Coronal Loops Might Not Be Loops At All

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).

“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 developed largely at NCAR/UCAR. The simulation is called MURaM. MURaM is a realistic simulation that extends from deep inside the Sun—about 10,000 km below the surface—up to almost 40,000 km into the corona. The physical conditions in that wide range vary widely in pressure and density. MURaM is significant because scientists hadn’t been able to model such a wide range of conditions before its development.

MURaM allowed scientists to observe the complete life cycle of a solar flare, starting from deep within the Sun, to the flare’s emergence on the surface, to its explosive release into space.

But the main result as far as this study is concerned is the data set created by MURaM. These data sets contain the magnetic field’s structure and the plasma that conforms to it. MURaM allows scientists to create observations of the Sun, including the corona, that are “artificial.” The Sun’s corona is optically thin, which means that it’s easy to see through. While that might sound like an observational advantage, that’s not necessarily the case.

That optical thinness means that structures like coronal loops can overlap one another when we observe the Sun. And it’s difficult to tell which might be in the foreground and which are in the background. The optical thinness also makes it difficult to discern how thick the loops are. Are they thick, kind of like a garden hose? Or are they thin, like a ribbon viewed on its edge?

There’s a third possibility when viewing these loops in the optically thin corona. Instead of a thin strand, loops could be an optical illusion caused by a fold in a sheet of plasma.

MURaM is one of the most powerful solar simulations ever created. It’s proven robust in many ways, but researchers are still cautious. In this paper, the team compared their MURaM data with actual images of the Sun from the Atmospheric Imaging Assembly (AIA) instrument on NASA’s Solar Dynamics Observatory (SDO.)

Images a to d, and f, are AIA images that highlight a particular part of a coronal loop. E is an image from the MURaM simulation. Image Credit: Malanushenko et al. 2022.

The MURaM simulation’s strength is that researchers can dissect the data from the simulation in a way that’s not possible with actual observations. MURaM produces data cubes that allow researchers to study regions of the Sun in a way that current observatories and instruments can’t.

“Because loops conform to field lines, scientists assume that the loops are plane-of-sky projections of thin magnetic flux tubes full of plasma of different temperatures and densities,” the authors write in their paper. “The areas between these flux tubes are also full of magnetic fields, but the plasma is fainter because it’s less dense. Or it could be a different temperature that doesn’t show up well in the wavelengths of the instrument sensing it.”

The authors studied volumetric emission to see these fainter areas between the flux tubes. Thin slices of MURaM allowed the researchers to dissect the data and look for structures and behaviour that aren’t apparent in real images. These slices show that some of what appear to be coronal loops aren’t loops but are made of ridges and wrinkles in the plasma.

These images from the study show what’s called volumetric emission. They’re thin slices of images from MURaM data cubes. Most of these images contain large-scale structures of complex shape, with numerous ridges and wrinkles, rather than structures corresponding to individual coronal loops. These structures are not easy to separate from one another. While some loops could be mapped to distinct, bright blobs, many loops do not seem to correspond with the isolated structures in the volume. Image Credit: Malanushenko et al. 2022.

This study raises questions about our understanding of the Sun. But one of the drawbacks of the study is that it’s based on a simulation. The authors acknowledge this, and that simulations play a more prominent and more significant role in astronomy and astrophysics as time goes on.

Can we trust MURaM?

The authors point out that MURaM is one of the most realistic simulations to date. We know this because it produces many things we can clearly observe about the Sun. “However, it is certainly vulnerable to criticism, like any other simulation. We nonetheless believe that our results are relevant to the actual corona…” they state.

What’s compelling about this discovery is that MURaM isn’t the only simulation to produce results supporting this study’s conclusion.

A study from 2005 based on a different simulation found that the footprint of some coronal loops “… are wrinkled and not geometrically simple…” The authors of this new research say that supports their conclusion that some coronal loops are, in fact, not loops.

Another study in 2014 based on coronal simulations also found emissions that don’t look like loops. Images from that simulation showed a diffuse component, several isolated blobs, and thin, bright, sheet-like structures similar to what MURaM showed in this new research. Wrinkles in those sheet-like structures produced loops that looked like the projection artifacts found in this new research.

This figure is from a 2014 paper based on different coronal simulations. It shows (a) a diffuse component, (b) several isolated blobs, and (c) thin, bright, sheet-like structures. Image Credit: Winebarger et al. 2014.

The next logical question to ask is: how many of the coronal loops are actual loops, and how many aren’t? New observational techniques will have to answer that, and those techniques will have to be carefully designed. It’ll also require new data handling techniques. But answering the question will lead to a better understanding of solar physics.

“We know that designing such techniques would be extremely challenging, but this study demonstrates that the way we currently interpret the observations of the Sun may not be adequate for us to truly understand the physics of our star,” Malanushenko said. 


Evan Gough

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