On Aug. 21, as millions of Americans prepare to watch a total solar eclipse from the ground — wearing their safety spectacles, natch — a small team of scientists will hop aboard a Gulfstream V jet in Tennessee. Time will be short. The plane, a research aircraft owned by the National Science Foundation, will race to intercept the path of the moon's shadow and follow it as long as it can. The astrophysicists, once they're above southern Kentucky, should have a four-minute window to record the ghost of the missing sun.
They won't have another shot for years. The next solar eclipse to pass over the United States occurs in 2024.
“The thing that makes me nervous is we only have those four minutes,” said Jenna Samra, an applied physics graduate student at Harvard University. “All this work for years and then we’d have to wait for the next eclipse.”
Samra and her colleagues are stalking the sun's corona, the wisps of plasma that billow around the star. On a typical day, the bright light of the photosphere (the visible part of the sun's surface) obscures the dimmer, though mysteriously hotter, corona.
But days with total solar eclipses are far from typical.
“The moon is a perfect occulter for the sun,” Samra said. The moon will blot out all of the sun's photosphere and little else, leaving room to view the corona at the solar edge. “Eclipses are really ideal times to do this science.”
The astrophysicists will image the corona using a device called a spectrometer. “We’re looking to identify emission lines in the corona in the near infrared region,” said Edward E. DeLuca, a solar physicist at the Smithsonian Astrophysical Observatory in Massachusetts and Samra's adviser.
Emission lines form when electrons in the plasma smash into charged atoms. The collision might knock another electron free, like a tooth from a sucker-punched jaw. Using the light emitted from this knock, the researchers can identify the atomic element involved. During a solar eclipse in 1868, French astronomer Pierre Janssen discovered the element helium when he detected a yellow emission line. DeLuca said lines in the infrared spectrum are able to reveal heavier elements: silicon, sulfur, iron and magnesium.
From there, the experts will check whether it's possible to use these lines to probe the sun's magnetic field. The corona can be a turbulent place, belching solar flares and coronal mass ejections into space. The corona gets its energy for violence from a magnetic field, which Samra likened to a twisted rubber band.
Except that, once released, the snap comes with huge explosions of plasma. These plasma explosions drive space weather. But they “can’t be fully explained or predicted yet,” Samra said.
The most intense space squalls have been felt on Earth. During the Carrington Event of 1859, a giant curl of plasma caused telegraph lines to go haywire, sputtering out messages in gibberish. Others caught on fire. If such an event were to happen today, a sudden coronal mass ejection could disrupt electric grids and cause trillions of dollars in damage.
“It's important to understand the magnetic field, if we want to understand how the corona really works,” Samra said. “It's intimately linked to everything that goes on.”
Yet taking pictures of the corona in the infrared spectrum won't be easy. The Gulfstream jet's true advantage is altitude, not speed. The plane cannot fly fast enough to keep up with the eclipse, which, as Samra put it, will be “screaming across the country.”
The problem is water, which interferes with infrared light. “We need to be above the atmosphere,” she said. “Water vapor in the atmosphere absorbs infrared radiation.”
As the plane flies over Kentucky, the sun will be about 65 degrees above the horizon. At that angle, even a neck craned out one of the Gulfstream's windows would be of little help to see the eclipse. What would help is the porthole in the top of the plane. But none of the scientists aboard will have time nor space to lie down, look up and take in the astronomical event.
Instead, they'll be tending to the spectrometer. The device must be chilled with liquid nitrogen. This, in turn, means the whole thing must be sealed inside a vacuum chamber to avoid clogging ice. What's more, the porthole is in the top right of the plane; the spectrometer had to be set up on the left hand side. A mirror will reflect the sun into the spectrometer.
Complicating matters further, the scientists might be in for a bumpy ride. Planes usually fly paths that take into account the wind. The Gulfstream's heading is dictated by the solar eclipse. To minimize bad angles and jostles, Samra had to design a tracking system to point the mirror on the eclipse for the four-minute totality.
“This is pretty high-pressure,” Samra said. If all goes well, the spectrometer will capture an infrared image per second.
As for DeLuca, he said that high-pressure experiments come with the astrophysical territory. He's been part of projects that required launching a rocket into space only to gather five minutes of data.
“We put ourselves in these positions over and over again. It is stressful. But I have faith in the system,” DeLuca said, “and practice.”