Acknowledgement. We thank Science for their permission to use an excerpt from:

Warwick, J. W., et al. 1982. Planetary Radio Astronomy Observations from Voyager 2 Near Saturn. Science 215 (4532), 582-587. (Excerpt from pp. 586-587.)

Copyright AAAS, January 29, 1982.

Planetary Radio Astronomy Observations from Voyager 2 Near Saturn

Ring plane event. The PRA instrument on Voyager 2 also detected an intense event at or near the time of ring plane crossing ( Figs. 4 and 6). Power in the PRA channels peaked at 0418:17 SCET on day 238 (26 August 1981). The distance of the spacecraft from Saturn at ring plane crossing was approximately 2.88 R_S, near the nominal 2.82 R_S location of the G ring.

The time profile of the PRA event was generally symmetrical about the peak. The central peak displays a half-power rise time of 6 seconds or less, and the overall pattern exhibits a 30-dB rise time of approximately 1 minute. These times correspond to distances of about 70 and 700 km, respectively, normal to the ring plane. At its peak, the ring plane event extended from frequencies of 10 Hz or less to approximately 1 MHz, and the spectrum peaked in the 56-Hz channel of the plasma wave science instrument (14). The spectral density over five decades in frequency is shown in the inset to Fig. 6. The emission showed no evidence of polarization in any of the PRA channels. A similar event may have occurred when Voyager 1 crossed the ring plane outbound (15); however, the essential features of that event are difficult to extract because of strong plasma wave phenomena and SKR emissions.

The ring plane event is distinct from both SKR and SED in onset, duration, spectral character, and polarisation (24); the associated mechanism, we presume, is also distinct. The plasma instrument on Voyager 2 measured a nominal plasma concentration of approximately 100 particles per cubic centimeter during the ring plane crossing (9). Thus the ambient plasma frequency was at least 100 kHz, which is well above the frequency at which the event spectrum peaked. Therefore the observed emissions evidently are not propagating electromagnetic disturbances originating at Saturn or in any of its rings, including the G ring. On the contrary, the phenomenon appears to originate at the spacecraft.

Warwick et al. (2) suggested that charged dielectric particles striking the PRA antenna booms could generate electrical events. Micrometer-size ice particles striking the spacecraft at relative velocities of 10 km/sec or more probably would vaporize and ionize. In the G ring most particles are believed to be 1 to 5 micrometers in diameter (16), and Clark (17) deduced that the composition of Saturn ring material is approximately 95 percent water ice and 5 percent ferric oxide. The spacecraft velocity relative to G ring material was about 14 km/sec at the time of ring plane crossing.

We propose the following simple model of the ring plane event. At or near the time of ring plane crossing, Voyager 2 strikes charged micrometer-size ice particles in the outer regions of the G ring. The 56-Hz spectral peak is taken to be an approximate measure of the impact frequency. Each impact produces a tenuous charged plasma enveloping part or all of the spacecraft. The plasma dissipates because of thermal motion and relative motion between the spacecraft and Saturn's corotating magnetic field, in which the plasma becomes embedded. A typical dissipation time is 0.5 msec. Since this is short compared to the inferred impact frequency, the phenomenon is dominated by single events. The step function increases in voltage associated with the production of charged plasma exhibit an f^-2 flux density spectrum; however, such a spectrum is modified at low frequencies by the impact rate itself and at high frequencies by dissipation effects and plasma physical phenomena. Additional modifications could result from spacecraft interactions with the plasma. Therefore, an impact discharge phenomenon could produce a spectrum like that shown in Fig. 6.

In this model, the Voyager 2 PRA and plasma wave science instruments act in tandem to yield in situ measurements of G ring material. The falloff in intensity and the change in spectrum away from the ring plane are attributed to variations in particle size and number density along the path of the spacecraft. The total vertical thickness of ~ 1500 km inferred from the duration of the ring plane noise event is much greater than the optical thickness reported for any of Saturn's major rings. Hence, these data indicate that the G ring possesses a tenuous halo that extends well beyond the nominal ring particle layer - much like the E ring with its 1800-km inferred thickness (18).

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