Simulation of the heliosphere: Generalized charge-exchange cross sections

The interaction of the solar wind with the very local interstellar medium d epends strongly on charge exchange between the neutral and ionized hydrogen outside of the heliopause, While the protons are diverted by the heliosphe re obstacle, the neutral atomic hydrogen sees no immediate change in enviro nment and continues its free streaming. The ensuing difference in flow velo cities results in resonant charge exchange that tends to divert the neutral flow around the heliosphere as well. Large mean free paths strictly requir e a kinetic approach, yet some idea of the interaction can be gleaned from a much less computationally complex fluid treatment. We show that the momen tum- and energy-exchange operators between the charge and neutral fluids ca n be evaluated in a near-exact fashion for the charge-exchange cross sectio n. This new formulation can be used both to provide a better fluid descript ion and to test how robust the fluid picture is.