Why these scientists fly all over the world to study the sun’s corona during total solar eclipses

By inergency On Apr 6, 2024

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For scientists focused on unlocking the secrets of the sun, it’s not a newfangled invention but a cyclical celestial event — and an appetite for world travel — that offers them an unmatched opportunity to observe our nearest star.

Though, the high-tech equipment helps, too.

During total solar eclipses, which are visible from a different location on Earth roughly every one to two years, the moon briefly obscures the sun’s disk (i.e. the big, round circle) and reveals its corona, or outer atmosphere that radiates beyond it like a halo.

“Total solar eclipses let us study and see the corona in ways that just would not be possible at any other time and in any other way,” said Amir Caspi, a solar astrophysicist at the Southwest Research Institute in Boulder, Colorado.

On April 8, the moon’s shadow will fall across North America in a path of totality that starts in Mexico and ends in eastern Canada. Barring technological glitches or bad weather, Caspi and other leading researchers are poised to acquire a treasure trove of new information about the corona.

Animation by Megan McGrew/PBS NewsHour

The corona is a million times dimmer than the sun’s disk, which makes it impossible to see without aid. To study the corona on a normal day, researchers rely on telescopes equipped with a camera and an occulting disk — essentially an artificial moon — that blocks out the sun’s ultra-bright face. Called coronagraphs, these instruments are stationed on the ground or in space.

French astronomer Bernard Lyot invented the coronagraph in the early 20th century “because he was frustrated that eclipses happen so infrequently that he came up with this design whereby you can basically make an artificial eclipse any time you want,” said Shadia Habbal, an astronomer and professor of solar physics at the University of Hawai’i at Manoa’s Institute for Astronomy.

But these tools have some limitations. Ground-based coronagraphs have to compete with the brightness of the daytime sky, which can make getting a clear look at the dimmer parts of the corona difficult. Coronagraphs in space typically use occulting disks that are slightly larger than the telescope’s view of the sun, Caspi explained, which means that they extend slightly beyond the solar surface and block the lower part of the corona from sight.

“The quality [of a coronagraph observation] is nothing like what you achieve with a total solar eclipse,” said Habbal, who also leads the Solar Wind Sherpas, an international team of scientists who travel the globe to document and observe the corona during total solar eclipses.

Animation by Megan McGrew/PBS NewsHour

Humans have been watching eclipses since ancient times. Astronomers have used the “clockwork of planetary motion” to predict these events for millennia, Habbal noted. Total solar eclipses, she said, have been key for humans to recognize that stars — including the sun — have an atmosphere that extends beyond their readily visible surface.

A scorching solar mystery

Today, researchers are investigating the mystery of the corona’s temperature, which is mind-blowingly hotter than the solar surface.

“It’s a conundrum from the point of [view of] physics,” Habbal said. “Usually, when a surface is hot, as you go away from the surface, then the temperature drops.”

The sun’s surface clocks in at about 6,000 degrees Celsius, compared to 1 to 2 million degrees in its atmosphere above. The cause of this disparity isn’t yet clear. But today’s researchers, far removed from the technological limitations of antiquity, can glean new insights by training modern observational tools on the sun during eclipses. These tools work by taking in specific types of light, which reveal all sorts of information about the celestial objects they came from.

Temperature isn’t uniformly distributed across the corona, a dynamic region that contains multiple different structures, Habbal said. Some of that material forms 2-million-degree loops that make their way back around toward the solar surface. Solar wind, on the other hand, is closer to 1 million degrees, and it hurtles into interplanetary space as it leaves the atmosphere.

During a total solar eclipse in 2017, Caspi and his colleagues collected images of the corona in the midwave infrared range using a special camera aboard one of NASA’s high-altitude jets. That imager documented wildly different temperature ranges, including some coronal structures that measured tens of thousands of degrees yet somehow glowed at about the same brightness in the midwave infrared as other structures that were millions of degrees, Caspi said. For this year’s experiment, his team plans to make new observations to help explain the physics behind this phenomenon.

Harnessing the eclipse to study the corona

Caspi and Habbal are each spearheading different solar eclipse research projects that will use a combination of ground-based observations, as well as the instruments aboard the two NASA jets.

Those jets will reach 50,000 feet — above 90 percent of the atmosphere — during the eclipse, Caspi said. That’s important, he noted, because our atmosphere absorbs or blocks various types of light while also emitting its own glow. At their ultra-high altitude, the jets will have a much clearer view of the eclipse.

The Caspi team’s instrument will capture several separate ranges of light across the infrared and visible stretches of the spectrum, allowing for more detailed imaging that could help explain what causes different parts of the corona to glow in the midwave infrared range, and why they appear as bright as they do.

Habbal’s team selected two imagers and two spectrometers to fly during the eclipse. The imagers will document a kind of heat map of the corona that reveals which parts are 1 or 2 million degrees Celsius.

Meanwhile, the spectrometers will be used to capture the corona’s chemical composition, or the various elements that can be detected there. During the flight, one of the spectrometers will take in near-ultraviolet light, a part of the light spectrum that Habbal said can’t be captured from the ground.

Caspi is also involved with Citizen CATE 2024, a program composed of 35 volunteer teams stationed from Texas to Maine. Each group will record the eclipse during totality, when the moon completely obscures the face of the sun — which will range between 3.5 to 4.5 minutes depending on their location. Those clips will later be stitched together to create a single continuous observation.

total-solar-eclipse-western-australia-2023

The SwRI-led Citizen Continental-America Telescopic Eclipse (CATE) 2024 project evaluated special cameras to measure the polarization of coronal light during the April 2023 total solar eclipse in Exmouth, Western Australia. In this image, the colors indicate the polarization or orientation of the light. The white features, called prominences, have no polarization. Image courtesy of SwRI/Citizen CATE 2024/Ritesh Patel/Dan Seaton

“Instead of having totality for four minutes, we’ll have totality for an hour,” Caspi said. This extended view should help his team observe particularly dim coronal structures, as well as dynamic processes that take more than a few minutes to play out, he added.

Meanwhile, Habbal will be stationed in Arkansas, one of her team’s ground observation sites, among two others in Texas and Mexico. Each site is located roughly 500 miles apart, and their observations will span about 10 minutes of the eclipse during totality. One facet of their ground-based research aims to discover whether temperatures in the corona may reach 3 million degrees

Making a career out of chasing solar eclipses may sound glamorous, but finally taking in these cosmic wonders with the naked eye is a small fraction of the job. Caspi witnessed his first total solar eclipse in 2023, but he said he glimpsed only 10 to 15 seconds of the 58-second totality because he was occupied with livestreaming the event.

The 2024 solar eclipse will mark Habbal’s 20th time, but she said that 40 percent of those previous experiences were “clouded out.”

“I strive to look at the [corona] during totality, but most of the time we’re busy making sure everything is working,” she said.

Unlike other, more forgiving opportunities for scientific inquiry, total solar eclipses are a “hit or miss” situation, Caspi noted. Once those fleeting moments of totality are over, scientists must wait for the next one to come around.

“With a solar eclipse, it’s like, ‘Well, if you miss it, you’ve missed it,” he said. “And if you miss it by even a minute, you’re done.”

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