The ion-neutral friction (including mass loading) happens to be insufficient to support the magnetic barrier at comet Halley under conventional parameters of ion-neutral interaction. It is shown that a centrifugal force resulting from the transverse plasma motion should be added to the momentum balance in order to explain the observed magnetic field structure. The radial profile of the transverse plasma velocity is derived from the best fit with magnetic field data. The transverse velocity reaches ~8 km s-1 at 10 000 km from the nucleus. This value depends little on both the ion-neutral momentum exchange rate and the comet production rate, whence one may conclude that it is centrifugal force which dominates in the momentum balance at this distance. ¿ American Geophysical Union 1992