Challenging a paradigm: Do we need active and inactive areas to account for near-nuclear jet activity?

We briefly describe an advanced 3D gas dynamical model developed for the simulation of the environment of active cometary nuclei. The model can handle realistic nucleus shapes and alternative physical models for the gas and dust production mechanism. The inner gas coma structure is computed by solving self-consistently (a) near to the surface the Boltzman Equation (b) outside of it, Euler or Navier-Stokes equations. The dust distribution is computed from multifluid "zero-temperature" Euler equations, extrapolated with the help of a Keplerian fountain model. The evolution of the coma during the nucleus orbital and spin motion, is computed as a succession of quasi-steady solutions. Earlier versions of the model using simple, "paedagogic" nuclei have demonstrated that the surface orography and the surface inhomogeneity contribute similarly to structuring the near-nucleus gas and dust coma, casting a shadow on the automatic attribution of such structures to "active areas". The model was recently applied to comet P/Halley, for which the nucleus shape is available. In the companion paper of this volume, we show that most near-nucleus dust structures observed during the 1986 Halley flybys are reproduced, assuming that the nucleus is strictly homogeneous. Here, we investigate the effect of shape perturbations and homogeneity perturbations. We show that the near nucleus gas coma structure is robust vis-a-vis such effects. In particular, a random distribution of active and inactive areas would not affect considerably this structure, suggesting that such areas, even if present, could not be easily identified on images of the coma.