A theoretical model has been developed to evaluate the magnetic field disturbances due to plasma clouds formed by hypervelocity (68 km/s) impacts of milligram mass dust particles on the Giotto spacecraft at comet P/Halley. The model is partly analytical and assumes unmagnetized ions and frozen-in electrons as suggested by the high time-resolution magnetic field observations. It treats the cloud development after an initial phase with unfrozen plasma ending a few milliseconds after impact. Fitting the model to the observed magnetic field signature of the impacts allows the determination of the center-of-mass velocity and the expansion velocity as well as the total number of particles in the expanding cloud, which leads to an estimate for the mass of the dust particle. ¿ American Geophysical Union 1992