Giotto magnetic field observations at a time resolution of 28.24 vectors per second in the innermost part of the interaction region of comet Halley with the solar wind have yielded the following results: (1) The outer boundary of the cavity, the ionopause, has a thickness of 25 km only, both inbound and outbound, and magnetic field magnitude drops of 20 nT and 18.3 nT, respectively. Pressure balance arguments lead to an outward plasma pressure decreases of at least 1.5¿10-10 Pa. Plasma densities reported by the Giotto mass spectrometers then lead to three alternatives: If no density jump occurs across the ionopause an electron temperature of at least 6000 K just inside the ionopause cannot be avoided. Smaller electron temperatures require an inward discontinuous increase in plasma density across the ionopause. Third, an additional hot ion populaton of sufficient pressure inside the cavity would make a high electron temperature unnecessary (D. T. Young, personal communication, 1987). (2) The ionopause boundary layers are characterized by ( j¿B) forces of 5.7¿10-16 N m-3 and 7.3¿10-16 N m-3 inbound and outbound, respectively. These magnetic forces must be balanced by the sum of the elastic frictional force, the forces due to massloading by photoions, due to ion molecule reactions and due to ionization by hot electrons. An electron pressure gradient may also contribute. (3) A magnetic field bulge of 80-km length is interpreted as a weak, almost perpendicular, subcritical shock propagating radially outward generated by a temporary pressure increase in the cavity presumably due to a transient population of hot ions and electrons in the cavity. (4) The hot ions and particularly electrons and additional energy input suggested by these observations point to the importance of dynamic phenomena in the innermost part of the interaction region, such as magnetic field line merging in the tail. ¿ American Geophysical Union 1988