. However, based on the size and electrical conductivity of the Earth's core, the field, if it were not continually being generated, would decay away in only about 20,000 years since the temperature of the core is too high to sustain permanent magnetism. In addition, paleomagnetic records show that the dipole polarity of the geomagnetic field has reversed many times in the past, the mean time between reversals being roughly 200,000 years with individual reversal events taking only a couple thousand years.

The geodynamo theory was proposed by the German-born American physicist Walter M. Elsasser and the British geophysicist Edward Bullard during the mid-1900s. Although various other mechanisms for generating the geomagnetic field have been proposed, only the dynamo concept is seriously considered today.

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