Plasma physics at comet Halley Giotto and Vega observations

The first results obtained from the Giotto JPA experiment are discussed. The cometary bow shock, observed at $1.13x10^6$ km form the nucleus, is characterized by an enhanced inonization of cometary particles probably due to electron collisions. Water-group ions are observed as pick-up ions at $2.36x10^6$ km from the nucleus and as Fermi accelerated ions behind the bow shock. The solar wind flow is observed to become gradually field aligned as the nucleus is approached. The termination region of the solar wind flow is identified between 135000 km and 61000 km.