Dust Spatial Distribution and Radial Profile in Halley's Inner Coma

Abstract

Mosaic images of the brightness distribution of solar radiation scattered by dust in Halley's coma are constructed using the spectra obtained by the three-channel spectrometer TKS during the approach phase of the Vega 2 spacecraft. They cover a sector having a radius of 40,000 km centered at the nucleus with an angular extent of 50°. The dust scattered brightness is plotted as a function of the distance pbetween the nucleus and the line of sight. This distance pis also called impact parameter. The brightness varies as the inverse of pin the inner coma when pis less than 3000 km and larger than 7000 km. In the 3000–7000 km distance range, the brightness varies as p −1.52. At distances larger than 7000 km, two dust jets are clearly visible with a contrast comparable to the gaseous jets which appear in the OH, NH, CN, C 2, and C 3images (Clairemidi, J., G. Moreels, and V. A. Krasnopolsky, 1990, Astron. Astrophys.231, 235–240; 1990, Icarus86, 115–128). In the inner coma, the spatial distribution of dust seems to be more isotropic and less contrasted than the distribution of gaseous emissive species. A model is developed to calculate the scattered intensity integrated along a line of sight at a projected distance pfrom the nucleus. 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. Dyachkov, 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, the variation of phase function with scattering angle, and fragmentation processes. A simple method is used to simulate fragmentation: a particle in a mass decade class splits into particles of the lower mass decade, assuming that the mass conservation law is fulfilled. A single α( R) fragmentation coefficient is introduced, Rbeing the distance between the dust particle and the nucleus. A good agreement with the measured dust-scattered intensity radial profile can be reached if an efficient fragmentation process at R< 1000 km with α( R) = 0.87 is introduced, with the fragmentation coefficient decreasing to zero at R= 6000 km. Different values of the density ρ and complex index ( n− ik) of the grains are used. Two good fits with the model results are obtained for ρ = 2.2 − 1.4 a/( a+ 1) g cm −3, where ais expressed in micrometers (Lamy, P. L., E. Grün, 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, B. N., C. Sagan, E. T., Arakawa, F. Suits, T. A. Callcott, and M. W. Williams, 1984, Icarus60, 127–137). However, if a color index variation with pis considered, the agreement is much better when the complex index of tholin (1.7 − 0.02 i) is adopted.