10 Jan Earth's core modeled in underground experiment
In an underground bunker that brushes up against a Wisconsin barnyard on one side, and a cornfield on the other, a team of scientists, using a molten ball of metal, is replicating the same magnetic field-generating dynamo that exists at the core of the Earth.
For a century, scientists have sought ways to explore the naturally occurring dynamos that exist at the center of the Earth, and other celestial bodies such as stars, galaxies and planets beyond our own. But it is impossible, according to Cary Forest, a University of Wisconsin-Madison physics professor, to stick a probe into the molten ball of iron at the center of the Earth, or into a distant star, to make the kind of direct measurements scientists need to explain these natural phenomena.
The alternative, he said, is to make your own.
And that is what Forest and his colleagues have done. With substantial help from the National Science Foundation, the Wisconsin physicist has constructed a device that holds a cubic meter of molten sodium and, under experimental conditions, creates the current and transfer of energy necessary to generate a magnetic field in the same manner that the Earth makes its magnetic field.
“The big picture is that in the universe there are all these objects that have their own magnetic fields,” said Forest, who with a team of graduate students and engineers has spent five years building his device, known as the Madison Dynamo Experiment. “Here we have constructed the simplest version of how those systems work.”
The Wisconsin project is one of half a dozen worldwide seeking to be the first to unlock the mysteries of the well-known, but poorly understood, phenomenon seen in the Earth and other rotating objects, including planets, stars and even entire galaxies. It is the largest working version in its class.
Like the others, the Madison Dynamo Experiment seeks to answer some very basic questions, Forest said: “How fast do the naturally occurring magnetic fields grow? When do they stop growing? What causes them to stop growing? That’s the big one. These are really, really fundamental questions that theory doesn’t address.”
Having an experimental system that can be manipulated, Forest explained, will help scientists tease out the subtleties of a system that now can only be observed and modeled. “It’s the details that are important, and with the Madison Dynamo Experiment, we can turn the knobs and see what happens,” he said.
Such manipulations may help scientists resolve outstanding issues. For example, the Earth has an observable magnetic field, a dipole that flows into the South Pole and out of the North Pole. But some scientists think there is a hidden toroidal or donut-shaped magnetic field inside the Earth that goes from east to west.
With the Madison Dynamo Experiment operational and generating data, the secrets of how those natural dynamos perform will begin to emerge and the limits of current theory can begin to be tested, Forest said.
Inside the steel sphere at the heart of the Madison Dynamo Experiment, two opposing propellers stir 2,235 pounds of molten sodium, a dull silver metal that, at room temperature, has the consistency of soft cheese.
“When it liquefies, it has the same properties as water,” Forest said.
Powered by two 100-horsepower electric motors, the propellers are used to stir the molten sodium in ways that elicit specified motions. The idea, Forest said, is to approximate the flows that exist at the Earth’s outer core. There, molten iron combines with our planet’s rotational energy to generate the self-perpetuating magnetic field that causes compasses to spin and shields the Earth from harmful cosmic rays.
“We’d like to see this big ball of molten metal flowing in a certain way to produce a magnetic field,” Forest said. “At the core of the Earth, it is thought that there are lots of little flows and swirls occurring that contribute to the generation of the planet’s magnetic field. In nature, the system works very well.”