Detection of the inner plasma pileup region at Comet Halley during the Vega 1 flyby by the radio sounding experiment

The two Vega spacecraft sounded the ionosphere of Comet P/Halley during their flybys at two coherent one-way downlink frequencies. The frequency recordings were processed with software adapted from a software code used for processing Ulysses radio science data. An electron density model was developed in order to model the electron content profile observed during the Vega 1 flyby +/-1500 s (+/-120,000 km) about the closest approach (8900 km). The electron density for distances larger than 11,000 km followed a modified r(-2) law, while the electron density in the ''pileup'' region between 8900 km and 11,000 km increased with distance according to a single power law r(4). The inferred electron density is a factor of 2 lower than that measured by the Vega BD 3 in situ experiment during inbound flight and agrees with the outbound observations. The electron density profile inferred by the radio science experiment is an average over all inhomogeneities along the ray path and is probably a better representation of the large-scale density profile originating from the comet nucleus than the in situ observations, which are affected by inhomogeneities along the trajectory.