The observations made by Sakigake revealed the asymmetric distribution of the interaction region of Halley's comet, and the center of the interaction region was shifted toward the post-encounter side on the spacecraft trajectory. Oya (1993) proposed the model that this asymmetric distribution is explained by the field aligned motion of picked up ions. Based on this model, we examined spatial and time scale of field aligned motion of cometary ions in the ion pick-up process by using numerical experiments and the shift of the interaction region is estimated. In the simulation results, it was shown that the braking process of the field aligned motion of picked up ions are understood by the wave particle interaction through the excitation of the MHD wave and pitch angle scattering caused by the turbulent magnetic field, and spatial scale of field aligned motion of cometary ion was estimated to 1.5 × 106 km. This result suggests that observed asymmetrical distribution of the interaction region of the comet is formed by the motion of picked up ions toward the field aligned direction until their merging into the solar wind, as it was proposed by Oya (1993).
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