The Ionospheric Extent of P/halley from Ground-Based Observations during the 1985-86 Apparition

The authors present spatial profiles of H₂O⁺ emission in the inner coma of comet P/Halley on eight dates during the 1985 - 1986 apparition. The profiles are derived from two-dimensional CCD spectrograms taken with the spectrograph slit oriented along the cometary radius vector. The authors calibrate the surface brightness profile of the λ6198 H₂O⁺ emission line in terms of the column density of H₂O⁺ ions. The profile from 14.4 March 1986 is compared with the Giotto Ion Composition Experiment measurement of the shape of the H₂O⁺ space density profile. A primary result is that the ionosphere becomes larger and steeper in the sunward direction with an increasing water production rate.