English: On June 27, 1996, the Galileo spacecraft flew near Jupiter's moon Ganymede for the first time. Fields and particles instruments on board made a startling discovery: This moon had a significant magnetic field which produced it's own magnetosphere embedded in the huge Jovian magnetosphere. The plasma wave instrument recorded waveform samples up to 80kHz which revealed classic magnetospheric structures. These samples have been slowed down by a factor of nine to shift the high frequencies down into the audible range for this recording.

The image is a dynamic spectrogram showing the intensity of waves as a function of frequency (vertical axis) and time (horizontal axis) in which red indicates high intensity waves and blue indicates low intensities. This spectrogram was obtained by Fourier transforming the actual waveform from the electric antenna at a sample rate of 201,600 samples per second. We have used the same waveform to generate an audio signal but have used a sample rate of about a factor of 9 slower in order to shift the 80-kHz bandwidth down into the audio frequency range. We have also used a technique called time-slicing to reduce the 45-minute recording to just one minute. The cursor moves across the spectrogram as the audio signal is played.

For more information, see

http://www-pw.physics.uiowa.edu/~wsk/galileo/ganymede/ganymede.html

http://solarsystem.nasa.gov/galileo/

http://space-audio.org/