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Astron. Astrophys. 341, 296-303 (1999)

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2. Ulysses impact data

The Ulysses spacecraft was launched in October 1990 and after its passage through the asteroid belt it was deflected in an out-of-ecliptic orbit during a flyby at Jupiter. This orbit is nearly perpendicular to the ecliptic plane and passes through the regions above the solar poles (Wenzel et al., 1992). The different stages of the mission are the southern polar passage at a distance of 2.3 AU in August 1994, the crossing of the ecliptic plane at 1.3 AU in February 1995 and the passage of the northern solar pole in the time span of June to September 1995 at a distance of about 2.2 AU. Data from the dust detector are studied until December 1995, when the spacecraft was at 3.0 AU and [FORMULA] ecliptic latitude (Grün et al. 1997; Krüger et al., 1998).

The dust detector is a multi-coincidence impact ionisation detector which measures submicrometer- and micrometer-sized dust particles. A detailed description is given by Grün et al. (1992a, 1992b). The detector has a geometric area of [FORMULA]. It is mounted [FORMULA] to the spin axis of the spacecraft which is pointing towards the Earth. This determines the detection geometry of the dust detector. The experiment measures particles in the mass range from [FORMULA] to [FORMULA] g with impact speeds from 2 to 70 km/s (Grün et al., 1992b). The uncertainty in the speed determination amounts to a factor of 1.6 and the mass of impacting particles is determined to a factor of 6. The experiment detects particles within a viewing cone of [FORMULA] relative to the sensor axis which yields a limit to the determination of the impact direction and hence to the determination of the orbital parameters of the detected grains (Grün et al., 1995a, 1995b).

Essentially three different components of dust were so far identified in the data. Streams of particles were detected during the time before and after the Jupiter encounter. These streams are limited to a mass interval from [FORMULA] to [FORMULA] g and they come from the Jovian environment (Grün et al., 1993). Particles that enter the solar system from interstellar space have been identified in the data from their high impact speed and from their impact directions (Grün et al., 1994). The latter are comparable to the direction of the interstellar neutral gas flux measured onboard the same spacecraft (Witte et al., 1993). The remaining set of dust impacts may be assumed to be interplanetary dust particles. This is especially the case for the ecliptic path of the mission. The particles of this latter component tend to have relatively high impact velocities and the masses are typically lower than the mass of the particles that were classified as interstellar. Hence [FORMULA]-meteoroids, as well as particles that were ejected by Lorentz-forces and also interstellar particles that were depleted from their original flux direction may contribute to this component. In our study we work with this latter "interplanetary" component. We will use the data set after excluding the time period where the Jupiter streams have been detected and after subtracting impacts that are classified as interstellar according to the criteria given by Baguhl et al. (1994), i.e. from their impact velocity and mass.

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© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998