The Sounds of Interstellar Space   
       
   Nov. 1, 2013:  Scifi movies are sometimes criticized when explosions in the   
   void make noise.  As the old saying goes, "in space, no one can hear you   
   scream." Without air there is no sound.   
       
   But if that's true, what was space physicist Don Gurnett talking about when he   
   stated at a NASA press conference in Sept. 2013 that he had heard "the sounds   
   of interstellar space?"   
       
   It turns out that space can make music... if you know how to listen.   
       
   http://www.youtube.com/watch?v=jgQ9THRckJ0   
       
   What does deep space sound like? A new ScienceCast video answers this   
   question. Play it   
       
   Gurnett is the James Van Allen professor of physics at the University of Iowa   
   and the principal investigator for the Plasma Wave Science instrument on   
   Voyager 1.  At the press conference, he played some plasma wave data for the   
   audience.  The sounds, he explained, were solid evidence that Voyager 1 had   
   left the heliosphere.   
       
   The heliosphere is a vast bubble of magnetism that surrounds the sun and   
   planets. It is, essentially, the sun's magnetic field inflated to enormous   
   proportions by the solar wind.  Inside the heliosphere is "home." Outside lies   
   interstellar space, the realm of the stars.   
       
   Auroras Underfoot (signup)For decades, researchers have been on the edge of   
   their seats, waiting for the Voyager probes to leave.  Ironically, it took   
   almost a year for NASA to realize the breakthrough had occurred. 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 from the   
   Plasma Wave Instrument reached his desk in the summer of 2013. The distant   
   tones were conclusive: "Voyager 1 had made the crossing."   
       
   Strictly speaking, the plasma wave instrument does not detect sound. Instead   
   it senses waves of electrons in the ionized gas or "plasma" that Voyager   
   travels through. No human ear could hear these plasma waves. Nevertheless,   
   because they occur at audio frequencies, between a few hundred and a few   
   thousand hertz, "we can play the data through a loudspeaker and listen," says   
   Gurnett.  "The pitch and frequency tell us about the density of gas   
   surrounding the spacecraft."   
       
   http://www-pw.physics.uiowa.edu/voyager/v1pws_interstellar_epo.html   
       
   Electron plasma oscillations: The evidence that Voyager 1 crossed into   
   interstellar space. Listen   
       
   When Voyager 1 was inside the heliosphere, the tones were low, around 300 Hz,   
   typical of plasma waves coursing through the rarified solar wind. Outside, the   
   frequency jumped to a higher pitch, between 2 and 3 kHz, corresponding to   
   denser gas in the interstellar medium. The transition music to Gurnett's ears.   
       
   So far, Voyager 1 has recorded two outbursts of "interstellar plasma   
   music"--one in Oct-Nov. 2012 and a second in April-May 2013.  Both were   
   excited by bursts of solar activity.   
       
   "We need solar events to trigger plasma oscillations," says Gurnett.   
       
   The key players are CMEs, hot clouds of gas that blast into space when solar   
   magnetic fields erupt.  A typical CME takes 2 or 3 days to reach Earth, and a   
   full year or more to reach Voyager.  When a CME passes through the plasma, it   
   excites oscillations akin to fingers strumming the strings on a guitar.   
   Voyager's Plasma Wave Instrument listens . and learns.   
       
   "We're in a totally unexplored region of space," says Gurnett.  "I expect some   
   surprises out there."   
       
   In particular, Gurnett is hoping for plasma waves not excited by solar storms.   
   He speculates that shock fronts from outside the solar system could be   
   rippling through the interstellar medium.  If so, they would excite new plasma   
   waves that Voyager 1 will encounter as it plunges ever deeper into the realm   
   of the stars.   
       
   The next "sounds" from out there could be surprising indeed.   
       
   Credits:   
       
   Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   More information:   
       
   Voyager 1 Reaches Interstellar Space -- Science@NASA   
       
   "Sounds of Space" -- from various spacecraft   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.96   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)