Why are the loops in the sun’s atmosphere so neat and tidy?

Movies taken during the eclipse may set the record straight on the corona’s structure

solar loop structures

A TANGLED SKEIN  Splendid loops in the corona protrude from the sun’s surface, seen in this 2014 ultraviolet image from NASA’s Solar Dynamics Observatory. They are anchored in the sun’s messy magnetic field — so it’s a mystery how they stay so smooth.

NASA/Solar Dynamics Observatory

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When the Aug. 21 solar eclipse unveils the sun’s normally dim atmosphere, the corona will look like an intricate, orderly network of loops, fans and streamers. These features trace the corona’s magnetic field, which guides coronal plasma to take on the shape of tubes and sheets.

These wispy coronal structures arise from the magnetic field on the sun’s visible surface, or its photosphere. Unlike the corona, the photosphere’s magnetism is a complete mess.

“It’s not a static surface like the ground, it’s more like an ocean,” says solar physicist Amir Caspi of the Southwest Research Institute in Boulder, Colo. “And not just an ocean. It’s like a boiling ocean.”

Because the corona’s loops and streamers all originate in the turbulent photosphere, their roots should get twisted and turned around.

“And yet these structures in the corona are not tangled and snarled and matted like kelp or seaweed in the ocean,” Caspi says. “They seem to still be these organized, smooth loops. Nobody understands why.”

To unknot the photosphere’s tangled mats, the corona must release some of the energy stored there, Caspi says. So during the eclipse, he and his colleagues will be looking for the release valves that set the corona free.

One possibility is that wave motion in the corona’s magnetic field lines helps untie the snarls. Magnetic waves in plasma, called Alfvén waves, are thought to ripple through the sun’s magnetic field lines like vibrations in a guitar string. Researchers have directly observed Alfvén waves in the lower corona, within about half a solar radius of the surface (SN: 4/11/09, p. 12), but not farther out where similar waves with higher amplitudes would travel. Those close-in waves were too weak to explain the corona’s features, but perhaps more distant waves could shake things up enough.

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REINING IN Magnetic structures keep coronal loops in check, as shown here in extreme ultraviolet light observed by NASA’s Solar Dynamics Observatory just before a coronal mass ejection. A. Vourlidas/SDO/NASA

Another option is that little hypothetical spurts of magnetic energy could help release the tangles. These nanoflares and nanojets would be like solar flares but with a billionth of the energy. By going off all the time, nanoflares and nanojets could collectively release enough energy to give the corona some structure, simulations have shown.

“Both are symptoms of tiny rearrangements of the magnetic field — magnetic reconnection,” says solar physicist Craig DeForest, also at the Southwest Research Institute. Solar flares and bigger outbursts called coronal mass ejections are also signs of magnetic reconnection, but they’re not frequent enough to account for the corona’s smoothness. “Nanojets and/or nanoflares in the middle corona would be a smoking gun that would explain why the corona is so organized,” DeForest says.

No one has actually seen any nanoflares or nanojets. Theories suggest that they’re too small and quick to see individually — but they should be visible as a cacophony of little pops when the solar eclipse reveals the lower corona.

The shaking from Alfvén waves and the flickers of nanoflares could not only loosen up the tangled skein of magnetism, but also transfer heat high up into the corona. Caspi, DeForest and their colleagues hope to see both effects on August 21, when they fly a pair of telescopes on twin NASA WB-57 high-altitude research jets along the path of the eclipse (SN Online: 8/14/17).

“We’re taking high-speed movies of the sun and analyzing them for things that look like waves,” Caspi says. “We’re just overall looking at the structure of the corona.”

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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