The far side of the sun seems inaccessible, obscured from our view by 1.4 million kilometers of hot, seething gas. But because the sun rotates every 27 days, that hidden face emerges without fail to shine upon us and, at times, launch dangerous storms our way. Now, researchers may have learned how to detect storms brewing on the far side of the sun, weeks before they swing toward Earth, thanks to a technique that literally hears the rumbling of big sunspots through the sun itself.
The technique, dubbed "helioseismic holography," relies upon acoustic vibrations that ring the sun like the solar system's largest bell. Gas churning near the sun's surface propels sound waves inward, where they skip and reflect until the entire sun hums in a complex, low-pitched cacophony. During their travels, the sound waves speed up when they cross regions where the strong magnetic fields that accompany sunspots and other storm centers push down the solar surface.
Looking for evidence of storms on the far side of the sun, solar physicists Charles Lindsey and Douglas Braun of the Solar Physics Research Corp. in Tucson, Arizona, analyzed data that might reveal the throbbings of the sun's surface. On 8 April 1998, a major storm swung into view on the eastern side of the solar disk. Tracing backward, the researchers found evidence that the storm had been brewing for more than a week, when the turbulent part of the sun faced away from the recording station. On 28 and 29 March, they calculated, the travel times of certain waves decreased by about 6 seconds--a mere hiccup during their 3.5-hour journeys from the near to the far side, but enough to create a splotch in the acoustic signatures, they report in the 9 March Science.
Although these first images are smudgy, the technique could provide advance warning that solar flares and storms will take aim at Earth--warnings that could help electrical utilities or satellite operators plan for possible disruptions and put key instruments into a safe mode, says Ernest Hildner, director of the Space Environment Center at the National Oceanic and Atmospheric Administration in Boulder, Colorado. But protecting humans in space may be the greatest benefit, says solar physicist William Wagner of NASA, especially with astronauts due to spend thousands of hours on space walks during the next decade to assemble the international space station.