A comprehensive comparison between the results of a multiscale three-dimensional adaptive MHD model of the comet Halley magnetosphere and in situ observations by the Giotto mission is presented. It is shown how a steady state model can describe the effects of the varying IMF direction on the cometary magnetosphere. The simulation results reproduce the observed profiles of the magnetic field and plasma velocity very well. There are only two discrepancies. First, the magnitudes of both the magnetic field strength and plasma velocity are slightly but systematically lower in numerical simulation as compared with the experiment. This can be attributed to some inaccuracy in our knowledge of plasma interaction parameters (ion-neutral momentum exchange rate, photoionization and recombination rates). Second, the observed and simulated magnetic field profiles show some differences in the anticipated ion pile-up region along the outbound trajectory. This means that the ion pile-up region on the outbound leg (if it existed) differed significantly from that on the inbound leg. ¿ 1999 American Geophysical Union