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T.M. Ho, N. Thomas (Physikalisches Institut, Universitaet Bern,, Switzerland), D.C. Boice (SwRI), B. Grieger, C. Koellein (MPAE, Germany), L.A. Soderblom (USGS)
We present an analysis of particle acceleration in the vicinity of comet 19P/Borrelly. The data were obtained by the Miniature Integrated Camera and Imaging Spectrometer (MICAS) experiment onboard the Deep Space 1 spacecraft which encountered comet Borrelly on September 22nd 2001. All images show a dust coma dominated by jets. In particular a major collimated dust jet on the sunward side of the nucleus was observed. To analyze these features we integrate the observed intensity in concentric envelopes around the nucleus. The same procedure has been used on the Halley Multicolour Camera images of comet 1P/Halley acquired on March 14th 1986. We are able to show that at Borrelly the dust brightness dependence as a function of radial distance is different to that of Halley. At large distances both comets show constant values as the size of the concentric envelopes increases (as one would expect for force free radial outflow). For Halley the integral decreases as one gets closer to the nucleus. Borrelly shows opposite behavior. The main cause for Halley's intensity distribution is either high optical thickness or particle fragmentation. But dust acceleration and/or fragmentation into submicron particles would explain the observed steepening of the intensity profile close to the nucleus of Borrelly. We have constructed a simple model of the brightness distribution near the nucleus by using multiple dust jets with gaussian intensity profiles. Further we implement dust velocities taken from hydrodynamical calculations into our model to simulate the acceleration zone. The results of our model will show the influence of dust acceleration processes on the observed brightness distribution in the first kilometers away from the nucleus surface. The role of particle fragmentation and/or sublimation of ice covered particles which also can play a role in this scenario can then be constrained.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.