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Y. V. Skorov, H. U. Keller (MPI for Aeronomy), H. Rickman (Uppsala Univ.)
A kinetic model of the innermost gas-dust coma region is proposed. We consider the Knudsen layer adjacent to the phase boundary, where the velocity distribution function relaxes to the Maxwell equilibrium distribution function and both the gas and the dust macro-characteristics vary several--fold. Two models of ice sublimation, determining initial conditions for a gas expansion in the non-equilibrium region, are presented: sublimation of water ice from a plane homogeneous surface; sublimation of water ice from under a porous dust mantle. Analytical relations between the characteristics of the gas flow on the boundaries of the non-equilibrium layer and the characteristics of the returning gas flow adsorbed by the surface are determined.
We also treat the expansion of dust grains into the coma, concentrating on the interaction between a non-equilibrium gas flow and a test particle. The dynamics of grains of a different shape (sphere, cylindrical disk, Gaussian particle) and size is explored by using simplifying assumptions for the variation of the drag force. The velocity of a particle at the exterior boundary of the Knudsen layer is then estimated.
The obtained characteristics of the dust-gas mixture are used as boundary conditions for the macroscopic equations of the heterogeneous dynamics. The dust velocity is found to be a crucial parameter specifying the expansion regime of the gas flow. Since in a subsonic regime the gas flow is accelerated by hot dust, but decelerated in a supersonic one, complicated transformations of heterogeneous flow may occur in the immediate vicinity of a cometary nucleus. For example under some plausible initial values of dust grains, the formation of a shock wave is found to be possible . It has to change the dust and gas densities and, hence, optical characteristics of the region in hand (as a formation of bright structures). This effect may be observe in frame of the forthcoming OSIRIS project.
The author(s) of this abstract have provided an email address for comments about the abstract: skorov@linmpi.mpg.de