No one can hear humans scream in space, but NASA can hear electrons whistle there. But how did the electrons get into our atmosphere, and why do they make noise? A new study has the answers, thanks to two robotic spacecraft: NASA’s Van Allen Probes mission and FIREBIRD II CubeSat.
First up, the noise question. The 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.
The resulting sound (which you can hear below) is both creepy and like something out of a 1960s sci-fi film. As for where the electrons and waves come from in the first place, recently the two spacecraft "found themselves at just the right places at the right time to witness first hand both the impulsive electron loss and its cause," according to NASA.
Known as whistler mode chorus, these [plasma] waves are created by fluctuating electric and magnetic fields. The waves have characteristic rising tones — reminiscent of the sounds of chirping birds — and are able to efficiently accelerate electrons.
“Observing the detailed chain of events between chorus waves and electrons requires a conjunction between two or more satellites,” said Aaron Breneman, researcher at the University of Minnesota in Minneapolis, and lead author on a new paper on the topic published in Geophysical Review Letters. “There are certain things you can’t learn by having only one satellite — you need simultaneous observations at different locations.”
From different vantage points, NASA gained a better understanding of the "chain of cause and effect of the loss of these high-energy electrons."
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.
Editor's note: This story has been updated since it was originally published in July 2017.