The Sun’s coronal loops could be an optical illusion

Some of the Sun’s most spectacular features are its coronal loops – glowing structures of hot plasma that extend thousands of miles above magnetically active regions of the Sun, forming what appear to be curved strands.

But appearances can be deceiving. Now, a team of solar physicists says these iconic structures may not be loops at all. Instead, the curls can be an illusion rooted in a more complex structure – a magnetic sheet or a curtain that’s pulled and crinkled. The team calls this the coronal veil, and they believe that bright coronal loops appear where the veil is crumpled and our field of vision passes through it more.

The idea came from exploring simulations of the Sun’s magnetic field published on March 2 in The Astrophysical Journal.

“I’ve spent my entire career studying coronal loops,” Malanushenko, a research scientist at the National Center for Atmospheric Research in Boulder, Colorado, and lead author of the study, said in a statement. “I didn’t expect this. When I saw the results, my mind exploded. This is a whole new paradigm for understanding the Sun’s atmosphere.


For decades, scientists generally assumed that coronal loops were what they looked like – strands of hot, glowing plasma. As plasma is made up of electrically charged particles, their movements are influenced by the Sun’s magnetic field. Physicists say that plasma is “frozen” in a magnetic field: the magnetic force guides the plasma along magnetic field lines, the same lines that iron filings trace around a magnetic bar. Therefore, it is no exaggeration to think that these luminous loops are thin strands of frozen plasma, following the curvature of the magnetic field.

However, there are a few issues with the strand hypothesis that call it into question. The first is that magnetic field lines tend to move away from their source, whether that source is a bar magnet or a group of sunspots. This means that if the coronal loops are strands that trace magnetic field lines, they should also unfold and widen above the surface of the Sun. But this is not what the observations show. “The consensus is that they get bigger with height, but not as much as we think they should,” Malanushenko said. Astronomy.

The other problem with the strand hypothesis has to do with how the Sun’s atmosphere becomes less dense farther from its visible surface. This means that the tops of the coronal curls must also be thinner and therefore less shiny than at their bases. Instead, they maintain relatively even brightness from top to bottom.

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