In about 100 days, on Aug. 21, a total solar eclipse will sweep across the continental United States for the first time since 1918. Astronomers are calling it the Great American Eclipse.
For solar astronomers, the eclipse offers 3 minutes (give or take) to collect as much data as possible about the sun's usually hidden outer atmosphere.
Researchers have been anticipating the event for years.
Some will take measurements from the sky; others have engaged vast networks of citizen scientists to track the eclipse as its shadow moves across the ground. They hope their findings will tell them more about the sun's magnetic field, the temperature of its outer atmosphere, and how energy moves through the star and out into space.
But no matter how carefully scientists plan, nature can disrupt the observations with something as trivial as a cloud momentarily passing through the wrong patch of sky.
"I've had those experiences, and it's heartbreaking," said Shadia Habbal, who studies the solar wind at the University of Hawaii's Institute for Astronomy.
A partial eclipse can be stunning, but that phenomenon is as different from a total eclipse as day is from night.
The sun is so bright that even when 99 percent of it is covered by the moon, the remaining 1 percent is still bright enough to make the sky blue, said Jay Pasachoff, an astronomer at Williams College in Massachusetts who has seen 33 total eclipses and 32 partial eclipses. During a total solar eclipse, the moon completely obscures the face of the sun, causing the daytime sky to darken by a factor of 1 million.
This time of totality lasts only a few minutes. Those who have seen it say it's unlike anything they've ever experienced.
"It's a really unique feeling, standing in the shadow of the moon," said Matt Penn, an astronomer at the National Solar Observatory in Tucson. "Crickets start to chirp. Birds start to roost. Chickens do weird things. And it's all in reaction to the strange light."
A total solar eclipse occurs somewhere on Earth about once every 18 months, and most eclipse-chasers have to travel far from home to see one.
On Aug. 21, however, what's known as the path of totality will cut a 60-mile-wide arc across the United States, beginning in Oregon at 10:15 a.m. local time and ending in South Carolina about an hour and a half later.
Experts estimate that 11 million people won't have to travel at all to observe it, and an estimated 76 million more will be within a 200-mile drive.
Scientists expect it will be the most-studied eclipse of all time.
Most researchers plan to study the sun's outer atmosphere, or corona. This is a vast region of superheated gas held in place by the sun's magnetic field.
Under normal circumstances, the corona isn't visible from the ground because it is overwhelmed by the brightness of the photosphere, the sun's main disk. But with the photosphere blocked, the corona will become a pale, spiky halo of streamers that appears to radiate from the blacked-out solar surface.
Composite images and measurements made during other eclipses reveal that the corona is composed of a complex swirl of gases much hotter than the surface of the sun. The surface is a toasty 6,000 kelvins (more than 10,000 degrees Fahrenheit), but the temperature of the corona averages 1 million kelvind (1.8 million degrees Fahrenheit).
"The fundamental question we are asking is, what is causing the atmosphere to heat up like that?" said Habbal. "This is one of the scientific mysteries regarding the sun that remains unanswered."
Habbal has led 14 eclipse expeditions since 1995, traveling as far as the Arctic. This year, she and her colleagues will make the most of the Great American Eclipse by viewing it from five distinct sites from Oregon to Nebraska.
Each group will wield custom-made cameras that can capture images of the corona in the spectrum of visible light. The teams will also take spectra measurements to see which elements are in the corona and how hot they are.
On the other side of the country, researchers from the Harvard-Smithsonian Center for Astrophysics are planning to study the corona from a plane flying at 49,000 feet.
The group, led by solar physicist Ed DeLuca, is building an instrument that will allow them to examine the solar atmosphere in infrared wavelengths. Their goal is to better understand the magnetic fields in this outer region of the sun -- in part because this is where coronal mass ejections originate.
"Measuring these magnetic fields is really useful for understanding how energy is stored in the corona and when we expect it to be released," DeLuca said. "Once we understand that, we can make better space weather predictions."
A coronal mass ejection sends millions of tons of the sun's material hurtling through space. If a well-aimed one hits Earth, it can mess with the planet's magnetosphere and inflict damage on satellites, astronauts and the power grid.
DeLuca is hoping the weather won't be a problem. The flight is happening over Tennessee, where thunderstorms have been known to go quite high, but they usually don't develop until later in the afternoon.
On the day of the eclipse, the researchers will have to make sure the light from the solar atmosphere comes through a 6-by-9-inch window on the right side of the plane. Then it will hit a telescope that feeds a spectrograph enclosed in a cryogenic vacuum chamber.
The plane will fly along with the shadow of the moon, giving the scientists an additional minute of observing time. Every minute counts when you have less than 5 minutes to collect data.
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