Here on Earth, we’re surrounded by noise — much of it created by other humans. Maybe it’s because of that clamor that we tend to imagine space as silent and peaceful.
But NASA’s Van Allen Probes, a pair of spacecraft launched in 2012, are posted to a territory that can hardly be called peaceful. The Van Allen Belts are two donuts of radiation, plasma, and electric and magnetic forces raging around the Earth. Those belts are also where some 800 of our satellites hang out while acting as crucial communication and navigation infrastructure.
The Van Allen Probes are there to keep an eye on space weather, which is caused by the sun spitting energy and charged particles out into its surroundings. Space weather is what causes auroras, but can also lead to power blackouts on Earth and satellite failures.
When space weather enters the Van Allen Belts, it adds to the cacophony of charged particles — also known as plasma — being twisted and turned by the electric and magnetic fields surrounding Earth. That constant churn creates plasma waves, just like wind scratching across a body of water creates waves.
NASA is interested in plasma waves because their motion, along with all the other forces at work in the Van Allen Belts, can make these regions grow and shrink and can determine how charged particles make their way through the region to reach Earth.
Each Van Allen Probe is equipped with an instrument called EMFISIS, or Electric and Magnetic Field Instrument Suite and Integrated Science. EMFISIS measures the frequency of plasma waves it encounters — and that frequency can be converted into sound waves.
The resulting records are stunningly otherworldly. Different parts of the Van Allen Belts have different types of plasma waves, which in turn create different sounds.
For example, chorus waves sound oddly like whale whistles, but are actually the result of electrons hitting plasma above the side of Earth currently experiencing nightfall. Plasmaspheric hiss waves, which are found relatively close to Earth, sound like white noise or static. Perhaps the weirdest-sounding waves are whistler waves, created by the electrical surges of lighting strikes.
The University of Iowa, which helps run the Van Allen Probes, offers a collection of wave songs if you want to hear more.