Observations of the Ion Pile-up in Comets Hale–Bopp and Hyakutake

Abstract

High resolution long-slit spectra of Comet Hale–Bopp (C/1995 O1) were obtained with the Hamilton Echelle Spectrograph of Lick Observatory in March and April 1997. H_2O^+ column density profiles derived from these spectra imply the existence of a shell of enhanced plasma density surrounding the nucleus on its sunward side at a radius of (1.03±0.12)×10^5 km. We propose that this is the ion pile-up detected at 1.1×10^4 km sunward of the nucleus of Comet Halley (1P/1682 Q1) by both the Giotto and Vega 1 spacecraft. Long-slit H_2O^+ spectra of Comet Hyakutake (C/1996 B2) indicate the presence of a similar ion pile-up at a radial distance of greater than 1.6×10^4 km from the nucleus. These observations suggest that the ion pile-up is a typical feature of the near-nuclear plasma environment of active comets. The standoff distance and width of the high density region roughly scale with the gas production rate of the comet but the exact functional dependence, which could be used to constrain the mechanism of pile-up formation, remains unclear.

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