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A.C. Levasseur-Regourd, E. Hadamcik (Univ. Paris VI / Aeronomie CNRS)
Most of our knowledge about the physical properties of cometary dust particles relies on the solar light they scatter. Their linear polarization phase curves present similar trends. However, even for a given phase angle and wavelength, some clear differences are pointed out. Some comets exhibit higher polarization levels (at least above 40 deg. phase angle) than other ones. Also, a high resolution images reveal some drastic changes in the jet-like structures and in the inner comae (Levasseur-Regourd, Space Sci. Rev. 90, 163, 1999).
Previous observations will be updated, together with their interpretation through models of scattering by irregular particles and through laboratory measurements. The observations of Comet Hale-Bopp 1995 O1 will be reviewed, and some parameters retrieved from the polarization images will be discussed.
Special emphasis will given to recent observations of Comet LINEAR 1999 S4. Polarization images obtained with the 0.8 m telescope at Haute-Provence Observatory (France) will be presented. They allow us to monitor the evolution of the scattering properties of LINEAR from the 24th to the 28th of July 2000, at the time where the comet was most likely breaking apart. The results will be tentatively compared to those previously deduced from light scattering observations of condensations attributed to mini-nuclei in the comae of Comets Grigg-Skjellerup and Hyakutake.