South-North and radial traverses through the interplanetary dust cloud

Identical in situ dust detectors are flown on board the Galileo and Ulysses spacecraft. They record impacts of micrometeoroids in the ecliptic plane at heliocentric distances from 0.7 to 5.4 AU and in a plane almost perpendicular to the ecliptic from -79 degrees to +79 degrees ecliptic latitude. The combination of both Ulysses and Galileo measurements yields information about the radial and latitudinal distributions of micron- and submicron-sized dust in the Solar System. Two types of dust particles were found to dominate the dust flux in interplanetary space. Interplanetary micrometeoroids covering a wide mass range from 10(-16) to 10(-6) g are recorded mostly inside 3 AU and at latitudes below 30 degrees. Interstellar grains with masses between 10(-14) and 10(-12) g have been positively identified outside 3 AU near the ecliptic plane and outside 1.8 AU at high ecliptic latitudes (>50 degrees). Interstellar grains move on hyperbolic trajectories through the planetary system and constitute the dominant dust flux (1.5 X 10(-4) m(-2) sec(-1)) in the outer Solar System and at high ecliptic latitudes. To compare and analyze the Galileo and Ulysses data sets, a new model is developed based on J. Geophys. Res. 98, 17029-17048, Divine's (1993, ''five populations of interplanetary meteoroids'' model. Both models describe the interplanetary meteoroid environment in terms of dust populations on distinct orbits. Taking into account the measured velocities and the effect of radiation pressure on small particles (described by the ratio of radiation pressure force to gravity, beta), we define four populations of meteoroids on elliptical orbits and one population on hyperbolic orbit that can fit the micrometeoroid flux observed by Galileo and Ulysses. Micrometeoroids with masses greater than 10(-10) g and negligible radiation pressure (beta = 0) orbit the Sun on low to moderately eccentric orbits and with low inclinations (less than or equal to 30 degrees). Populations of smaller particles with mean masses of 10(-11) g (beta = 0.3), 10(-13) g (beta = 0.8), and 5 X 10(-15) g (beta = 0.3), respectively, have components with high eccentricities and have increasingly wider inclination distributions with decreasing mass. Similarities among the orbit distributions of the small particle populations on bound orbits suggest that all are genetically related and are part of an overall micrometeoroid complex that prevails in the inner Solar System. The high-eccentricity component of the small particle populations may actually be beta-meteoroids which are not well characterized by our measurements. Our modeling suggests further that the interstellar dust flux is not reduced at Ulysses' perihelion distance (1.3 AU) and that it contributes about 30% of the total dust flux observed there

Dust measurements in the Jovian magnetosphere

Dust measurements have been obtained with the dust detector onboard the Galileo spacecraft inside a distance of about 60R(J) from Jupiter (Jupiter radius, R-J = 71, 492 km) during two periods of about 8 days around Galileo's closest approaches to Ganymede on 27 June and on 6 Sept 1996. The impact rate of submicrometer-sized particles fluctuated by a factor of several hundred with a period of about 10 hours, implying that their trajectories are strongly affected by the interaction with the Jovian magnetic field. Concentrations of small dust impacts were detected at the times of Ganymede closest approaches that could be secondary ejecta particles generated upon impact of other particles onto Ganymede's surface. Micrometer-sized dust particles, which could be on bound orbits about Jupiter, are concentrated in the inner Jovian system inside about 20R(J) from Jupiter

