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Tuesday, August 10, 1999 Published at 10:49 GMT 11:49 UK

Hectic hunt for Sun's secrets

Rare chance to probe corona

Special report
Special report
11 August
As the eclipse approaches scientists are preparing to make the most of a rare opportunity to learn more about the way the Sun works. So while the rest of us are admiring the spectacle, scientists will be filling the two minutes of totality with frantic activity.

First on their list of priorities will be the corona, the Sun's outer atmosphere. This pearly layer is normally too faint to see. Only at times of total solar eclipse do scientists get the chance to observe it in all its glory.

BBC News' Pallab Ghosh reports on what scientists are hoping to see
Space probes such as the Solar and Heliospheric Observatory (Soho), and the Japanese satellite Yohkoh, have been gathering invaluable data on the way the sun works from outer space. But the corona is so delicate that its finer detail is more accurately observed from Earth, as the moon blots out the blinding light of the Sun's surface.

Hot topic

What puzzles scientists is why the corona is so hot. While the visible surface of the sun, called the photosphere, is around 5,000 degrees Centigrade, the temperature of the corona is far hotter - nearer 2,000,000 degrees.

[ image: SOHO has revolutionised our knowledge of the sun]
SOHO has revolutionised our knowledge of the sun
This is odd because the gases and ions which make up the corona are further from the furious nuclear fusion reactions at the Sun's core than the visible surface . Logically, the corona should be cooler.

One theory for the excess heat is that energy from the super-hot centre of the Sun is carried to the corona by the star's magnetic field. Another theory favours the interaction between magnetic and steady electrical fields.

In a field in Bulgaria researchers from Oxford will be trying to find the answer by filming the eclipse on a fast video camera.

The vibrating sun

They are looking for evidence of rapid vibrations in the Sun's magnetic field which it is thought could be heating the corona.

The camera, called the Coronal Imaging System, will record over 50 images of the corona's fine detail every second - that's around 7,000 pictures during the time the corona is visible.

Professor Ken Phillips, from the Rutherford Appleton Laboratory, said: " We only have two minutes of totality to gather all the data, so it is vital there no hitches. We're going to the Black Sea because the chances of clear skies are very good."

Much closer to home, in Cornwall, another Oxford scientist is hoping to map the Sun's magnetic field. Alan Ridgeley will use an infra-red spectrometer to measure the intensity of the field.

Because the Sun is nearing the peak of its 11-year activity cycle, the corona should be more active than during previous eclipses.

Race to see rare eruption

Yet another scientist is hoping to race the Moon's shadow across the globe in an effort to see a coronal eruption.

[ image: Solar activity is near maximum]
Solar activity is near maximum
Called Coronal Mass Ejections (CMEs), these massive eruptions fling huge balls of writhing plasma hundreds of thousand of kilometres into space at speeds of up to 500,000 km/h.

They can confuse satellites and even damage electricity power grids, if they hit the Earth.

Frederic Clette of the Royal Observatory of Belgium is organising an attempt to catch a rare image of a CME by taking photographs right along the eclipse track.

Professional and amateur observers will take repeated pictures of the corona through polarised filters which cut through the scattering effect of dust in the Earth's atmosphere.

By using a chain of observatories Mr Clette hopes to make maximum use of the whole duration of the eclipse - it will take 90 minutes for the Moon's shadow to dash the length of Europe.

Amateur radio and gravity waves

Further afield, scientists at Nasa's Marshall Space Flight Centre, are inviting ham radio operators from around the world to participate in a scientific experiment by recording signals from European radio stations during the eclipse.

The purpose is better to understand the nature of ionospheric absorption, which may lead to improved radio reception in the future.

Nasa will also be using a gravity sensor during the eclipse, also based at the Space Flight Centre.

The experiment will test the extraordinary findings of Nobel laureate Maurice Allais who detected anomalies in the movement of Foucault's Pendulum during total eclipses in the 1950s.

Allais later said: "This was quite inexplicable within the framework of currently accepted theories of gravity."

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Amateur radio experiment

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