In a new study, a team of physicists has recruited nearly 1,000 undergraduate students at the University of Colorado Boulder to help answer one of the sun’s most enduring questions: How does a star’s outer atmosphere, or “corona,” get so hot?
The research represents an almost unprecedented feat of data analysis: From 2020 to 2022, a small army of first- and second-year students examined the physics of more than 600 actual solar flares—giant volcanic eruptions of energy from the sun’s raging corona.
The researchers, including 995 undergraduate and graduate students, published their findings on May 9. Astrophysical Journal. The findings suggest that solar flares may not be responsible for the warming of the solar corona, as a popular astrophysics theory suggests.
“We really wanted to assure these students that they are doing real science research,” said James Mason, lead author of the study and an astrophysicist at Johns Hopkins University’s Applied Physics Laboratory.
Study co-author Heather Lewandowski agreed, noting that the study would not be possible without the undergraduate students, who contributed an estimated 56,000 hours of work to the project.
“It was a tremendous effort from everyone involved,” said Lewandowski, professor of physics and fellow at JILA, a joint research institute between CU Boulder and the National Institute of Standards and Technology (NIST).
The study addresses a conundrum that has left even top astrophysicists scratching their heads.
Telescope observations indicate that the sun’s corona hums at temperatures in the millions of degrees Fahrenheit. By contrast, the Sun’s surface is much cooler, only registering thousands of degrees.
“It’s like standing right in front of a campfire, and as you move away, it gets even hotter,” Mason said. “Does not make sense”.
Some scientists suspect that very small flares, or “nanoflares,” which are too small for even the most advanced telescopes to detect, might be responsible. If such events exist, they may appear across the Sun on an almost constant basis. The theory goes that they can build up to make the corona warm. Think of boiling a pot of water using thousands of individual matches.
Mason said the students’ findings cast doubt on that theory, though he thinks it’s too early to say for sure.
“I wish our result had been different,” Mason said. “I still feel nanoflares are an important driver of coronal heating.” “But the evidence from our paper says otherwise. I’m a scientist. I have to go where the evidence points.”
Epidemic peak times
The effort began at the height of the COVID-19 pandemic.
In spring 2020, CU Boulder, like most universities across the country, moved its courses entirely online. However, Lewandowski ran into a quandary: She was teaching a class on action research called “Experimental Physics 1” that fall, and she had nothing for her students to do.
“These were peak times of the pandemic,” Lewandowski said. “It’s hard sometimes to remember what life was like at the time. These students were so isolated. They were really stressed out.”
Mason, then a research scientist at the Laboratory for Atmospheric and Space Physics (LASP) at CU Boulder, offered an idea.
The world has long wanted to research the mathematics of solar flares. In particular, he attempted to examine a data set of thousands of flares that occurred between 2011 and 2018 and were detected by instruments in space. They included the National Oceanic and Atmospheric Administration’s (GOES) Operational Geostationary Environmental Satellite Series, NASA’s Miniature Solar X-ray Spectrometer (MinXSS), a CubeSat mission designed and built at LASP.
The problem: There were too many torches to check out alone.
That’s when Mason and Lewandowski turned to the students for help.
Mason explained that you can infer details about the behavior of nanoflares by studying the physics of large flares, which scientists have observed first-hand for decades.
To do just that, the students split into groups of three or four and chose an ordinary glow that they wanted to analyze over the course of the semester. Then, through a series of lengthy calculations, they added up the amount of heat that each of these events could flow into the sun’s corona.
Their calculations painted a clear picture: the Sun’s nanoparticles probably wouldn’t be powerful enough to heat the corona to millions of degrees Fahrenheit.
What makes the corona so hot is not clear. A competing theory suggests that waves in the sun’s magnetic field carry energy from the sun’s interior to its atmosphere.
But the study’s actual results aren’t its only important findings. Lewandowski said her students were able to obtain opportunities rare for scientists and engineers so early in their careers—to learn firsthand about the collaborative and often chaotic way scientific research works in the real world.
“We still hear students talking about this course in the halls,” she said. “Our students have been able to build a community and support each other at a really challenging time.”
CU Boulder co-authors of the new study include Alexandra Wirth, postdoctoral researcher at JILA; Colin West, Assistant Professor of Physics; Alison Youngblood, a LASP astrophysicist now at NASA’s Goddard Space Flight Center; Donald Wodraska, Team Leader for Data Systems at LASP; and Courtney Beck, data systems software engineer at LASP and the Collaborative Institute for Research in Environmental Sciences (CIRES).
Funding for the research came from NASA through the MinXSS mission and the US National Science Foundation through the STROBE Center for Science and Technology and the JILA Physics Frontier Center.