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News and Trends in the Geosciences

 May 2000

News Notes
 Solar Research

The sun sounds off

Wise investments by NASA and NSF have come to fruition through recent progress made in the field of helioseismology. Scientists have proven a 10-year-old theory that the acoustical properties of the sun can be modeled to detect solar flares originating on its far side. The new forecasting technique can provide up to two weeks advance notice of an approaching solar storm. Previous technology provided warning of a storm only when a flare rotates into Earthís view. By that time itís only a matter of hours to days before the intense radiation affects Earth ó posing a hazard to astronauts orbiting Earth.

Scientists discovered propagating sound waves in the sunís hot, gaseous interior during the 1960s. Convection near the sunís surface causes turbulent flows, or solar flares, to erupt on the surface and produce a broad spectrum of acoustical noise. The disturbance causes pressure waves to move into the sunís interior. As the waves travel through the sun, they reflect off its interior surface, causing ripples on the exterior of the star where they strike. When they strike the surface of the sun that faces Earth, the ripples can be measured with the Michelson Doppler Imager (MDI) on NASAís Solar and Heliospheric Observatory (SOHO) spacecraft.

In work published in the March 10 Science, Charles Lindsey of the Solar Physics Research Corp. in Tucson, Ariz., and D.C. Braun of Northwest Research Associates in Boulder, Colo., explain how they used computer modeling to trace the surface disturbances through the sunís interior and determine the time and location of a flareís focal point. This technique is most useful when a flare occurs on the backside of the sun, where it cannot be detected by any other means until it crosses the sunís eastern horizon and becomes visible from Earth.

   NASA's Skylab space station captured this 
   image, the largest solar flare ever recorded,
   on Dec. 19, 1973.

By the time the solar storm is visible, its irradiative effects are already less than a day away from being felt on or near Earth. Because the sun completes one rotation every 27 days, scientists can now pinpoint the time and location of a flare accurately enough to forecast it within a few hours, when the storm will show itself as it rotates into view.

The radiation from solar flares can cause power outages on Earth, and also disrupt satellites and shuttles in space. According to Lindsey, this new forecasting technique provides ďa medium-term warning of
changes in space weather.Ē This could be particularly advantageous to scientists planning shuttle missions, he says. For astronaut safety, he adds, ďNASA wouldnít want to plan extra-vehicular activity in two weeks if there were a flare on the far side of the sun today.Ē

Laura Wright

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