The induced magnetosphere of comet Halley 4. Comparison of in situ observations and numerical simulations

A comprehensive comparison between the results of a multiscale three-dimens ional adaptive MHD model of the comet Halley magnetosphere and in situ obse rvations 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 cometar y magnetosphere. The simulation results reproduce the observed profiles of the magnetic field and plasma velocity very well. There are only two discre pancies. First, the magnitudes of both the magnetic field strength and plas ma velocity are slightly but systematically lower in numerical simulation a s compared with the experiment. This can be attributed to some inaccuracy i n our knowledge of plasma interaction parameters (ion-neutral momentum exch ange rate, photoinization and recombination rates). Second, the observed an d simulated magnetic field profiles show some differences in the anticipate d 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.