Sound waves clue to the rotation of the Sun

TOM WILKIE

Science Editor

The Sun is ringing like a celestial bell and, by "listening" to the sound reverberating through it, scientists from Birmingham University believe that its core may be rotating more slowly than it did a decade ago.

At the National Astronomy Meeting in Liverpool yesterday, Dr Yvonne Elsworth said that information about the stellar interior came from listening to the Sun in "breathing mode" - where the surface of the whole star moves in and out, rather as someone's chest expands and contracts as they inhale and exhale.

Researchers had once believed that the core of the Sun rotated twice or three times as fast as the outer surface layers, Dr Elsworth said, but observations of the sound-waves resonating within the Sun showed that the interior was in step with the surface.

Using BiSON, the Birmingham Solar Oscillation Network, the researchers can measure how the surface of the Sun moves in and out to a precision of one part in a hundred billion.

Ironically, none of the measuring instruments is located in Britain. "The weather is not good enough in Britain," Dr Elsworth said. "Our brightest day is appalling."

The BiSON network consists of six instruments around the world: in Tenerife; South Africa; Western and Eastern Australia; Chile and California. "Chile is brilliant," Dr Elsworth said.

In principle, BiSON is rather like the British Empire and the Sun should never set on it, but in practice, Dr Elsworth said, the researchers get measurements for about 80 per cent of the time.

According to Dr Bill Chaplin, one of Dr Elsworth's colleagues in the Department of Physics and Space Science at the university, the researchers have been re-analysing the good quality data recorded since 1981.

"We have found that perhaps there may be tentative evidence for the core rotation rate having decreased since 1981."

Although there is a well-known 11-year solar cycle - linked among other things to sun-spot activity, there is also a less well known 22-year cycle in magnetic activity in the Sun and any decrease in the core rotation rate may be linked with this, Dr Chaplin speculated.

The Sun as a whole is spinning much more slowly than younger stars are known to do, and it is thought that the braking mechanism is a form of magnetic drag connected with the stream of charged sub-atomic particles being ejected in the solar wind.

Dr Chaplin stressed, however, that the data were only revealing hints of a core slowdown. "We need more data. You need good coverage to beyond the year 2000," he said.

Dr Chaplin believes the results from BiSON may also help solve the long- standing "solar neutrino problem" - experiments on earth are detecting only about one-third of the quantity of a particularly exotic sub-nuclear particle, the neutrino, that nuclear theory would predict. BiSON's results show that we understand solar physics fairly well, Dr Chaplin said, so the "loss" of neutrinos must occur en route to Earth as the original particles transmute themselves into other, unobserved types of neutrino.