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We present new analyses of the photometric lightcurve of Comet 1P/Halleyduring its 1985/86 apparition. As part of a world-wide campaign coordinated bythe International Halley Watch (IHW), narrowband photometry was obtained withtelescopes at 18 observatories. Following submissions to and basic reductionsby the Photometry and Polarimetry Network of the IHW, we computed productionrates and created composite lightcurves for each species. These were used tomeasure how the apparent rotational period (~7.35 day), along with its shape,evolved with time during the apparition. The lightcurve shape systematicallyvaried from double-peaked to triple-peaked and back again every 8-9 weeks, dueto Halley's non-principal axis (complex) rotation and the associated componentperiods. Unexpectedly, we found a phase shift of one-half cycle also took placeduring this interval, and therefore the actual beat frequency between thecomponent periods is twice this interval or 16-18 weeks. Preliminary modelingsuggests that a single source might produce the entire post-perihelionlightcurve variability and associated evolution. The detailed evolution of theapparent period varied in a non-smooth manner between 7.2 and 7.6 day, likelydue to a combination of synodic effects and the interaction of solarillumination with isolated source regions on a body in complex rotation. Theneed to simultaneously reproduce each of these characteristics will providevery strong additional constraints on Halley's component periods associatedwith its complex rotation. To assist in these and future analyses, we created asynthetic lightcurve based directly on the measured data. We unexpectedlydiscovered a strong correlation of ion tail disconnection event start timeswith minima in the comet's gas production, implying that a decrease inoutgassing is another cause of these events.

The Evolving Photometric Lightcurve of Comet 1P/Halley's Coma During the 1985/86 Apparition