Galileo dust data from the jovian system: 1997–1999

Abstract

The dust detector system on board the Galileo spacecraft recorded dust impacts in circumjovian space during the craft's orbital mission about Jupiter. This is the eighth in a series of papers dedicated to presenting Galileo and Ulysses dust data. We present data from the Galileo dust instrument for the period January 1997–December 1999 when the spacecraft completed 21 revolutions about Jupiter. In this time interval data were obtained as high resolution realtime science data or recorded data during 449 days (representing 41% of the entire period), or via memory readouts during the remaining times. Because the data transmission rate of the spacecraft was very low, the complete data set (i.e. all parameters measured by the instrument during impact of a dust particle) of only 3% (7625) of all particles detected could be transmitted to Earth; the other particles were only counted. Together with the data of 2883 particles detected during Galileo's interplanetary cruise and 5353 particles detected in the jovian system in 1996, complete data of 15 861 particles detected by the Galileo dust instrument from 1989 to 1999 are now available. The majority of the detected particles were tiny grains (about 10 nm in radius), most of them originating from Jupiter's innermost Galilean moon Io. They were detected throughout the jovian system and the highest impact rates exceeded 100 min - 1 (C21 orbit; 01 July 1999). With the new data set the times of onset, cessation and a 180 ∘ shift in the impact direction of the grains measured during 19 Galileo orbits about Jupiter are well reproduced by simulated 9 nm particles charged up to a potential of + 3 V , confirming earlier results obtained for only two Galileo orbits (Horányi, M., Grün, E., Heck, A., 1997. Modeling the Galileo dust measurements at Jupiter. Geophys. Res. Lett. 24, 2175–2178). Galileo has detected a large number of bigger particles mostly in the region between the Galilean moons. The average radius of 370 of these grains measured in the 1996–1999 period is about 2 μ m (assuming spherical grains with density 1 g cm - 3 ) and the size distribution rises steeply towards smaller grains. The biggest detected particles have a radius of about 10 μ m .