The formation of a magnetic-field-free cavity at comet Halley

One of the most important results of the Giotto mission to comet Halley was the discovery of a magnetic cavity surrounding the nucleus in which the magnetic field strength was apparently zero¹. The boundary of the cavity, the ionopause, occurred at 4,760 km (3,840 km) and the maximum magnetic field strength was ∼55 nT (65 nT), thousands of km outside the rather sharply defined transition to zero field on the inbound (outbound) part of the trajectory, respectively. We show here with the aid of a simple model that such a magnetic field structure can be explained as being largely the result of ion-neutral friction²rather than simple pressure-balance alone ^3,4.