Decreased values of cosmic dust number density estimates in the Solar System

Experiments to investigate the effect of impacts on side-walls of dust detectors Such as (he present NASA/FSA Galileo/Ulysses instrument are reported. Side walls constitute 27% of the internal area of these instruments, and increase field of view from 140 degrees to 180 degrees. Impact of cosmic dust particles onto Galileo/Ulysses Al side walls was simulated by tiring Fe particles. 0.5-5 mu m diameter, 2-50 km s(-1), onto an Al plate, simulating the targets of Galileo and Ulysses dust instruments. Since side wall impacts affect the rise time of the target ionization signal, the degree to which particle fluxes are overestimated varies with velocity. Side-wall impacts at particle velocities of 2-20 km s(-1) yield rise times 10-30% longer than for direct impacts, so that derived impact velocity is reduced by a factor of similar to 2, Impacts on side wall at 20-50 km s(-1) reduced rise times by a factor of similar to 10 relative to direct impact data. This would result in serious overestimates or flux of particles intersecting the dust instrument at velocities or 20-50 km s(-1). Taking into account differences in laboratory calibration geometry we obtain the following percentages for previous overestimates (if incident particle number density values, from the Galileo instrument [Grun et al., 1992. The Galileo dust detector. Space Sci. Rev. 60, 317-340]: 55% for 2 km s(-1) impacts, 27% at 10 km s(-1) and 400% at 70 km s(-1). We predict that individual particle masses are overestimated by similar to 10-90% when side-wall impacts occur at 2-20 km s(-1) and underestimated by similar to 10-10(2) at 20-50 km s(-1). We predict that wall impacts at 20-50 km s(-1) can he identified in Galileo instrument data on account of their unusually short target rise times. The side-wall calibration is used to obtain new revised values [Kruger et al.. 2000. A dust cloud of Ganymede maintained by hypervelocity impacts of interplanetary micrometeoroids, Planet. Space Sci. 48, 1.157-1471. 2003, Impact-generated dust clouds surrounding the Galilean moons. Icarus 164, 170-187] of the Galilean Satellite dust number densities of 9.4 x 10(-5), 9.9 x 10(-5), 4.1 x 10(-5), and 6.8 x 10(-5) m(-3) at 1 satellite radius from lo. Europa, Ganyimede. and Callisto, respectively, Additionally, interplanetary particle number densities detected by the Galileo mission are found to be 1.6 x 10(-4), 7.9 x 10(-4), 3.2 x 10(-5). 3.2 x 10(-5),and 7.9 x 10(-4) m(-3) at heliocentric distances of 0.7, 1, 2. 3,and 5 AU, respectively. Work by Burchell et al. [1999b. Acceleration of conducting polymer-coated latex particles as projectiles in hypervelocity impact experiments. J. Phys. D: Appl. Phys. 32, 1719-1728] Suggests that low-density "fluffy" particles encountered by Ulysses will not significantly affect our results-further calibration would be useful to confirm this. (c) 2005 Elsevier Inc. All rights reserved.