There is no sound in space, at least not the kind of sound we as humans can detect. There is, however, something akin to sound if you have the right tools to listen to it. NASA‘s Goddard Space Flight Center does, in the form of the Van Allen probes. This pair of robotic spacecraft have been in orbit of Earth for almost five years, sending back data on the electromagnetic and radiological conditions in space. The agency has released a number of audio recordings generated by plasma wave data from the probes. The sounds are both eerie and fascinating.
While space is largely devoid of matter (hence why it’s called space), there are particles zipping around. That’s especially true near a planet with a magnetic field like Earth. One of the most common and varied types of plasma wave around Earth is known as the whistler-mode wave. These waves travel through the plasma around Earth, and are recorded by the probes using an instrument called EMFISIS, short for Electric and Magnetic Field Instrument Suite and Integrated Science. By shifting the frequency down, we can hear it as sound.
Whistler-mode waves closer to Earth, which you can hear in the clip above, are a product of electrical activity in the atmosphere. When lightning strikes, it triggers whistler-mode waves. Some of those electron waves escape beyond the atmosphere and bounce around in Earth’s magnetic field — specifically in the plasmasphere, which is a dense envelope of “cold” plasma. Lightning produces a wide range of frequencies, but the higher frequencies travel faster. This is what gives whistler-mode waves their distinctive whistling quality.
Beyond the plasmasphere, whistler-mode waves have a higher “chirping” sound. Scientists refer to these types of waves as a chorus, and they aren’t connected to lightning strikes on Earth. Instead, chorus whistler-mode waves are created when electrons are pushed toward the night side of Earth. When the lower energy electrons from the wave hit Earth’s plasma envelope, they produce sharp signals interpreted as a rising tone.
The last example from the Van Allen probes is what’s known as a “plasmaspheric hiss.” These are whistler-mode waves that travel inside the plasmasphere and sound like breathing radio static. Scientists aren’t entirely certain what causes them. It may be related to lightning strikes like the whistler waves. Others suspect a leaking of chorus waves.
The Van Allen probes aren’t just out there to record electromagnetic music. These spacecraft are gathering important data on how waves and particles interact. This could be vital in predicting space weather and improving telecommunications.
Image credit: NASA’s Goddard Space Flight Center/Brian Monroe