No one can hear humans scream in space, but apparently two robotic probes can hear electrons whistle there.
These electrons are bouncing around the planet's electric and magnetic fields thanks to the motion of plasma waves. The waves are formed when electric and magnetic fields collide with "clumps" of particles, like ions and electrons, and that sometimes results in the particles moving at faster-than-usual speeds. When they pick up speed, that's when we're able to hear them.
Using the Van Allen probes, spacecraft that orbit the Earth and study the radiation belt around the planet, NASA was able to hear the electrons as they bounced around. And the result is both creepy and like something out of a 1960s sci-fi film, or, if you're feeling imaginative, a tropical rainforest ... in space, of course.
Soundscapes of space
The sounds these particles and waves make hinge on where they are around the Earth. Take the whistler-mode waves, for instance. These waves of plasma make different sounds in the plasmasphere, an area around Earth that is packed with cold plasma, than they make outside of it. First, listen to what whistler waves sound like before they arrive in the plasmasphere:
It's as if the technician is warming up the soundboard for a laser show at a planetarium. Created by lightning strikes, whistler-mode waves can have a range of frequencies, hence the up-and-down quality of the sounds. Some of these waves escape the atmosphere and travel along the planet's magnetic fields.
If these whistler-mode waves get outside of the plasmasphere, their sound changes. The plasma here is warmer and less densely packed together, and that results in a noise that is something drastically different. And it gets a different name, too. The whistler-mode wave is now a chorus wave. You'll hear why in the clip below.
Now you're suddenly in an aviary filled with noisy birds. Chorus waves are the result of low-energy electrons striking plasma and sharing their energy with particles already in the plasma. That creates the rising tone you hear.
When whistler-mode waves travel inside the plasmasphere, scientists call it a plasmaspheric hiss. NASA describes the sound as radio static, but to me it sounds more like the sounds of someone breathing in a space suit or scuba gear.
There are a couple of theories as to what causes the plasmaspheric hiss, according to NASA. One theory ties the hiss to lightning strikes, while another suggests it's actually chorus waves that have seeped into the plasmasphere.
Lest you think NASA is just giving us something new to listen to while we're zoning out, the sounds do have scientific value. Understanding how plasma waves and particles interact with the plasmasphere can lead to a better understanding and predicting of space weather. This can help us protect our satellites and telecommunication in space.