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

by Dr Tony Phillips

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.

Lots of theories...Not enough data...

There are many ideas to explain the extraordinary warmth of the Sun's corona. The leading theories fall into three categories: miniature solar flares, atmospheric waves, and electrical dissipation.

A typical solar flare releases 10²⁷ ergs per second, equivalent to millions of 100-megaton hydrogen bombs exploding at the same time. Although solar flares are powerful, there aren't enough of them to maintain the corona's high temperature. Instead, some solar physicists, including Ron Moore and Jason Porter of the NASA/Marshall Space Flight Centre, suspect that miniature solar flares, called microflares, are responsible. Microflares are small explosions that release less than 1% of the energy in a full-fledged flare. Although they're relatively small, microflares may be so numerous that as an ensemble they could provide the energy needed to keep the corona superheated.

Unfortunately, microflares are so small that they're near the limits of what current telescopes can see. The number of observed microflares doesn't quite add up to the energy budget of the corona but there could be many more very faint ones that can't yet be detected. The new TRACE satellite is providing better sensitivity and resolution than previous instruments, and solar physicists are looking forward to 2004, when Japan will launch Solar B with a more powerful array of telescopes to test these theories.

Another way to heat the corona involves magnetic waves called Alfvén waves. The solar atmosphere is permeated with magnetic fields that are especially intense around sunspot groups. The sun is a rumbling, boiling, dynamic place, so the magnetic field lines are constantly shaking back and forth. These oscillations send waves of magnetised plasma (ionised gas) propagating outward into the corona.

"You can think of an Alfvén wave as acting something like a bullwhip," explains Hathaway. "A magnetic Alfvén wave starts out in the dense atmosphere near the photosphere and moves upward into the corona where its amplitude grows in the tenuous gas. Eventually the wave breaks and it dumps its energy in the form of heat. It's like cracking a whip. A small shake near your hand (the photosphere) becomes a big crack near the tip (the corona)."

A breaking ocean wave deposits some of its energy on the beach. Scientists think that acoustic waves and magnetic Alfvén waves from the sun's lower atmosphere might propagate upward into the corona where they "break" and convert wave energy into thermal energy that heats the gas there.

Solar sound waves might work in much the same way. The Sun is churning with seismic activity that produces a roar of gaseous compression waves in the solar atmosphere. Sound waves that propagate from the dense lower atmosphere into the rarefied corona break, like an ocean wave on the beach, and convert wave energy into thermal energy.

Hathaway and colleagues have studied acoustic vibrations in the sun's atmosphere with periods slower than about 1 minute. They find that the total energy in such waves cannot deliver enough heat to the corona to keep it warm. That doesn't mean that sound waves can't do the job. The instrument that Hathaway et al used for their study was not sensitive to vibration periods faster than about 60 seconds. If there is a great deal of energy in shorter period sound waves and if they can propagate all the way out to the corona - two very big ifs - then acoustic waves could be a important heating source.

"In principle, in situ measurements by spacecraft are great for characterising all kinds of waves, but none of the satellites in orbit now are close enough to reveal what's really happening in the corona," continued Hathaway. "The Solar Probe Mission could change all that. Solar Probe will have a special heat shield that lets it fly right through the corona within a few solar radii of the sun. With direct measurements we might finally understand what's going on in there."

Meanwhile, here on Earth, impatient scientists aren't waiting for the next wave of high-tech spacecraft to solve the problem. Hundreds of astronomers and solar physicists positioned themselves along the path of totality of the August 11, 1999, solar eclipse to gain a fleeting view of the corona with state-of-the-art observing hardware. Prof. Jay Pasachoff of William's College led an expedition of over 30 scientists and students to Romania where the eclipse lasted longest, 2 minutes and 29 seconds. Using high-time-resolution digital cameras, Pasachoff's team observed the Sun through filters sensitive to the Fe XIV green line in hopes of detecting the signatures of high-frequency Alfvén waves in the corona.

The sun isn't the only place where temperature seems to defy common sense. Just as the temperature of the corona increases with altitude so does the temperature of Earth's stratosphere. The lowest region of our atmosphere is called the troposphere. It contains about half the total mass of Earth's atmosphere and is the layer where weather takes place. In the troposphere the air becomes colder as one ascends to higher altitudes. The primary source of heat for the troposphere is the sun-warmed surface of the Earth, which is why the troposphere is warmest near the surface. Just above the troposphere is the stratosphere, where the temperature begins to increase again. The relatively high temperature of the stratosphere is caused by the presence of an ozone layer near an altitude of 25 kilometres. The ozone molecules absorb high-energy ultraviolet rays from the sun, which warm the atmosphere at that level.

"It was a complete and total success," reported Pasachoff when the eclipse was over. "We viewed the two and a half minutes of totality in a completely clear sky. We played back data from our hard drives, and we can see that we have fabulous scientific data. It should keep my students and me busy for years."

Pasachoff's group was not alone. Scientists from many countries captured high resolution images of the corona during totality in search of microflares, magnetic vibrations and other phenomena. High-caliber data obtained during the eclipse may help scientists evaluate another possible mechanism for coronal heating: electrical dissipation.

It's been known since the days of Faraday and Maxwell that if you wave a magnet back and forth in the vicinity of a conducting wire, a current is induced in the wire. The same thing takes place in the sun's atmosphere. Oscillating magnetic fields generate currents that flow through the highly ionised gases above the photosphere and in the corona. How does that heat the corona? When current flows through a resistor some of the energy is dissipated as heat. A common light bulb is a good analogy. Electricity moves through a partially conducting filament, the filament glows and it also become very hot. Again, the question hinges on better observations of magnetic fields and plasmas in the corona. Scientists know that there is some resistive dissipation of energy in the corona, but they can't be sure how much.

There is no shortage of ideas about what may heat the corona. Microflares, magnetic and acoustic waves, and electrical dissipation are all good candidates, but the observed energy flux into the corona from each of these mechanisms is about an order of magnitude too low to account for coronal heating. More and better data are needed to finally reveal the culprit.

"My bet is that it's going to be some mixture," says Hathaway, "but only time will tell! When we do know, we'll have solved one of the big mysteries in astrophysics."