On April 20, visitors to the small town of Exmouth at the northwestern tip of Australia will witness a rare astronomical event: During a total solar eclipse, the moon will pass entirely in front of the sun, blocking most of its light and turning the seaside community an eerie, twilight blue at 11:30 a.m. local time.

Researchers from CU Boulder and the National Solar Observatory (NSO) will be there to watch it happen.

Kevin Reardon, professor adjunct in the Department of Astrophysical and Planetary Sciences (APS) and scientist at the NSO, and first-year doctoral student Sarah Bruce are traveling to Exmouth for the event. They’ll be joined by Sanjay Gosain and John Williams from the NSO. The team will bring a telescope and other instruments to observe a phenomenon astronomers rarely see from the ground—bright tendrils of white light streaming from the corona, a hot region surrounding the sun that extends millions of miles into space. 

The duo’s observations could help scientists get one step closer to answering a space mystery that has eluded them for decades: How does the corona get so hot, reaching temperatures of millions of degrees Fahrenheit?

“One of the great things about eclipses is the excitement that comes to town,” Reardon said. “So many people drop in on these remote locations that may not be used to being the center of attention, and it’s an atmosphere of real celebration.”

To add to that excitement, he and Bruce, who has never seen a total solar eclipse before, will be in a race against time. The event’s totality, the moment when the sun, moon and Earth all align perfectly, will last just one minute. The researchers won’t be able to waste a second as they start their cameras and collect their data.

The trip is practice for an even more dramatic event in spring 2024—when a solar eclipse with a totality lasting four minutes passes over Texas and a wide swath of the United States. 

“What I’m excited to see is the way an eclipse changes the appearance of everything around you,” Bruce said. “I’ve heard people say that the illumination from the narrow crescent of the solar disk as the eclipse progresses makes everything look rendered like you’re in a video game.”

Day becomes night

Scientists like her, however, are interested in eclipses for more than just their other-worldly ambience.

These events are perfect opportunities for researchers to observe the “white-light corona”—a spiky crown of light around the sun. During the totality, this feature is about as bright as the full moon, or bright enough that you can see it with the naked eye.

“One of the most striking things about an eclipse are the long streamers coming out from the sun,” Reardon said. “Those are created by light scattering off of the diffuse free electrons in the corona.”

And, he noted, such streamers can tell you a lot about the sun, including how fast the corona is heating up and where.

Reardon explained that sun watchers have long wondered why the sun’s outermost atmosphere is so much hotter than its visible surface, which tops out at a measly 11,000 degrees Fahrenheit. He and Bruce are hoping to learn more about the processes that make the corona so toasty—and how these physics can lead to bursts of energy from the sun that may damage satellites in orbit around Earth and even fry electronics on the ground. 

Other eyes will be watching the eclipse, too: The pioneering Parker Solar Probe, which is orbiting the sun, will sample the corona on April 20, allowing the researchers to compare their findings to data collected from space. On the ground, the NSO’s Daniel K. Inouye Solar Telescope (DKIST) in Hawaii will take observations of other coronal emissions from outside the path of the eclipse.

“Everything is aligned,” Reardon said. 

He and Bruce, however, have a long few weeks ahead of them. They’ll fly first to Perth, Australia, then make the 12-hour drive to Exmouth, a remote town of less than 3,000 residents more commonly known for its coral reefs and marine life. 

Bruce, who earned her bachelor’s degree from the University of Texas at Austin in 2022, can’t wait to set up the team’s telescope in Australia.

“I just love having the hands-on element of astronomy and getting to build things,” she said. “I'm really excited to actually use instruments in the field.”

Reardon experienced his first totality in Hawaii when he was a college undergrad in the 1990s and has seen another six eclipses since then. He’s grateful he has a chance to share the same “unexpected” experience with another rising astrophysicist.  

“The moment when you go into totality, and day becomes night, and you can see the corona burst out—you can't imagine what that that's like,” he said.