Bulk properties and velocity distributions of water group ions at comet Halley: Giotto measurements.

Abstract

In the region upstream of comet Halley, pickup heavy ions of cometary origin were directly observed by the Implanted Ion Spectrometer on Giotto. Here the authors discuss diffusion of this population in pitch angle and in energy, during the approach to the comet and on the outbound leg. The two data sets are compared and qualitative ideas on scattering timescales are inferred. In addition the bulk parameters of these distributions have been computed and the authors present a comparison of the observed speed in the solar wind frame and the observed density with expectations. We find that pitch angle scattering occurs more slowly than expected with filled shells appearing at 2.5x10⁶km, and that significant energy diffusion does not occur until the bow shock region. Also the shell distributions downstream of the shock flow at the bispherical bulk speed (related to the Alfvén speed) along the magnetic field with respect to the solar wind in accordance with conservation of energy between the pickup ions and the wave turbulence.