  Various hydrodynamic models of the heliospheric interface have been presented
meanwhile, numerically simulating the interaction of the solar wind plasma
bubble with the counterstreaming partially ionized interstellar medium. In
these model approaches the resulting interface flows are found by the use of
hydrodynamic simulation codes trying to consistently describe the dynamic and
thermodynamic coupling of the different interacting fluids of protons, H-atoms
and pick-up ions. Within such approaches, the fluids are generally expected to
be correctly described by the three lowest velocity moments, i.e., by shifted
Maxwellians. We shall show that in these approaches the charge-exchange-induced
momentum coupling is treated in an unsatisfactory representation valid only at
supersonic differential flow speeds. Though this flaw can be removed by an
improved coupling term, we shall further demonstrate that the assumption of
shifted Maxwellians in some regions of the interface is insufficiently well
fulfilled both for H-atoms and protons. Using a Boltzmann-kinetic description
of the proton- and H-atom- distribution functions coupled by charge exchange
processes we emphasize the fact that non-negligible deviations from shifted
Maxwellians are generated in the interface. This has to be taken into account
when interpreting inner heliospheric measurements in terms of interstellar
parameters.
