Comet Giacobini-Zinner: In Situ Observations of Energetic Heavy Ions

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Abstract
Conclusive evidence is presented for the existence of energetic (∼535,0000 to 150,000 electron volts), heavy (>-12 atomic mass units), singly charged cometary ions within ∼1.5 x 106 kilometers of comet Giacobini-Zinner. The observations were made with the University of Maryland/Max-Planck-Institut ultralow-energy charge analyzer on, the International Cometary Explorer spacecraft. The most direct evidence for establishing the mass of these ions was obtained from an analysis of the energy signals in one of the solid-state detectors; it is significant at the three-sigma level. Maximum fluxes were recorded ∼1 hour before and ∼1 hour after closest approach to the cometary nucleus. Transformation of the particle angular distributions observed at ∼50,000 kilometers radial distance from the comet during the inbound pass into a rest frame in which the distributions are nearly isotropic requires a transformation velocity that is consistent with the local solar wind velocity if one assumes that these particles are primarily singly ionized with a mass of 18 ± 6 atomic mass units. The existence of a frame of reference in which these water-group ions were isotropic implies that they underwent strong pitch angle scattering after their ionization. Particle energies in the rest frame extend to substantially higher values than would be expected if these ions were locally ionized and then picked up by the solar wind, implying that the ions were accelerated or heated. The derived ion density, ∼0.1 per cubic centimeter, is consistent with a crude model for the production and transport of pickup ions.