The Sun’s coronal loops may be an optical illusion - Beyond The World
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The Sun’s coronal loops may be an optical illusion

The arcs of plasma that dance above the Sun could be wrinkles in a massive plasma veil.

The Sun’s coronal loops are some of its most stunning characteristics. These are incandescent structures of heated plasma that arc for millions of kilometers over magnetically active regions of the Sun, generating what appear to be curving threads.

However, appearances can be deceiving. According to a group of solar physicists, these famous formations may not be loops at all. Instead, the loops could be an illusion caused by a more complicated structure, such as a magnetic sheet or curtain being tugged and wrinkled. This is referred to as the coronal veil by the scientists, and they believe that bright coronal loops emerge where the veil is wrinkled and our line of sight passes through more of it.

The discovery comes from studying simulations of the Sun’s magnetic field, which were published on March 2 in The Astrophysical Journal.

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

Lifting the veil

For decades, scientists have assumed that coronal loops are exactly what they appear to be: strands of hot, glowing plasma. Because plasma is made up of charged particles, its movements are governed by the Sun’s magnetic field. Plasma is “frozen in” to a magnetic field, according to physicists: the magnetic force leads plasma along magnetic field lines, the same lines that iron filings trace out around a bar magnet. As a result, it’s not a stretch to believe that these bright loops represent small strands of frozen-in plasma that are following the curvature of the magnetic field.

The strands hypothesis, on the other hand, has a handful of flaws that throw it into question. One is that magnetic field lines tend to fan out more from their source, whether it’s a bar magnet or a cluster of sunspots. If coronal loops are strands that trace magnetic field lines, they should widen out and become wider high above the Sun’s surface. However, observations reveal that this is not the case. “The consensus is that they do expand with height but not nearly as much as we think they should,” Malanushenko told Astronomy.

The strands hypothesis also has a problem with how the Sun’s atmosphere grows less dense as it moves away from its visible surface. This implies that the tops of coronal loops should be narrower and thus less luminous than their bases. Instead, they keep a reasonably constant brightness from top to bottom.

These physical models demonstrate the difference between the traditional description of coronal loops as strands of plasma (left) and the new, proposed explanation that they are wrinkles in a plasma veil (right).

These physical models demonstrate the difference between the traditional description of coronal loops as strands of plasma (left) and the new, proposed explanation that they are wrinkles in a plasma veil (right).Anna Malanushenko

However, under the veil hypothesis, the loops do not correspond to compact plasma strands, but rather to a perspective effect created by wrinkles in a sheet of plasma. The effect is similar to that of a thin veil: when the material bunches up so that we can see it edge-on or is folded so that we can see through many layers, it absorbs more light and obscures our vision of what’s behind it. Of However, because plasma in the solar corona emits light rather than absorbs it, such wrinkles appear brighter rather than darker to us.

Hard to test

According to Malanushenko, one of the reasons the wrinkled veil hypothesis hasn’t been seriously examined before is that we don’t have much experience with thin sheets of illuminating gases in everyday life.

However, there are some astrophysical precedents in the night sky, most notably the Veil Nebula, which is made up of the leftovers of an expanding cloud of debris from a supernova that occurred 10,000 to 20,000 years ago in the constellation Cygnus. The item appears to be made of ropelike threads, but the most frequent explanation is that the expanding shock wave of hot gas generates a thin layer that we see only when it is wrinkled and bunched up along our line of sight.

The Veil Nebula, imaged here by the Hubble Space Telescope, lies about 2,100 light-years away in Cygnus.

The Veil Nebula, imaged here by the Hubble Space Telescope, lies about 2,100 light-years away in Cygnus. ESA/Hubble & NASA, Z. Levay

For the Sun, the team provides qualitative support for the veil hypothesis using examples from MURaM, a widely used model of the solar magnetic field developed by researchers at the Max Planck Institute for Solar System Research in Göttingen, Germany, and the University of Chicago.

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

When seen in cross section, structures that looked to be coronal loops from one perspective were instead swirling sheet-like shapes.

In this simulation of the Sun’s magnetic field, features that look like coronal loops (left) are formed from swirling sheets of plasma (right).

In this MURaM simulation of the Sun’s magnetic field, features that look like coronal loops (left) are formed from swirling sheets of plasma (right).

The team openly admits that much more study is required to validate their idea — and there are numerous hurdles to doing so observationally, according to Malanushenko. The research believes the structures are so complicated that even with many observations, determining which loop is whose and determining the geometry of the veil, if it exists, would be difficult, if not impossible.

Direct measurements of the coronal veil using satellites are also currently beyond our capabilities. NASA’s Parker Solar Probe, launched in 2018, will make the closest approach to the Sun by a spacecraft, approaching within 4 million miles (6.4 million kilometers) of its visible surface. But to directly sample coronal loops or make in situ measurements, a craft would have to get about 1,000 times closer.

For the time being, the team intends to proceed by undertaking additional modeling and comparing the results to observations. One method for distinguishing between the loop and veil hypotheses is to examine the brightness contrast of apparent loops as well as the distance between them. Strands should contrast sharply with their surroundings, whereas a veil would result in more diffuse emission between the bright wrinkles.

According to Malanushenko, advances in modeling may provide a more robust observational method. “Given the magnitude of this influence, we should be cautious.” We should look for observational evidence.”

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