Pickup ions in the unshocked solar wind at comet Halley

Abstract

The Tünde‐M experiment on board the VEGA 1 spacecraft detected energetic cometary ions as far as 10 million kilometers from comet Halley. The measured ion fluxes increased with decreasing radial distance from the nucleus, in agreement with theoretical expectations. Sharp enhancements spaced at approximately 4‐hour intervals were superimposed on the general increase of the ion fluxes on the inbound leg of VEGA 1. Periodic neutral structures originating from gas production by active areas on the nucleus are suggested to explain the recurrent intensity peaks. These neutral density enhancements would then produce the ion density enhancements seen by Tünde‐M as the spacecraft traversed the structures. This explanation requires a considerably larger neutral expansion velocity (≈6 km/s) than was actually measured near the nucleus in order to allow the rotation period of the nucleus to generate the observed period of the enhancements. No obvious correlations of the ion fluxes with the limiting energy for pickup were found. However, at a distance of about 107 km from the comet, a weak anticorrelation was found between the ion flux and the angle between the interplanetary magnetic field and solar wind direction. Cometary ion distribution functions in the solar wind reference frame were derived from measured counting rates. These distributions are approximately Maxwellian for energies between about 100 keV and 150 keV. The derived temperatures are between about 5 and 8 keV for most cometocentric distances, although in a region between 10 and 8 million km from the nucleus the ion spectra were harder, with temperatures ranging from 10 to 20 keV.