Changes in the Physical Environment of the Inner Coma of 67P/Churyumov–Gerasimenko with Decreasing Heliocentric Distance

Abstract : The Wide Angle Camera of the OSIRIS instrument on board the Rosetta spacecraft is equipped with several narrow-band filters that are centered on the emission lines and bands of various fragment species. These are used to determine the evolution of the production and spatial distribution of the gas in the inner coma of comet 67P with time and heliocentric distance, here between 2.6 and 1.3 au pre-perihelion. Our observations indicate that the emission observed in the OH, O i, CN, NH, and NH₂ filters is mostly produced by dissociative electron impact excitation of different parent species. We conclude that CO₂ rather than H₂O is a significant source of the [O i] 630 nm emission. A strong plume-like feature observed in the CN and O i filters is present throughout our observations. This plume is not present in OH emission and indicates a local enhancement of the CO₂/H₂O ratio by as much as a factor of 3. We observed a sudden decrease in intensity levels after 2015 March, which we attribute to decreased electron temperatures in the first few kilometers above the surface of the nucleus.