A photochemical model of ultraviolet atomic line emissions in the inner coma of comet 67P/Churyumov-Gerasimenko

Abstract

Alice ultraviolet spectrometer onboard Rosetta space mission observed several spectroscopic emissions emanated from volatile species of comet 67P/Churyumov-Gerasimenko (hearafter 67P/C-G) during its entire escorting phase. The measured emission intensities, when the comet was at around 3 AU pre-perihelion, have been used to derive electron densities in the cometary coma assuming that H I and O I lines are solely produced by electron impact dissociative excitation of cometary parent species (Feldman et al., 2015). We have developed a photochemical model for comet 67P/C-G to study the atomic hydrogen (H I 1216, 1025, & 973 Å), oxygen (O I 1152, 1304, & 1356 Å), and carbon (C I 1561 & 1657 Å) line emissions by accounting for major production pathways. The developed model has been used to calculate the emission intensities of these lines as a function of nucleocentric projected distance and also along the nadir view by varying the input parameters, viz., neutral abundances and cross sections. We have quantified the percentage contributions of photon and electron impact dissociative excitation processes to the total intensity of the emission lines, which has an important relevance for the analysis of Alice observed spectra. It is found that in comet 67P/C-G, with a neutral gas production rate of about 1027 s−1 when it was at 1.56 AU from the Sun, photodissociative excitation processes are more significant compared to electron impact reactions in determining the atomic emission intensities. Based on our model calculations, we suggest that the observed atomic hydrogen, oxygen, and carbon emission intensities can be used to derive H2O, O2, and CO, abundances, respectively, rather than electron density in the coma of 67P/C-G, when the comet has a gas production rate of 1027 s−1 or more.