Spatial distribution of H 2O + in comet P/Halley

Abstract

An analysis of the H 2O + distribution and near-nucleus ion physics is presented using our comet P/Halley spectroscopic and imaging data near the time of the Vega 1 spacecraft encounter 1986 March 02–06. An extensive data reduction procedure was necessary to extract accurate H 2O + column densities. This process is outlined in the paper and required such steps as flat fielding, sky removal, continuum subtraction, contamination corrections, absolute flux calibration, and conversion to ion column densities. The spectroscopic data used the H 2O + 0,8,0 red peak close to the nucleus and the sum of the 0,8,0 and 0,6,0 band farther from the nucleus. The filter imaging data employed a filter centered on the H 2O + 0,8,0 band at 6192 Å. A contour plot of the H 2O + column density, 9′ × 9′ in size, gives a quantitative map of the ion distribution near the Vega 1 encounter. Spectroscopic and imaging ion density profiles are also presented for radial cuts along the tail and transverse cuts various distances downstream from the nucleus. A collision zone of radius ∼25,000 km, within which collisions dominate the sweeping effect of the solar wind, is verified. The central part of this collision zone, ∼12,000 km in radius, shows a depletion of the ion density by about a factor of 3, in agreement with Vega 1 spacecraft data. This is probably caused by electron recombination. Our spatial distribution of H 2O + ions agrees well with the 1/ R 2 falloff reported by Vega 1. Our derived H 2O + number densities are about half those reported by Giotto, suggesting that fluorescence efficiencies for H 2O + may be too high by a factor of 2. A production rate of H 2O + ions is calculated from the average velocity and column density integral across the tail. To arrive at reasonable ion outflow velocities from the collision zone, an H 2O + production rate lower than theoretical expectations by about a factor of 10 is required. Much of this discrepancy may be resolved by including the ion destruction in the inner part of the collision zone. Further ameliorating factors are our proposed lower g factor and/or a somewhat lower photodissociation rate. Analysis of the tailward ion flow shows a constant acceleration beyond 130,000 km from the nucleus, with values ∼100 cm sec −2 (∼0.1 g).