Relationship between wave energy and free energy from pickup ions in the Comet Halley Environment

Abstract

The free energy available from the implanted heavy ion population at comet Halley is calculated by assuming that the initial unstable velocity space ring distribution of the ions evolves toward a bispherical shell. Ultimately this free energy adds to the turbulence in the solar wind. We obtain upstream and downstream free energies separately for the conditions observed along the Giotto spacecraft trajectory. The results indicate that the waves are mostly upstream propagating in the solar wind frame. The total free energy density always exceeds the measured wave energy density because, as expected in the nonlinear process of ion scattering, the available energy is not all immediately released. An estimate of the amount which has been released can be obtained from the measured oxygen ion distributions and again it exceeds that observed. The difference may be expressed in terms of ``damping'' of the waves and enables the damping rate to be calculated. We extend the theoretical analysis to calculate the k spectrum of the cometary-ion-generated turbulence. It provides an almost complete quantitative explanation of the observed spectrum. The differences at long wavelengths could be the result of the firehose instability or induced scattering on solar wind ions.