The objective of the present study is to understand the interaction of the solar wind with new born ions in far upstream regions of a comet where the level of intrinsic turbulence is moderately low. Based on the assumption that quasi-linear theory is adequate and applicable, we investigate the pitch angle diffusion process and the time evolution of the newborn ion distribution function. Numerical solutions to the quasi-linear diffusion equation, including the effect of resonance broadening and that of continuous creation of newborn ions due to ionization process, are obtained under several assumptions and approximations. It is found that theoretical results are consistent with the Giotto observations recently reported by Neugebauer et al. (1989) which enable us to understand the formation of partial shells which were frequently observed in far upstream regions. The pitch angle diffusion process is also examined by the hybrid simulation. The results of the simulation are in qualitative agreement with those obtained from the quasi-linear analysis. ¿American Geophysical Union 1990