Some of the sun’s iconic coronal loops may be ghostly illusions

Wrinkles in the sun’s outer atmosphere might trick the eye into seeing glowing arches

Some of the sun’s coronal loops (bright yellowy structures in this image) might be optical illusions caused by “wrinkles” in the solar atmosphere, a new study suggests.

Solar Dynamics Observatory/GSFC/NASA

Hot strands of plasma arch out from the sun’s surface. These iconic features are known as coronal loops. But many of the loops we see might not actually be there, scientists now report.  

Some, they say, might be an illusion created by dense “wrinkles” in a curtain of plasma known as the coronal veil. They base this idea on unexpected structures that emerged in computer models. Those computer programs were designed to simulate the sun’s atmosphere — including its outermost region, the corona.

Researchers proposed these phantom loops March 2 in The Astrophysical Journal. If true, these ghostly loops may change how scientists go about measuring some properties of our star.

“It’s kind of inspiring to see these detailed structures,” says Markus Aschwanden. These loops, he says, “are so different than what we anticipated.” Aschwanden was not involved in the study. This astrophysicist works at Lockheed Martin’s Solar & Astrophysics Lab in Palo Alto, Calif.

Loopy science

Only in the past few years have cientists begun to develop a better understanding of the sun’s complex corona. Its loops have been used to measure many of the corona’s traits, including its temperature and density. In fact, coronal loops may be key to figuring out why the sun’s atmosphere is so much hotter than its surface. Astronomers also have puzzled over why the loops appear to be so orderly when they rise from the sun’s turbulent surface.     

Solar physicist Anna Malanushenko works at the National Center for Atmospheric Research in Boulder, Colo. She’s part of a team that tried to isolate individual coronal loops in 3-D computer simulations. That computer program had been developed to model the life cycle of a solar flare — powerful magnetic outbursts that shoot bright spurts of radiation into space. Coronal loops appear to align themselves to the sun’s magnetic field, like metal shavings around a bar magnet. So the researchers expected to see neatly oriented strands of plasma.

But they didn’t. The plasma instead formed a curtainlike structure winding out from the sun’s surface. Parts of it folded in on itself like a wrinkled sheet. The computer model suggested that many of the supposed coronal loops weren’t real. Plasma structures formed along the magnetic fields. But these were neither thin nor as compact as had been expected. They more closely resembled clouds of smoke.

The team changed the point of view by which the computer visualized these wrinkles in the modeled veil. This changed the plasma’s shape. From certain viewing angles, what had been wrinkles now instead looked like coronal loops.

The observations were mind-blowing, says Malanushenko. “The traditional thought was that if we see this arching coronal loop that there is a garden hose–like strand of plasma.” But the computer analysis now suggests the plasma’s structure is much more complex. It has complicated boundaries and a ragged structure.

Still, not all coronal loops are necessarily ghosts within a coronal veil. “We don’t know which ones are real and which ones are not,” Malanushenko says. “And we absolutely need to,” she says, to understand the sun’s atmosphere.

It’s also not clear how the apparent coronal veil might impact previous analyses of the sun’s atmosphere. “On one hand, this is depressing,” Malanushenko says. Why? Her team’s new findings cast doubt on what solar scientists had thought they understood. On the other hand, she finds this new uncertainty exciting. Astronomers will need to develop a way to observe the veil and confirm its existence, she says. And, she adds, “Whenever we develop new methods, we open the door for new knowledge.”

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