Dust Stream Measurements from Ulysses' Distant Jupiter Encounter

In 1992 the impact ionisation dust detector on board the Ulysses spacecraft discovered periodic burst-like streams of dust particles within 2AU from Jupiter. The streams occurred at approximately monthly intervals (28 ± 3 days) and the maximum impact rates exceeded, by three orders of magnitude, the rates typically measured in interplanetary space [3]. These fluctuations were a complete surprise because no periodic phenomenon for small dust particles in interplanetary space was known before. The dust streams gave a completely new picture of interplanetary dust because they showed for the first time that dust originating from the environment of a planet can reach interplanetary space. The particles arrived at Ulysses in collimated streams radiating from close to the line of sight to Jupiter, suggesting a jovian origin. The 28-day periodicity was explained by the particle interaction with the interplanetary magnetic field [4]. Derived particle sizes were ∼ 10 nm and the particle speeds exceeded 200 km s [6]. With such high speeds, the jovian system turned out to be a source for interplanetary and even interstellar dust. Later Galileo measurements showed strong particle interaction with Jupiter’s magnetosphere [5, 2] and Io was identified as the grain source [1]. 12 years after its initial Jupiter flyby Ulysses approached the planet a second time in February 2004 with a closest approach distance of 0.8 AU. The first dust stream was detected in November 2002 at a distance of 3.3 AU from Jupiter which was the most distant stream detected so far. The maximum impact rates, measured around equatorial plane crossing of Jupiter, were three times larger than in 1992 (Figure 1). At least 17 dust streams were detected by December 2004, confirming grain properties recognised during the first flyby. The measured impact directions are consistent with a grain origin from the jovian system. The streams occur at about 26 day intervals closely matching the solar rotation period. Close to the equatorial plane the streams occur at a 13 day period, in agreement with theoretical predictions [4]. The impact direction of the streams is correlated with the polarity and strength of the interplanetary magnetic field. Taken all dust stream measurements since 1992 collected with three spacecraft together (Ulysses, Galileo, Cassini) the streams were detected over a large latitude range from the equator to the polar regions of Jupiter (−35◦ to +75◦ jovigraphic latitude).