The Sun's Sizzling Corona

Scientists continue to ponder one of the Sun's most closely guarded secrets.

On a cold winter's day a good place to be is by a roaring fire with a cup of hot cocoa. If the fire feels a bit too hot - no problem - just move away. As with most heat sources, the temperature in the vicinity of a fireplace falls with increasing distance from the hearth.

This simple rule works for fireplaces, but not so well for the atmosphere of our planet's main source of heating - the Sun. A thermometer at the at the sun's surface (called the photosphere) would register a sizzling 6000 degrees C - a bit too warm for comfort. Unlike the family fireplace, however, moving away from the photosphere doesn't lower the temperature. In the 3000 km zone between the photosphere and the corona, the temperature jumps from 6000 C to over one million degrees. In apparent defiance of common sense, the further away you go from the Sun's surface into the solar atmosphere, the warmer the gas becomes!

A 130 year old mystery...

For most of recorded history, the corona was simply a mysterious pearly white glow around the Sun and Moon during a total eclipse. It wasn't until the 17th and 18th centuries that astronomers were certain that the corona surrounded the Sun and not the Moon. The first hint that the corona might be really interesting surfaced during the total solar eclipse of August 7, 1869, which was visible from Alaska to North Carolina, when observers discovered a weak emission line in the green part of the spectrum of the corona. The wavelength of the line didn't correspond to lines in the spectra of any known elements on Earth. Some scientists thought it might be a new element called Coronium. Others were convinced that it was simply the spectral signature of an already known element, but that the wavelength of the "green line" hadn't been properly measured. For the next 70 years solar astronomers followed eclipses and made increasingly precise observations of the corona's spectrum. One by one, elements like argon, oxygen, and calcium were rejected as candidates. Finally, in 1939, the mystery was solved when Grotrian and Edlen showed that the green line was emitted by a species of highly ionised iron, Fe XIV.

Jonathan Kern Only in the fleeting darkness of a total solar eclipse is the light of the solar corona easily visible from Earth. Caltech scientist Jonathan Kern captured this image from Romania during the August 11, 1999, total solar eclipse. The dark sky during an eclipse contains some blue light scattered from the bright corona and from the sunlit horizon. The blue sky colour shows up in this unusual picture because of the long exposure on colour film. The coronal image was obtained using a radially graded filter in the image plane so that the inner corona is not over-exposed.

The solution of one mystery - the origin of the green line - led to another more perplexing conundrum. Fe XIV ("iron fourteen") is an iron atom that has lost 13 of its 26 orbital electrons. Stripping iron of so many electrons requires an enormous amount of energy - such an ion can only exist in a very hot gas. Careful studies of spectra obtained during eclipses show that the temperature of the corona is about two million degrees C, hundreds of times hotter than the surface of the Sun. Since the corona cools rapidly, losing its heat as radiation and the solar wind, something has to be pumping energy up from the surface. But what? That's what scientists would like to know.