You’ve been drawing the sun’s corona ever since you were in pre-K — and that’s probably the last time it made any sense. The sun is the 865,000-mile ball of gas that was the scribbly yellow circle in your drawing. The corona is the veil of luminous plasma streaming millions of miles into space, where you drew straight yellow rays. Things were never so simple again.
Studying the mysteries of the corona is not easy, for the same reason that looking at the sun itself isn’t easy: the brilliance of the solar fires washes out everything else. Coronagraphs — black masks fitted in telescopes and other observing instruments — can cover up the solar disk and allow astronomers to focus just on the plasma. But diffraction of the incoming light makes the pictures imperfect.
It is only during a total eclipse, when the moon itself acts as the greatest coronagraph of all, that a truly good look at the corona becomes possible. That’s exactly what will happen on August 21, when the event that is being called The Great American Eclipse tracks across the U.S. in a path of totality that will run from western Oregon to eastern South Carolina, traveling from coast to coast in just over 90 minutes. Those will be 90 minutes that scientists from NASA, the University of Hawaii, the Southwest Research Institute, and multiple other labs and universities plan to spend well, scrutinizing the corona until the moon passes by and the sun once again forbids such a clear gaze from Earth.
Perhaps the greatest mystery of the corona concerns its temperature. The heat on surface of the sun tops out at about 10,000º F (5,537º C). But the corona — which streams into the frigidity of space — blazes at millions of degrees, or exactly the opposite of the differential that would be expected. So-called solar tornadoes, vortices of magnetic turbulence that swirl up from the surface, might be to blame. So too may be deeper, magnetic tsunamis within the sun, transferring their heat outward. Observations during the eclipse could help settle the issue between the two, or reveal another, entirely unexpected cause.
Magnetic field lines will also be visible in the coronal veil, as they loop out of the sun and fall back, leaving glowing lines traced in their path. Without the benefit of an eclipse, astronomers must infer these magnetic fingerprints. During an eclipse they need merely look up.
The 2017 spectacle will also afford an opportunity to compare the current appearance of the corona to the way it looked during the 2012 eclipse — and the difference should be significant. In 2012, the sun was going through one of the phases known as the solar maximum — when it is especially active and turbulent. At the moment, the sun is in a solar minimum phase. The spiky corona of five years ago will likely give way to a quieter one, with a great deal of complex activity near the solar equator but much less in the north and south hemispheres.
It is impossible to say now what discoveries might be made during the fleeting 90 minutes of eclipse totality, but history offers precedent. It was during the August 18, 1868 eclipse, for example, that French astronomer Jules Janssen, studying the spectral lines in the fleetingly visible corona, noticed the signature of a buoyant gas that exists on Earth but had not yet been discovered here. The newly identified gas, appropriately, would take its name from Helios, the ancient Greek personification of the sun.
This time around, there may or may not be a discovery as fundamental as what we now know as helium. No matter what is found, however, the odds are good that on August 21, the world’s solar astronomers will be at least a little bit smarter at the end of the critical 90 minutes than they were before it.