UNITED STATES: Instruments aboard Voyager 1, which has moved past the edge of the solar system, and into the interstellar medium, have detected the sounds of plasma waves, according to research published Monday in the journal ‘Nature Astronomy’.
Heliosphere
Until recently, every spacecraft in history had made all of its measurements inside our heliosphere. Heliosphere is the magnetic bubble inflated by our Sun and its plasma radiation pressure. It is the protective layer for our solar system. But on August 25, 2012, NASA’s Voyager 1 changed that. As it crossed the heliosphere’s boundary, it became the first human-made object to enter – and measure – interstellar space. Now eight years into its interstellar journey, Voyager 1’s data is yielding new insights into what that frontier is like.
If our heliosphere is a ship sailing interstellar waters, Voyager 1 is a life raft just dropped from the deck, determined to survey the currents. For now, any rough waters it feels are mostly from our heliosphere’s wake. But farther out, it will sense the stirrings from sources deeper in the cosmos. Eventually, our heliosphere’s presence will fade from its measurements completely. Stella Ocker, a Ph.D. student at Cornell University in Ithaca, New York, and the newest member of the Voyager team, said, “We have some ideas about how far Voyager will need to get to start seeing more pure interstellar waters, so to speak, but we’re not entirely sure when we’ll reach that point.”
Interstellar medium
When one pictures the stuff between the stars – astronomers call it the “interstellar medium,” a spread-out soup of particles and radiation. One might imagine a calm, silent, serene environment. That would be a mistake. “I have used the phrase ‘the quiescent interstellar medium’ – but you can find lots of places that are not particularly quiescent,” said Jim Cordes, space physicist at Cornell and co-author of the paper.
Like the ocean, the interstellar medium is full of turbulent waves. The largest come from our galaxy’s rotation, as space smears against itself and sets forth undulations tens of light-years across. Smaller (though still gigantic) waves rush from supernova blasts, stretching billions of miles from crest to crest. The smallest ripples are usually from our own Sun, as solar eruptions send shockwaves through space that permeate our heliosphere’s lining.
These crashing waves reveal clues about the density of the interstellar medium. Its value will change our understanding of the shape of our heliosphere, how stars form, and even our own location in the galaxy. As these waves reverberate through space, they vibrate the electrons around them. These vibrating electrons ring out at characteristic frequencies depending on how crammed together they are. The higher the pitch of that ringing, the higher the electron density.
In November 2012, three months after exiting the heliosphere, Voyager 1 heard interstellar sounds for the first time. Six months later, another “whistle” appeared – this time louder and even higher pitched. The interstellar medium appeared to be getting thicker, and quickly. These momentary whistles continue at irregular intervals in Voyager’s data. They’re an excellent way to study the interstellar medium’s density, but it does take some patience. (You can hear the sound here )
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This may be a pathbreaking engineering feat which can transform our understanding of cosmos. In the meantime, Voyager 1’s Plasma Wave Subsystem keeps sending back data farther and farther from home, where every new discovery has the potential to make us reimagine our home in the cosmos.
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