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

Grunet, D. A., et al. 1989. First plasma wave observations at Neptune Science 246 (4936), 1494-1498. (Excerpt from pp. 1495-1496.)

Copyright AAAS, December 15, 1989.

First Plasma Wave Observations at Neptune

Ring plane. At the ring plane crossing of Saturn it was discovered that the plasma wave instrument and planetary radio astronomy instrument could detect small, micrometer-sized dust particles striking the spacecraft (10). When a small particle strikes the spacecraft at a velocity exceeding a few kilometers per second, the particle is instantly vaporized and heated to a high temperature, producing a cloud of ionized gas that expands away from the impact site. As the ionized gas cloud contacts the electric antenna, some of the charge is collected by the antenna, thereby causing a voltage pulse. Laboratory experiments show that the charge released is proportional to the mass of the dust particle. The amplitude of the voltage pulse is therefore proportional to the mass of the impacting particle.

Since Neptune was known to have a ring system, it was anticipated that the plasma wave instrument would detect dust impacts at the ring plane crossings. Indeed this was the case. Two ring plane crossings occurred during the Neptune flyby, one on the inbound leg shortly before closest approach, and the other on the outbound leg shortly after closest approach. The locations of the two ring plane crossings are indicated at the top of Fig. 1. At each ring plane an intense broadband burst of noise can be seen in nearly all channels, extending from below 10 Hz to above 10 kHz. This noise is caused by dust impacts on the spacecraft. The peak intensity detected at the inbound and outbound ring plane crossings occurred at 0253:19 + 4 s and 0516:07 + 4 s SCET, respectively. Two components can be seen. The most intense component is sharply peaked at the ring plane and has a duration of about 10 min. The second weaker component extends over a broad region, beginning about 1 hour before the first ring plane crossing and ending about 1 hour after the second ring plane crossing. Impact noise was even detected over the polar region, between the two ring plane crossings. These data suggest that a dense, thin disk of dust about 1000 km thick exists along the equatorial plane, surrounded by a tenuous halo extending many tens of thousands of kilometers on either side of the equatorial plane. To provide higher resolution of the dust impacts, a series of wideband plasma wave frames was collected at each of the ring plane crossings. These frames give 48-s samples of the antenna voltage waveform at a sample rate of 28,800 samples per second. A typical impact waveform is shown in Fig. 4. The general characteristics of the waveforms are similar to those observed at Saturn and Uranus. Based on these similarities we assume that the particles detected at Neptune are of similar size (that is, diameters of a few micrometers or less). Further study will be required to more accurately determine the mass and size distribution. Because individual impacts can be detected, accurate measurements can be made of the impact rate as the spacecraft passed through the ring plane. During the inbound ring plane crossing the maximum impact rate was about 280 impacts per second, and during the outbound, ring plane crossing the maximum impact rate was about 110 impacts per second. By using the nominal values for the spacecraft velocity and cross-sectional area of the spacecraft, we find that these impact rates imply maximum number densities on the order of 1e-3 particles per cubic meter.

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