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A plasma wave receiver on a spacecraft is an instrument capable of measuring characteristics of space plasma waves detected by electric or magnetic antennas. Unlike radio waves which are free-space propagating electromagnetic waves, plasma waves are intimately coupled with the thin charged gas through which they propagate. Oscillating charged particles in the plasma produce oscillating electric and magnetic fields which are detected with antennas and measured by the plasma wave receiver. Typically the receiver either takes very rapid samples of the oscillating field amplitude across a wide frequency band (waveform measurement) or a set of narrow frequency detectors integrate received power over a longer accumulation time (spectral measurement). The first type of measurement results in time-domain data that show the waveform detected on the antenna, while the second type results in frequency-domain data showing the wave power in a sequence of narrow frequency channels.
One popular interesting feature of plasma waves is that the characteristic frequencies involved can range through frequencies audible to humans. So the waveform measurements of plasma wave receivers may be played back directly as audio signals without any modification, generally sounding like noise but occasionally revealing very interesting and even melodic details. For this reason, plasma waves are sometimes called space audio. Although these waves propagate through a medium of charged particles and there is a very tiny pressure component, they are not detectable with the human ear in space, however the associated oscillating fields are easily detected with antennas.
It may have been Donald Gurnett, University of Iowa Professor of Physics, who invented the Plasma receiver. Gurnett has been intimately involved in the key space probes sent since 1962 (Ijun III, Voyager I and II, Galileo and Cassini–Huygens amongst others).
Vibrations in the audible frequency range are perceived by humans when air vibrates against their eardrum. Air, or some other vibrating medium such as water, is indispensable in the perception of sound by the human ear. Without it acting as a transmitter, the sound produced by the source will not be perceived by a human. There is no air in outer space, nor there is any other type of medium capable of transmitting any vibration from a source to a human ear. However, there are sources in outer space that do vibrate at frequencies that would be audible by a human, if only there were some sort of transmitting media to carry those vibrations from the source to a human eardrum.
One such source, capable of vibrating at audible frequencies (45 to 20,000 vibrations per second) is plasma. Plasma is a collection of charged particles, such as free electrons or ionized gas atoms. Examples of plasma are solar flares, solar wind, neon signs and fluorescent lamps. Plasma interacts with electrical and magnetic fields in ways that can result in vibrations in many frequencies, including the audible range.
It appears that Gurnett designed the first plasma receiver, an instrument capable of detecting the vibrations in outer space plasma. These interplanetary plasma vibrations can be transformed into sound waves or air vibrations audible to a human ear. NASA provided recordings of these interplanetary and outer space plasma vibrations to composer Terry Riley and Kronos quartet founder David Harrington, which inspired the composition of "Sun Rings", a multimedia 85-minute piece for string quartet and choir. "Sun Rings" was performed November 3, 2006, at the Veteran's Auditorium, in Providence, Rhode Island.
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