THE ION POPULATION BETWEEN 1300 KM AND 230000 KM IN THE COMA OF COMETP HALLEY/

During the encounter of the spacecraft Giotto with Comet Hailey the tw o sensors of the ion mass spectrometer (IMS), HERS and HIS, measured t he mass and the three-dimensional velocity distributions of cometary i ons. HIS looked mainly at the cold, slow part of the distribution clos e to the nucleus, HERS at the more energetic pick-up ions further out. After a thorough recalibration of the HIS flight spare unit and an ex tensive data analysis we present here continuous ion density-, composi tion-, velocity- and temperature profiles for the water group ion (mas s range 16-19 amu/e) along Giotto's inbound trajectory from 230000 to 1300 km from the comet nucleus. The two sensors are in very good agree ment in the region where their measurements overlap thus giving an exc ellent data base for the discussion of theoretical comet models. The m ost prominent feature where models and observations disagree is the so called pile up region between 8000 and 15 000 km from the nucleus.