Sun’s coronal loops may be an optical illusion

(ORDO NEWS) — One of the most impressive features of the Sun is its coronal loops – incandescent structures of hot plasma that stretch for thousands of kilometers above the magnetically active regions of the Sun, forming, as it were, curving filaments.

But looks can be deceiving. Now a group of solar physicists are arguing that these iconic structures may not actually be loops at all.

Instead, the loops may be an illusion rooted in a more complex structure – a magnetic sheet or curtain that stretches and crumple. The team calls this a coronal veil, and they believe that bright coronal loops appear where this veil is crumpled, and our line of sight passes through most of it.

This insight came from studying simulations of the Sun’s magnetic field, published March 2 in The Astrophysical Journal.

“I’ve spent my entire career studying coronal loops,” Malanushenko, a researcher at the National Center for Atmospheric Research in Boulder, Colorado, and lead author of the study, said in a statement. “I never expected this. When I saw the results, my mind exploded. This is a completely new paradigm for understanding the atmosphere of the Sun.”

Lift the veil

For decades, scientists have generally assumed that coronal loops are what they look like – filaments of hot, glowing plasma. Because plasma is made up of particles with an electrical charge, their motion is influenced by the Sun’s magnetic field.

Physicists say that plasma is “frozen” in a magnetic field: The magnetic force directs the plasma along magnetic field lines – the very lines that iron filings draw around a bar magnet. Therefore, it is not surprising that these bright loops are thin filaments of “frozen” plasma that follow the curvature of the magnetic field.

However, there are several problems with the filament hypothesis that cast doubt on it. The first is that magnetic field lines tend to fan out farther from their source, be it a bar magnet or a group of sunspots.

This means that if coronal loops are filaments that trace magnetic field lines, then they must also branch out and become wider high above the Sun’s surface. But observations show otherwise. “The general consensus is that they do expand with height, but not as much as we think,” Malanushenko told Astronomy.

Another problem with the filament hypothesis is that the Sun’s atmosphere becomes less dense further from its visible surface. This means that the tops of the coronal loops must also be thinner and therefore not as bright as their bases. Instead, they maintain a relatively uniform brightness from top to bottom.

Suns coronal loops may be an optical illusion 2

But these inconsistencies disappear under the veil hypothesis, where the loops do not correspond to compact plasma filaments, but are a perspective effect caused by wrinkles in the plasma sheet.

The effect is similar to a thin veil: When the material is wrinkled so that we see it from the edge, or folded so that we look through several layers, it absorbs more light and blocks our view of what is behind it. Of course, since the plasma in the solar corona emits light rather than absorbs it, such folds appear brighter to us, not darker.

Difficult to check

The fact that we don’t have much experience with thin sheets of hot gases in everyday life is probably one of the reasons why the wrinkled veil hypothesis has not been taken seriously before, Malanushenko says.

But there are some astrophysical precedents in the night sky – most famously the Veil Nebula, which consists of the remnants of an expanding cloud of supernova debris in the constellation Signus 10,000 to 20,000 years ago.

The object is made up of rope-like filaments, but the generally accepted explanation is that the expanding shock wave of heated gas forms a thin layer that is only visible to us when it is crumpled and crumpled along our line of sight.

Suns coronal loops may be an optical illusion 3

For the Sun, the team qualitatively supports the veil hypothesis with examples from a widely used model of the solar magnetic field called MURaM, developed by researchers at the Max Planck Center for Solar System Research in Göttingen, Germany, and the University of Chicago.

“I was very excited that in the simulation I could take a scalpel and cut the model into different parts, isolate individual loops and study them,” says Malanushenko. “And what I saw was nothing like what I expected.”

Features that looked like coronal loops from one angle, when viewed in cross section, were not bundles of filaments, but twisted sheet-like elements.

Suns coronal loops may be an optical illusion 1

The team readily admits that there is still a lot of work to be done to test their hypothesis – and there are many difficulties to do it observationally, Malanushenko says. The team believes the structures are so complex that even from multiple viewpoints, it would be very difficult, if not impossible, to tell which loop is which, and to determine the veil geometry, if one exists.

Conducting direct measurements of the coronal veil using spacecraft is also beyond our capabilities at present. NASA‘s Parker Solar Probe, launched in 2018, will make the closest spacecraft ever to the Sun, coming within 4 million miles (6.4 million kilometers) of its visible surface. But for direct sampling of coronal loops or in situ measurements, the spacecraft would have to get close to about 1,000 times.

For now, the team plans to continue the work by running additional simulations and comparing the results with observations.

One strategy to distinguish between loop and veil theories is to look at the brightness contrast of visible loops as well as the space between them. The threads should stand out sharply against the general background, while the veil will result in more diffuse radiation between bright folds.

It is possible that improved modeling will allow for a stronger observational strategy, Malanushenko says. “Given the strength of this impact, yes, we have to be careful. We have to look for observational evidence.”

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