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Science Tribune - News - June 1998


Cool Sun reveals a magnetic personality

David Cromwell

James Rennell Division for Ocean Circulation, Southampton Oceanography Centre, Southampton, SO14 3ZH, UK
E-mail : ddc@soc.soton.ac.uk

The mystery of coronal heating

Our nearest star, the Sun, is the benign nuclear reactor upon which all our lives depend. Deep inside its core, atomic nuclei are fused together at unbelievably high temperatures, releasing the energy which reaches the Earth as life-giving sunlight. Should the solar nuclear reactor ever shut down - and thankfully that's unlikely for at least a few more billion years! - we'd know about it after a lapse of about 8 minutes: the travel-time for light to reach the Earth from the Sun, 93 million miles away.

You might think that, given such a nearby star, astronomers would have pretty well wrapped up everything we need to know about the Sun. But no. There remain many questions about how this superhot furnace works.

One of them hinges on the enormous temperature difference between the Sun's visible surface and the giant atmosphere - the "corona" - above it (a). "The outer part of [the Sun's] atmosphere forms a giant halo of eerie light around the Sun during a solar eclipse and consists of giant, super-hot loops that extend high above the Sun's surface," explains Professor Eric Priest, a solar physicist based at St Andrews University. A question that has been puzzling astronomers for over half a century is why the Sun's surface is relatively cool - a mere 6,000 degrees C or so - while the Sun's corona is much hotter, over a million degrees C.

Super-hot coronal loops

A team of solar physicists from the UK, France and the USA has just come a step closer to resolving this solar conundrum (1). According to them, it's all because our cool Sun is bursting with magnetic energy. "A clash of magnetic fields is likely to cause the heating of gigantic and super-hot loops of material that arch high above the Sun's surface," says Priest who led the team. "They tangle like spaghetti in the solar atmosphere and break, causing dozens of explosions that release energy along the loop. These turbulent explosions occur in tiny regions of intense electric current that heat the atmosphere in the same way as the electric current in a light bulb or an electric fire." This could be the mechanism which keeps the corona hot.

Using data from an X-ray telescope aboard the Japanese/US/UK space satellite called Yohkoh, which means "Sunbeam," the team has measured how the temperature varies along giant coronal loops. The observations were compared against three different theories of coronal heating. The theory which stood up best was one predicting a uniform heating along the loop caused by "a clash of magnetic field lines." This finding does not solve the puzzle of the corona's intense heat but it does point to a heating mechanism.

Solar flares - extreme coronal heating!

Under extreme conditions, as yet not understood, the giant coronal loops studied by solar physicists can sometimes burst open catastrophically in an enormous solar flare - the most energetic event in the solar system. Solar flares disrupt communications satellites and power systems here on Earth. And although it has never happened to date, a solar flare - which emits high-energy particles - could prove fatal for any astronaut who happens to be working outside the safe confines of a spacecraft.

Physicists believe that a solar flare is triggered by "magnetic reconnection" when the fields of magnetic energy in coronal loops abruptly change their configuration. The huge release of energy is like a twisted rubber band unwinding or breaking. It may be an extreme example of the "quiescent coronal heating" investigated by the team. Again, everything seems to hinge on the details of how the Sun's surface and corona interact, on how energy is transported from the solar surface into the outer atmosphere.

A new spacecraft being used to study this transfer is the US space agency NASA's Transition Region And Coronal Explorer (Trace) satellite. Launched in April 1998, it will be able to detect changes in solar activity occurring in short periods of time or over short distances on the Sun (b). Already, it has revealed that large areas of the Sun can heat up or cool down significantly, and thus appear and disappear in just a few minutes.

As ever in science, new questions arise even as old ones appear closer to being answered. Meanwhile, the Sun shines nonchalantly on !


(a) An excellent description of the solar corona, including links to eclipse images and movies of the sun in soft X-rays and visible light can be found at: http://www.windows.umich.edu/sun/atmosphere/corona.html

(b) A description of the Trace satellite mission can be found at: http://www.lmsal.com/TRACE/welcome.html


1. Priest ER et al. Nature 393, 545-547, 1998.