Nonequilibrium Processes and Their Effect on the Structure of Near-Nucleus Cometary Coma: II. Models of Dusty Gas Flows

We discuss the problem of constructing adequate mathematical models for a dusty gas flow in a cometary coma. A kinetic approach is used to reveal specific regimes (modes) of the flow and to determine the boundaries of these regimes depending on the ratio of gas molecule and dust particle masses, as well as on the ratio of numerical concentrations of these fractions. In each mode the appropriate gas dynamic equations are given, which describe the evolution of the flow in terms of a set of macroscopic variables. A comparison of the obtained models with some widely used models is made. We explored which modes could be realized in diffeent areas of comae of typical active comets and showed that the traditional calculations [ 1, 2] are based on the models that cannot be justified for the initial gas and dust parameters chosen in these calculations. The main conclusion from this study is that all macroscopic flow regimes cannot be described by a single universal macroscopic model. Furthermore, our approach does not permit one to obtain independent equations for the velocities of the light and heavy components (a multivelocity model) without introducing a number of additional assumptions. It is difficult, however, to argue in favor of these assumptions from the physical point of view.