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This animation combines two ways of displaying the Voyager Plasma Wave Science (PWS) observations of electron plasma oscillations which provide the basis for concluding that the spacecraft is now in interstellar space. The graphic is called a spectrogram that shows the amplitude of waves (in which reds are the most intense and blues the least intense) as a function of frequency (vertical axis) and time (horizontal axis). In many respects, this spectrogram is like a voice print which shows the evolution of the spectrum of sounds as a function of time. The sound track reproduces the amplitude and frequency of the plasma waves observed. The vertical white bar that moves across the spectrogram links the sound track to the graphic.
The frequency range shown from about 1.75 kiloHertz to 3.5 kiloHertz is a portion of the actual frequency range detected by PWS and is well within the audio frequency range. Importantly, the frequency is directly related to the number of electrons per unit volume in the vicinity of Voyager and corresponds to about 1 electron per 10 cubic centimeters or a cube about 1 inch on a side. The time scale for this presentation represents 225 days or a bit more than 7 months, while it only takes about 12 seconds to play the audio file. Hence, the time compression is about 1.6 million to one. It should be noted that this compression was done in such a way as to not change the frequencies. For a brief sample of raw uncompressed audio, see http://youtu.be/CFklHHWbYrk.
In this animation, there are two events of interest. In the October-November 2012 time frame there is a tone near 2.1 kHz which gradually increases in frequency. Again, in the April-May 2013 time frame there is another event, somewhat more intense and at a higher frequency near 2.6 kHz. We conclude that these two events indicate an ongoing trend to higher frequencies. The second graphic frame which appears in the animation includes a dashed line showing this increase in frequency and suggests that the density of electrons is continually increasing over this time interval as Voyager moves outwards from the heliopause (which was crossed on 25 August 2012).
You can shift-click to download a copy of the mp4 video. Also, see the most recent update to these observations.
For more information on the Voyager project, see
For more information on the Voyager plasma wave investigations, see
For more information on space audio, see
University of Iowa news release
Voyager 1 spacecraft reaches interstellar space
University of Iowa video release
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The Radio and Plasma Wave Group, Department of Physics & Astronomy, College of Liberal Arts & Sciences.