During the hour following the bow shock of the outbound leg of Giotto's encounter with comet P/Grigg-Skjellerup, asymmetric cometary ion distributions were observed in velocity space. Various explanations for the observed asymmetry include sudden changes in the magnetic field or magnetic mirroring. Several workers have noted that the parallel Alfv¿nic Mach number was close to unity during this interval, a result of the near orthogonal orientation of the interplanetary magnetic field with respect to the solar wind. Newly formed cometary ions would be implanted into the upstream diffusion gap and would therefore be more likely to diffuse through resonant interactions with downstream propagating waves. Starting with this observation, a simple finite difference representation of Boltzmann's equation has been utilized to model the velocity space distributions. The results of the simulation show that the asymmetry is a consequence of the geometry of the implanted ion distribution and the lifetime of the cometary ions in the solar wind and that no other mechanisms are needed to explain the observations. ¿ 1998 American Geophysical Union