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October 24, 2000

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Unlocking Secrets of Magnetic Fields' Power


Laura Pedrick for The New York Times
Dr. Masayuki Ono, left, and Dr. Rob Goldston of the Princeton Plasma Physics Laboratory inspect the National Spherical Torus Experiment device.

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On Earth, magnetic fields generally make for a dull show. Yes, compass needles point along the magnetic field lines sprouting from Earth's poles, and iron filings on a piece of paper trace the same kind of field lines when they are produced by a bar magnet. Those magnetic fields, however, just exist without doing much, and they have little influence on the surrounding atmosphere.

But in the high chaparral of space, magnetic fields become squirming, snapping, dynamic entities that drive huge storms, heat the atmosphere of the Sun to millions of degrees, shelter the Earth from showers of energetic particles and help sculpt the observable landscape of the cosmos. The turbulent, electrically charged gases of space, called plasmas, stick to magnetic fields and ride them like cowboys on a bronco.

When the fields use their energy to raise the temperature of the plasmas or fling them at nearly the speed of light, "it's like having plastic explosive in space," said Dr. James Drake, a physicist at the University of Maryland. "You're releasing that energy in an explosive manner."

The question that scientists have been trying to answer for decades is exactly how the magnetic field of the Sun releases its energy so suddenly to create a solar flare or an eruption of gases into space. The favorite contender has been a poorly understood process called reconnection, in which taut field lines brush together and, in effect, cut each other and then whip around, like snapped rubber bands, in a new configuration.

Unfortunately, reconnection research has been stymied for decades over the physics of the process and how fast it can occur. But this week, at a meeting of the American Physical Society's division of plasma physics in Quebec City, Quebec, Dr. Drake is describing research that he and his collaborators say shows not only that reconnection can take place quickly, but also that it is a simpler and more universal phenomenon than suspected.

"This really is a tremendous step," said Dr. Spiro Antiochos, an astrophysicist at the Naval Research Laboratory, "and the real breakthrough there is that he's done it without any a priori assumptions."

While earlier theorists simplified some complex realities of space, Dr. Antiochos said, the new work starts from first principles.

The push to understand reconnection is not limited to the theorists. Recent satellite measurements have probed magnetic reconnection in space, and a few Earth-bound experiments have made progress as well, with an eye toward applications like a reactor that harnesses thermonuclear energy the way the Sun does.

At the Princeton Plasma Physics Laboratory, the Magnetic Reconnection Experiment is an effort to shed light on the basic physics of reconnection, while the National Spherical Torus Experiment in the same laboratory is exploring the possibility that the phenomenon could be used to help confine hot plasmas that might someday produce large amounts of energy from thermonuclear fusion.

Scientists working on both Princeton experiments will also give talks in Quebec City. Magnetic reconnection "happens all over the universe," said Dr. Rob Goldston, director of the Princeton Plasma Physics Laboratory. "We're interested in these plasmas because they're scientifically interesting. But at the same time, they have some potential to make fusion more practical."

Magnetic fields are produced by electrical currents, either at the atomic level (as in bar magnets) or by the motion of charged particles like electrons and ions in wires or Earth's core. Once a magnetic field has been created, electrically charged particles tend to spiral around its field lines and slide along them. So charged particles from space get funneled toward Earth's poles, where field lines sprout like cowlicks. The particles crash into the atmosphere there, creating the northern and southern lights.

Near Earth's surface, the atoms of oxygen and nitrogen contain an equal number of electrons and protons and have no net charge. So they flow through Earth's magnetic field without hindrance. In the tenuous gases of space, however, the atoms (mostly hydrogen, with traces of other elements) the electrons and proton-heavy nuclei generally become separated, creating a plasma that cannot ignore a magnetic field.

Plasmas tend to stick to magnetic field lines, causing an eternal tug of war: relatively strong magnetic fields can hold plasmas in place, while dense plasmas can shove magnetic fields around. Far above the atmosphere in Earth's "magnetosphere," in deep space and in the magnetized plasmas of the Sun, the two entities are tightly joined.

"What happens on the Sun and in interplanetary space and the Earth's magnetosphere is that the magnetic field has a physical identity," Dr. Antiochos said. "You could actually think of field lines like real, physical wires. They move with the gas."

Those jumping field lines thread galaxies, rise in vast arches above the Sun and drift through space on the solar wind. When they near Earth, they meet the outer reaches of its magnetic field in the magnetosphere, which forms a sort of protective cocoon that is swept into a long teardrop shape by the wind.

As malleable as the fields are, they face one great barrier to changing their configurations in response to plasma flows: magnetic field lines have no ends. They exist only as loops, so physicists have long suspected that major changes involve reconnection, or a kind of cutting and pasting that allows one set of field lines to splice into adjacent ones.

Because field lines, like rubber bands, have tension, the reconnection should in some cases cause them to snap back, flinging plasma around like slingshots and perhaps explaining impulsive phenomena like solar flares. Two sets of field lines would always snap in opposite directions, producing two counter plasma jets.

"The plasma releases its energy very much like a traditional explosion, except that it's not isotropic," or the same in all directions, Dr. Drake said.

The problem that physicists have had since they began studying reconnection in the 1950's comes in understanding how it happens so fast. Theoretical calculations have generally predicted that flares should take days or even years to go off, rather than minutes, as observed.

Just as there is a limit to the speed of sound waves in Earth's atmosphere, called "mach 1," there seemed to be a limit on the speed of field lines during the snapback, limiting the speed of reconnection.

But Dr. Drake, using computer computations and working with Dr. Amitava Bhattacharjee of the University of Iowa, Dr. Michael Hesse of the NASA Goddard Space Flight Center in Greenbelt, Md., and others, found that the rebound's speed may actually depend on the size of the reconnecting region. The smaller the region, the faster it snaps back.

Theorists think that the splicing takes place on very tiny scales. Therefore, the high speeds would allow the field lines to get out of the way quickly. "There has been about 20 to 30 years of controversy," Dr. Bhattacharjee said, referring to calculations that predicted the slow pace. Now, he said, the theoretical predictions are beginning to match what is seen in nature.

The group "has really got a breakthrough there," said Dr. Eric Priest, an applied mathematician and solar physicist at the University of St. Andrews in Scotland.

The work, which will also be presented in The Journal of Geophysical Research, comes as other researchers are making strides in understanding reconnection in complicated field geometries in three-dimensional space, Dr. Priest said.

The observers have been doing their part, too. Dr. Marit Oieroset and Dr. Tai Phan of the University of California, Berkeley, and their co- workers have collected evidence that spacecraft have drifted through reconnection regions where Earth's magnetosphere meets the solar wind observing the counterstreaming plasma jets set loose by the violent reconnection process, for example.

On the ground, the doughnut- shaped National Spherical Torus Experiment, also at Princeton, is an effort to put reconnection to work to confine a hot plasma. The device, which builds on related experiments at the University of Washington in Seattle, generates strong magnetic fields and then gets them to reconnect in such a way that they wrap around the plasma without touching the cold walls of the vessel.

In a plasma composed of fuel capable of undergoing thermonuclear fusion, that research could lead to more economical reactor designs. Dr. Masayuki Ono, the project director, compared generating reconnection in the device to "a giant spark plug" to start a hot plasma.

Dr. Ono said that as much as scientists would like to understand reconnection fully, the process could become useful even before they learn all there is to know. "People learned how to use spark plugs in the internal combustion engine before they understood it," he said.

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