MHD-Boltzmann simulations of the solar wind-interstellar medium interaction

The supersonic solar wind carves out a bubble within the local interstellar cloud. The Sun is moving supersonically with respect to the cloud, giving rise to a solar wind termination shock, an interstellar bow shock and a tangential discontinuity called the heliopause which separates the solar and interstellar plasmas. The local interstellar medium is a partially ionized plasma, being comprised predominantly of neutral hydrogen gas. While the ionized components are constrained by magnetic fields, the neutral atoms travel large distances unimpeded, their dominant interaction being charge exchange with a plasma proton. Due to these large mean free paths the neutral hydrogen component needs to be modeled kinetically rather than hydrodynamically. In this paper we present some background to the solar wind-interstellar medium interaction problem and some details of the Monte Carlo algorithm used to model neutral atom trajectories. We will then compare kinetic and hydrodynamic models for hydrogen atoms in the heliosphere and present some latest predictions for neutral atom fluxes which IBEX will measure.