What Does the Earth Sound Like?

  • By Tim De Chant
  • Posted 11.01.12
  • NOVA

By now, most of us know what the Earth looks like from space. But what it sounds like is another matter. Scientists have long known that radiation belts surrounding our planet—also called the Van Allen belts—produce a storm of electromagnetic waves across a range of frequencies. When reproduced as sound instead of radiation, the result is a string of chirps, chatters, whoops, and whistles reminiscent of an extraterrestrial jungle from a sci-fi movie. Yet despite their otherworldly tenor, they are perhaps the most earthly of sounds.

Listen to the story.

Radiation from the Van Allen belts, when converted, sounds otherworldly.

Physicist Craig Kletzing, a professor at the University of Iowa, and his colleagues at NASA recently recorded the clearest example yet of this chorus from the Radiation Belt Storm Probes, a pair of satellites surveying the radiation fields that envelop the Earth. Just days after the satellites' launch, they were beaming back high-resolution data that matches the quality of CD audio.

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The twin Radiation Belt Storm Probes will study the Van Allen belts that surround the Earth in greater detail than ever before. Enlarge Photo credit: Courtesy NASA

The spacey sounds weren't a part of an experiment. "We're in our commissioning phase now," Kletzing says. It's the part of the mission where scientists and engineers make sure everything is in working order. That involves sending wake-up signals to the satellite, turning on one system after another, and waiting for satisfactory replies as each comes on line. As successive sensors reply, their signals are recorded and analyzed to make sure they are in working order. "That's actually been going very, very well," Kletzing adds.

Most properly functioning sensors return patterns that are pleasing to scientists and gibberish to others, but the Radiation Belt Storm Probes' earliest signals could be music to anyone's ears. Kletzing and his colleagues took electromagnetic radiation as recorded by the Electric and Magnetic Field Instrument Suite and Integrated Science sensor, known as EMFISIS for short, and converted the frequencies into sound waves. On their own, each electromagnetic wave reproduced as sound isn't that appealing. A 1 kHz electromagnetic wave, for example, sounds exactly like the tone of a TV test pattern. But combined, the natural swells and dips make for an alluring soundtrack.

Eavesdropping on the chorus

The radiation belts are bands of particles—mostly protons and electrons—trapped by magnetic fields surrounding our planet. They are commonly called the Van Allen belts, named after discoverer James Van Allen, a scientist on the early Explorer and Pioneer missions. The closest of these contains a relatively stable number of high-energy protons that are produced when cosmic rays smash into the Earth's atmosphere. Beyond that is another, bigger belt of mostly high-energy electrons, predictably called the outer radiation belt. This region contains particles blown in by magnetic storms. Since the Earth's magnetic field is weaker in the outer radiation belt, and because of the irregularity of magnetic storms, its density of particles is more variablity than the inner belt's.

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Two radiation belts encircle the Earth. The inner belt is more stable and higher in energy while the outer is more variable. Enlarge Photo credit: Courtesy NASA

Particles throughout the radiation belts are so high energy that they emit waves of electromagnetic radiation. The chorus comes from the radio part of the spectrum—specifically between 10 Hz and 10 kHz. They're not sound waves (there is no sound in space, after all), but because human hearing ranges from 20 Hz to 20 kHz, the chorus's radio emissions can be easily transformed into audio. Ham radio operators, for example, have been eavesdropping on—or rather intruded upon by—the chorus for decades.

In the mid-1970s, scientists felt they had a good grasp on the basic physics behind the radiation belts. But research conducted in the 1990s suggested otherwise, Kletzing says. "Suddenly it became quite apparent that they were vastly more variable than we had ever thought."

Forecasting space weather

Kletzing and other space physicists hope the new, paired satellites will provide more information about this variability. Previously, research satellites were sent up singly and were only able report on conditions in their immediate vicinity. The twin satellites of the Radiation Belt Storm Probes mission will be able to provide data in stereo, allowing researchers to gather more detailed and more spatially explicit information about the radiation belts. Eventually, the hope is this data will lead to more accurate models that forecast space weather—the changes in radiation, particles, and magnetic fields that surround Earth—in much the same way that data from Doppler radar has enabled meteorologists to make more accurate and sophisticated predictions about terrestrial weather. "We're way behind the weather guys," Kletzing admits. "But, you know, we're making steady progress."

Though forecasting space weather may sound like an esoteric goal, it has become increasingly necessary as we conduct more business in space. "We've actually gotten a lot more dependent on using space than I think we sometimes realize," Kletzing says.

Astronauts on manned space missions need to be alerted to changing space weather conditions so they can take shelter in the advent of a radiation storm. Satellites, too, are vulnerable to radiation storms. Unlike astronauts, they can't hide from the worst of it, so manufacturers have to plan for a certain amount of wear and tear due to radiation. That shortens satellites' lifespans, sometimes unpredictably so. Now, most satellite constellations like GPS include working spares in case an in-service unit fails—a costly insurance policy. In the future, knowing precisely how much radiation a satellite must endure throughout its lifetime could help manufacturers plan replacement schedules better, reducing the need for expensive spares.

For now, such advanced prediction of space weather is a ways off. But the Radiation Belt Storm Probes—and their crystal clear recordings of the peaks and valleys of the radiation that surrounds Earth—are already gathering clearer data than ever before, hopefully paving the way to a better understanding of the radiation belts and, eventually, accurate forecasts of space weather.

Tim De Chant is the senior digital editor of NOVA Online.

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