Kinetic vs. multi-fluid models of H atoms in the heliospheric interface: a comparison

The goal of this paper is to illuminate similarities and differences in the kinetic and multi-fluid models of the heliospheric interface, the region of solar wind interaction with the Local Interstellar Cloud (LIC), and then to explore physical reasons for these differences. We present a detailed comparison of two types of models. The first type is based on a kinetic description of the interstellar H atom flow, which is required for this problem due to the fact that the mean free path of H atoms is comparable to the characteristic size of the heliospheric interface. The second type of model is based on a voluntary assumption that the flow of H atoms can be described hydrodynamically by a set of Euler equations (one-fluid approach) or by 2, 3, and 4 sets of Euler equations for different populations of H atoms (thus 2-, 3-, or 4- fluid models). It is shown that differences are significant between kinetic and multi-fluid models in observationally meaningful measurements such as the location of the termination shock and heliopause, the filtration of H atoms through the heliospheric interface, and the velocities and temperatures of H atoms. Therefore, the multi-fluid models may lead to incorrect interpretation of observational data.