By Mara Johnson-Groh
NASA’s Goddard Space Flight Center
Space isn’t silent. It’s abuzz with charged particles that — with the right tools — we can hear. Which is exactly what NASA scientists with the Van Allen Probes mission are doing. The sounds recorded by the mission are helping scientists better understand the dynamic space environment we live in so we can protect satellites and astronauts.
This is what space sounds like.
To some, it sounds like howling wolves or chirping birds or alien space lasers. But these waves aren’t created by any such creature – instead they are made by electric and magnetic fields.
If you hopped aboard a spacecraft and stuck your head out the window, you wouldn’t be able to hear these sounds like you do sounds on Earth. That’s because unlike sound — which is created by pressure waves — this space music is created by electromagnetic waves known as plasma waves.
Plasma waves lace the local space environment around Earth, where they toss magnetic fields to and fro. The rhythmic cacophony generated by these waves may fall deaf to our ears, but NASA’s Van Allen Probes were designed specifically to listen for them.
The Waves instrument, part of the Electric and Magnetic Field Instrument Suite and Integrated Science — EMFISIS — instrument suite on the Van Allen Probes, is sensitive to both electric and magnetic waves. It probes them with a trio of electric sensors as well as three search coil magnetometers, which look for changes in the magnetic field. All instruments were specifically designed to be highly sensitive while using the least amount of power possible.
As it happens, some electromagnetic waves occur within our audible frequency range. This means the scientists only need to translate the fluctuating electromagnetic waves into sound waves for them to be heard. Effectively, EMFISIS allows scientists to eavesdrop on space.
When the Van Allen Probes travel through a plasma wave with fluctuating magnetic and electric fields, EMFISIS studiously records the variations. When the scientists compile the data they find something that looks like this:
Whistler Waves Recorded by NASA’s Van Allen Probes. Credit: University of Iowa
This video helps the scientists visualize the sounds coming from space. The warmer colors show us more intense plasma waves as they wash over the spacecraft. For these particular waves generated by lightning, the higher frequencies travel faster through space than those at lower frequencies. We hear this as whistling tones decreasing in frequency. These particular waves are an example of whistler waves. They are created when the electromagnetic impulses from a lightning strike travels upward into Earth’s outer atmosphere, following magnetic field lines.
Below 0.5kHz (the very bottom of the graph in the video) the sound is filled with what are known as proton whistlers. These types of waves are generated as a result of lightning strike-triggered whistlers interacting with movement of protons, not electrons. Recently, NASA’s Juno mission recorded high frequency whistlers around Jupiter — the first time they’ve been heard around another planet.
In addition to lightening whistlers, a whole menagerie of phenomena has been recorded. In this video we hear a whooping noise made by another type of plasma wave — chorus waves.
Chorus Waves Recorded by NASA’s Van Allen Probes. Credit: University of Iowa
Plasma wave tones are dependent on the way waves interact with electrons and how they travel though space. Some types of waves, including these chorus waves, can accelerate electrons in near-Earth space, making them more energetic. Here is another typical example of chorus waves.
Chorus Waves Recorded by NASA’s Van Allen Probes. Credit: University of Iowa
NASA scientists are recording these waves not for musical interests, but because they help us better understand the dynamic space environment we inhabit. These plasma waves knock about high-energy electrons speeding around Earth. Some of those freed electrons spiral earthward, where they interact with our upper atmosphere, causing auroras, though others can pose a danger to spacecraft or telecommunications, which can be damaged by their powerful radiation.
