Extraterrestrial chorus heard in radiation belts

SAN FRANCISCO — Is it alien birds singing alongside crickets? Or the sound of radio waves sweeping through Earth’s magnetosphere? A recently released recording is a little bit of both.

The soundtrack captures “chorus” waves, electromagnetic disturbances that ripple through belts of charged particles that surround Earth. The chorus becomes audible to the human ear when translated into sound waves, as heard in a recording made by space physicists at the University of Iowa.

Ham radio operators have known about this chorus for decades, but scientists now have a lot more data on it thanks to a pair of satellites known as the Van Allen probes. NASA launched them in August to fly through and study Earth’s two main radiation belts, called the Van Allen belts — an inner one made mostly of protons and an outer one made mostly of electrons. The electronics on most spacecraft get fried if they spend too much time in these belts, but the Van Allen probes are built with components that won’t fritz out when charged particles hit them.

Already scientists are uncovering surprises from the mission, some of which they reported December 4 at a meeting of the American Geophysical Union. The radiation belts turn out to be much more active than anyone had suspected, having shifted their location and intensity each time the probes swept through on their nine-hour orbits. New measurements also show exactly where the highest energy protons lurk in the inner belt.

And then there’s the chorus. An instrument that measures electromagnetic fields and waves recorded the chorus — “striking and beautiful,” said University of Iowa physicist Craig Kletzing. The chorus usually appears on the dawn side of the planet; the probes have now recorded it in 16-bit audio. “Now we have all the components to know how the waves are moving,” Kletzing said. “We didn’t have that before.”

Besides their aural beauty, choral waves and other Van Allen findings will have practical use. Engineers need that data to better design satellites to make it through the dangerous belts.

“Any space vehicle has to penetrate these regions to get outside,” said Joseph Mazur of the Aerospace Corporation in Chantilly, Va. “If you don’t spend a lot of time there that’s fine — but we really need to measure and specify what’s going on in the inner belt.”