Direct acceleration of pickup ions at the solar wind termination shock: The production of anomalous cosmic rays

We have modeled the injection and acceleration of pickup ions at the solar wind termination shock and investigated the parameters needed to produce th e observed anomalous cosmic-ray (ACR) fluxes. A nonlinear Monte Carlo techn ique was employed that in effect solves the Boltzmann equation and is not r estricted to near-isotropic particle distribution functions. This technique models the injection of thermal and pickup ions, the acceleration of these ions, and the determination of the shock structure under the influence of the accelerated ions. The essential effects of injection are treated in a m ostly self-consistent manner, including effects from shock obliquity, cross -held diffusion, and pitch-angle scattering. Using recent determinations of pickup ion densities, we are able to match the absolute flux of hydrogen i n the ACRs by assuming that pickup ion scattering mean free paths, at the t ermination shock, are much less than an AU and that modestly strong cross-h eld diffusion occurs. Simultaneously, we match the flux ratios He+/H+ or O/H+ to within a factor similar to 5. If the conditions of strong scattering apply, no pre-termination-shock injection phase is required and the inject ion and acceleration of pickup ions at the termination shock are totally an alogous to the injection and acceleration of ions at highly oblique interpl anetary shocks recently observed by the Ulysses spacecraft. The fact that A CR fluxes can be modeled with standard shock assumptions suggests that the much discussed "injection problem" for highly oblique shocks stems from inc omplete (either mathematical or computer) modeling of these shocks rather t han from any actual difficulty shocks may have in injecting and acceleratin g thermal or quasi-thermal particles.