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S.M. Krimigis, D.G. Mitchell, D.H. Hamilton (Department of Physics, Univ. of Maryland, College Park, MD), S. Livi (Max-Planck Institut fur Aeronomie, Katlenburg-Lindau, Germany), J. Dandouras (Centre de Etude Spatiale des Rayonnements, CNRS/Universite Paul Sabatier, Toulouse, France)
The Magnetospheric Imaging Instrument (MIMI) is a neutrals and charged particles measurements system on the Cassini Orbiter designed to perform both global imaging and in-situ measurements to study the overall configuration and dynamics of Saturn's magnetosphere. Sensors cover the range from a few keV/e to several tens of Mev for ions, and >15 keV for electrons, and also provide images of Energetic Neutral Atoms (ENA's). During Venus swingby on June 24, 1999, MIMI sensors acquired data for ~4 hours, as the spacecraft approached the planet from the antisolar direction along the dusk flank, with closest approach occuring at ~2030UT. Typical magnetosheath flows of heated plasma were observed throughout most of the traversal until the spacecraft crossed the putative bow shock at ~2040UT, coincident with an ion spike. Intense fluxes of ions were seen after this time, apparently in the upstream region, with spectra characteristic of such events, and extending to energies of several tens of keV. Evidence of both heated solar wind (He++), and ions of Venusian origin (He+, C+, O+) was obtained. Further, fluxes of ENA's were seen, as expected from modeling of solar energetic protons impinging on the Venusian atmosphere and becoming neutralized through charge-exchange. Details of the observations will be presented and discussed within the context of previously obtained measurements during Galileo's flyby (Williams, D.J., et al, Science, 253, 1525,1991).