Abstract. During the Voyager 1 and 2 flybys of the outer planets it has been demonstrated that the plasma wave instrument can detect small dust particles striking the spacecraft. In this paper, we examine the Voyager plasma wave data for dust impacts in the interplanetary medium at heliocentfic radial distances ranging from 6 to 60 astronomical units (AU). The results show that a small but persistent level of dust impacts exists out to at least 30 to 50 AU. The average number density of these particles is about 2 x 10 -8 m '3 and the average mass of the impacting particles is believed to'be a few times 10 -ll g, which corresponds to particle diameters in the micron range. Possible sources of these particles are planets, moons, asteroids, comets, and the interstellar medium. Of these, comets appear to be the most likely source. The number densities are only weakly dependent on ecliptic latitude, which indicates that the particles probably do not originate from planets, moons, or asteroids. Comparisons with interstellar dust fluxes measured in the inner regions of the solar system by the Ulysses spacecraft indicate that the particles are not of interstellar origin.

We report the detection of a dense ionized layer in the upper atmosphere of Mars caused by the impact of dust from comet Siding Spring. The observations were made by the ionospheric radar sounder on the Mars Express spacecraft during two low‐altitude passes approximately 7 h and 14 h after closest approach of the comet to Mars. During these passes an unusual transient layer of ionization was detected at altitudes of about 80 to 100 km with peak electron densities of (1.5 to 2.5) × 105 cm−3, much higher than normally observed in the Martian ionosphere. From comparisons to previously observed ionization produced by meteors at Earth and Mars, we conclude that the layer was produced by dust from the comet impacting and ionizing the upper atmosphere of Mars.

During the Voyager 2 flyby of Neptune a series of 16 whistlerlike events were detected by the plasma wave instrument near closest approach. These events were observed at radial distances from 1.30 to 1.99 RN and magnetic latitudes from −7° to 33°. The frequencies ranged from 6.1 to 12.0 kHz, and the dispersions fit the Eckersley law for lightning‐generated whistlers. Lightning in the atmosphere of Neptune is the only known source of such signals. The frequency range of the whistlers (up to 12 kHz) indicates that the local electron densities are substantially higher (Ne > 30 to 100 cm−3) than indicated by the in situ plasma measurements. The dispersion of the whistlers is very large, typically 26,000 s Hz1/2. On the basis of existing plasma density models and measurements, the dispersions are too large to be accounted for by a single direct path from the lightning source to the spacecraft. Therefore multiple bounces from one hemisphere to the other are required. The most likely propagation path probably involves a lighting source on the dayside of the planet, with repeated bounces through the dense dayside ionosphere at low L values.

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 2Δz = 962 km. A less dense “halo” with a north‐south thickness of 2Δz = 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 RS. 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.

The Voyager 2 plasma wave instrument detected many familiar plasma waves during the encounter with Neptune, including electron plasma oscillations in the solar wind upstream of the bow shock, electrostatic turbulence at the bow shock, and chorus, hiss, electron cyclotron waves, and upper hybrid resonance waves in the inner magnetosphere. Low-frequency radio emissions, believed to be generated by mode conversion from the upper hybrid resonance emissions, were also observed propagating outward in a disklike beam along the magnetic equatorial plane. At the two ring plane crossings many small micrometer-sized dust particles were detected striking the spacecraft. The maximum impact rates were about 280 impacts per second at the inbound ring plane crossing, and about 110 impacts per second at the outbound ring plane crossing. Most of the particles are concentrated in a dense disk, about 1000 kilometers thick, centered on the equatorial plane. However, a broader, more tenuous distribution also extends many tens of thousands of kilometers from the equatorial plane, including over the northern polar region.