We’ve all seen the gorgeous images and videos of coronal loops. They’re curved magnetic forms that force brightly glowing plasma to travel along their path. They arch up above the Sun, sometimes for thousands of kilometres, before reconnecting with the Sun again.

But a new study says that some of what we’re seeing aren’t loops at all. Instead, they’re a type of optical illusion. Do we know the Sun as well as we think we do?

The Sun’s corona is the outermost layer of its atmosphere. It’s made of plasma, which contains lots of charged particles. That means that it readily responds to electromagnetic fields. The Sun has a powerful magnetic field that varies by place and time. Sometimes that magnetism drives the plasma high above the corona, forming fantastical structures called coronal loops that eventually reconnect to the Sun’s surface. Some of these structures can last for days or even for weeks.

“This is an entirely new paradigm of understanding the Sun’s atmosphere.”

Anna Malanushenko, lead author.

But if this new study is correct, then it means that many of what appear to be loops aren’t loops.

The new study is published in The Astrophysical Journal. Its title is “The Coronal Veil.” Anna Malanushenko, a scientist at the National Center for Atmospheric Research, is the lead author.

The study is based on an advanced 3D simulation of the Sun’s corona. It allowed scientists to isolate individual coronal loops by slicing the corona into distinct sections. The researchers found some loops, but their results showed that many of what looked like loops weren’t loops at all.

“I have spent my entire career studying coronal loops,” said lead author Malanushenko. “I was excited that this simulation would give me the opportunity to study them in more detail. I never expected this. When I saw the results, my mind exploded. This is an entirely new paradigm of understanding the Sun’s atmosphere.”

Coronal loops glow brightly in extreme UV radiation due to their temperature. Their shape conforms to our understanding of magnetism, so concluding that they’re loops is logical. The new study doesn’t wholly refute the existence of loops. But it does refute some of them.

This new research makes scientists take another look at the Sun and its behaviour. The authors of this paper are puzzled because the coronal loops don’t seem to conform to what they know about magnetism.

The Sun’s magnetic field lines are powerful, but they still have to weaken further from the source. So if coronal loops are indeed loops that flow along the Sun’s field lines, they should spread apart the further they get from the Sun. But that’s not what happens. Images of the Sun show that the loops are still thin and bright, even high above the Sun.

“… coronal loops appear to lack expected visual expansion with height, as the confining magnetic field weakens with altitude on average in the corona,” the authors point out in their paper.

So if they’re not loops, what are they?

According to the research, some of them are illusions that they’re calling the “coronal veil.” And their existence is making the authors question what they thought they knew about the Sun.

Coronal loops, observed in the ultraviolet radiation Fe IX 17.1 nm (171 Å) by the TRACE spacecraft on 6 November 1999, extending 120 000 km off the Sun's surface. (Credit: TRACE/NASA).
Coronal loops, observed in the ultraviolet radiation Fe IX 17.1 nm (171 Å) by the TRACE spacecraft on 6 November 1999, extending 120 000 km off the Sun’s surface. (Credit: TRACE/NASA).

“This study reminds us as scientists that we must always question our assumptions and that sometimes our intuition can work against us,” Malanushenko said.

Some of the loops are what they’re calling projection artifacts. But it’s not easy to discern between the actual coronal loops and artifacts. “We demonstrate the difficulty of discerning from observations whether a particular loop corresponds to a strand in the volume or a projection artifact. We demonstrate how apparently isolated loops could deceive observers, even when observations from multiple viewing angles are available,” the authors write.

This research is based on a ground-breaking simulation

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