The dominant forces resisting the transport of magnetic field into the inner coma of a comet are ion mass loading from and friction with the expanding neutral atmosphere. A magnetic cavity is thereby created. Close to it the frictional force is most important. Careful interpretation of the magnetic field profile measured during the Giotto flyby of comet P/Halley reveals the existence of an inward directed component of plasma flow of a few km/s, which drops to zero at the boundary of the cavity. The energy transferred from the neutral gas to the plasma by friction and mass loading is responsible for the strongly elevated ion temperatures outside the magnetic cavity. Fitting of the observed magnetic profile and ion temperature distribution yields quantitative determinations of some crucial parameters of the coma. ¿American Geophysical Union 1987