November 4, 2013
‘Interstellar Music’ Serves Notice That Voyager Has Left The Heliosphere
[ Watch the Video: Voyager 1 Encounters An Interstellar Love Song ]
redOrbit Staff & Wire Reports - Your Universe OnlineThere is no air in space, and there cannot be sound without air. So what did NASA researchers mean when they recently stated that they had detected “interstellar music” which provided strong evidence that Voyager 1 had left the magnetic bubble surrounding the sun and planets known as the heliosphere?
That “interstellar music” is actually plasma waves detected by the probe’s Plasma Wave Science Instrument. As University of Iowa professor Don Gurnett, a space physicist and the chief investigator overseeing the Plasma Wave Science Instrument, explained during a September press conference, the instrument does not actually detect sound.
Instead, it detected electron waves in the plasma (ionized gas) which Voyager traveled through. While those plasma waves cannot be detected by the human ear, they occur at audio frequencies between a few hundred and a few thousand hertz, Gurnett explained. For that reason, he and his colleagues are able to listen to the data through a loudspeaker, determining the density of the gas surrounding the spacecraft through the pitch and frequency.
[ Watch the Video: The Sounds of Interstellar Space ]
Gurnett played some of the plasma wave data for those in attendance at the September press conference, explaining that they indicated that the US space agency’s probe had left the heliosphere, which is essentially the sun’s magnetic field expanded to massive proportions by the solar wind. NASA officials have been waiting for the Voyager probes to exit the heliosphere for decades, and apparently Voyager 1 did so months before they realized it.
“It took almost a year for NASA to realize the breakthrough had occurred,” the US space agency said. “The reason is due to the slow cadence of transmissions from the distant spacecraft. Data stored on old-fashioned tape recorders are played back at three to six month intervals. Then it takes more time to process the readings. Gurnett recalls the thrill of discovery when some months-old data ... reached his desk in the summer of 2013.”
He said that the sounds conveyed by the data conclusively demonstrated that Voyager 1 “had made the crossing.” While the probe was within the heliosphere, the tones detected by the Plasma Wave Science Instrument were low (approximately 300 hertz). Those sounds are typical of plasma waves coursing through solar wind.
Outside the heliosphere, the frequency rose to a higher pitch (between two and 3 kilohertz), corresponding to the denser gas located in the interstellar medium. Thus far, NASA said that Voyager 1 has only recorded two outbursts of this so-called “interstellar plasma music” – one that took place in October-November of 2012, and another which occurred in April-May of 2013. Both incidents were excited by solar activity.
“We need solar events to trigger plasma oscillations,” Gurnett explained. In particular, coronal mass ejections (CMEs) serve as a catalyst for the events. Those hot gas clouds, which are sent into space following the eruption of a solar magnetic field, typically take at least a full year to reach Voyager, NASA officials explained. When a CME passes through the plasma, it “excites oscillations akin to fingers strumming the strings on a guitar,” they added.
“We’re in a totally unexplored region of space. I expect some surprises out there,” Gurnett said. Specifically, he is hoping to find plasma waves that are not excited by solar storms. He believes that shock fronts from outside the solar system could be traveling through the interstellar medium, and if so, they could excite new plasma waves that the Voyager probes will encounter as it travels further outside of the heliosphere.