Far-ultraviolet spectroscopy of Comet Halley (1986 VI)

Far-ultraviolet spectra were obtained of Comet Halley (1986 III) on 26 February 1986 and on 13 March 1986 using an imaging spectrograph launched aboard a sounding rocket. Most of the far-ultraviolet emissions are excited by fluorescence with solar line emission; therefore, the fluorescence excitation rates have been re-evaluated as functions of heliocentric velocity using high-resolution solar fluxes and the latest values for the oscillator strengths. The spectra are analyzed with particular emphasis placed on identifying the weak features present. The upper limits on the production rates of OCS and CS2 determined from upper limits placed on sulfur emissions are found to be consistent with production rates inferred by other observers. The upper limits on the brightness of the H2 λ1608 emission have been used to place upper limits on the column density of H2 in the coma. These upper limits are ≅3 times the column densities determined by models that assume that H2 is produced solely by photodissociation of H2O. The presence of the O I λ1356 emission in the comet spectra is evidence of the importance of electron impact processes in the inner coma, since this emission is probably excited from the ground state of neutral oxygen by electron impact. The spatial distribution of the CO Fourth Positive Band emissions and the C I λ1561 and C I λ1657 emissions are examined. The shapes of the C emission profiles cannot be adequately described by models which assume that C is produced by photodissociation of CO and other carbon bearing species