Dynamo Theory and the Magnetic Field of the Earth

At a Glance

• Source Liquid iron-nickel outer core
• Theory Dynamo (geodynamo) theory
• Shape Roughly a dipole, like a bar magnet
• Strength at surface About 25 to 65 microtesla
• Role Shields the Earth from solar wind
• Exam link UPSC Prelims, Geography: Interior of the Earth and Magnetic Field

Where Does It Fit?

This is the fifth topic of the category on the interior of the Earth and its magnetic field. The previous topic named the discontinuities, including the boundary of the liquid outer core, and this one explains how that core makes the magnetic field. The next topic covers magnetic poles, declination and inclination. Previous topic: Seismic Discontinuities of the Earth. Next topic: Magnetic Poles, Declination and Inclination.

What Is the Magnetic Field of the Earth?

In short: the Earth acts as a giant magnet whose invisible field surrounds the planet and extends far into space.

The Earth has a magnetic field that is detected by a compass needle, which aligns itself along the field lines. The field is roughly like that of a bar magnet placed near the centre and tilted by about 11 degrees from the axis of rotation. The magnetic field lines leave the Earth in the southern hemisphere and re-enter in the northern hemisphere, so the north end of a compass needle points to the region near the geographic North Pole. The strength at the surface is about 25 to 65 microtesla, strongest near the magnetic poles and weakest near the equator. The field is not a permanent magnetism of the rocks, because the interior of the Earth is far above the temperature, called the Curie point, at which iron loses its magnetism.

What Is the Dynamo Theory?

The currently accepted explanation of the field is the dynamo theory, also called the geodynamo. It requires three conditions that exist in the Earth: a large volume of electrically conducting fluid, a source of energy to move it, and rotation. The liquid iron and nickel of the outer core conduct electricity. Heat from the cooling core and the slow freezing of the inner core drives convection in the outer core, as hot metal rises and cooler metal sinks. The rotation of the Earth twists these flows through the Coriolis effect, organising them into spiral columns. Moving conductors in a magnetic field produce electric currents, and these currents generate a magnetic field that reinforces the original one, so the dynamo sustains itself as long as the energy supply continues.

Why Is the Outer Core Central to the Theory?

The outer core is about 2,200 km thick and liquid, as shown by the absence of S waves, which is described in Seismic Waves and Their Types. This is the only layer that is both fluid and a good conductor. The solid inner core is hot, but because it is solid it cannot circulate, and the mantle is a poor conductor. Planets that have a liquid metallic core and spin, such as Mercury, Jupiter and Saturn, also have magnetic fields, while Venus, which rotates very slowly, and Mars, whose core has cooled, have no global field today. As an analytical point, the dynamo theory is supported by observation and computer models, but the details of the flow in the core cannot be seen directly, so it remains a model that is continually refined using data from satellites such as the European Space Agency’s Swarm mission.

Is the Field Constant?

The magnetic field is not constant. It changes in strength and direction over years, called secular variation, as the flows in the core change. The magnetic poles drift, the field has weakened by about 9 per cent over the last 170 years or so, and over geological time the field has reversed many times, as discussed in Geomagnetic Reversal and Geomagnetic Poles. A region of weak field, the South Atlantic Anomaly, extends over the South Atlantic and parts of South America, and satellites passing through it experience higher radiation. These facts show that the geodynamo is a changing system and not a fixed property.

Why Does the Magnetic Field Matter?

• Protection: it deflects charged particles of the solar wind and cosmic rays, forming the magnetosphere that shields life and the atmosphere, explained in Magnetosphere, Auroras and Van Allen Radiation Belts.
• Navigation: compasses and many migrating animals, such as birds and sea turtles, are believed to sense it.
• Geological record: magnetic minerals in rocks record the field at the time of their formation, which gave evidence for seafloor spreading and plate movement.
• Technology: space weather affects satellites, power grids and radio communication.

Why Is This Topic Important?

• Origin Core and magnetism
• Evidence Basis for paleomagnetism
• Space weather Current affairs link

What Came Next?

The next topic describes magnetic poles, declination and inclination. Next topic: Magnetic Poles, Declination and Inclination.

Key People and Terms

• Geodynamo Process making the field
• Convection Heat-driven circulation of fluid
• Secular variation Slow change of the field
• Curie point Temperature above which magnetism is lost
• Dipole Field with two poles
• South Atlantic Anomaly Region of weak field

Exam Corner

Mains Angle

Questions ask for the origin of the field. These points add depth.

• Origin Convection in the outer core
• Change Secular variation and reversals
• Role Shield against the solar wind