The plasma velocity along the Sun-comet Halley line is derived by using in situ measurements of the magnetic field along the Giotto trajectory. Including this velocity profile in the momentum balance turns out to be insufficient in order to support the magnetic barrier with conventional parameters of ion-neutral interaction. The observed magnetic barrier of comet Halley is shown to be consistent with the following parameters of interaction: kin=5.8¿10-9 cm3 s-1 (with Q=6.9¿1029 s-1) or Q=19¿1029 s-1 (with kin=1.1¿10-9 cm3 s-1). An estimated electric field of convection enables one to find a shielding factor of 0.03--0.07 in the maximum of the magnetic barrier of comet Halley. ¿ American Geophysical Union 1993