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Planetary Dynamo On The Desk
by Terry Devitt
Madison - October 20, 1999 - For more than 100 years, scientists have been trying to tease out the secrets of the natural, magnetic field-generating dynamos that exist in the Earth and virtually all other celestial objects from stars to galaxies.
But the phenomenon, which occurs deep within the Earth at its outer core of molten iron, or at vast distances from our planet, has proved inaccessible, a mystery shielded by miles of rock or distances often measured in light years.
Caption: Physics professor Cary Forest shows off a large stainless steel sphere used for the Madison Dynamo Experiment, which is expected to demonstrate how molten iron combines with the planet's rotational energy to generate the self-perpetuating magnetic field responsible for effects that range from spinning compass needles to shielding our planet from dangerous cosmic rays. Photo by Jeff Miller Now, however, scientists may soon get the opportunity to study the phenomenon firsthand. The finishing touches are being applied to a university experiment that will attempt to recreate -- in a 1-meter-wide stainless steel sphere -- the same conditions that give rise to the self-perpetuating magnetic fields seen in nature.
"This is something that hasn't been done before," says Cary Forest, a physics assistant professor.
Forest leads one of several groups from around the world racing to be the first to mimic a well-known, but poorly understood, feature seen in the Earth and other rotating objects such as stars, galaxies and planets.
"You can't measure it in the Earth. You can only look at it from far away," says Forest.
At the heart of the Madison Dynamo Experiment is perhaps the world's most unusual blender, a large stainless steel sphere fitted with two opposing propellers.
By filling the sphere with 200 gallons of molten sodium and spinning the propellers in opposite directions, Forest hopes to recreate the same kinds of flows that exist at the Earth's outer core.
Caption: The sphere is fitted with two opposing propellers. By filling the sphere with 200 gallons of molten sodium and spinning the propellers in opposite directions, Forest hopes to recreate the same kinds of flows that exist at the Earth's outer core. Photo - Jeff Miller There, molten iron combines with the planet's rotational energy to generate the self- perpetuating magnetic field responsible for effects that range from spinning compass needles to shielding our planet from dangerous cosmic rays.
"The most successful outcome would be that we spin it and a magnetic field spontaneously grows," Forest says.
Such an outcome would permit scientists access to the fine details of a natural system that cannot be directly observed. In recent years, elaborate high-speed computer models have been developed to simulate the flow of the molten fluids at the Earth's core, but to date there is virtually no experimental system to observe or manipulate.
"There are unknown issues, questions that can only be answered experimentally," Forest says. "Just observing a magnetic field won't tell us much. It's the details that are important, and with this experiment we can turn the knobs and see what happens."
For example, the Earth has an observable magnetic field, a dipole, that flows into the South Pole and out of the North Pole. But inside the Earth, some scientists believe there is a yet-to-be discovered toroidal or donut- shaped magnetic field that goes from east to west. Seeing a similar phenomenon in the laboratory, according to Forest, would help confirm this picture of what is thought to be happening at the center of the Earth.
These intricate, hidden details of naturally occurring dynamos, says Forest, have relevance to a raft of fields including geophysics, solar physics, plasma physics and astrophysics.
"This will be a working model that has relevance to all of those things," he says of the experiment.
According to Forest, physicists would like to answer the fundamental question of how these magnetic fields are generated in the first place.
In principle, nature's electromagnetic dynamos consist of a source of free energy that drives a motion. For example, at the outer core of the Earth, the flow of molten iron creates an electric field which, in turn, generates the planet's magnetic field.
"In the Earth, the source of free energy is heat from the core and the rotational energy of the planet," Forest says. The turbulent motion that's generated within the molten iron at the core of the Earth -- and the one that scientists hope to recreate with propellers in their stainless steel sphere -- is one that "self excites" or produces a spontaneous magnetic field, Forest says.
In nature, as in the laboratory, another magnetic field is required to jump- start an electric current and transform it into a self-perpetuating magnetic field. The Earth's magnetic field was helped along early in the history of our planet, scientists think, by a preexisting magnetic field in the solar system. In the Madison Dynamo Experiment, electric fields generated within the swirling molten sodium will get a kick-start from the Earth's small magnetic field.
Forest says the stainless steel sphere at the heart of the experiment will have no wires to direct current flow, and there will be no insulators or pipes within the sphere to control the flow of molten liquids.
"In principle, we're going to learn something about a very basic system," he says. "We can measure everything we need to and test theory to its extremes."
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