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

Scarf, F. L., et al. 1982. Voyager 2 Plasma Wave Observations at Saturn. Science 215 (4532), 587-594. (Excerpt from pp. 591-594.)

Copyright AAAS, January 29, 1982.

Voyager 2 Plasma Wave Observations at Saturn

The ring plane crossing. When Voyager 1 crossed the ring plane and magnetic equator near the Dione L shell, the plasma wave instrument detected a broad intensity peak in the whistler-mode turbulence and in the (n + 1/2)f_c bands (1) ( Fig. 3). About 45 minutes earlier, a Voyager 1 waveform frame confirmed the presence of strong chorus and whistler-mode hiss and weaker cyclotron harmonic emissions, together with a large number of intense impulsive noise bursts that were clearly detectable only in the wideband data [Fig. 5B in (1)]. In commenting on the sounds derived from the "static" or impulses which accompanied the chorus, Scarf et al. suggested possible interpretations involving "(a) lightning from the atmosphere, (b) discharges from the spacecraft, dust, and/or ring material, (c) impacts on the spacecraft, (d) Doppler-shifted ion acoustic waves" (l4, p. 2). We anticipated that Voyager 2 would provide much more definitive information, and arrangements were made to record six waveform frames in the equatorial region, including one frame that would contain data taken during the actual ring plane crossing.

Figure 2 shows that fairly low wave levels were detected throughout the region near Voyager 2 closest approach except right at the ring plane crossing (0418). The Voyager 2 plasma probe reported a very low plasma density (N << 0.1 cm^-3 or f_p << 2.8 kHz) for a period of hours around this time, but within 0.4 R_S of the equator, they detected a high density of very cold plasma. The absence of strong chorus and hiss near the Voyager 2 magnetic equator crossing is likely to be associated with the fact that the fluxes of resonant electrons were very low (3, 15); we will consider the intense broadband noise bursts detected in the immediate vicinity of the ring plane.

Figure 7A shows an expanded meridian plane plot of the Voyager 2 trajectory near closest approach. The heavy segment emphasizes the interval from 0405 to 0430; the time axis of the 16-channel spectrum analyzer plot for this interval is vertical (Fig. 7B), in accordance with the actual spacecraft motion from north to south. This shows that with the exception of an extremely intense and brief broadband noise burst centered at the ring plane crossing, the wave levels below 30 kHz were almost at background for 25 minutes as Voyager 2 moved from 9050 km above the ring plane to 7800 km below the plane. The central noise burst, which almost saturated the plasma wave receiver, lasted for approximately 2 or 3 minutes, corresponding to a north-south extent +/- 700 to 1000 km, which is about equal to the anticipated thickness of an outer ring system such as the E ring or the G ring (16).

It is not possible to explain the intense noise burst of Fig. 7 in terms of external plasma wave phenomena. At 0418, f_c was above 25 kHz, but the noise spectrum extended smoothly across this characteristic plasma frequency, suggesting that electrical impulses associated with local phenomena on the spacecraft were detected. [The moderate radio noise or 3f_c/2 emission in the 31- and 56-kHz channels of Fig. 7 may obscure the fact that the ring plane noise spectrum extends above f_c, but this peak was still detected in these high-frequency channels. Moreover, the planetary radio astronomy receiver on Voyager 2 observed strong signals to much higher frequencies (17), and the combined spectrum verifies that the ring plane noise has no resonances or cutoffs at the local characteristic plasma frequencies.]

Unambiguous identification of this noise phenomenon comes from analysis of the waveform data. Figure 8B shows the calibrated 56-Hz electric field amplitude plot along with marks indicating the locations of the six waveform frames. The ring plane was crossed during the last part of the wideband frame that went from 0417:35 to 0418:23. At this time we also detected the highest E field reading since launch in the 56-Hz channel [E (peak)~ 1000 times the "ambient" value].

The sound recording derived from this waveform frame provides a convincing way to identify the source of the intense turbulence. We summarize the audio analysis by stating that the sounds, which resemble a hailstorm, are those of impacts on the spacecraft. Further clarification comes from examining the highest-resolution electric-field time profile. The inset in Fig. 8A shows an E field plot with 35, microsecond between samples. This figure shows that the ring plane crossing noise consists of a sequence of short (tau ~= milliseconds or less) impulses. Since the relative velocity between the orbiting ring material and Voyager 2 exceeded 14 km/sec at this point, any dust particles impacting the spacecraft would break up into ionized fragments (18). We propose that the observed signals are electrical impulses associated with charged dust particles, and we note that the peak impact rate was greater than 200 per second. Since ground-based observations show that particles in the G and E rings are in the range 5 micrometer or less (19), the total particle mass flux was low even though the impact rate was high.

The ring plane spectral density (Fig. 8A) is preliminary and subject to correction because it includes effects of impulses that are strong enough to be clipped by the waveform receiver. A preliminary power spectrum made up from only unclipped impulses is flatter below 100 to 200 Hz, and it falls off less steeply than the ~ (f)^-2.5 behavior shown here. However, the spectrum in Fig. 8 is correct in the sense that it demonstrates how the ring plane noise is peaked below 100 to 200 Hz, consistent with a maximum impact rate of hundreds of hits per second. We conclude that Voyager 2 actually passed through an extension of Saturn's G ring and that the ring material hit the spacecraft, producing the noise characteristics of Figs. 7 and 8.

Although these conclusions are derived from analysis of audio frequency waves, our techniques do not have the ambiguity associated with early microphone dust-detector systems in which thermal creaks were counted as dust impacts; on Voyager, creaks, thruster firings, and other spacecraft noises are frequently detected, but they have recognizable frequency spectra and characteristic intensity levels and repetition rates different from the ones shown in Figs. 7 and 8. It is also significant that the Voyager plasma wave instrument found no detectable changes in wave level or noise bursts associated with passage into Saturn's shadow at 0409 (Fig. 7) and we conclude that there were no detectable thermal creaks as the spacecraft temperature stabilized in shadow.

The waveform frames extending out to 0425 had a relatively small number of impulses, and the audio analysis suggests the characteristics of "static" (Fig. 3A) rather than the "hailstorm" effect associated with the sounds from the ring plane crossing frame. This distinction is similar to one derived from the Voyager 1 waveform data, in which the frame nearest the ring plane crossing (at 5.4 R_S) had sounds suggesting impacts, whereas static sounds appeared on the frames recorded on 12 November 1980 at 0143 (24 R_S), 1257 (12.5 R_S), and 1830 (6.9 R_S), and on 13 November at 0108 (3.6 R_S) (14). However, high-resolution analysis of the E waveform amplitude data shows that the static sounds come from undispersed or local impulses associated with E(t) profiles essentially identical to the one contained in the inset of Fig. 8A. This result suggests that dust impacts cause impulses even at locations far from the rtngs, and it may mean that Saturn's gravity attracts dust over very large distances. Similar impulses have never been detected when Voyager 2 was in cruise, and they are not clearly evident in the Jupiter data analyzed to date; our initial analysis of Jupiter wave observations concentrated on frames having intense plasma wave activity, however, and weak bursts of undispersed impulsive noise would not appear.

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