Magnets aren’t miracles, but neither are they a phenomenon that physicists completely understand. Particularly big magnets, like the sun. Until recently, the annals of research failed to completely explain how massive currents blooming on the sun’s surface burst into solar flares, releasing incredible volumes of energy in short time frames.
Peter Sweet was vexed by this problem when, in 1956, the English physicist traveled to Stockholm for a meeting of the International Astronomical Union. He presented a partial solution: When two magnetic fields meet, a current sheet forms between them, and plasma (fiery blobs of energy) erupts at the seam. An American physicist named Eugene Parker saw Sweet’s presentation, and worked out the math on his flight back to the states. For fifty years, their Sweet-Parker model has been crucial for explaining not just solar flares, but other large-scale magnetic activity, like Earth’s aurora.
However, Sweet-Parker is too slow. Under that model, solar flares would take weeks to burst. “Imagine you have many persons in a room, but just one door to exit,” says Luca Comisso, a heliophysicist—sun scientist—at Princeton University. “The rate at which they can leave is fixed, so it takes a long time for them all to leave.” But solar flares discharge their energy in minutes. The problem is Sweet-Parker assumes magnetic fields remain stable when they meet. Like sophisticated guests at a society ball, the accumulated quanta of energy would exit the current sheet in orderly fashion.
Comisso says it’s not that kind of party. Magnetic field behave more like fraternity ragers being busted by the cops: People crawling out windows, leapfrogging through doors, busting down walls to escape. He and some co-authors recently published an alternative theory, on the open physics exchange arXiv. “Current sheets are not stable in time, they evolve, get narrow, become more intense,” says Comisso. This dynamic activity causes the huge, burning plasmas carried by the current sheets to intensify. “Plasmoids are like small blobs in this current sheet that grow until they break,” he says. “At a certain point they become big enough to burst, and destroy their current sheet and you have an explosion of current energy.”
Comisso and his co-authors built on 10 years of research by themselves and others on plasmoid instability to develop their mathematical solution. The theory calculates a given plasmoid’s size, and the size it would need to be in order to destroy its current sheet. “We can characterize the properties of plasmoid instability, and identify which blob of plasmoids will become big first,” he says. Developed more fully, their theory could become a the basis for things like early warning systems for the satellite-wrecking waves of energy emanating from bursted solar flares.
Nuclear physicists working on fusion energy might find the theory useful, as well. A tokamak is a type of fusion reactor that uses electromagnetic coils to control donut-shaped plasmas of energy. But heating the plasma to fusion-hot temperatures—about 10 times hotter than the center of the sun—is complicated. Because just like on the sun’s surface, the current sheets between magnetic fields in the tokamak want to burst. This releases energy, lowering the temperature, making safe, stable fusion impossible. But, if scientists can predict when and where plasmoids will burst, they can use some external force, like radiofrequency waves, to keep the current sheet stable. And if they figure all that out? Well, talk about a miracle.
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