TOTALLY AMAZING Keon Gibson, an intern for the National Center for Atmospheric Research, shot the moment of totality through a telescope atop Casper Mountain, Wyo., on August 21. The corona stretches around the moon-blocked sun. The star system Regulus is visible at bottom left.
Keon Gibson
CASPER MOUNTAIN, Wyo. — It’s nothing like a sunset. It’s cold and dark, but it’s not like nighttime, or even twilight. The moon just snaps into place over the last slivers of the sun, turning the sun into a dark hole. The only illumination — a flat, ghostly, metallic sort of light — is from peaked gossamer streamers stretching out toward the edges of the sky.
I’ve been writing about eclipse science and interviewing researchers who study that eerie halo for the better part of a month. I thought I knew what to expect from my first total solar eclipse.
I had no idea.
I’m at a Baptist summer camp called Camp Wyoba about a half hour’s drive up a mountain from Casper, Wyo., with a group of engineers and solar physicists. Most come from the National Center for Atmospheric Research, or NCAR, in Boulder, Colo.
Our presence here is a stroke of luck: Retired NCAR researcher William Mankin’s wife Mary Beth is a Baptist pastor. When they realized the camp would be in the path of the total eclipse, the Mankins suggested holding an event, complete with a scientific lecture the night before and a church service in the morning. They also invited Mankin’s former NCAR colleagues to bring their experiments — and their families.
The day before the eclipse, scientists tested their equipment in a field at the top of Casper Mountain near camp, while a group of kids played dodgeball nearby. But by afternoon, the team’s luck seemed to be flagging. One of their telescopes started malfunctioning in a way they hadn’t seen before. They had less than 24 hours to fix it. “It’s a very bad thing if we can’t get it going,” said instrument leader Steven Tomczyk.
Tomczyk and his colleagues schlepped three telescopes and a spectrometer the size of a coffee table up here to try to solve one of the greatest mysteries of the sun’s corona: Why this ethereal solar atmosphere is so much hotter than the sun’s surface?
The visible surface of the sun is about 5,500° Celsius. Higher up in the sun’s atmosphere, though, the temperature jumps to 10,000° C and then makes a sudden leap to millions of degrees. It’s a real puzzle why. Most materials transfer heat via atoms smacking into each other or through swirling, churning currents. In the corona, which is made of a diffuse charged gas called plasma, particles are so far apart that neither scenario seems likely.
Solar physicists are pretty sure that the corona’s magnetic field is somehow to blame for the heat up (SN Online: 8/16/17), but it’s so weak that it has never been measured directly. So the team in Wyoming hopes to chip away at understanding that magnetic field. Their experiments will take steps toward measuring its strength and shape so that a future telescope can make a more complete measurement.
The spectrometer will measure the corona in infrared wavelengths between 1 and 6 micrometers — the first time it has been measured fully in this range. Infrared light is a good probe of the magnetic field because stronger magnetic fields change the way light is emitted in that range. Atoms in the corona are so hot that they give up many of their electrons — iron atoms have been known to lose up to half of their original count. The remaining electrons are often excited to higher energy levels, and when they drop back into their original state, they emit a particle of light in a particular wavelength. That photon shows up as a peak in the spectrum.
Magnetic fields make the higher energy levels split into two new levels, so electrons dive from two different platforms and emit different particles of light. That makes the peak split in two as well. The stronger the magnetic field, the farther the distance between the peaks.
The spectrometer won’t directly view the sun — it’s inside a trailer. A hole in the trailer wall leads to an angled mirror, which will track the eclipsed sun as it moves across the sky and direct the light into the instrument.
There, a beam splitter will split the light in two and direct it through a series of gold-plated mirrors. Ultimately, the light beams will be recombined. If all goes well, the shape of the light wave at the end will allow the team to calculate the sun’s infrared spectrum. They’re looking for already known peaks in the spectrum — one from silicon that has lost eight electrons, for instance, was observed in 2003 when the sun wasn’t eclipsed — and ones theorized in the 1990s but never observed.
“We’re at the ragged edge of our signal to noise,” says James Hannigan, who’s in charge of the spectrometer. “I’m really not sure what we’re gonna see.”
This eclipse is this instrument’s maiden voyage; it was designed in the 1990s but completed only a few months ago. It has had some last-minute headaches, too, Hannigan says. The beam splitter, a sort of half-transparent mirror, had to be polished until its height varied no more than 80 nanometers — or 80 billionths of a meter. It was so difficult to do that the piece of equipment arrived at Hannigan’s house only nine days before the eclipse. “It’s a little more harried than I would have liked,” Hannigan says. “I would have loved to have been testing this thing for the last month and a half, but so it goes.”
Outside, Tomczyk and the rest of the crew are testing the three telescopes. One will take a picture of the entire corona in infrared wavelengths out to 10 solar radii away from the sun’s surface. That will provide context for the other measurements, letting the team figure out the strength of the field in different parts of the corona.
Another is actually two telescopes linked together: one infrared and one that measures visible wavelengths. Both send data to a spectrograph, which splits light into all its component wavelengths. The visible light telescope’s job is to take a quick spectrum of the layers of the sun’s atmosphere between the photosphere and the corona, an area called the chromosphere.
The chromosphere is only visible for a few seconds at the beginning and end of an eclipse. For those few seconds, the visible telescope will take a picture once every 1/125th of a second. “It will help us understand how the atmosphere is changing with height, which helps connect the corona to the surface,” says Philip Judge, one of the experiment’s principal investigators.
The third telescope — a polarization camera that will measure the magnetic field’s shape — is the one that’s acting up.
“We’ve been rehearsing this dance over the last couple of days,” says Ben Berkey, who works for NCAR in Hawaii. They’ve practiced every motion they’ll make during the eclipse: Check that the sun is in each telescope’s field of view; remove the lens caps at just the right moment, to get as much time watching the corona as possible without frying the delicate instruments; and so on.
“If things are boring, that’s not a bad thing necessarily,” Tomczyk says during one run-through.
“But you won’t be bored,” says Paul Bryans, one of the science leads. “You’ll be watching the partial eclipse.”
By 4 p.m., the problem with the polarimeter is solved: The computer storing the data needed its hard drive reconfigured. The team is so nervous about losing the data that they plan to make four copies of the hard drives before leaving Casper Mountain, and send them back to NCAR in four different cars, just in case. “They’re precious,” Tomczyk says.
The morning of the eclipse dawns cool and clear.
It’s already getting chilly when Judge bellows, “Two-minute warning!” The team jumps into action, taking peeks at the last tiny slices of sunlight through eclipse glasses.
The moment of totality is sudden and absolute. The corona pops into view all at once, pointing its silvery arms at the treetops and the sky. People cheer; some children scream. Someone lends me a pair of binoculars, and through them I can see the chromosphere, glowing red and purple. I can see Mercury, nestled right up next to the corona.
And just as suddenly, it’s over. Judge counts down the seconds to the end of totality, and right on schedule, the sun returns. It’s incredible how much light that tiny dot of sunlight provides. I had been told that a 99 percent eclipse is nothing at all like a total eclipse. I get it now.
Tomczyk and the crew, meanwhile, are already backing up their data and taking the telescopes off their tripods. All the instruments worked, although they’ll have to take the data back to Boulder and process it to know if they got all they’d hoped for.
“Who knows what we’ll see,” Tomczyk says. “I feel exhausted. And relieved.”
Editor’s note: This story was updated August 24, 2017, to correct the photo caption. The bright spot in the lower left corner is the star system Regulus, not Mercury.
