Earth: Geodynamo

Earth: Geodynamo

Published: November 25, 2007, 12:00 am
Updated: March 24, 2009, 1:21 pm
Lead Author: Sten Odenwald
Topics: Magnetosphere Space Science The Solar System

Introduction

Earth's magnetic field appears to be generated by a magnetic dynamo process near the core of the Earth in its outer liquid region. The electrical currents that flow there cause a magnetic field to form, much as a current flowing through a wire wrapped around a nail.

Supercomputer model of Earth's magnetic field. (Courtesy Gary Glatzmaier) Supercomputer model of Earth's magnetic field. (Courtesy Gary Glatzmaier)

Supercomputer model of Earth's magnetic field. (Courtesy Gary Glatzmaier)

This field is not static in time or space. Geologic evidence shows that it reverses its polarity every 250,000 to 500,000 years. In fact, the geomagnetic field is currently decreasing in overall strength by 5% per century, suggesting that in a few thousand years at the present rate, it may temporarily vanish during a next field reversal episode. Past reversals have not caused biological impacts because even without its magnetic field, the Earth's atmosphere is very effective in shielding the surface from cosmic rays able to do biological damage. The currents in the core are also not stable in space, but cause the location of the poles at the surface to wander over time at a rate of 5 to 10 kilometers per year in a roughly northerly direction. This drift causes measurable changes in compass bearings, forcing mapmakers to periodically update their maps to accommodate this drift.

At the present time, Earth's magnetic field is weakening in strength by 5% every 100 years. It may be near zero in another few thousand years at this rate.  Earth's field often changes its strength significantly in a short time (measured in thousands of years) and often does not vanish everytime. Scientists have a lot to learn about the exact behavior of Earth's complex field. Until then, making predictions about what it may look like in a few thousands years from now is a matter of guessing, not forecasting.

Supercomputer modeling of the magnetohydrodynamics of conducting, rotating spheres by Gary Glatzmaier have revealed that these reversals are a feature of the dynamo process itself. With a year of computing on Pittsburgh's CRAY C90, 2,000 hours of processing, Glatzmaier and collaborator Paul Roberts of UCLA took a big step toward some answers. Their numerical model of the electromagnetic, fluid dynamical processes of Earth's interior reproduced key features of the magnetic field over more than 40,000 years of simulated time. The results appeared in the September 21, 1995 issue of Nature magazine.

Related EoC Articles

External Links

  • Gary A. Glatzmaier, Professor of Earth and Planetary Sciences, University of California, Santa Cruz.

Further Reading

Preview Image

"A Magnet in Space." - The Earth is sometimes referred to as a magnet in space. To begin our exploration of the Earth's magnetic field, let us assume that the Earth is a spherically shaped magnet. The magnetic field around a spherical magnet is essentially the same as if a bar magnet were located inside the Earth as seen [in the image].  To imagine the magnetic field lines around a spherical magnet, we can use the field lines around a circular magnet and then rotate the circular magnet about the vertical axis, creating donut shaped fields layered one on another.  (Source: NASA, "StarGazers - Solar Terrestrial Probes/Living with a Star Education & Public Outreach Website.")

Citation

Odenwald, Sten, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2008. "Earth: Geodynamo." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 25, 2007].
<http://www.cosmosportal.org/articles/view/135513/>

 

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