GALILEO PLASMA WAVE INVESTIGATION:
OBSERVATIONS AT CALLISTO
Of the four Galilean satellites, Callisto appears to have the weakest interaction with the Jovian magnetosphere. This is perhaps due to its relatively large distance from the planet (about 25 Jovian radii), or perhaps due to its surface characteristics.
The Callisto interaction appears to consist of a number of broadband, bursty electrostatic emissions, although one cannot rule out the possibility that these bursts are associated with the ambient magnetosphere. The brief high-frequency bursts are thought to be upper hybrid resonance bands indicating a plasma density of about 100 per cubic centimeter. Since this is much larger than the surrounding Jovian plasma density, this feature suggests that Callisto is a source of plasma, although not nearly as prolific as Io!
The only features in the magnetic field spectrogram are due to known sources of interference on the Galileo spacecraft. This fact is used to identify the bursty features in the electric field spectrogram as electrostatic waves as opposed to electromagnetic waves.
Listen to sounds of Callisto's plama waves. The Galileo plasma wave receiver has a special mode which records the actual waveforms of the signals detected by the instrument's electric field antenna. In order to make it possible to "listen" to the ~45 minutes of signals around closest approach, we've had to shorten the recording by using very short slices of the waveform and cutting out relatively long periods between each pair of slices. We've also had to play back the waveform at about a factor of 10 slower since the original recording had a bandwidth of 80 kHz and the human ear can only hear sounds up to about 10 or 20 kHz.