CU Boulder students disprove popular theory on how the sun's atmosphere gets so hot

Jun. 13—Hundreds of undergraduate students at the University of Colorado Boulder found evidence disproving a popular theory used by scientists to explain one the biggest mysteries in astronomy.

Approximately 1,000 undergraduate students at CU Boulder found strong evidence against the theory that solar flares, or intense bursts of energy in the sun's atmosphere, are the reason the sun's corona, or outer atmosphere, is so hot compared to the sun's surface.

James Mason, research scientist at the Johns Hopkins University Applied Physics Laboratory, said the mystery — called the coronal heating problem — is one of the biggest problems in astronomy. The mystery is that the sun seems to violate the laws of thermodynamics because it gets hotter farther away from the surface.

"It's just like being next to a heater in a room or a campfire and walking away from it and it getting hotter," Mason said. "That's not how heat works, and so it seems to break the laws of thermodynamics."

While the sun isn't actually breaking the laws of thermodynamics, Mason said there is some sort of mechanism that scientists don't fully understand that's transporting heat from the solar surface out into the atmosphere.

There are two theories for how this happens, one involving nanoflares that CU Boulder students found evidence against, and another that involves waves carrying heat through the magnetic field.

Students in groups of three or four chose a specific solar flare to study and created a report on it. Once the energy of the flares was computed, the results found that the amount of energy the flares produce combined with the frequency they occur did not produce enough energy to account for the heat of the sun's corona.

"This is evidence against nanoflares being able to explain how the solar corona is one million kelvin while the surface is only 5,000 or 6,000 (kelvin)," Mason said.

William Solorio, a rising senior at CU Boulder, was a freshman when he contributed to the project in his physics class. He said he enjoyed the project because he was exposed to what it's like to do real research.

"Having that class was like an introduction to that type of research and was a tool for students to test the waters and see if they like it," Solorio said. "No one really knows what the final answer should be, so going through that uncharted water was very interesting."

Solorio said the project was one of the main factors in his interest in the field and wanting to continue his education to get a doctorate.

CU Boulder Physics professor Heather Lewandowski decided to have students take on the project in her lab classes due to the pandemic. Because students couldn't be in person, she decided to try something different and find a project where students could participate in actual research.

Lewandowski said it was exciting to have students participate in real research and see their hard work cumulate into a result contributes to the scientific community.

"Here students really got a sense of what real research was, and they could identify themselves as real scientists," Lewandowski said. "I think that growth of identifying as a scientist was a really important outcome."

Mason said the results were surprising, but also a delight. He said the sun's corona has been an extremely active area of research for decades, and while it hasn't been solved, scientists now have strong evidence against nanoflares. More research is needed because evidence against one theory is not de facto evidence for the other theory, Mason said.

"If we can't explain how the sun's atmosphere, the corona, is so hot, then we don't really know how the star is transporting energy," Mason said. "It means we don't know how any stars are transporting energy in their atmospheres, so it's important for us to figure that out so we can understand how stars work, how they transport energy and how they produce space weather and affect the Earth."