"Space is not empty, nor is it silent," announces NASA's Van Allen Probes mission. During mission observations of the near-Earth space environment, scientists recorded three types of eerie music created by interactions between charged electron particles and electric and magnetic field lines. In the complicated map of zones that constitute the near-Earth space environment, two whistles come from close to Earth in the plasmasphere and one from beyond the plasmasphere, further away from Earth.

Charged electron particles and plasma waves

NASA's Van Allen discovery gives us awesome recordings of electron particle whistles to listen to, but the trick is understanding where and how these haunting whistles are made.

To pique your interest, here's a sample whistle called the chorus.

On Earth, waves carry sound. NASA's example of waves carrying sound is ocean waves: Live by the ocean, and the sound of waves forms the backdrop to your day and lulls you to sleep at night. In the region of space around Earth, a region that is within Earth's magnetosphere — Earth's protective bubble — waves flow through plasma instead of through oceans as here on Earth. Plasma waves contain electromagnetic field lines and charged ions and electron particles. When the electromagnetic field lines "plow through clumps of ions and electrons," as NASA writer Mara Johnson-Groh puts it, particles are accelerated and waves are produced.

Inside story on different plasma and whistles

Electron particle whistles sound different depending on the kind of plasma the wave is produced in — whether warm or cold — and depending on what kind of ions and electrons are present. For example, lightning particles escaping into cold, near-Earth space plasma create a whistle sounding rather like something found in the "Star Trek" Enterprise.

But in the warmer, thinner plasma region, the whistle sounds like a chorus of whales or tropical birds.

NASA's Van Allen Probes mission has observed electron particle interactions and made recordings of three eerie whistles. These three are the whistler wave (lightning interacting with magnetic field lines in near-Earth space); the chorus wave, found in plasma the furthest from Earth; and the hissing wave, also produced nearest to Earth.

These three kinds of plasma waves are categorized as whistler-mode waves, remembering that plasma waves contain electromagnetic field lines and charged ions and electron particles.

The Whistler wave electron whistle

Earth's charged lightning particles escape into the near-Earth plasmasphere and interact wildly with north-south polar magnetic field lines. This interaction produces a broad range of electromagnetic frequencies — where high frequencies travel at higher speeds — resulting in a whistle with sharply falling (descending) pitch. This plasma wave noise is called Whistler wave. Founded on the interaction between highly charged lightning particles with electromagnetic lines, Whistler waves are intense and "bounce like bumper cars along Earth's magnetic field lines," as Johnson-Groh says.

The chorus electron whistle

Earth's farthest plasma region holds various plasma. It has a somewhat warmer temperature, and a "tenuous," or weakly constituted property. The plasma wave produced in this warmer, thinner plasma is not wild and energetically piercing as is the plasma that produces Whistler waves. The whistle of this plasma wave sounds relaxed, with a rising (ascending) pitch that contrasts against the whistler wave are piercingly energetic, descending pitch.

Chorus waves are intriguing because they are "created when electrons are pushed towards the night side of Earth — which in some cases, may be caused by magnetic reconnection." Magnetic reconnection is a violent process in which magnetic lines splice to other magnetic lines, kindling, among other things, thermal energy and electron particle acceleration.

Magnetic reconnection in the plasmasphere causes "a dynamic explosion of tangled magnetic lines on the dark side of Earth," as Johnson-Groh describes it. Through magnetic reconnection, some released "low energy electrons hit the plasma," interacting with the ions and electrons composing the plasma, "imparting their energy and creating a unique rising tone" for this electron plasma wave whistle.

The hiss electron whistle

The plasmasphere, the plasma region nearest Earth, produces a second whistle-mode wave of undetermined cause. The plasma is dense and cold in the plasmasphere. The whistle produced by cold, slow particle and plasma interactions is steady and slow, with gradual peaks and dips of whistling.

Sounding similar to radio static, this whistle-mode sound is called the hiss.

The two theories explaining the plasmasphere hiss start with particles entering the plasmasphere from another zone. One theory suggests lightning particles forcefully injected into the plasmasphere cause the hiss (as well as causing the whistler wave). Another theory suggests chorus wave particles — pushed to the violent night-side — leak into the plasmasphere from the warmer plasma zone and cause the hiss.

Electron particle waves and space weather

The Van Allen Probes mission is "to understand the dynamics of plasma waves to improve predictions of space weather." The Probes were launched in 2012 (nasa.gov/missions/, Van Allen Probes) as a response to the discovery in 2008 (science.nasa.gov/, "Giant Breach in Earth's Magnetic Field") of an unexpected hole in Earth's magnetosphere — protective magnetic egg-shaped bubble — and of unexpected behavior of Earth's magnetic field lines, which were reconnecting in unpredictable ways.

The magnetosphere hole and magnetic line behavior discoveries were of scientific interest because an in tact magnetosphere prevents the penetration to Earth's environment of solar blasts — space weather — from coronal mass ejections, solar flares, and other solar activity. A magnetosphere hole and unpredictable magnetic line behavior degrade the level of protection the magnetosphere provides against the encroachment of solar space weather, which can disable GPS satellites, communications satellites, military satellites and scientific satellites while putting astronauts in space at risk.

Audible electron particle science

Equipped with the Electric and Magnetic Field Instrument Suite and Integrated Science, or EMFISIS, two Van Allen spacecraft were launched into Earth orbit to "measure electric and magnetic waves." The two EMFISIS recorded "changes in the frequency of the electric and magnetic fields" as they interacted with plasmatic ions and electrons, frequencies scientists later shifted to the audible range of the electromagnetic spectrum. As a bonus of understanding the dynamics of plasma waves, we "can listen to the sounds of space," or electron particle whistles, in the near-Earth region of the magnetosphere.