Charge-exchange-induced perturbations of ion and atom distribution functions in the heliospheric interface

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.