Sun Science: 150,000 Plasma Swirls the Size of Oregon Could Be Superheating Two-Million-Degree Corona

Scientists have discovered enormous swirls on the surface of the sun that appear to generate Alfvén pulses—the "holy grail in plasma-astrophysics." These swirls, scientists say, may help solve one of the biggest mysteries of the sun: Why the outer layer of the atmosphere—the corona—is so much hotter than its surface.

Researchers from the U.K.'s University of Sheffield discovered there are about 150,000 of these swirls on the sun at any given moment. They are about the size of the state of Oregon and appear to transport energy from the surface to the solar atmosphere. The findings, published in Nature Communications, provide the first observational evidence of energetic wave pulses that appear to be superheating the corona.

The surface of the sun reaches temperatures of around 10,000 degrees Fahrenheit. Yet 1,000 miles above, in the outermost layer of the solar atmosphere, they go above two million degrees Fahrenheit. Why and how this happens is unknown.

Alfvén waves are very difficult to detect because they are oscillations of magnetic field lines—similar to the movements of guitar strings, only they are invisible, senior author Robertus Erdélyi told Newsweek. "They have been observed before, but only indirectly and certainly not in the form of swirls. Now, we detected them in plenty and that is a great surprise even to us. Alfvén waves are the holy grail in plasma-astrophysics. Their discovery was awarded the Nobel Prize in 1970. And now, we observe them on the sun too."

Magnetic plasma waves has long been suggested as one of the mechanisms heating the sun's corona. In their research, the team discovered plasma swirls on the surface appear to generate short-lived Alfvén pulses. There are around 150,000 of these swirls on the sun at any given moment, the team says.

It is thought that Alfvén pulses enter the solar atmosphere by traveling along the magnetic field lines all the way past the chromosphere—the layer between the surface and the corona. Researchers estimate that the energy carried by Alfvén pulses is about 10 times higher than is needed to heat the upper solar chromosphere, lead author Jiajia Liu said in a statement.

There are still many unanswered questions about Alfvén pulses and how they heat the sun's corona. "They seem to do this heating very efficiently," Erdélyi said. "We just do not know yet how. That's the next big question in plasma-astrophysics."

He also said there are other heating candidates, including different types of magnetized plasma waves, as well as a process called magnetic reconnection—where field lines reconnect and release energy. "However, at present, there are simply not enough observational justification to prove the latter," Erdélyi said. The next steps will be to use bigger and better telescopes to better understand these swirls and how they might produce Alfvén waves, he added.

This article has been updated for increased accuracy of the scientific terms used. Erdélyi's title has been updated to senior author.

sun eclipse
An eclipse of the sun captured by NASA. NASA Goddard