In the same way, astronomers are now able to measure millions of sound waves that propagate throughout the Sun, causing it to vibrate or ring like a bell. This technique, known as helioseismology, is the solar equivalent of terrestrial seismology.

On Monday 7 April, Dr. John Leibacher (U.S. National Solar Observatory) will highlight recent results from helioseismology studies during a presentation to the UK/Ireland Solar Physics Meeting in Dublin. These will include new views of the rapidly changing "sub-surface solar weather" and the far side of the Sun, as well as prospects for seeing finer and deeper details within the Sun and other stars.

"Unimaginable 25 years ago, helioseismology today allows us to 'see' into the otherwise invisible interior of the Sun," said Dr. Leibacher. "This has enabled us to overthrow some theories, corroborate others, and pose many more new questions as we finally get a glimpse of how things work.

"We are now testing fundamental theories of physics and astrophysics, substantially advancing our knowledge of the Sun's structure and dynamics," he added. "We are also beginning to measure significant temporal variations ranging from the scale of the eleven-year solar sunspot cycle right down to 'solar weather' variations on the scale of a day.

"Recent observations have been producing some remarkable results on flows of gas that we can image below the surface of the Sun. For example, we are now seeing strong subsurface winds flowing into groups of sunspots, which change from day to day. These sunspots are the sources of the strong magnetic fields which give rise to explosions on the surface. These, in turn, produce all sorts of terrestrial effects, from the aurora borealis, to fluctuations in navigational satellite signals, to power outages, and in the longer term they may influence Earth's climate.

"With the discovery of these flows, we may be getting close to understanding the real nature of sunspots, with the tantalising prospect of being able to predict their occurrence. We can already utilise helioseismic imaging to detect sunspot groups on the far side of the Sun, before they rotate onto the visible hemisphere.

"It has been an exhilarating ride and we are excited to see what the next turn will reveal," he said.

Notes for editor

Helioseismology is the study of "Sunquakes". By observing the properties of pressure waves that propagate throughout the Sun's interior and appear at its visible surface, scientists can measure the internal structure and sub-surface "weather" of this otherwise inaccessible physical laboratory.

These vibrations are seen as the Doppler shifting of light emitted at the Sun's surface with periods near five minutes. Just as we hear the pitch of a police car's siren change when it moves towards or away from us, so sensitive ground- or space-based instruments are able to determine the motion of different parts of the solar surface. Modern observatories can detect relative motions of a few cm/sec on the Sun, which is 150,000,000 km from the Earth.

Worldwide networks of ground-based telescopes (one of which is based at the University of Birmingham in the UK) conduct a detailed study of solar internal structure and dynamics by obtaining nearly continuous observations of the Sun's five-minute pulsations. Other observations are made with the ESA-NASA SOHO spacecraft.

One of the additional advantages of such 24-hour-a-day solar observing is that phenomena such as the very rare transit of Mercury across the Sun in May 2003 should be visible in its entirety, although the transit will finish just at sunrise in the UK.

Dr. Leibacher is the director of the Global Oscillation Network Group programme, with sites in California and Hawaii, Western Australia, Rajasthan (India), the Canary Islands and La Serena (Chile). He is also Chair of the Solar Physics Division of the American Astronomical Society.

FURTHER INFORMATION AND IMAGES CAN BE FOUND ON THE WEB AT:

* Solar subsurface weather. http://lcd-www.colorado.edu/solar-storms

* Solar farside imaging. http://www.colorado-research.com/~dbraun/farside-gong

* Transit of Mercury across the Sun in May 2003. http://sunearth.gsfc.nasa.gov/eclipse/OH/transit03.html

* Global Oscillation Network Group. http://gong.nso.edu