On the role of solar EUV, photoelectrons, and auroral electrons in the chemistry of C( 1 D ) and the production of C I 1931 Å in the cometary coma: A case for comet 1P/Halley

The observations of C I 1931 Å emission in the ultraviolet spectra of comets show that a large fraction of carbon atoms in the cometary coma are produced in the metastable 1 D excited state. A coupled‐chemistry transport model is employed to examine in detail the various processes of production and loss of the C( 1 D ) atoms in the coma of comet 1P/Halley. Different mechanisms forming CI 1931 Å line are subsequently studied. The impacts of solar UV‐EUV photons, photoelectrons, and auroral electrons of solar wind origin, on the chemistry of C( 1 D ) and the C I 1931 Å emission are evaluated. Dissociation of CO by solar photon and electron impact are the dominant mechanisms of the production of C( 1 D ). Contrary to the findings of the earlier works, the dissociative e ‐CO + recombination is not a significant source of C( 1 D ) in the inner (<10 4 km) coma. Quenching of C( 1 D ) by H 2 O is the main loss mechanism of C( 1 D ) in the inner most coma (<10 3 km), while radiative deexcitation dominates at larger radial distances. The density of C( 1 D ) in the inner coma is found to be controlled predominantly by the CO and H 2 O density distribution. Deep in the coma (<300 km) the photodissociative excitation of CO is the major source of 1931 Å emission, while beyond 500 km the resonant scattering of solar radiation by C( 1 D ) atoms dominates. The contribution of the solar resonance fluorescence to the 1931 Å IUE slit‐averaged intensity is about 80% in the normal and auroral conditions. Results of the model case studies performed to assess the impact of changing C( 1 D ) + H 2 O quenching rate coefficient, C( 1 D ) to C( 3 P ) production ratio, neutral and electron temperatures, and extended CO source are discussed. The modeled intensity of carbon 1931 Å emission is found to be a factor of 3–6 smaller compared to the IUE‐observed intensity.