SIMULATION OF THE HELIOSPHERE - MODEL

The problem of the interaction of the solar wind with the very local i nterstellar medium (VLISM) is complicated by the role played by collis ions between the plasma components of the heliosphere and VLISM and th e neutral component of the VLISM. We outline the inherent approximatio ns in fluid descriptions of the problem and give formulas for the near -exact charge exchange and elastic collision transfer integrals for pa rticles, momentum and energy for two drifting Maxwellians with differe nt drift speeds and temperatures. To lowest order, all Boltzmann colli sion operators can be evaluated analytically in terms of exponentials and error functions. Analytic approximations that have relative errors of less than 2.62% compared with the exact expressions can be impleme nted in large simulation codes. Our formulation avoids approximations used previously by others and leads to a simplification of the formula tion and increased accuracy for numerical simulations of the heliosphe re/VLISM interaction.