THE EVOLVING PHOTOMETRIC LIGHTCURVE OF COMET 1P/HALLEY'S COMA DURING THE 1985/86 APPARITION

We present new analyses of the photometric lightcurve of Comet 1P/Halley during its 1985/86 apparition. As part of a worldwide campaign coordinated by the International Halley Watch (IHW), narrowband photometry using standardized filters was obtained with telescopes at 18 observatories. Following submissions to and basic reductions by the Photometry and Polarimetry Network of the IHW, we further reduced the resulting fluxes to production rates and, following temporal binning, created composite lightcurves for each species. These were used to measure how the apparent rotational period (similar to 7.35 days), along with its shape, evolved with time during the apparition. The lightcurve shape systematically varied from double-peaked to triple-peaked and back again every 8-9 weeks, due to Halley's non-principal axis (complex) rotation and the associated component periods. Unexpectedly, we found that a phase shift of one-half cycle also took place during this interval, and therefore the actual beat frequency between the component periods is twice this interval or 16-18 weeks. Preliminary modeling suggests that a single source might produce the entire post-perihelion lightcurve variability and associated evolution, and an additional source. is probably also required to explain additional features before perihelion. The detailed evolution of the apparent period varied in a nonsmooth manner between 7.2 and 7.6 days, likely due to a combination of synodic effects and the interaction of solar illumination with isolated source regions on a body in complex rotation. The need to simultaneously reproduce each of these characteristics will provide very strong additional constraints on Halley's component periods associated with its complex rotation. To assist in these and future analyses, we created a synthetic lightcurve based directly on the measured data and how the lightcurve shape evolved week to week. This synthetic lightcurve was successfully compared to other data sets of Halley and provides a valuable estimate of Halley's activity even when no narrowband photometry measurements were obtained. We unexpectedly discovered a strong correlation of start times of ion tail disconnection events. with minima in the comet's gas production, implying that a decrease in outgassing is another cause of these events.