Interstellar hydrogen atom distribution function in the outer heliosphere

We study the evolution of the velocity distribution function of interstellar hydrogen atoms in the heliospheric interface, the region of solar and interstellar wind interaction. The velocity distribution is a key tool to evaluate uncertainties introduced by various simplified models of the interface. We numerically solve the kinetic equation for neutral gas self-consistently with the hydrodynamical equations for plasma. Neutral and plasma components efficiently coupled by charge exchange. This interaction disturbs the atom velocity distribution, which is assumed to be Maxwellian in the circumsolar local interstellar medium. It is shown that besides "original" interstellar atoms, there are three other important atom populations originating in the heliospheric interface. We present velocity distribution functions of these populations in various locations of the interface region and discuss their kinetic properties.