The impulsive noise that the plasma wave and radio astronomy instruments detected during the Voyager 2 swing by Saturn was attributed to dust grains striking the spacecraft. This report presents a reanalysis of the dust impacts recorded by the plasma wave instrument using an improved model for the response of the electric antenna to dust impacts. The fundamental assumption used in this analysis is that the voltage induced on the antenna is proportional to the mass of the impacting grain. Using the above assumption and the antenna response constants used at Uranus and Neptune, the following conclusions can be reached. The primary dust distribution consists of a “disk” of particles that coincides with the equator plane and has a north‐south thickness of 2Δ z = 962 km. A less dense “halo” with a north‐south thickness of 2Δ z = 3376 km surrounds the primary distribution. The dust particle sizes are of the order of 10 µm, assuming a mass density of 1 g/cm³. The corresponding particle masses are of the order of 10 −9 g, and maximum number densities are of the order of 10 −2 m −3 . Most likely, the G ring is the dominate source since the particles were observed very close to that ring, namely at 2.86 R S . Other sources, like nearby moons, are not ruled out especially when perturbations due to electromagnetic forces are included. The calculated optical depth differs by about a factor of 2 from photometric studies. The current particle masses, radii, and the effective north‐south thickness of the particle distribution are larger than what Gurnett et al. (1983) reported by about 2, 1, and 1 orders of magnitude, respectively. This is attributed to the fact that the collection coefficient used in this study is smaller than what was used in Gurnett et al.'s earlier publication.
During the Voyager 2 flyby of Neptune the plasma wave and radio astronomy instruments detected numerous impulsive signals that were interpreted as micron‐sized particles striking the spacecraft. This paper presents an analysis of the particle impacts observed by the plasma wave instrument. From analysis of wideband antenna voltage waveforms a peak impact rate of 443 s −1 was observed near the inbound equator crossing, which occurred at a radial distance of 3.45 R N , and a peak impact rate of 151 s −1 was observed near the outbound ring plane crossing, which occurred at a radial distance of 4.20 R N . The inbound peak is offset by 146 ± 4 km north of the equatorial plane, and the outbound peak is offset by 948 ± 65 km south of the equatorial plane. A Gaussian fit of the impact rate profile has a width (to the e −l points) of 537 ± 22 km for the inbound “core” component and 2,073 ± 392 km for the outbound component. These impact rates correspond to maximum number densities of a few times 10 −3 m −3 . Analysis of the voltage amplitudes induced on the antenna indicates that the particles have masses ranging from 10 −10 g to a few times 10 −9 g, with an uncertainty of up to a factor of 10. Assuming a mass density of 1 g/cm 3 , these particles would have radii in the range 5‐10 μm, with an uncertainty of about a factor of 2 to 3. In addition to the high impact rates observed near the two equator crossings, an impact rate of at least 0.6 s −1 was observed over the entire region inside about 8 R N , including the northern polar region. Likely sources for these particles include (1) several of the small satellites (1989N1, 1989N2, 1989N3, and 1989N4) discovered near Neptune by Voyager and (2) the rings. Electromagnetic forces may play an important role in diffusing the particles away from the equatorial plane.
A strong heliospheric radio emission event has been detected by Voyagers 1 and 2 in the frequency range of 2 to 3 kilohertz. This event started in July 1992 and is believed to have been generated at or near the heliopause by an interplanetary shock that originated during a period of intense solar activity in late May and early June 1991. This shock produced large plasma disturbances and decreases in cosmic ray intensity at Earth, Pioneers 10 and 11, and Voyagers 1 and 2. The average propagation speed estimated from these effects is 600 to 800 kilometers per second. After correction for the expected decrease in the shock speed in the outer heliosphere, the distance to the heliopause is estimated to be between 116 and 177 astronomical units.
Using data from the Voyager 1 plasma wave instrument, a series of direction‐finding measurements is presented for the intense 1992–93 heliospheric 2‐ to 3‐kHz radio emission event, and several weaker events extending into 1994. Direction‐finding measurements can only be obtained during roll maneuvers, which are performed about once every three months. Two parameters can be determined from the roll‐induced intensity modulation, the azimuthal direction of arrival (measured around the roll axis), and the modulation index (the peak‐to‐peak amplitude divided by the peak amplitude). Measurements were made at two frequencies, 1.78 and 3.11 kHz. No roll modulation was observed at 1.78 kHz, which is consistent with an isotropic source at this frequency. In most cases an easily measurable roll modulation was detectable at 3.11 kHz. Although the azimuth angles have considerable scatter, the directions of arrival at 3.11 kHz can be organized into three groups, each of which appears to be associated with a separate upward drifting feature in the radio emission spectrum. The first group, which is associated with the main 1992–93 event, is consistent with a source located near the nose of the heliosphere. The remaining two groups, which occur after the main 1992–93 event, have azimuth angles well away from the nose of the heliosphere. The modulation indexes vary over a large range, from 0.06 to 0.61, with no obvious trend. Although the variations in the directions of arrival and modulation indicies appear to reflect changes in the position and angular size of the source, it is also possible that they could be caused by refraction or scattering due to density structures in the solar wind.
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