Comparison of observed and calculated implanted ion distributions outside comet Halley's bow shock

This paper compares calculated and measured energy spectra of implanted H{sup +} and O{sup +} ions on the assumption that the pickup geometry is quasi-parallel and about 1% of the waves generated by the cometray pickup process propagate backward (toward the comet). The model provides a good description of the implaneted O{sup +} and H{sup +} energy distribution near the pickup energies. The thickness of the implanted ion velocity distribution shells was nearly constant between 2.50 {times} 10{sup 6} km and 1.20 {times} 10{sup 6} km (just outside the shock) along the inbound Giotto trajectory. The explanation is that the velocity diffusion coefficient and characteristic diffusion time vary approximately as 1/r and r, respectively, and therefore their product (which determines the velocity shell thickness) remains nearly constant.