CLEMSON – While tens of thousands were experiencing the Aug. 21 total solar eclipse on the expansive grounds of Clemson University, professor Sean Brittain and several students from the department of physics and astronomy were collecting scientific data on the roof of the Watt Family Innovation Center.
Using a telescope and computer software, Brittain and his team gathered images of the sun’s outer atmosphere – the corona – during the 2 minutes and 37 seconds of totality. These images will be compiled with thousands of others taken by 68 identical telescopes placed along the 2,500-mile path of totality, in an effort dubbed the Citizen CATE (Continental-America Telescopic Eclipse) Experiment. They will provide 90 minutes of unprecedented, continuous footage to better understand the details of the sun’s corona.
All 68 telescopes were manned by volunteer citizens, including retirees, school kids, teachers, astronomers and college students. Fifty-eight of those sites had clear weather on Aug. 21 and were able to collect data for the experiment. Clemson University was one of the lucky ones.
“It went terrific,” Brittain said. “The weather cooperated and we were able to see the total eclipse clearly. There were seven sites in South Carolina, and all of them were able to get good data, which was encouraging. The equipment all worked very well, too. We’ve already sent our data to Citizen CATE, and they’ve been able to check it out.”
Luckily, Brittain and his team got to step back and observe totality for themselves as the computer and telescope did the work for them.
“The way it worked was, we waited until totality started, then we removed the telescope’s solar filter and the computer started recording images for the whole two and a half minutes,” Brittain said. “We were able to set a timer for two minutes, so we knew when totality was about to end. When the timer went off, we put the solar filter back on so that when the sun came out, the telescope would be protected. So we could just go and enjoy the eclipse itself, and that was really nice.”
Starting at the first observation site in Portland, Oregon, this same setup was replicated as totality passed over each of the 68 sites. When the moon’s shadow came into view on the horizon – moving at roughly 1,500 miles per hour and covering about 70 miles in diameter – the observers removed their solar filters and their computers started imaging. Observations wrapped up at the final site in Charleston at 2:46 p.m. EST.
“Some of the sites were observing the eclipse at the same time,” Brittain said. “The reason for that is, if one site didn’t work, if something broke or if a cloud moved right in front of the sun at the wrong time, the other sites could still get images on either side of that site. So out of the 68 sites, 58 of them were able to collect data. Only losing 10 out of 68 – that’s pretty good.”
From the data, researchers hope to be able to pinpoint where polar plumes – streams of plasma that project from the sun – emerge, as well as how they are connected to the sun’s magnetic field. Before Citizen CATE, solar features like these were difficult to discern, because researchers could only view the corona down to 70,000 miles above the sun’s surface. The CATE data will reach within about 20,000 miles of the solar surface.
“You can’t see the corona when the sun is not covered, because the solar surface – the photosphere – is about a million times brighter than the corona,” Brittain said. “If you went out today with your solar glasses on, all you’d see is a perfect circle that’s nice and smooth. If you block out that circle, then you see the corona, which is a million times fainter than the photosphere. You can take a blocking disk, called a coronagraph, and put it in the telescope to block the sun and see the stuff around it. The thing about that, though, is that there are edge effects that come from using coronagraphs that blur the light.”
While a coronagraph attempts to block out the sun, it can’t compare to the real deal.
“The moon allowed us to get within about five percent of the surface of the sun or about 20,000 miles above the surface of the sun,” Brittain said. “Twenty-thousand miles sounds like a long way, but the corona extends 13 million miles out. The eclipse allowed us to push this in closer to the sun by about a factor of three.”
When strung together, the 90 minutes of footage from all the sites in the Citizen CATE Experiment will give researchers a chance to watch the polar plumes rise, spread out and dissipate – a dynamic view of the sun’s corona that will help test various computer models of this effect.
“We have lots of computer models for this, though they’re really complicated,” Brittain said. “You can’t put all the physics in your model or else it would have to run forever. So you have to make assumptions and come up with shortcuts that allow your model to run. One of the things you always want to know is: Does the model reproduce reality? The way you know that is by comparing it to actual data. This experiment has the dataset to test these large models in new ways.”
Because of funding by the National Science Foundation, NASA and other organizations, all of the 68 observation sites will get to keep the equipment they were granted for participating in the experiment. Brittain says Clemson plans to incorporate the equipment into physics and astronomy curriculum, as well as in public outreach events.
“One of the cool things you can do with the equipment is measure the rotation rate of the sun by taking data over several days of sun spots and seeing them move, based on different latitude rates and where the sun spots are,” Brittain said. “You could observe at night without the solar filter and take data on eclipsing stars, binary stars, transiting exoplanets and these kinds of things. There are lots of fun things that can be done with the equipment even without an eclipse, and we plan to do that.”
A preliminary video of the images collected from the Aug. 21 eclipse can be viewed on Citizen CATE’s website, with a full version to come as the 4,000-plus images are sorted.