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

Abstract

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 1D excited state. A coupled‐chemistry transport model is employed to examine in detail the various processes of production and loss of the C(1D) 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(1D) 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(1D). Contrary to the findings of the earlier works, the dissociative e‐CO+ recombination is not a significant source of C(1D) in the inner (<104 km) coma. Quenching of C(1D) by H2O is the main loss mechanism of C(1D) in the inner most coma (<103 km), while radiative deexcitation dominates at larger radial distances. The density of C(1D) in the inner coma is found to be controlled predominantly by the CO and H2O 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(1D) 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(1D) + H2O quenching rate coefficient, C(1D) to C(3P) 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.