DUST SPATIAL-DISTRIBUTION AND RADIAL PROFILE IN HALLEYS INNER COMA

Mosaic images of the brightness distribution of solar radiation scatte red by dust In Halley's coma are constructed using the spectra obtaine d by the three-channel spectrometer TKS during the approach phase of t he Vega 2 spacecraft. They cover a sector having a radius of 40,000 km centered at the nucleus with an angular extent of 50 degrees. The dus t scattered brightness is plotted as a function of the distance p betw een the nucleus and the line of sight. This distance p is also called impact parameter. The brightness varies as the inverse of p in the inn er coma when p is less than 3000 km and larger than 7000 km. In the 30 00-7000 km distance range, the brightness varies as p(-1.52). At dista nces larger than 7000 km, two dust jets are clearly visible with a con trast comparable to the gaseous jets which appear in the OH, NH, CN, C -2, and C-3 Images (Clairemidi, J., G. Moreels, and V., A. Krasnopolsk y, 1990, Astron. Astrophys. 231, 235-240; 1990, Icarus 86, 115-128). I n the inner coma, the spatial distribution of dust seems to be more is otropic and less contrasted than the distribution of gaseous emissive species. A model is developed to calculate the scattered intensity int egrated along a line of sight at a projected distance p from the nucle us. The model is based upon Mie theory and uses the data of the impact particle counter SP-2 on board the Vega spacecraft (Mazets, E. P., R. Z. Sagdeev, R. L. Aptekar, S. V. Golenetskii, Yu. A. Guryan, A. V. Dy achkov, V. N. Elyinskii, V. N. Panov, G. G. Petrov, A. V., Savvin, I. A. Sokolov, D. D. Frederiks, N. G. Khavenson, V. D. Shapiro, and V. I. Shevchenko, 1987, Astron. Astrophys. 187, 699-706), extrapolated from 8030 to 440 km. The model takes into account the fountain effect due to the competition between solar gravitation and radiation pressure, t he variation of phase function with scattering angle, and fragmentatio n processes. A simple method is used to simulate fragmentation: a part icle in a mass decade class splits into particles of the lower mass de cade, assuming that the mass conservation law is fulfilled. A single a lpha(R) fragmentation coefficient is introduced, R being the distance between the dust particle and the nucleus. A good agreement with the m easured dust-scattered intensity radial profile can be reached if an e fficient fragmentation process at R < 1000 km with alpha(R) = 0.87 is introduced, with the fragmentation coefficient decreasing to zero at R = 6000 km, Different values of the density rho and complex index (n - ik) of the grains are used. Two good fits with the model results are obtained for rho = 2.2 - 1.4 a/(a + 1) g cm(-3), where a is expressed in micrometers (Lamy, P. L., E. Grun, and J. M. Perrin, 1987, Astron. Astrophys. 187, 767-773) and both indices 1.387 - 0.031 i (Mukai, T., S. Mukai, and S. Kikuchi, 1987, Astron. Astrophys. 187, 650-652) or 1. 7 - 0.02 i (Khare, E. N., C. Sagan, E. T., Arakawa, F. Suits, T. A. Ca llcott, and M. W. Williams, 1984, Icarus 60, 127-137). However, if a c olor index variation with p is considered, the agreement is much bette r when the complex index of tholin (1.7 - 0.02 i) is adopted, (C) 1997 Academic Press.