GALILEO PLASMA WAVE INVESTIGATION:
OBSERVATIONS AT GANYMEDE
In the December 12, 1996 issue of Nature Galileo Plasma Wave observations are shown which provide evidence that Jupiter's moon Ganymede possesses its own magnetosphere. This page summarizes the data used in "Evidence of a Magnetosphere at Ganymede from Galileo Plasma Wave Observations" by D. A. Gurnett, W. S. Kurth, A. Roux, S. J. Bolton, and C. F. Kennel.
This electric field spectrogram shows the very strong interaction between Ganymede and the Jovian magnetosphere. The wealth and diversity of the wave signatures shown here provide evidence of a small magnetosphere surrounding Ganymede. The band of noise labeled fUH is at the upper hybrid resonance frequency and can be used to determine a plasma density of approximately 100 particles per cubic centimeter. The broadband bursts at the beginning and end of the interaction period are typical of the plasma wave signature for a magnetopause, or boundary of a magnetosphere. The banded emissions after closest approach are electron cyclotron harmonic emissions which are known at Earth to contribute to the generation of the aurora. The bright, broadband emission centered on closest approach and the emissions identified as "chorus" in the spectrogram are called whistler-mode emissions. The maximum frequency of these emissions enable the determination of a maximum in the Ganymede magnetic field traversed by Galileo of about 400 nanoTesla. The narrowband radio emissions extending primary to the right of the Ganymede interaction in the spectrogram are the first known radio emissions from a planetary satellite; these are similar to radio emissions studied at Earth and the outer planets, including Jupiter.
This magnetic field spectrogram shows the electromagnetic character of the whistler-mode emissions also seen in the spectrogram above.
This Quicktime Movie (3.9MB) allows you to hear the plasma waves observed by the Galileo Plasma Wave Receiver as it flew past Ganymede. 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.
A separate audio file (WAV 1.3MB) is also available.
We are grateful to Larry Granroth and Joe Groene at the University of Iowa for generating the spectrograms and sound files. Sugi Sorensen and Mike Martin of JPL's Data Distribution Lab kindly generated the Quicktime movie.