Production and emissions of atomic carbon and oxygen in the inner coma of Comet Halley: Role of electron impact

A coupled chemistry-transport model is developed to study the production and loss of neutral atomic carbon and oxygen in the inner coma (less than or equal to 10(4) km) of Comet Halley. Subsequently, the generation of OI 1304 Angstrom and OI 1356 Angstrom and CI 1561 Angstrom and CI 1657 Angstrom emissions is studied. Calculations are performed utilizing solar EUV, utilizing photoelectrons, and assuming the precipitation of high energy ''auroral'' electrons of solar wind origin, the evidence for which has been inferred from many observations, as the sources for the production of these atoms and emissions, including only the gas phase species in the cometary coma. It is revealed that the electron impact dissociation of parent species could be a potentially important source of C and O production and their emissions in the inner cometary coma. Our calculated O density profile is in agreement with that calculated by R. R, Hedges, Jr, (1990, Icarus 83, 410-433). The present study has demonstrated the importance of contributions from sources other than the solar fluorescence for deriving the O and C production rates from their emission line brightness, It is found that the electron impact and solar fluorescence contribute 34 and 66%, respectively, to the OI 1304 Angstrom emission, However, when the auroral electrons are also included, the relative contributions of electron impact and solar fluorescence are 72 and 28%, respectively. In the case of CI 1657 Angstrom the electron impact contribution amounts to 24 (18)% with (without) inclusion of auroral electrons. (C) 1996 Academic Press, Inc.