Acknowledgement. We thank Science for their permission to use an excerpt from:
Gurnett, D. A., et al. 1986. First plasma wave observations at Uranus. Science 233 (4759), 106-109. (Excerpt from p. 109.)
Ring plane. As the spacecraft passed through the ring plane at about 1715 on 24 January, an intense burst of low-frequency noise coincided almost exactly with the time of the ring plane crossing. This noise, which lasted about 6 minutes, can be seen in Fig. 4 at frequencies below about 1 kHz, increasing in intensity with decreasing frequency. The characteristics of this noise are similar to the noise observed during the Voyager 2 ring plane crossing at Saturn, when many micrometer-sized dust particles struck the spacecraft (8, 20). Investigation of the broadband data at the Uranus ring plane confirms that the noise is caused by dust impacts. The waveform from the broadband receiver (Fig. 6) consists of impulses lasting a few milliseconds and is similar to that for the dust impacts at Saturn. At the time of maximum intensity, around 1715:30, the impact rate is about 30 to 50 impacts per second as judged from initial estimates.
As presently understood, the impulsive voltage produced by a dust impact is caused by the charge released when a particle strikes the spacecraft. Laboratory measurements (21) show that when a small particle strikes a solid surface at a high velocity the particle is instantly vaporized and ionized, producing a small cloud of plasma that expands away from the impact site. As the plasma cloud sweeps over the electric antenna, some of the charge is collected by the antenna, thereby causing a voltage pulse. Laboratory measurements show that the charge released is directly proportional to the mass of the particle. The pulse amplitude is therefore proportional to the mass.
Because Voyager 2 passed through the ring plane at about 4.5 R_U, well beyond the visible rings, the dust impacts detected by the PWS instrument are not directly associated with the rings. It is possible, of course, that these dust particles may represent a disklike extension of the visible ring system out to where Voyager crossed the ring plane. Miranda could be another possible source. As judged from the similarity to the dust impacts at Saturn, the impacts at Uranus are probably caused by particles in the micrometer size range. The number density, n, can be estimated from the relation R=nUA, where R is the counting rate, U is the spacecraft speed, and A is the area of the spacecraft. From nominal values, the maximum number density is about 1e-3 particles per cubic meter. From the duration we estimate the north-south thickness, L, of the dust impact region to be about 4000 km. The columnar number density, nL, is then on the order of a few thousand particles per square meter. For micrometer-sized particles this number density is too small to be detected with the imaging system.
Last updated Feb-27-1997