Three years of Ulysses dust data: 1993-1995

The Ulysses spacecraft is orbiting the Sun on a highly inclined ellipse (i = 79 degrees). After its Jupiter flyby in 1992 at a heliocentric distance of 5.4 AU, the spacecraft reapproached the inner solar system, flew over the Sun's south polar region in September 1994, crossed the ecliptic plane at a distance of 1.3 AU in March 1995, and flew over the Sun's north polar resion in July 1995. We report on dust impact data obtained with the dust detector onboard Ulysses between January 1993 and December 1995. Wt publish and analyse the complete data set of 509 recorded impacts of dust particles with masses between 10-(16) g-10(-7) Together with 968 dust impacts from launch until the end of 1992 published earlier (Grun et al., 1995c), information about 1477 particles detected with the Ulysses sensor between October 1990 and December 1995 is now available. The impact rate measured between 1993 and 1995 stayed relatively constant at about 0.4 impacts per day and varied by less than a factor of ten. Most of the impacts recorded outside about 3.5 AU are compatible with particles of interstellar origin. Two populations of interplanetary particles have been recognized: big micrometer-sized particles close to the ecliptic plane and small sub-micrometer-sized particles at high ecliptic latitudes. The observed impact rate is compared with a model for the flux of interstellar dust particles which gives relatively good agreement with the observed impact rate. No change in the instrument's noise characteristics or degradation of the channeltron could be revealed during the three-year period

Three years of Galileo dust data: II. 1993-1995

Between January 1993-December 1995, the Galileo spacecraft traversed interplanetary space between Earth and Jupiter and arrived at Jupiter on 7 December 1995. The dust instrument onboard the spacecraft was operating during most of the time and data from the instrument were obtained via memory readouts which occurred at rates between twice per day and once per week. All events were classified by an onboard program into 24 categories. Noise events were usually restricted to the lowest categories (class 0). During Galileo's passage through Jupiter's radiation belts on 7 December 1995, several of the higher categories (classes 1 and 2) also show evidence for contamination by noise. The highest categories (class 3) were noise-free all the time. A relatively constant impact rate of interplanetary and interstellar (big) particles of 0.4 impacts per day was detected over the whole three-year time span. In the outer solar system (outside about 2.6 AU) they are mostly of interstellar origin, whereas in the inner solar system they are mostly interplanetary particles. Within about 1.7 AU from Jupiter intense streams of small dust particles were detected with impact rates of up to 20,000 per day whose impact directions are compatible with a Jovian origin. Two different populations of dust particles were detected in Jovian magnetosphere: small stream particles during Galileo's approach to the planet and big particles concentrated closer to Jupiter between the Galilean satellites. There is strong evidence that the dust stream particles are orders of magnitude smaller in mass and faster than the instrument's calibration, whereas the calibration is valid for the big particles. Because the data transmission rate was very low, the complete data set for only a small fraction (2525) of all detected particles could be transmitted to Earth; the other particles were only counted. Together with the 358 particles published earlier, information about 2883 particles detected by the dust instrument during Galileo's six years' journey to Jupiter is now available

Dust measurements during Galileo's approach to Jupiter and Io encounter

About a hundred dust impacts per day were detected during the first week in December 1995 by Galileo during its approach to Jupiter, These impacts were caused by submicrometer-sized particles that were just above the detection limit. After the closest approach to Io on 7 December, impacts of these small particles ceased, This effect is expected for dust grains emitted from Io that exit the field of view of the instrument after the flyby. The impact rate of bigger micrometer-sized dust grains continued to increase toward Jupiter. These dust particles are in orbit about Jupiter or are interplanetary grains that are gravitationally concentrated near Jupiter

Discovery of Jovian Dust Streams and Interstellar Grains by the Ulysses Spacecraft

On 8 February 1992, the Ulysses spacecraft flew by Jupiter at a distance of 5.4 AU from the Sun. During the encounter, the spacecraft was deflected into a new orbit, inclined at about 80-degrees to the ecliptic plane, which will ultimately lead Ulysses over the polar regions of the Sun1. Within 1 AU from Jupiter, the onboard dust detector2 recorded periodic bursts of submicrometre dust particles, with durations ranging from several hours to two days, and occurring at approximately monthly intervals (28 +/- 3 days). These particles arrived at Ulysses in collimated streams radiating from close to the line-of-sight direction to Jupiter, suggesting a jovian origin for the periodic bursts. Ulysses also detected a flux of micrometre-sized dust particles moving in high-velocity (greater-than-or-equal-to 26 km s-1) retrograde orbits (opposite to the motion of the planets); we identify these grains as being of interstellar origin