Seeing through the sun

researchers using the Solar and Heliospheric Observatory (SOHO) spacecraft have developed a method of seeing through the sun to its far side.

"This new method allows more reliable advance warning of magnetic storms brewing on the far side that could rotate with the sun and threaten the Earth," said NASA-supported scientist Phil Scherrer of Stanford University, USA.

Magnetic storms resulting from violent solar activity disrupt satellites, radio communications, power grids and other technological systems on Earth. Advance warning can help planners prepare for operational disruptions. The sun rotates once every 27 days, as seen from Earth, and this means the evolution of active regions on the far side of the sun previously has not been detectable. Knowing whether there are large active regions on the opposite side of the sun may greatly improve forecast of potential magnetic storms.

The new observation method uses SOHO's Michelson Doppler Imager (MDI) instrument to trace sound waves reverberating through the sun to build a picture of the far side.

The sun is filled with many kinds of sound waves caused by the convective motion of gas in its surface layers. The far side imaging method compares the sound waves that emanate from each small region on the far side with what was expected to arrive at that small region from waves that originated on the front side. An active region reveals itself because its strong magnetic fields speed up the sound waves. The difference becomes evident when sound waves originating from the front side and from the back side get out of step with one another.

"The original far-side imaging method only allowed us to see the central regions, about one-quarter to one-third of its total area," Scherrer said. "The new method allows us to see the entire far side, including the poles."

SOHO is a cooperative project between the European Space Agency and NASA.