THE SENSITIVITY OF THE SIZE DISTRIBUTION TO THE GRAIN DYNAMICS - SIMULATION OF THE DUST FLUX MEASURED BY GIOTTO AT P HALLEY/

A cometary dust emission model, based on rigorous keplerian dynamics, is developed and, for the first time, the dependence of fluence on the probability distribution of the dust ejection velocity vector is demo nstrated. The results are compared with the fluences measured by the D IDSY experiment on board the GIOTTO spacecraft during the Halley's fly -by in 1986. A fit of the total fluence is obtained and an interpretat ion of the observed differences, between pre and post fly-by, is propo sed. From the best-fitting process we conclude that the dust ejection from P/Halley was strongly anisotropic and mainly Sun-ward oriented wi th an angular dispersion of 18.4 degrees, for the adopted gaussian dis tribution. The most probable velocity at the fly-by is 50 +/- 5 m s(-1 ) for 1 mm sized grains and the power index of the velocity size-depen dence is -0.5. Both these results agree with those of dust-gas drag mo dels. Moreover, the dust velocity presents a wide dispersion (35 +/- 5 m s(-1)), which explains the velocity size-dependence derived by Neck -Line photometry. For grains larger than 20 mu m, the power index of t he differential size distribution is constant (alpha = -3.5 +/- 0.2). Since alpha > -4, most of the dust mass is released in the form of lar ge grains. The dust to gas ratio is chi = 4 +/- 1. The last two conclu sions agree with the output of previous DIDSY fitting processes and ar e compatible with inverse dust tail models; they must be considered th e best constrained results coming from the DIDSY experiment. Our resul ts imply that future in-situ cometary experiments will have to measure both mass and velocity vector for each grain, in order to determine t he dust size distribution.