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

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1. Introduction

The in-situ detection of [FORMULA]-meteoroids, i.e. dust particles in hyperbolic orbits away from the Sun has been reported for the first time by Berg & Grün (1973) and by Zook & Berg (1975), based on measurements of the Pioneer 8 and 9 spacecraft. Grün & Zook (1980) invented the expression [FORMULA]-meteoroid, as opposed to the so called [FORMULA]-meteoroids, which are particles that were detected to come from the apex direction of the spacecraft. They determined the flux of [FORMULA]-meteoroids at 1 AU to be [FORMULA] [FORMULA] for particles of masses [FORMULA] (Berg & Grün, 1973). A flux of [FORMULA]-meteoroids has also been identified in the data of the micrometeoroid detector onboard Helios 1 at solar distances between 0.31 and 0.98 AU in the ecliptic (Grün et al., 1980).

The existence of particles that escape from the Solar system in hyperbolic orbits, is known for the surrounding of comets. Particles are ejected from the Solar system in unbound orbits when the repelling force from the solar radiation is of the same order of magnitude as solar gravity. The orbital parameters for the released particle depend on the exact conditions at the time of the dust ejection from the comet (see for instance Krésak, 1976). The orbital elements of the parent body as well as the acting radiation pressure force determine the orbit of the released particle. The detection of [FORMULA]-meteoroids points to the fact that a similar process must occur near the Sun. Dust particles in unbound orbits can be produced in the case of mutual collisions of micrometeoroids and larger meteoroids, as well as from the sublimation of dust particles near the Sun (cf. Mukai, 1985, 1996; Schwehm, 1977). Mukai (1996) has shown that absorbing particles may be forced into orbits of higher eccentricity when their size is reduced by sublimation and radiation pressure force increases. Also a collisional evolution of dust particles is expected to take place in the interplanetary dust cloud inside 1 AU (Leinert et al., 1983; Grün et al., 1985) as well as very close to the Sun (Ishimoto & Mann, 1996).

Since the study of [FORMULA]-meteoroids will reveal their possible formation process it helps towards a better understanding of the mass balance and collisional evolution of the interplanetary dust cloud. We will investigate the detection of [FORMULA]-meteoroids from the measurements of the dust experiment onboard ULYSSES (Grün et al., 1992a). As was shown in a previous study by Baguhl (1993), the Ulysses dust detector has detected [FORMULA]-meteoroids during the first phase of the mission 1990-1992 in its ecliptic orbit from 1 to 1.6 AU. We now check for a possible detection of [FORMULA]-meteoroids at high ecliptic latitudes. Our analysis is based on the data set which is published in the Ulysses/Galileo data page (Grün et al., 1995a) including measurements up to the end of 1995 which covers both passages of the solar poles. Using some basic assumptions we will develop a method to describe the orbits of [FORMULA]-meteoroids that could cross the Ulysses orbit. We calculate the detection probabilities for these possible fluxes of [FORMULA]-meteoroids for the detection geometry of the dust detector on the Ulysses-spacecraft (Grün et al., 1995b), and compare them to the earlier study by Baguhl (1993). Applying this method to the data set, we derive the flux rate of [FORMULA]-meteoroids for the ecliptic and for the out-of-ecliptic part of the Ulysses orbit and we estimate the orbital parameters of the identified [FORMULA]-meteoroids.

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© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998