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(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(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.