Paper detail

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

Alice ultraviolet spectrometer onboard Rosetta space mission observed several spectroscopic emissions emanated from volatile species of comet 67P/Churyumov-Gerasimenko (hear after 67P/C-G) during its entire escorting phase. We have developed a photochemical model for comet 67P/C-G to study the atomic hydrogen (HI 1216, 1025, & 973 Ang), oxygen (OI 1152, 1304, & 1356 Ang), and carbon (CI 1561 & 1657 Ang) 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 with 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 important relevance for the analysis of Alice observed spectra. It is found that in comet 67P/C-G, which is having a neutral gas production rate of about 10$^{27}$ 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 H$_2$O, O$_2$, and CO, abundances, respectively, rather than electron density in the coma of 67P/C-G, when the comet has a gas production rate of $\ge$ 10$^{27}$ s$^{-1}$.