NASA’s twin Radiation Belt Storm Probes (RBSP) took a musical interlude and listened to the Earth singing to itself. This “Earthsong,” as NASA calls it, was recorded by the two spacecraft as they orbited inside the highly radioactive Van Allen Belts that surround the Earth. The “song” is in the form of radio waves generated by the belts and the study of it may provide a clue toward answering the question of how to protect satellites and astronauts from deadly radiation storms.
The Earthsong is better known to ham radio operators as the “chorus.” They've been listening to it from the ground for years and it’s rising and falling tones sound like a dawn chorus of chirping birds, from which it derives its name.
"This is what the radiation belts would sound like to a human being if we had radio antennas for ears," said Craig Kletzing of the University of Iowa, whose team built the RBSPs’ Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) receivers used to pick up the signals.
The chorus isn't acoustic, but rather electromagnetic in nature. It’s made up of radio waves in the 0 to 10 kHz range and is caused by intense plasma waves, called chorus waves, in the Van Allen Belt. However, they are more than just a pretty sound. One of the most important waves in the Van Allen Belts, they may be the cause of “killer electrons” – electrons that are caught up in the chorus wave and boosted to a high enough energy to damage electronic equipment or even astronauts.
This fits in very well with the brief of the Radiation Belt Storm Probes. They’re designed to detect these waves by means of the EMFISIS antennae, which are tuned to detect very low frequency (VLF) emissions. The transmissions they send back are 16-bit, which is the same quality as a CD. Furthermore, with these two spacecraft, stereo is possible. That isn't just to make the music sound richer, it also helps to build up a complete picture of the phenomenon, where exactly it occurs and how.
Launched August 30, 2012, the RBSPs’ mission is to study the highly radioactive Van Allen Belts and the part they play in space weather. This goes beyond pure science because severe space weather, such as a massive solar flare could imperil astronauts or even threaten to knock out the power grid on half the Earth in an instant. Given that the chorus wave may contribute to such severe events, it may be that the Earthsong is more like the song of the mythical Sirens that were a prelude to disaster.
An MP3 recording of the Earthsong is available here.
The NASA video below discusses the chorus wave.