How 1,000 CU Boulder undergraduate students helped answer one of the most enduring questions about the sun 

For a new study, a team of physicists recruited more than 1,000 CU Boulder undergraduates to help answer one of the most enduring questions about the sun: How does the star’s outermost atmosphere, or “corona,” get so hot? 

The research represents a nearly-unprecedented feat of data analysis: From 2020 to 2022, the small army of mostly first- and second-year students examined the physics of more than 600 real solar flares— gigantic eruptions of energy from the sun’s roiling corona. 

The researchers included roughly 1,400 undergrads who contributed an estimated 56,000 hours of work to the project. Their results suggest that solar flares may not be responsible for superheating the sun’s corona, as a popular theory in astrophysics suggests. 

“It was a massive effort from everyone involved,” said Heather Lewandowski, study co-author and fellow of JILA, a joint research institute between CU Boulder and the National Institute of Standards and Technology (NIST). 

The project began in summer 2020 at the height of the COVID-19 pandemic. Lewandowski was teaching a class on hands-on research called “Experimental Physics I” that fall, and she had nothing for her students to do. She decided to join forces with James Mason, the lead author of the study, who was then a researcher at the Laboratory for Atmospheric and Space Physics (LASP). 

Mason, an astrophysicist, had long wanted to dig into a mystery that has puzzled even senior scientists. 

Telescope observations suggest that the sun’s corona sizzles at temperatures of millions of degrees Fahrenheit. The surface of the sun, in contrast, is much cooler, registering only in the thousands of degrees. 

“That’s like standing right in front of a campfire, and as you back away, it gets a lot hotter,” said Mason, now at the Johns Hopkins University Applied Physics Laboratory. “It makes no sense.” 

Some scientists suspect that especially tiny flares, or “nanoflares,” which are too small for even the most advanced telescopes to spot, may be responsible. If such events exist, they may pop up across the sun on a nearly constant basis. And, the theory goes, they could add up to make the corona toasty. Think of boiling a pot of water using thousands of individual matches. 

To find out what role, if any, such nanoflares play in making the corona so hot, the scientists turned to the undergrads for help. Mason explained that you can infer details about the behavior of nanoflares by studying the physics of larger flares, which scientists have observed directly for decades. 

Over three semesters, students in Lewandowski’s class split into groups of three or four and picked a flare to investigate from a large dataset. The flares occurred between 2011 and 2018 and had been spotted by instruments in space. Through a series of lengthy calculations, the students quantified how much heat each of these explosive events might have poured into the sun’s corona. 

Their findings painted a clear picture: The sum of the sun’s nanoflares likely wouldn’t be powerful enough to heat up its corona to millions of degrees Fahrenheit. 

“We really wanted to emphasize to these students that they were doing actual scientific research,” Mason said. 

What is making the corona so hot still isn’t clear. 

The study’s scientific findings, however, aren’t its only important results, Lewandowski said. 

“We still hear students talking about this course in the halls,” she said. “Our students were able to build a community and support each other at a time that was really tough.” 

Image: An active region on the sun emits a solar flare—a powerful burst of radiation. Image by NASA/Solar Dynamics Observatory.