Dubbed "solar ultrasound," the waves are approximately 300 times deeper than the deepest pitch audible to the human ear, at a frequency of 100 millihertz (10-second period).

"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 TRACE data collected in January 2003.

The waves are most likely created by the sudden collapse of magnetically induced electric currents (magnetic reconnection) or by lower frequency sound waves that crash like ocean waves as they make their way up from the surface of the Sun. Both of the sources are likely candidates for the source of the solar atmosphere's mysterious extra heat, making the new waves a valuable tool for exploring a decades-old mystery.

At up to 100,000 deg C (180,000 deg F), the chromosphere, or middle solar atmosphere, is nearly 20 times hotter than the 6,000 deg C (11,000 deg F) surface of the Sun. The solar corona, at 1,000,000 deg C (1,800,000 deg F), is about 10 times hotter still, or 200 times hotter than the surface of the Sun. Although scientists have been studying the process for more than 50 years, the reason for this difference in temperature remains elusive.

"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."

The Sun is filled with lower-pitched waves, at about 3 mHz (5-minute period), that are used to probe the solar interior and even to make images of the far side of the Sun. The solar ultrasound is too high pitched to be directly related to these more well-known "photospheric oscillations."

Sound waves cannot travel through interplanetary space, so they are detected remotely as small fluctuations in the brightness of solar ultraviolet emissions. The TRACE spacecraft, built by Lockheed-Martin for NASA's Explorer program, is an ultraviolet telescope in orbit around Earth. The solar ultrasound is at the limit of detectability by TRACE -- so faint that individual waves cannot be resolved. Instead, DeForest sleuthed for patterns in the background noise of the telescope.

"Each individual wave train has an amplitude of about one-tenth of the smallest brightness value that TRACE can see," he says. "But when we average many images together in the right way, a pattern emerges that we can recognize as the signature of trapped waves." The pattern emerges through three-dimensional Fourier analysis, a mathematical technique that isolates individual types of motion from the morass of activity above the solar surface.

Although the waves appear faint, they are quite energetic. "These ripples seem to be carrying about 1 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. Very loud."

"The discovery of coherent waves at such high frequencies in the upper solar atmosphere challenges our understanding of the magnetic structures in the quiet Sun," says Dr. Joseph Gurman, TRACE mission scientist at NASA's Goddard Space Flight Center.

"This work proves that we need new tools to understand the propagation of energy into and out of this part of the solar atmosphere where much of the activity that can affect life here on Earth originates."

The open data policy of the TRACE mission boosts scientific productivity by allowing researchers and the general public to download solar data collected by the spacecraft. This allows individuals to re-use the data for purposes beyond those of the original mission.

Future instruments will be able to better detect the waves. While TRACE is a simple telescope that can detect only changes in brightness, the waves are likely to have a much stronger Doppler signature. One problem remains, however; Earth's ozone layer is opaque to the ultraviolet light used to see and "hear" the solar ultrasound, making the light difficult to detect from the ground.

SwRI scientists are designing rocket- and balloon-borne instruments to observe from above the bulk of Earth's atmosphere to catch a better glimpse of the wave spectrum for probing the solar atmosphere.

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