POLARIMETRIC CCD IMAGING OF COMET 47P ASHBROOK-JACKSON AND VARIABILITY OF POLARIZATION IN THE INNER COMA OF COMETS/

Polarimetric observations of comet 47P/Ashbrook-Jackson at low phase a ngle have been performed between 20 and 22 August 1993 from Pic du Mid i Observatory by using the CCD-polarimetry method. The polarization ma ps, as compared with the brightness maps, enhance some details and asy mmetry in the inner coma, i.e. a circumnucleus halo and jet structure. Values of polarization, obtained for various fields of view centered on the nucleus, are compared to results from previous observations per formed in 1977 on the same comet by Kiselev and Chernova (1981). The d etected evolution of polarization with increasing distance from the nu cleus could be due to the presence of dust jets, as is suggested on th e polarimetric map obtained on the 21 August 1993. These results are s imilar to those obtained by the same method on comet Levy C/1990 K1 fo r the same phase angle domain. The variability of the polarization aro und the nucleus is compared with the results obtained for others comet s observed by remote sensing, and through insitu measurements performe d on comet 26P/Grigg-Skjellerup by the OPE instrument onboard the Giot to spacecraft. The nature of grains that fill the regions respectively of high and low polarization, detected in the inner coma of several c omets for distances to the nucleus typically smaller than 3000 km, are then tentatively discussed.