Cometary ion distributions near the pickup energy outside comet Halley's bow shock

Abstract

This paper compares calculated and measured energy spectra of implanted H+ and O+ ions on the assumption that the pick-up geometry is quasi-parallel and about 1% of the waves generated by the cometary pickup process propagates backward (towards the comet). The model provides a good description of the implanted O+ and H+ energy distribution near the pickup energies. The thickness of the implanted ion velocity distribution shells was nearly constant between 2.50 x 106 km and 1.20 x 106 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, therefore their product (which determines the velocity shell thickness) remains nearly constant.