Angular and energy distribution of low energy cometary ions measured in the outer coma of Comet Halley

During the early phase of the Giotto encounter with comet Halley, at distances from the nucleus greater than 350,000 km, the neutral mass spectrometer was operated in a mode allowing the measurement of low energy ions. Data reveal two important features of the outer coma: the presence of a sharp discontinuity in the plasma flow at 550,000 km from the nucleus which results in a significant decrease of the plasma flow accompanied by an increase in temperature; and the detection of newly born ions identified as O(+) and CO(+), at distances from the comet greater than 800,000 km.