Diamagnetic effect of the ion pile-up region of comet Halley

The ion pile-up region in the comet Halley ionosphere observed on the inbound leg of the Giotto trajectory is accompanied by a decrease of the magnetic field strength. This magnetic field dip is explained by means of a diamagnetic effect of the plasma density increase in the pile-up region. No similar magnetic effect of the enhanced plasma density was observed near the anticipated position of the ion pile-up region for the outbound leg of the trajectory. Thus, we conclude that the ion pile-up in the inner cometary coma, as it was observed on the inbound leg of the Giotto trajectory, may be a transient (spatially and/or temporarily) phenomenon rather than a permanent steady state feature.