December 13, 2004 ‘Ultrasound’ May Explain Solar Weather Mystery

Astronomers have identified ultrasoundlike waves in our sun’s atmosphere that could explain some strange aspects of solar weather. An analysis of data from NASA’s TRACE spacecraft published in the current issue of Astrophysical Journal Letters suggests that the waves could be responsible for the star’s unexplained extra heat.The surface of the sun reaches a blistering 6,000 degrees Celsius. The chromosphere, or middle solar atmosphere, is even more scorching at 100,000 degrees Celsius, whereas the solar corona is the hottest part of all, with temperatures nearing a million degrees Celsius. Just what causes these wide discrepancies in temperature has intrigued researchers for decades. Craig DeForest of the Southwest Research Institute and his colleagues analyzed data from the TRACE ultraviolet telescope and found evidence of waves with a frequency of 100 millihertz, which corresponds to a sound 300 times deeper than the lowest noise audible to the human ear. “These ripples seem to be carrying about one kilowatt of power per square meter on the surface of the sun,” says DeForest. “That is similar to the sonic energy you might find coming out of the speakers at a rock concert. “The researchers did not get a very detailed look at the waves, because they are close to TRACE’s detection limit. They expect that future instruments will allow them to investigate the waves more fully. “By examining these waves more closely, we should be able to discern the source of energy release in the solar atmosphere, just like you can tell by listening whether the car is running in a dark garage,” says DeForest. “In both cases, something is releasing energy into the environment, and that release has a recognizable sonic signature.” — Sarah Graham

Now, in a letter published on December 10th in Astrophysical Journal Letters, researchers report that the Sun's atmosphere is filled with ultrasound-like waves at a frequency of about 100 millihertz - every ten seconds. "At 10-second period, these waves qualify as ultrasound because individual atoms on the Sun experience only a few collisions during the brief passage of each wave, just as with ultrasound here on Earth," says Dr. Craig DeForest, a senior research scientist in the SwRI Space Science and Engineering Division. DeForest found the signature in data collected in January 2003 in the TRACE program.