Eclipse watchers will go after the biggest solar mystery: Why is the corona so hot?

A total solar eclipse shines a light on the sun's elusive atmosphere. When the moon blocks the sun, it’s finally possible to see how this diffuse cloud of plasma, called the corona, is magnetically sculpted into beautiful loops. The material there is about a trillionth the density of the solar surface. From its delicate and diaphanous appearance, you might expect the corona to be where the sun goes to cool off.

That couldn’t be more wrong. The corona is a mysteriously sizzling inferno where the temperature jumps from a mere few thousand degrees to several million degrees. Why?

“It’s one of the longest unanswered questions in all of solar physics,” says Paul Bryans of the High Altitude Observatory at the National Center for Atmospheric Research in Boulder, Colo. “There are a bunch of different ideas about what’s going on there, but it’s still highly debated.” Data collected during the Aug. 21 solar eclipse may bring scientists closer to settling that debate.

The sun simmers at about 5,500° Celsius at its visible surface, the photosphere. But the gas just above the photosphere is heated to about 10,000° C. Then in the corona, the temperature makes an abrupt jump to several million degrees.

“It’s counterintuitive that as you move away from a heat source, it gets warmer,” Bryans says. The corona’s diffuseness makes its heat even stranger — the most basic ways to heat a material rely on particles crashing into each other, but the corona is too tenuous for that to work.

An eclipse first brought this abnormal arrangement to light. German astronomer Walter Grotrian observed spectral lines — the fingerprints of elements that show up when light is split into its component wavelengths — emitted by the corona during a total solar eclipse in 1869.

Astronomers at first assumed those lines were due to a new element they dubbed coronium. But Grotrian realized that iron atoms stripped of several of their electrons by the heat were responsible. These iron lines in the corona are still used to measure its temperature: The more electrons lost, the hotter the material in the corona (SN Online: 6/16/17).

Such extreme temperatures have something to do with the corona’s magnetic field, which is probably where all that energy is stored. Once the energy is there, the corona has a hard time radiating it away, so it builds up. Most of the ways that materials release energy — stripping electrons from atoms, accelerating those electrons so they release X-rays and ultraviolet particles of light — are already maxed out in the corona.

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“We know there’s energy coming in, and it’s hard to get it out unless you get very hot,” says Amir Caspi of the Southwest Research Institute in Boulder, Colo. “What we don’t understand is how that energy gets into the corona in the first place.”

Physicists have several ideas. Maybe loops of magnetic field lines in the corona vibrate like guitar strings, heating things up, sort of like how a microwave oven heats food. Maybe the magnetic anchors of those loops on the sun’s surface braid and twist the magnetic field above them, dumping in energy that is then continually radiated away like the heating element in a toaster.

Or maybe tiny explosions called nanoflares or jets called spicules carry energy away from the photosphere and into the corona. The formation of new coronal loops that connect to existing ones could dump in enough extra energy to heat the plasma up.

During the solar eclipse, dozens of groups of scientists across the country will deploy telescopes equipped with filters to pick out polarized light, infrared light or those electron-deprived iron atoms in search of answers. Bryans and his colleagues will be on a mountaintop near Casper, Wyo., in the path of totality. There, the team will take images at a fast clip in both visual and infrared wavelengths to map how the corona changes as the moon moves across the sun. (I will be in Wyoming with this team on the day of the eclipse and will be sharing more about how the experiments went.)

“We can look at how things change as we move from the surface up into the atmosphere,” Bryans says. “How that changes is tied to understanding how the corona is heated.”

Probably all of those mechanisms scientists have thought up contribute to the corona’s extreme heat. It’s difficult to declare just one the most important. But ultimately, the solar eclipse is the best chance scientists have to test them. It’s the only time the corona is the star of the solar show.