Some of the most spectacular features on the Sun are its coronal loops — incandescent structures of hot plasma that arc for thousands of miles above magnetically active regions of the Sun, forming what appear to be curving strands.
But appearances can be deceiving. Now, a team of solar physicists say 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 is being pulled and wrinkled. The team call this the coronal veil, and they think that bright coronal loops appear where the veil is wrinkled and our line of sight runs through more of it.
The insight came from exploring simulations of the Sun’s magnetic field published 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 generally assumed that coronal loops are what they look like — strands of hot glowing plasma. Because plasma consists of particles with an electric charge, their movements are influenced by the Sun’s magnetic field. Physicists say that plasma is “frozen in” to a magnetic field: The magnetic force guides plasma along magnetic field lines, the same lines that iron filings trace out around a bar magnet. Therefore, it’s not much of a stretch to think that these bright loops are thin strands of frozen-in plasma, following the curvature of the magnetic field.
However, there are a couple issues with the strands hypothesis that call it into question. One is that magnetic field lines tend to fan out further from their source — whether that source is a bar magnet or a group of sunspots. That means if coronal loops are strands that trace magnetic field lines, they should also fan out and get wider high above the Sun’s surface. But that’s not what observations show. “The consensus is that they do expand with height but not nearly as much as we think they should,” Malanushenko told Astronomy.
The other issue with the strands hypothesis is related to how the Sun’s atmosphere becomes less dense further away from its visible surface. This means that the tops of coronal loops should also be thinner and therefore not as bright as at their bases. Instead, they maintain a relatively even brightness from top to bottom.