nature.net
March 2008
Journey to the Core
By Robert Anderson
Eighteen hundred miles beneath our feet, the outer core, a Mars-size sphere of churning molten iron, generates Earth’s magnetic field. Exactly how it works is only now yielding to sophisticated computer models. One question we’d like the answer to is why the magnetic poles wander and periodically switch, with magnetic north becoming magnetic south. That event that happens on average every 250,000 years, and we’re long overdue: the last geomagnetic reversal was 800,000 years ago.
The earliest written record of magnetic compasses comes from China and dates back nearly a thousand years. Ever since the discovery that a small, floating magnet could point the way, the mysteries of Earth’s magnetic field have attracted some of humanities greatest thinkers. At Trinity College in Dublin you can find a synopsis of the progress in Magnetism through the Ages. William Gilbert, personal physician to England’s Elizabeth the First, was the first to recognize that the Earth behaves much like a giant bar magnet, roughly aligned with the poles. David P. Stern, a retired physicist from the Lab of Particles and Fields at the Goddard Space Flight Center in Greenbelt Maryland, has created a Web page The Great Magnet, the Earth to honor Gilbert, a man who helped usher in the modern era of scientific experimentation, with the his classic work “De Magnete,” published in 1600. Scroll down to the index of the site and you’ll find a chronology of the evolving theory about our planet’s magnetic field, including a synopsis of Gilbert’s work and its impact. Another great site to get the basics on geomagnetism is at the British Geological Survey. Scroll down to “Education and General Interest” to find a list of topics, including an overview on the science and monitoring of our planet’s magnetism.
At the United States Geological Survey’s National Geomagnetism Program there are plenty of data on the evolving magnetic field for those who depend on the information, but of most interest to the layperson are the movies showing how the field has shifted over the centuries. From the menu to the left, select “Movies” and then “Declination” and you’ll find a visual record that dates back 1590, a remarkably old data set first collected by early seafaring navigators.
After viewing the movies at the site above, it’s clear that over time, compass readings are always changing. Another clear indicator is the wandering magnetic North Pole. Go to the Geologic Survey of Canada’s Geomagnetism page and click on “North Magnetic Pole.” In addition to the early history of expeditions to find it, you’ll learn that it has migrated some more than 680 miles in the time since its discovery (click on “Long term movement” to see the path it is taking). Its position can vary considerably from short-term magnetic storms, and its daily excursions often trace fifty-mile oval paths about its current average position in the Arctic Sea, north of Canada. It’s headed across the pole to Siberia.
The most remarkable variation in Earth’s magnetic field is its propensity to reverse poles. Some 400 reversals have been documented during the past 330 million years, and they probably go back several billion years to a time soon after the iron-nickel core formed. Click on “Magnetic reversals” in the Geologic Survey of Canada’s site given above for a succinct explanation of how the dynamo in the Earth’s liquid outer core that generates the field can become unstable and flip polarity. David P. Stern’s site mentioned above also gives the basics. In a nutshell: the heat in the Earth’s interior drives convection currents in the liquid metal of the outer core, which generate powerful electric currents, which in turn sustain the planet’s magnetic field.
The Earth’s spin and the influence of its solid iron core, spinning at a slightly faster rate, complicate an already difficult computer-modeling problem. (See the confirmation of the out-of-sync inner core spin at this Lamont-Doherty Earth Observatory page.)
Nevertheless, the virtual core is already a reality. In 1995, geophysicists, Gary Glatzmaier, now at the University of California in Santa Cruz, and Paul Roberts, at the University of California, Los Angeles, had created the first successful model of a self-sustaining magnetic field—one that even flipped polarity spontaneously after a 36,000-year run. Go to the PBS NOVA program site “Magnetic Storm” (http://www.pbs.org/wgbh/nova/magnetic/about.html) and click “See a Reversal” in the box to the right. There you will find slideshow with movies that show the model’s evolving magnetic field as it switches. At the Pittsburgh Supercomputing Center page When North Goes South, Glatzmaier and Roberts’s model is displayed in all its complexity, complete with 3D animations of twisting magnetic field lines.
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The core of the planet, shrouded by the mantle and crust, has given up its secrets slowly for the simple reason that it cannot be studied directly. But what if we could go to the core? Before moving on to magnetic fields on other planets, I recommend deep-cave explorer Bill Stone’s Technology, Entertainment, Design (TED) lecture entitled “Journey to the Center of the Earth” in which he proposes to do just that. Make of his proposal what you will, but it certainly gets you thinking about the difficulties.
Other planets—and stars—generate magnetic fields, but not all. For a quick comparison of magnetic fields throughout the Solar System return to David P. Stern’s site. For a somewhat technical look at the work being done by Glatzmaier and his collaborators on the fields emanating from Saturn and the Sun go to their page at the University of California Santa Cruz. Magnetic fields turn out to be key to understanding the interiors of all worlds, not just our own.
Copyright © Natural History Magazine, Inc., 2008