IDENTIFICATION OF SMALL DUST IMPACTS IN THE ULYSSES DUST DETECTOR DATA

Since October 1990, 3 weeks after the launch of the Ulysses spacecraft, the dust detector onboard recorded impacts of cosmic dust particles. Besides dust impacts, the detector recorded noise from a variety of sources. So far, a very rigid scheme had been applied to eliminate noise from impact data. The data labeled ''big'' dust impacts previously led to the identification of interstellar dust and of dust streams from Jupiter. The analysis presented here is concerned with data of signals of small amplitudes which are strongly contaminated by noise, Impacts identified in this data set are called ''small'' impacts. It is shown that dust impacts can be clearly distinguished from noise for most of the events due to the multi-coincidence characteristics of the instrument. 516 ''small'' impacts have been identified. For an additional 119 events, strong arguments can be given that they are probably small dust impacts. Thereby, the total number of dust impacts increases from 333 to 968 in the time period from 28 October 1990 to 31 December 1992. This increase permits a better statistical analysis, especially of the Jupiter dust streams which consist mostly of small and fast particles. Additional dust streams have been identified between the already known streams before and after Jupiter flyby. The dependence of the deflection from the Jupiter direction, the stream intensity and width on Jupiter distance support the assertion that they have been emitted from the Jovian system. The masses of the 635 ''small'' dust particles range from 6 x 10(-17) to 3 x 10(-10) g with a mean value of 1 x 10(-12) g, which compares to a range from 1 x 10(-16) to 4 x 10(-9) g with a mean value of 2 x 10(-11) g for the previously identified 333 ''big'' dust particles.