When you observe a solar eclipse—with great care, of course—what you see is a thin, red crescent outlining the blocked-out Sun and, extending beyond it, a stark white mane. This is the corona, an aura millions of miles thick of superheated plasma.
It's natural to assume the corona is cooler than the sun's blazing surface. But in fact it gets hotter as it ranges outward. The Sun's surface temperature is about 10,000 degrees Fahrenheit. The corona can get as hot as 5.4 million degrees Fahrenheit. This phenomenon, known as the "coronal heating problem," "remains one of the great unsolved problems in space science," says James A. Klimchuk, research astrophysicist at the NASA Goddard Space Flight Center.
Scientists still don't quite understand why the corona is so toasty. But at least they accept that it is hotter. Which was not the case for a long chapter in science history. It took a little-known Swede, Hannes Alfvén, an electrical engineer by training, to convince the world of the corona's hotness.
Around the early 1940s, there was no coronal heating problem. That's because scientists were confident the corona's temperature was lower, or not drastically higher, than the sun's surface temperature. One estimate,…
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