The pickup of newborn ions in the solar wind by means of low-frequency, electromagnetic waves, either due to the intrinsic turbulence of the solar wind or induced turbulence due to instabilities excited by the newborn ions, is discussed. The pickup process is envisioned to occur in three stages, formation of a ring-beam distribution, rapid pitch angle scattering of this initial distribution into a thin shell, and slower velocity diffusion that spreads out the shell. The process of shell formation and evolution are studied by means of numerical simulations, first for the situation of relatively low levels of turbulence such as would occur naturally in the solar wind and then for higher levels of turbulence characteristic of those encountered near cometary bow shocks. The results of the numerical experiments are compared with theory.

The initial velocity spreading and growth of electromagnetic fluctuations due to the interaction of a small initial density of newborn ions interacting with the solar wind in the absence of significant background turbulence is shown to be well described by linear- and second-order theory. When the initial density of newborn ions is sufficiently small or the level intrinsic turbulence is large enough so that there is no appreciable growth and self-interaction of generated waves with the newborn ions, these ions pitch angle scatter to form a thin, partial shell distribution. The time scale for the inital scattering is shown to be related to the gyroperiod in the perturbed wave field. On a longer time scale that is consistent with a simplified quasi-linear theory, the ions in this shell diffuse in velocity space to fill it out in pitch angle, as well as cause it to spread in energy. However, the diffusion process slows considerably, long before a filled-in velocity sphere is formed. These results are then used to estimate the time scales for shell formation and diffusion in several situations of recent observational interest, the interstellar helium data obtained by AMPTE and cometary ion pickup distributions obtained by various spacecraft at comets Giacobini-Zinner and Halley. ¿ American Geophysical Union 1988