Scientists 'look through' the Sun

Active regions on the Sun - it is probably the same on the far side

By BBC News Online science editor Dr David Whitehouse

Scientists can "see" explosive regions on the far side of the Sun for the first time, allowing them to give bad weather warnings in space.

The Solar and Heliospheric Observatory (Soho) spacecraft is now able to analyse ripples on the Sun's visible surface to reveal activity on the hidden side. It can give up to a week's warning.

Coronal mass ejections can knock out power on Earth

Before this discovery astronomers were often surprised when a group of previously hidden explosive regions rotated suddenly into view as the Sun turned.

The new technique uses the Michelson Doppler Imager (MDI) instrument on Soho and is said by scientists to create a window to the far side of the Sun.

"We've known for 10 years that in theory we could make the Sun transparent all the way to the far side," said Dr Charles Lindsey of the Solar Physics Research Corporation in Tucson. He is a member of the team which published its research in the journal Science.

"But we needed observations of exceptional quality. In the end we got them, from MDI on Soho."

Potentially explosive areas on the Sun's surface are called active regions. These are areas of strong magnetic fields much larger than the diameter of the Earth. They produce explosions, called flares, and eruptions of plasma (hot, electrically charged gas), called coronal mass ejections (CMEs).

The radiation and plasma from these events can sweep past the Earth, disrupting spacecraft, radio communications and power systems. With a far-side preview of sunspots, nasty shocks from the Sun may now be avoidable.

Million measurements

The MDI instrument on Soho measures the motion of the Sun's surface at a million points. Computers then interpret the motions in terms of sound waves travelling through the Sun. The waves are affected by the various layers of gas and different motions that they encounter.

The new technique, called helioseismic holography, finds the effect on the sound waves of active regions on the far side of the Sun.

An active region reveals itself because it possesses very strong magnetic fields that speed up the sound waves. Waves that pass through an active region have a round trip travel time about twelve seconds shorter than the average of six hours.

To test the technique astronomers analysed Soho data for 28 March 1998 that suggested a new sunspot group on the far side. Ten days later it was seen as the Sun rotated.