Featured Research

from universities, journals, and other organizations

First measurement of magnetic field in Earth's core

Date:
December 19, 2010
Source:
University of California - Berkeley
Summary:
Measurements of the magnetic field at the earth's surface can tell only so much about the dynamo producing it in the planet's core. Geophysicists have now used precise astronomical position data to calculate tidal damping in the core and determine for the first time the magnetic field in the center of the planet. The magnetic field strength is 25 Gauss, or 50 times stronger than the magnetic field at the surface that makes compass needles align north-south.

3-D model illustrating layers of Earth.
Credit: iStockphoto/Baris Simsek

A University of California, Berkeley, geophysicist has made the first-ever measurement of the strength of the magnetic field inside Earth's core, 1,800 miles underground.

The magnetic field strength is 25 Gauss, or 50 times stronger than the magnetic field at the surface that makes compass needles align north-south. Though this number is in the middle of the range geophysicists predict, it puts constraints on the identity of the heat sources in the core that keep the internal dynamo running to maintain this magnetic field.

"This is the first really good number we've had based on observations, not inference," said author Bruce A. Buffett, professor of earth and planetary science at UC Berkeley. "The result is not controversial, but it does rule out a very weak magnetic field and argues against a very strong field."

The results are published in the Dec. 16 issue of the journal Nature.

A strong magnetic field inside the outer core means there is a lot of convection and thus a lot of heat being produced, which scientists would need to account for, Buffett said. The presumed sources of energy are the residual heat from 4 billion years ago when the planet was hot and molten, release of gravitational energy as heavy elements sink to the bottom of the liquid core, and radioactive decay of long-lived elements such as potassium, uranium and thorium.

A weak field -- 5 Gauss, for example -- would imply that little heat is being supplied by radioactive decay, while a strong field, on the order of 100 Gauss, would imply a large contribution from radioactive decay.

"A measurement of the magnetic field tells us what the energy requirements are and what the sources of heat are," Buffett said.

About 60 percent of the power generated inside the earth likely comes from the exclusion of light elements from the solid inner core as it freezes and grows, he said. This constantly builds up crud in the outer core.

The Earth's magnetic field is produced in the outer two-thirds of the planet's iron/nickel core. This outer core, about 1,400 miles thick, is liquid, while the inner core is a frozen iron and nickel wrecking ball with a radius of about 800 miles -- roughly the size of the moon. The core is surrounded by a hot, gooey mantle and a rigid surface crust.

The cooling Earth originally captured its magnetic field from the planetary disk in which the solar system formed. That field would have disappeared within 10,000 years if not for the planet's internal dynamo, which regenerates the field thanks to heat produced inside the planet. The heat makes the liquid outer core boil, or "convect," and as the conducting metals rise and then sink through the existing magnetic field, they create electrical currents that maintain the magnetic field. This roiling dynamo produces a slowly shifting magnetic field at the surface.

"You get changes in the surface magnetic field that look a lot like gyres and flows in the oceans and the atmosphere, but these are being driven by fluid flow in the outer core," Buffett said.

Buffett is a theoretician who uses observations to improve computer models of the earth's internal dynamo. Now at work on a second generation model, he admits that a lack of information about conditions in the earth's interior has been a big hindrance to making accurate models.

He realized, however, that the tug of the moon on the tilt of the earth's spin axis could provide information about the magnetic field inside. This tug would make the inner core precess -- that is, make the spin axis slowly rotate in the opposite direction -- which would produce magnetic changes in the outer core that damp the precession. Radio observations of distant quasars -- extremely bright, active galaxies -- provide very precise measurements of the changes in the earth's rotation axis needed to calculate this damping.

"The moon is continually forcing the rotation axis of the core to precess, and we're looking at the response of the fluid outer core to the precession of the inner core," he said.

By calculating the effect of the moon on the spinning inner core, Buffett discovered that the precession makes the slightly out-of-round inner core generate shear waves in the liquid outer core. These waves of molten iron and nickel move within a tight cone only 30 to 40 meters thick, interacting with the magnetic field to produce an electric current that heats the liquid. This serves to damp the precession of the rotation axis. The damping causes the precession to lag behind the moon as it orbits the earth. A measurement of the lag allowed Buffett to calculate the magnitude of the damping and thus of the magnetic field inside the outer core.

Buffett noted that the calculated field -- 25 Gauss -- is an average over the entire outer core. The field is expected to vary with position.

"I still find it remarkable that we can look to distant quasars to get insights into the deep interior of our planet," Buffett said.

The work was funded by the National Science Foundation.


Story Source:

The above story is based on materials provided by University of California - Berkeley. The original article was written by Robert Sanders, Media Relations. Note: Materials may be edited for content and length.


Journal Reference:

  1. Bruce A. Buffett. Tidal dissipation and the strength of the Earth’s internal magnetic field. Nature, 2010; 468 (7326): 952 DOI: 10.1038/nature09643

Cite This Page:

University of California - Berkeley. "First measurement of magnetic field in Earth's core." ScienceDaily. ScienceDaily, 19 December 2010. <www.sciencedaily.com/releases/2010/12/101216142541.htm>.
University of California - Berkeley. (2010, December 19). First measurement of magnetic field in Earth's core. ScienceDaily. Retrieved September 23, 2014 from www.sciencedaily.com/releases/2010/12/101216142541.htm
University of California - Berkeley. "First measurement of magnetic field in Earth's core." ScienceDaily. www.sciencedaily.com/releases/2010/12/101216142541.htm (accessed September 23, 2014).

Share This



More Earth & Climate News

Tuesday, September 23, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Obama: No Nation Gets Pass on Climate Change

Obama: No Nation Gets Pass on Climate Change

AP (Sep. 23, 2014) — In a forceful appeal for international cooperation on limiting carbon pollution, President Barack Obama warned world leaders at the UN Climate Summit on Tuesday that the globe's climate is changing faster than efforts to address it. (Sept. 23) Video provided by AP
Powered by NewsLook.com
Hundreds of Thousands Hit NYC Streets to Protest Climate Change

Hundreds of Thousands Hit NYC Streets to Protest Climate Change

AFP (Sep. 22, 2014) — Celebrities, political leaders and the masses rallied in New York and across the globe demanding urgent action on climate change, with organizers saying 600,000 people hit the streets. Duration: 01:19 Video provided by AFP
Powered by NewsLook.com
Raw: Protesters Stage Wall Street Climate Sit-in

Raw: Protesters Stage Wall Street Climate Sit-in

AP (Sep. 22, 2014) — A day after over 100,000 people marched against climate change, more than 1,000 activists blocked parts of Manhattan's financial district. Over 100 people, including a person wearing a white polar bear suit, were arrested Monday night. (Sept. 22) Video provided by AP
Powered by NewsLook.com
French FM Urges 'powerful' Response to Global Warming

French FM Urges 'powerful' Response to Global Warming

AFP (Sep. 22, 2014) — French Foreign Minister Laurent Fabius on Monday warned about the potential "catastrophe" if global warming was not dealt with in a "powerful" way. Duration: 01:08 Video provided by AFP
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
 
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:  

Breaking News:

Strange & Offbeat Stories

 

Plants & Animals

Earth & Climate

Fossils & Ruins

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:  

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile iPhone Android Web
Follow Facebook Twitter Google+
Subscribe RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins