Van Allen belts belt out music

If you ask Craig Kletzing, the recordings echo the chirping of crickets. To his wife, they sound like a chorus of alien birds.

But there is no life where these sounds are made, in the dazzling and dangerous stream of highly charged particles that surrounds our planet. For years, Kletzing, a physics professor at the University of Iowa, has been monitoring the radio waves that undulate through the void around Earth. When the data are turned into sound files, the result is an eerie cosmic symphony.

Although space is a vacuum, it is neither empty nor quiet. Just above our atmosphere exist two belts of energetic particles fromthe sun that get trapped by Earth’s magnetic field. This phenomenon is vital to making our planet livable; the captured electrons and protons zip back and forth between Earth’s magnetic poles instead of streaming through the atmosphere to bombard the surface. But the zones where these particles dwell, called the Van Allen belts, are still dangerous: The trapped particles pose a threat to satellites and astronauts at the International Space Station, and the belts play a role in space weather that can destroy power grids on the ground.

In 2012 NASA launched the Van Allen Space Probes, twin robotic craft that orbit the Earth and monitor this roiling envelope of charged particles. The probes carried a suite of instruments called EMFISIS, short for Electric and Magnetic Field Instrument Suite and Integrated Science. EMFISIS is designed to detect radio waves rippling around the Earth.

“It ’s like sticking a microphone out into space, but instead of listening to sound waves we’re listening to electromagnetic waves,” said Kletzing, EMFISIS’s lead investigator.

Humans can’t hear all the activity in the Van Allen belts. Our ears respond only to soundwaves, which we sense via the vibration of molecules that are disturbed by thewaves as they propagate through the air. Space is airless — practically void of matter — and therefore soundless.

But the electromagnetic waves are in the same frequency range as the part of the sound spectrum that is audible to humans. Itwas a simple matter to translate those radiowaves as MP3s.

“There’s a side ofme that listens to it and says ‘Wow, what interesting wave forms,’ ” Kletzing said. “But there’s also a piece that just listens.”