The sun: Waves of plasma are travelling much faster than they should be

Astronomers have found waves made up of eddies of plasma inside the sun, and they can’t explain why they are travelling three times faster than similar waves

By Leah Crane

An odd type of wave has been discovered that travels backwards through the plasma that makes up the sun. But of three possible mechanisms to explain these waves, none fits the data, so they remain a mystery.

Chris Hanson at New York University Abu Dhabi in the United Arab Emirates and his colleagues found the waves, called high-frequency retrograde vorticity waves, using decades of data from both ground and space-based telescopes. The HFR waves are formed of groups of small eddies, or vortices, travelling around the sun in the opposite direction to its rotation.

However, the vortices move around the sun three times faster than other, similar waves – more rapidly than can be explained by any of our models of plasma motion within the sun.

The researchers tested three possible explanations: that the waves were caused by magnetic fields within the sun; that they come from other ripples in the sun called gravity waves; or that they are due to compression of plasma. None of these ideas matches the data.

“To find a set of waves that has no current explanation is… exciting and intriguing, because the challenge now remains to explain what they are,” says Hanson. “We are missing an ingredient in our understanding of the sun.”

There may be a similar type of wave in Earth’s oceans, where ripples called Rossby waves have been found to travel more quickly than researchers can explain. “In the absence of an explanation of the fast-moving solar HFR or oceanic Rossby waves, we can only say that it is worthwhile to investigate the similarities,” says Hanson. The parallel may make it easier to solve this mystery on the sun by studying it on Earth.

Hanson says it is still likely that some combination of magnetism, gravity and compression is causing these waves. “It’s very difficult to imagine a scenario where one of these isn’t playing some kind of role in enhancing the mode speed,” he says. The researchers couldn’t think of any other plausible mechanisms, and they hope that more detailed modelling will be able to explain these strange waves in the future.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-022-01632-z

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