GALILEO PLASMA WAVE INVESTIGATION:
OBSERVATIONS AT IO
Since the early 1960's Io has been known to be an important member of the Jovian system because of its control over some of the Jovian decametric radio emissions. However, it wasn't until the discovery of the very high level of volcanic activity by Voyager that the full breadth of the Io influence on the Jovian system was realized. Io spews out literally tons of material (mainly sulfur and oxygen) per second. One of the results of this activity is the formation of a donut-shaped cloud of plasma surrounding Jupiter near Io's orbit called the Io plasma torus. One of the first results of the Galileo plasma wave investigation at Jupiter was to measure the density of the plasma in the Io torus and to detect very high densities very close to Io.
The narrowband line in this electric field spectrogram which gradually increases in frequency with time over most of the time interval is called the UHR or upper hybrid resonance band. This spectral feature provides a direct measurement of the density of plasma in the Io torus which is in the range of a few thousand per cubic centimeter over most of the torus, about a factor of two larger than observed by Voyager 1 in 1979. The sharp peak in this line at the time of Io closest approach represents a density of about 45,000 per cubic centimeter at Io closest approach and serves to confirm Io as the source of the plasma in the torus. The waves labeled whistler-mode emissions are apparently permanent constituents of the torus wave spectrum. However, there are several enhancements of these waves during the interval which have been shown to be associated with motions of the plasma which are responsible for moving the plasma coming from Io out into the outer portion of Jupiter's magnetosphere.
This is the magnetic field spectrogram, showing the magnetic component of the whistler-mode emissions seen above.