On the distribution of pickup ions as observed by the Vega spacecraft at comet Halley Original Research Article

Two electrostatic analysers of the PLASMAG-1 plasma instrument package detected energy/charge spectra of ions (without mass separation) aboard the Vega spacecraft when encountering comet Halley. They could observe only two relatively small sections of velocity space showing a mixed effect of decelerated solar wind particles and cometary pickup ions downstream of the bow shock. The separation of the different ion components was attempted by comparing the energy spectra with simple plasma distributions. The effect of cometary pickup ions observed by Vega-1 was different from that observed by Vega-2 at cometocentric distances >0.5 million km. These differences could be interpreted when using the bispherical shell distribution based on magnetic field vectors measured by the magnetometer.