Some Iconic Coronal Loops of the Sun May Be Illusions

The coronal loop, a clear plasma hot filament that bulges into the Sun’s atmosphere, is an iconic image of the Sun. But most of the coronal loops we are supposed to see may not exist at all. Some coronal loops may be illusions created by denser “wrinkles” in the plasma curtain, dubbed coronal loops, researchers speculated on March 2 in The Astrophysical Journal. If true, the discovery, caused by an unexpected plasma structure seen in computer simulations of the solar atmosphere, could change the way scientists measure some of the properties of our stars.

“Looking at this detailed structure is very inspiring,” said Markus Aschwanden, an astrophysicist at Lockheed Martin’s Solar and Astrophysics Laboratory in Palo Alto, California, who was not involved in the study. “They were very different than we expected.”

Scientists have begun to better understand the complex atmosphere of the Sun, or corona, only in recent years (SN: 12/19/17). Coronal loops have been used to measure many coronal properties, including temperature and density, and may be key to understanding why the Sun’s atmosphere is much warmer than its surface (SN: 08/20/17). But astronomers have long wondered how the loop looks so orderly when it occurs on the turbulent surface of the Sun (SN: 08/17/17).

So solar physicist Anna Malanushenko and her colleagues tried to isolate individual coronal loops in a 3D computer simulation that was originally developed to mimic the life cycle of a solar flare. The team is expected to see neatly oriented plasma filaments because the coronal loops appear to be aligned with the Sun’s magnetic field, like a metal razor around a bar magnet.

Instead, the plasma appears as a curtain -like structure that writhes from the surface of the Sun, which folds over itself like a wrinkled leaf. In the simulation, most of the supposed coronal loops turn out not to be real objects. Although there is a structure along the magnetic field, it is not as thin and not as dense as expected.

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They look more like plumes of smoke. When the team changed the way they saw these veil wrinkles in the simulation, their shape and orientation changed. And from a certain point of view, wrinkles resemble coronal loops.

The sightings were spectacular, said Malanushenko of the National Center for Atmospheric Research in Boulder, Colorado. “Traditionally, it has been thought that if we look at this curved coronal loop, then this is a plasma jet, similar to a garden hose.” The structures in the simulations are more complex and display complex boundaries and blurred structures.

However, not all coronal loops are necessarily illusions in the coronal lid. “We don’t know which ones are right and which ones aren’t,” Malanushenko said. “And we really need to talk to study the solar atmosphere.”

It is also unclear how the proposed coronal veil could affect previous analyzes of the solar atmosphere. “On the one hand, it’s sad,” Malanushenko said of how the new decision casts doubt on previous perceptions. Instead, he finds the uncertainty interesting. Astronomers need to develop ways to observe the veil and confirm its existence. “Every time we develop a new method, we open the door to new knowledge.”