Interstellar hydrogen atom distribution function in the outer heliosphere

We study the evolution of the velocity distribution function of interstella r hydrogen atoms in the heliospheric interface, the region of solar and int erstellar wind interaction. The velocity distribution is a key tool to eval uate uncertainties introduced by various simplified models of the interface . We numerically solve the kinetic equation for neutral gas self-consistent ly with the hydrodynamical equations for plasma. Neutral and plasma compone nts efficiently coupled by charge exchange. This interaction disturbs the a tom velocity distribution, which is assumed to be Maxwellian in the circums olar local interstellar medium. It is shown that besides "original" interst ellar atoms, there are three other important atom populations originating i n the heliospheric interface. We present velocity distribution functions of these populations in various locations of the interface region and discuss their kinetic properties.