Dark side of the sun

The sun is not round. It is more like a bag of jelly. Next year's eclipse will plunge Britain into mid-morning darkness and give scientists a chance to learn more about this second generation star. John Davies reports

It was impossible not to feel a mysterious and terrifying emotion. It was just as if the light of the sun had been extinguished for ever and the end of the world was at hand." That was the Daily Mail back in 19, writing about a phenomenon that is due to make its return to mainland Britain next year for the first time in over 70 years: the total eclipse of the sun.

If the skies are clear and you are in the 100-kilometres wide "path of totality" that extends over the Scilly Isles, Cornwall, parts of South Devon and Alderney in the Channel Islands late in the morning of August 11 1999, look up: as darkness falls in mid-morning you will be able to observe the last solar eclipse of the old millennium. When the moon gradually edges in front of the sun until there is only a thin silvery ring of sunlight surrounding the dark outline of the moon, will you experience an emotion similar to that of the 19 Daily Mail writer? After all, without this "second generation" star, which is estimated to last another four and a half thousand million years before collapsing, we would indeed be experiencing the end of the world.

For some people, though, eclipses are not just once-in-a-lifetime events. A total eclipse of the sun can be seen somewhere on earth about once every 18 months. There's one next week, for instance: on February 26, the moon's shadow will cross the northwestern tip of South America and the southern Caribbean, and not only amateur "eclipse chasers" but serious solar scientists will be turning their telescopes towards the sun.

Cambridge University solar physicist Helen Mason, whose special field is the sun's corona - the outer region of the sun that is still visible as a white halo around the moon's outline during a total eclipse - is heading for Guadeloupe. "For my PhD thesis I studied eclipse observations taken in 1952 - but in 20 years of solar physics research this will be the first eclipse I've been to. It has been my lifelong ambition to see one," says Mason, assistant director of research at Cambridge's applied mathematics department.

Also in Guadeloupe will be Francisco Diego of University College London's physics and astronomy department who has been photographing total eclipses since he saw his first one as a teenager in his native Mexico. "You see the twilight all around you," he says. "People claim they have seen stars - I have seen planets, never stars. But perhaps that's because I am very busy at that moment taking pictures, so I don't have much time to look at the sky. Those two or three minutes go like seconds."

For finding out about the sun itself, however, eclipse observations are not as important as they used to be. Information is now being sent back from satellites, and for the last two years there has been Soho - the Solar and Heliospheric Observatory. Built by the European Space Agency and launched by Nasa, this spacecraft hovers around a point where the gravitational pulls of the sun and earth are in balance, one and a half million kilometres out from the earth. From there it can monitor the sun continually and send back data on everything from the solar wind - the virtually continuous outflow of gas from the sun - to the chemical elements in its interior, building up a picture that is radically changing science. We know that the sun is not round, as previously thought, but constantly changing shape, like a bag of jelly. Thanks to Soho we also know that it is fairly stable in size. And we have information that is crucial to the electrical power industries in certain parts of the world.

George Simnet, professor of astrophysics at Birmingham University, is particularly interested in coronal mass ejections, - "truly enormous bubbles of gas that are blown off from the sun typically a couple of times a day". Since SoHo's launch a better picture of these ejections has emerged. "The frequency of these big events has been somewhat surprising to us," says Simnet. "Although we see these things going off sideways (from the surface of the sun) and they look spectacular, there are just as many coming towards us. When a really big blast of ionised (electrically charged) gas hits the earth's magnetic field, it squeezes it inwards. This induces currents in the upper atmosphere which in turn can induce currents in transcontinental power lines. So the power industries in the northern US and Canada are very interested in these findings. Because they are relatively close to the geomagnetic North Pole and have very long transmission lines, they get huge current surges that can blow out transformers and cause all sorts of disruptions." Advance warning gives the industries time to take defensive action.

"In the past we have had instruments that have given snapshots of what's been going on," says Helen Mason. "The beauty of Soho is that it has a comprehensive set of instruments going right through from the sun's interior to the solar wind and that it's observing 24 hours a day." Data from Soho is helping her answer the question of why the sun's corona is so hot - some million degrees Celsius compared to the 6,000 or so degrees of the surface below it. "Below the surface of the sun there's a convection zone which kind of jumbles up the sun's magnetic field," she explains. "It's like when you twist an elastic band up far enough, it's going to spring back eventually. When a magnetic field reconfigures and goes back to its normal state, then it lets off a lot of energy. (This) is heating the corona."

Richard Harrison at the Rutherford Appleton Laboratory in Oxfordshire is the principal investigator for another of the 11 instruments Soho carries, the coronal diagnostic spectrometer (CDS), which looks at the extreme ultraviolet light radiated by gas in the sun's atmosphere. "The whole point of Soho is to give a totally new view," he says. "My team is looking at the spectral lines that indicate trace elements in the sun - elements like iron, magnesium and silicon. But the basic desire is to understand generally how the sun's atmosphere works - we're trying to use the instruments together, looking for the processes that might be heating the solar atmosphere or accelerating the solar wind or driving one of these coronal mass ejection events. It's not just solar physics, it's stellar physics we're addressing here."

Dozens of scientists in UK universities are now working on data supplied by Soho instruments. Given all this activity, perhaps it is not surprising that someone like George Simnet does not feel any need to travel to the Caribbean to watch a solar eclipse. "I'm today somewhat cynical about people who make a big fuss about eclipses now we have these instruments in space," he remarks.

Has he seen a total eclipse himself? "Yes, in northern Montana in February 1982. I was in Colorado, so I did go quite a long way. But it wasn't for any scientific reason, I was just interested. It was very cold but it was a beautiful morning. But once I'd seen one I'd no particular desire to make the effort to go and see another. My scientific colleagues who rush off just to catch a couple of minutes of observations through the atmosphere - it's just a boondoggle."

John Parkinson, director of the school of science and mathematics at Sheffield Hallam University, disagrees. Even though he has seen it six times now, Parkinson still finds the eclipse a remarkable phenomenon. As he puts it: "I am still amazed to realise that here we are perched on a little bit of rock close to a star that gets occluded by another rock (the moon) which appears to have the same apparent size. The coincidence I just find mind-boggling, knowing what I do about the structure of the galaxy."