Identification of β-meteoroids from measurements of the dust detector onboard the Ulysses spacecraft

We investigate the detection of β-meteoroids (i.e. dust particles that leave the Solar system in unbound orbits from the direction of the Sun) in the data set of the Ulysses dust experiment. Analysis of the detection geometry of the experiment for the time span from launch until the end of 1995 shows that the detection of β-meteoroids is possible during 3 phases of the mission: in the first 100 days in the beginning of the mission in the ecliptic part of the orbit at heliocentric distances smaller than 1.6 AU, during the south polar passage, and at a time interval of approximately 150 days around the north polar passage. For these three intervals we can identify 48 particles in hyperbolic orbits with perihelion distances smaller than about 0.5 AU which may be classified as β-meteoroids. The mass distribution of the detected β-meteoroids covers a relatively small interval in the ecliptic path, but for the high latitude path it shows no significant difference from the mass distribution of other detected particles of presumably interplanetary origin. The flux of β-meteoroids derived from the data amounts to 1.5 ± 0.3 · 10−4m−2s−1 between 1.0 and 1.6 AU in the ecliptic plane, and amounts to 9.0 ± 6.3 · 10−5m−2s−1 between 1.8 and 2.7 AU at solar ecliptic latitudes between 67◦ and 79◦ during the north polar passage.