"Combining ground-based eclipse results with the observations from satellites in the visible, ultraviolet, X-ray and radio parts of the spectrum will provide the most complete view of the solar atmosphere ever seen," noted astronomer Jay Pasachoff of Williams College writes in the August issue of Scientific American magazine. "Some might find it disconcerting that the sun, arguably the best studied of all celestial objects, is so incompletely understood. But I see the lingering questions as a wonderful excuse to share one of the greatest experiences in nature."
Ninety-three million miles from Earth, the sun is enormous by any human standard with a diameter of 860,000 miles and the mass of 330,000 planet Earths. By volume, the sun could swallow 1.3 million Earths. The sun generates so much gravity that a 170-pound human would weigh about 4,600 pounds on the sun's visible surface.
In the sun's hellish core, which extends out to about a quarter of the sun's diameter, the concentrated weight of the star creates enormous pressure and temperature -- 27 million degrees Fahrenheit -- enough to trigger nuclear fusion that consumes some 600 million tons of hydrogen per second.
It's been doing that for the past five billion years, balancing the inward pull of gravity with the outward push of fusion energy in the core. And even though the sun is a relatively small, run-of-the-mill yellow star, it still has enough gas in the tank, so to speak, to burn for another five billion years. But eventually, the core will run out of elements to fuse. At that point, gravity will take over and the core will collapse to become a compact, slowly cooling white dwarf. Along the way, the star's outer layers will be blown away into space and Earth will be burnt to a cinder.
That's the long-range forecast, based on theoretical analyses and decades of Earth- and space-based observations that have slowly peeled away many of the sun's mysteries. And as it turns out, some of the most profound are not in the sun's interior. The core is thought to be surrounded by an opaque "radiative zone" where the outward torrent of high-energy photons generated in the core work their way up toward the surface, constantly being absorbed and randomly re-emitted during endless collisions in this dense region.
Particles are packed so tightly that it takes photons about 170,000 years to make their way across the radiative zone and into the upper convective zone. By this point, the temperature has dropped to about 3.6 million degrees, powering titanic bubbles of electrically charged plasma that roil and rise to the visible surface of the sun -- the 10,000-degree photosphere.
As the name suggests, the photosphere is where light finally streams away into space in all directions, reaching Earth about eight minutes later. And this is where one might expect the temperature to steadily drop as the photons and charged particles are blasted away.
But somehow, during the very short trip from the photosphere, through a thin layer known as the chromosphere and into the sun's outer atmosphere, or corona, the temperature suddenly shoots up to some two million degrees.
In this same region, the sun's magnetic field accelerates electrically charged particles -- the solar wind -- to enormous velocities. Titanic magnetic storms can erupt, triggering high-energy flares and huge coronal mass ejections that can batter Earth's magnetic field, producing spectacular auroras, disrupting power grids and communications.
To learn more about what generates the temperature of the corona and the mechanisms responsible for solar storms, researchers need to study the inner corona and the transition zone where these processes are at work.
But that is extraordinarily difficult to do. The corona is a million times fainter than the sun's visible surface and as such, it is effectively invisible. It cannot be seen in daylight from Earth's surface or even in space without creating an artificial eclipse in a so-called coronagraph to block out the sun's glare.
Such instruments use a circular shield to cover up the sun's disk, blocking its glare and revealing the corona. But to keep from blinding sensitive detectors, the instruments must block the sun's light out to more than a solar diameter. The inner regions remain hidden.
Except during a total solar eclipse.
"What's particularly special about a natural eclipse is that the moon is a perfect occulter, it blocks the surface of the sun just perfectly so you can see very low into the solar atmosphere," said Carrie Black, associate program director of the National Science Foundation's Division of Atmospheric and Geospace Sciences.
During the Aug. 21 eclipse, the moon's shadow will cross the entire United States, from the coast of Oregon to the coast of South Carolina. Scientists from across the nation and around the world will be positioned along the 2,600-mile-long "path of totality" to study the corona in extraordinary detail.
"It is in those inner expanses (of the corona) that we seek an answer to one of the most nagging puzzles in astrophysics: Why does the sun's temperature increase as you move away from its surface?" writes Pasachoff.
"Within the sun, the temperature starts at 15 million degrees Celsius (27 million degrees F) at the center and steadily falls as you move outward, dropping to 5,500 degrees C (10,0000 degrees F) at the solar photosphere, the surface that emits sunlight into space. But then the trend turns around. The tenuous gas just above the visible surface climbs back up to 10,000 degrees C (18,000 degrees F) and abruptly leaps to millions of degrees. Scientists still debate the details of how that occurs."
There are two leading theories.
"One involves the loops of gas on the edge of the sun, the so-called coronal loops, and there are a couple of ways those loops can be made to vibrate and heat up all the gas around it to a million degrees," Pasachoff said in an interview with CBS News. "Then there's another class of models that has to do with billions of little, tiny, what are called nanoflares, going off all the time that could contribute.
"One of our main experiments is looking, at very high frequency, 10 times a second, at the spectral lines that come only from the hot coronal gas. And we're looking for how often those loops vibrate at different frequencies and where the peak vibrations are. By doing that, we hope to help pin down which of the models is most important."
In what amounts to a scientific assault, a dozen spacecraft will be monitoring the sun from space throughout the eclipse, multiple aircraft will carry instruments high above the thick lower atmosphere to probe infrared emissions and other phenomena and balloon-borne instruments will monitor how the atmosphere is affected by lower temperatures in the moon's shadow along the path of totality.
Some 200 citizen-scientists stationed at 68 sites will photograph the sun with identical equipment to generate a carefully calibrated movie showing the sun's inner corona, collecting data to learn how material is accelerated in magnetically driven plumes extending from the corona above the sun's poles.
"The idea is you imagine somebody across the table from you drinking a milkshake through a straw, you could track the particles moving along the straw and figure out their velocity," said Matt Penn, heading up the Citizen CATE (Continental-America Telescopic Eclipse) project.
"So that's what we're doing in the CATE images, tracking particles along these magnetic plumes, figuring out the velocity and hopefully the acceleration profile, how do they go from zero to 100 (kilometers per second) in this part of the corona."
Another project, the Eclipse Megamovie, aims to stitch together thousands of images from volunteers and the public at large to chart out the corona changes over time.
"Radio-wave studies have allowed us to closely observe very rapid variations of the corona, but now we expect to study such processes directly using visible light and thus enrich our knowledge of the sun's dynamic atmosphere considerably," according to the Megamovie website.
"The data gathered via the Eclipse Megamovie Project will be made available publicly and are expected to allow scientists to analyze the sun's corona for many years to come."
Photos can be uploaded to the project via an Android app, available now, or an Apple iOS app expected shortly.
Pasachoff plans to observe his 66th solar eclipse from Eugene, OR, where a large team of researchers will be stationed. And this time around, he hopes to take a few moments to simply enjoy the view.
During past eclipses, "I was so busy photographing during totality that I barely had time to look up to see it," he writes in Scientific American. "But now, with computer automation, I can enjoy a few seconds to savor the eclipse while cameras are clicking away and electronic sensors are uploading their data to computers. I look forward to the view."