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Dynamo in a box
Naomi Lubick

Dan Lathrop is building a planet in his lab. He custom-ordered a 3-meter-tall metal sphere, which will perch inside a metal box built in his brick-walled lab at the University of Maryland in College Park. He and his co-workers will drop 15,000 kilograms of sodium metal pellets inside the sphere — that’s 15 metric tons of a substance that can burst into flame when it contacts oxygen and explode when water touches it. Lathrop will heat the sodium (which will be fully enclosed and protected in its metal casing) and, with a mechanical kick, send the sphere spinning around its liquefied metal core.

If everything goes as planned, the sphere will self-start its very own dynamo, a process where heat and motion establish a magnetic field.

Dan Lathrop stands on the base of a stainless steel box, in its partially completed state last summer, that was built to hold a 3-meter sphere (upper left) mimicking a hot planet with convection. Lathrop and others hope the experiment will establish its own geodynamo. Sphere image courtesy of Dan Meade, Central Fabricators (Ohio); background photo by Naomi Lubick.

For about a decade, geoscientists and physicists around the world have been trying to replicate Earth’s dynamo in the lab, to understand how the dynamo started and how it has kept going at about the same strength for several billion years, and what that means for its magnetic field. Understanding Earth’s dynamo is also important for understanding the universe, says Jonathan Aurnou, a planetary physicist at the University of California, Los Angeles.

Space exploration has revealed that more and more celestial bodies have dynamos — including other planets in our solar system, our sun and other stars, and even other galaxies. Dynamo-generated magnetic fields shield planets against cosmic radiation, and “magnetic fields appear to be an important player for the global evolution of a habitable zone,” Aurnou says. Mars, for example, seems to have lost its magnetic field at an early age, about 4 billion years ago. “It may well be that once it lost its field, the solar wind was able to strip away its atmosphere,” he says. “Scientists are still trying to get a handle on how this works.”

An example of a simpler dynamo is a hydroelectric dam, where the energy of falling water is converted to an alternating current. Another simple example is a bicycle light, Aurnou says. The mechanical energy of pedaling transforms into electrical energy (with the help of magnets and wire) that runs the light. The key for a dynamo is that it also acts back on the system that gives it power to begin with: If the light were large enough, Aurnou says, “you would feel it in your legs,” as it sucks energy away from the pedaling (or mechanical energy) moving the bike.

A geodynamo in a planet is similar, but without the wires. A hot, convecting planetary center made of conductive metals drags electrically charged atoms in organized bundles following flowing currents. The resulting magnetic field in turn influences the convection patterns, partially trapping heat inside the sphere and stabilizing itself.

The upcoming experiment in Lathrop’s lab, with sodium metal standing in for Earth’s molten iron core, is the apex of their three previous and successively larger experiments. Moving from 15 to 110 kilograms of sodium and increasing diameters, Lathrop’s team has paved the way for the 3-meter experiment, the largest yet of any group’s research effort.

Results from the lab’s most recent rotating convection experiments, Lathrop says, have indicated how much a magnetic field might control the amount of heat from a hot core that leaves a planet during convection. A fairly weak field could reduce the escaping heat by 5 to 10 percent, by slowing the flow inside. “When the dynamo in Earth started early in the planet’s history, that might have had the ability to change how much heat could escape,” he says, affecting the development of the planet and its core.

Because so many other bodies in the solar system — and most likely elsewhere — have dynamos, there must be “a lot of different ways to self-generate one,” Lathrop hypothesizes. “Even one of the moons of Jupiter has a dynamo,” he says, but it’s more difficult to replicate in the lab than in nature. “We’re trying to find out what are the key ingredients,” he says.

If Lathrop has the right recipe, the sodium sphere’s dynamo should start up in less than a minute — much faster than the 10,000 years it took for Earth’s because of its size. But whether or not the largest sphere starts a self-sustained dynamo, the experiment will still be useful in informing the models.

“We can simulate magnetic field generation in our models, but we don’t have yet enough computer power to simulate turbulence,” says Gary Glatzmaier, a geophysicist at the University of California, Santa Cruz, who specializes in modeling. “These experiments, if they’re driven hard enough, will have turbulence,” he continues. Even if Lathrop’s magnetic field does not maintain itself, researchers will be able to watch it “decay away” in a turbulent regime, Glatzmaier says. “There are certain representations we need in our models to represent how effective turbulence would be; a lot of this information could come from these experiments.”

Glatzmaier says that researchers planned the first dynamo experiments about a decade ago, with their first results emerging about five years ago. Past experiments that successfully created dynamos, such as one in Karlsruhe, Germany, have used helical pipe fittings to set up the “wiring” inside a sphere, but the sodium inside Lathrop’s 3-meter version will be freely flowing. If it works, this freedom may enable Lathrop’s team to address open questions that the constrained experiments could not: What happens after a dynamo is established, and what causes reversals? Lathrop says the experiment will allow the researchers to see changes in flow and how the relationship between the flow and the dynamo evolves.

A French team has a cylindrical free-flowing experiment, but the largest free-flowing experiment operating at the moment is in Cary Forest’s lab at the University of Wisconsin, Madison. Forest uses a propeller to maintain movement in his experimental sphere, which is similar to Lathrop’s planned dynamo, except that it is stationary and smaller. At 1 meter in diameter and containing 500 kilograms of sodium, the experiment is large enough that it should “self-excite,” or start its own dynamo, says Forest, who hopes to get a self-sustaining dynamo by the beginning of next year.

However, even though self-excitation is an important part of the experiment, Forest says, “there are a whole lot of questions that you can address” without it. The rotational twist on Lathrop’s sphere “makes it very interesting,” he says. “I’m really looking forward to seeing data from it.”

Lathrop has “a good chance of generating a dynamo,” Aurnou says. “Mechanically stirring an open volume of fluid has been done here and in Europe, but nobody’s gotten sustained dynamo action.” Adding rotation to the mix, as in a planet, may be the key to lab success, he says, but even if it does not work, Lathrop has still “pushed the field forward a big step.”

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