Researchers and ‘Small Army’ Upend Popular Idea About the Sun’s Superheated Atmosphere

An image of the Sun, emphasizing bright spots where light and energy are being emitted
NASA’s Solar Dynamics Observatory captured this image of an X2.0-class solar flare bursting off the lower right side of the Sun on Oct. 27, 2014. The image shows a blend of extreme ultraviolet light with wavelengths of 13.1 and 17.1 nanometers. (Credit: NASA/Solar Dynamics Observatory)

A new study to help solve an enduring mystery about the Sun gives new meaning to the phrase, “It takes a village.”

Physicists and more than 1,400 undergraduate students sought to chip away at answering the longstanding question of how the Sun’s outermost atmosphere, or corona, gets so hot. In a nearly unprecedented feat of data analysis, the researchers and their small army of mostly first- and second-year students examined the physics of hundreds of solar flares — enormous eruptions of energy from the Sun’s corona that one popular theory purports are responsible for the corona’s heat. And their results suggest that popular idea may not be right after all.

“We really wanted to emphasize to these students that they were doing actual scientific research,” said James Mason, lead author of the study and an astrophysicist at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland.

Heather Lewandowski, a study co-lead and physicist at the joint research institute JILA, said the study wouldn’t have been possible without the undergraduate students, who contributed an estimated 56,000 hours of work to the project: “It was a massive effort from everyone involved.”

The researchers and 995 undergraduate and graduate students published their findings May 9 in The Astrophysical Journal.

Telescope observations have suggested the Sun’s corona sizzles at temperatures close to 2 million degrees Fahrenheit (1.1 million degrees Celsius), strikingly hotter than its surface that’s just 1,000 miles below, which registers around 10,000 degrees F (5,500 degrees C).

“That’s like standing right in front of a campfire, and as you back away, it gets a lot hotter,” Mason said. “It makes no sense.”

Why that is has puzzled physicists for decades. Some researchers suspect the culprit may be minuscule flares called nanoflares, too small for even the most advanced telescopes to spot. If such events exist, they may pop up across the Sun on a nearly constant basis and, the theory goes, could add up to make the corona toasty, like boiling a pot of water using thousands of individual matches.

The Solar Ultraviolet Imager (SUVI) on GOES-16 captured this imagery of a large solar flare on Sept. 10, 2017. This was one of hundreds of solar flares the team analyzed in a new study that involved more than 1,000 students.

Credit: National Oceanic and Atmospheric Administration

One way to determine if the nanoflare explanation holds up is by mathematically examining the frequency of flares of various energy around the Sun. Mason had long wanted to approach the issue this way, particularly examining a data set of thousands of flares between 2011 and 2018 that space instruments observed.

The problem was that there were just too many flares to examine on his own.

That’s when he and Lewandowski turned to students. In spring 2020, at the start of the COVID-19 pandemic, Lewandowski and Mason saw an opportunity to give three semesters’ worth of physics students at the University of Colorado Boulder an opportunity to get their hands dirty with real physics work that fits perfectly into a curriculum for classes that had just gone virtual.

Split into groups of three or four, the young scientists each picked a flare they wanted to analyze during the semester. Then, through a series of lengthy calculations, they added up how much heat each event could pour into the Sun’s corona.

They examined more than 600 solar flares, and their calculations were clear: The sum of energy from nanoflares alone likely isn’t powerful enough to heat up the corona to millions of degrees, casting doubt on the nanoflare theory. Mason said he thinks it’s still too early to eliminate the idea outright, though.

“I was hoping our result was going to be different,” he said. “I still feel like nanoflares are an important driver of coronal heating, but the evidence from our paper suggests the opposite. I’m a scientist. I have to go where the evidence is pointing.”

A competing theory suggests waves in the Sun’s magnetic field called Alfvén waves may carry energy from inside the Sun to its atmosphere, gradually heating it. The study’s results point to this alternative explanation as the important driver of coronal heating, but whether that’s actually the case is still unclear.

Regardless, the study has left an indelible imprint on the students who took part in producing it. The opportunity to learn firsthand about the collaborative and often messy way scientific research works in the real world, Lewandowski said, is rare for scientists and engineers so early in their careers.

“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.”

This story has been adapted from a release from the University of Colorado Boulder.

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