Shredding plasma sheets could solve solar flare puzzle
A new theory may explain how stars release powerful, unstable bursts of energy. Richard A. Lovett reports.
Giant sheets of plasma bursting into pieces may help produce high-energy magnetic reconnections that set off the enormous explosions that create solar flares and other powerful astrophysical events, scientists say.
Magnetic reconnections occur when magnetic field lines, trapped in hot, ionized gases, break and rejoin, thereby releasing a great deal of magnetic energy, says Luca Comisso, a physicist at Columbia University, New York.
These energy releases, he says, begin when tight sheets of unstable plasma act as transmission routes for large electrical currents. In a solar flare, for example, the plasma sheet might be 10,000 kilometers long, but only 100 meters thick.
There’s just one problem. Such a conducting pathway might sound like an electrical superhighway by earthly standards, but given the amounts of energy involved in astrophysical events, it’s still too small to produce the rapid outbursts known to occur. “We see a lot of events like solar flares, gamma ray bursts, and [emissions] close to black holes that involve a sudden emission of magnetic energy, very, very fast,” Comisso says.
In a pair of papers published this year and last, however, Comisso and colleagues believe they have found the answer: the plasma in these sheets is unstable. “These sheets break. They do not stay stable,” Comisso says. “A sheet is a very narrow layer where a lot of current is localized. It is hard to imagine that such structures will remain stable.”
He compares it to stirring milk into coffee to form a latte. If you do it carefully, the milk initially forms sheets that produce beautiful spiral patterns. Bur then the sheets break and the milk diffuses into the coffee.
In the case of plasmas, the sheets break into blobs, each of which can then create a new magnetic reconnection pathway. “The sheet is one reconnection site. If you break it into many pieces, it’s multiplying the number of points where the field lines can change their topology,” Comisso says.
Understanding this process, he adds, is important because it may someday help us predict dangerous solar storms, which can damage satellites, shut down communications, and threaten the Earth’s power grid.
It’s also interesting to astrophysicists, he says, because similar processes may be at work in producing spikes in high-energy jets emerging from the giant black holes that appear to lie at the heart of most galaxies, including our own.
Another application is in the development of fusion reactors called tokamaks, doughnut-like devices in which magnetic fields are used to contain high-energy plasmas in which fusion occurs. By understanding what makes plasma fields become unstable, it may be possible either to design more stable fusion reactors or to create control systems that rapidly reconfigure the system to damp out any instabilities that do emerge.
“But to do this, you need to know what is going on,” Comisso says. “This requires knowing the physics behind the instabilities.”
Scott McIntosh, director of the High Altitude Observatory at the National Center for Atmospheric Research in Boulder, Colorado, hails Comisso’s work as “solid, fundamental basic research”.
And he says, it’s testable, either with existing instruments or the upcoming Daniel K. Inouye Solar Telescope (DKIST), currently under construction on a 3084-meter peak in Hawaii. Both of these, but especially the DKIST, McIntosh says, should have the resolution either to confirm or refute Comisso’s hypothesis via detailed solar measurements.
The new theory, McIntosh adds, may also aid our understanding of the solar corona and the solar wind. It may also help us understand the formation of solar jets called spicules, which are abundant in the lower reaches of the sun’s atmosphere.
But it may be a while, McIntosh says, before we can fold the new theory into predictions of space weather. The problem is that in major solar events, a lot of things appear to happen simultaneously. “In some ways you’re looking at the individual straws,” he says, referring to the proverb about the straw that broke the camel’s back, “and trying to see if one goes unstable to bring down the whole camel.”