Authors: V. Izmodenov, M. Gruntman, Yu.G. Malama

Title: Interstellar Hydrogen Atom Distribution Function in the Outer Heliosphere

Abstract.

The so-called hot model and its modifications are usually used for simulations of the interstellar hydrogen atom distribution in the heliosphere. Such simulations are essentially dependent on the assumption of the known velocity distribution of interstellar atoms at infinity, usually assumed to be shifted Maxwellian. Simulations of the global heliosphere require self-consistent treatment of the interstellar plasma-neutral gas coupling. Such simulations are often performed under assumption that hydrogen can be described by the Maxwellian distribution. However, interstellar hydrogen is efficiently coupled to the interstellar plasma via charge exchange. The mean free path of the neutrals with respect to charge exchange is comparable to the size of the heliospheric interface. This interaction modifies hydrogen velocity distribution function, and the kinetic description of interstellar hydrogen atom flow is thus required.

We present here for the first time computer simulations of the velocity distribution function of interstellar hydrogen in the outer heliosphere. These distributions allow one to quantitatively evaluate uncertainty, or error, introduced by the simplifying assumption of the Maxwellian distributions. We use an axis-symmetric model of the solar wind/LISM interaction with the self-consistent treatment of plasma-gas coupling. The Monte-Carlo technique is used to calculate velocity distribution functions of neutral atoms.