Magnetohydrodynamic Instabilities at Comet P/Halley: Giotto Observations (*).

We examine the plasma parameters observed by the ion mass spectrometer of the Giotto JPA experiment, downstream the bow shock and up to the closest approach of comet P/Halley. From the analysis of the observations we have identified two regions where the ΔV between the proton and water group ion bulk velocities is first parallel and later perpendicular, respectively, to the magnetic-field direction. In the parallel region, a strong MHD turbulence is observed that we suppose to be generated by a firehose instability mechanism driven by the velocity difference. At about 5·105 km from closest approach, the center of mass switches from solar-wind protons to the cometary ions, while the velocity difference becomes perpendicular to the magnetic field, causing the quenching of the isstability and the disappearing of the plasma fluctuations.